Chirality (chemistry)

Chirality (chemistry)

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Encyclopedia
A chiral molecule ˈ is a type of molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 that lacks an internal plane of symmetry
Molecular symmetry
Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...

 and thus has a non-superimposable mirror image
Mirror image
A mirror image is a reflected duplication of an object that appears identical but reversed. As an optical effect it results from reflection off of substances such as a mirror or water. It is also a concept in geometry and can be used as a conceptualization process for 3-D structures...

. The feature that is most often the cause of chirality
Chirality
Chirality is a property of asymmetry important in several branches of science. It may refer to:* Chirality , a property of molecules having a non-superimposable mirror image...

 in molecules is the presence of an asymmetric carbon atom
Asymmetric carbon
An asymmetric carbon atom is a carbon atom that is attached to four different types of atom or four different groups of atoms. Knowing the number of asymmetric carbon atoms, one can calculate the maximum possible number of stereoisomers for any given molecule as follows:As an example, malic acid...

.

The term chiral in general is used to describe an object that is non-superimposable on its mirror image.
Achiral (not chiral) objects are objects that are identical to their mirror image. Human hand
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...

s are perhaps the most universally recognized example of chirality: The left hand is a non-superimposable mirror image of the right hand; no matter how the two hands are oriented, it is impossible for all the major features of both hands to coincide. This difference in symmetry becomes obvious if someone attempts to shake the right hand of a person using his left hand, or if a left-handed glove is placed on a right hand. The term chirality is derived from the Greek word for hand, χειρ (cheir). It is a mathematical approach to the concept of "handedness".

In chemistry, chirality usually refers to molecules. Two mirror images of a chiral molecule are called enantiomer
Enantiomer
In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...

s or optical isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

s. Pairs of enantiomers are often designated as "right-" and "left-handed".

Molecular chirality is of interest because of its application to stereochemistry
Stereochemistry
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules....

 in inorganic chemistry
Inorganic chemistry
Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry...

, organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

, physical chemistry
Physical chemistry
Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts...

, biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...

, and supramolecular chemistry
Supramolecular chemistry
Supramolecular chemistry refers to the area of chemistry beyond the molecules and focuses on the chemical systems made up of a discrete number of assembled molecular subunits or components...

.

History


The term optical activity is derived from the interaction of chiral materials with polarized light. A solution of the (−)-form of an optical isomer rotates
Levorotation and dextrorotation
Dextrorotation and levorotation refer, respectively, to the properties of rotating plane polarized light clockwise or counterclockwise , seen by an observer whom the light is approaching...

 the plane of polarization of a beam of polarized light in a counterclockwise direction (levorotatory), vice-versa for the (+) (dextrorotatory) optical isomer. The property was first observed by Jean-Baptiste Biot
Jean-Baptiste Biot
Jean-Baptiste Biot was a French physicist, astronomer, and mathematician who established the reality of meteorites, made an early balloon flight, and studied the polarization of light.- Biography :...

 in 1815, and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals. Louis Pasteur
Louis Pasteur
Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

 deduced in 1848 that this phenomenon has a molecular basis. Artificial composite materials displaying the analog of optical activity but in the microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 region were introduced by J.C. Bose in 1898, and gained considerable attention from the mid-1980s. The term chirality itself was coined by Lord Kelvin in 1873.

The word "racemic
Racemic
In chemistry, a racemic mixture, or racemate , is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was "racemic acid", which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.- Nomenclature :A...

" (mix of both chiralities) is derived from the Latin word "racemus" for "bunch of grapes"; the term having its origins in the work of Louis Pasteur who isolated racemic tartaric acid
Tartaric acid
Tartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds; is commonly combined with baking soda to function as a leavening agent in recipes, and is one of the main acids found in wine. It is added to other foods to...

 from wine.

Symmetry


The symmetry
Symmetry
Symmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...

 of a molecule (or any other object) determines whether it is chiral. A molecule is achiral (not chiral) when an improper rotation
Improper rotation
In 3D geometry, an improper rotation, also called rotoreflection or rotary reflection is, depending on context, a linear transformation or affine transformation which is the combination of a rotation about an axis and a reflection in a plane perpendicular to the axis.Equivalently it is the...

, that is a combination of a rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

 and a reflection in a plane, perpendicular to the axis of rotation, results in the same molecule - see chirality (mathematics)
Chirality (mathematics)
In geometry, a figure is chiral if it is not identical to its mirror image, or, more precisely, if it cannot be mapped to its mirror image by rotations and translations alone. For example, a right shoe is different from a left shoe, and clockwise is different from counterclockwise.A chiral object...

. An equivalent definition is that a chiral molecule lacks a plane of symmetry. For tetrahedral
Tetrahedral molecular geometry
In a tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1 ≈ 109.5° when all four substituents are the same, as in CH4. This molecular geometry is common throughout the first...

 molecules, the molecule is chiral if all four substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

s are different.

A chiral molecule is not necessarily asymmetric (devoid of any symmetry element
Symmetry element
A symmetry element is a point of reference about which symmetry operations can take place. In particular, symmetry elements can be centers of inversion, axes of rotation and mirror planes.-See also:* Symmetry* Group theory* Crystallography...

), as it can have, for example, rotational symmetry
Rotational symmetry
Generally speaking, an object with rotational symmetry is an object that looks the same after a certain amount of rotation. An object may have more than one rotational symmetry; for instance, if reflections or turning it over are not counted, the triskelion appearing on the Isle of Man's flag has...

.

By configuration: R- and S-


For chemists, the R / S system is the most important nomenclature system for denoting enantiomers, which does not involve a reference molecule such as glyceraldehyde
Glyceraldehyde
Glyceraldehyde is a triose monosaccharide with chemical formula C3H6O3. It is the simplest of all common aldoses. It is a sweet, colorless, crystalline solid that is an intermediate compound in carbohydrate metabolism...

. It labels each chiral center R or S according to a system by which its substituents are each assigned a priority, according to the Cahn–Ingold–Prelog priority rules (CIP), based on atomic number. If the center is oriented so that the lowest-priority of the four is pointed away from a viewer, the viewer will then see two possibilities: If the priority of the remaining three substituents decreases in clockwise direction, it is labeled R (for Rectus, Latin for right), if it decreases in counterclockwise direction, it is S (for Sinister, Latin for left).

This system labels each chiral center in a molecule (and also has an extension to chiral molecules not involving chiral centers). Thus, it has greater generality than the system, and can label, for example, an (R,R) isomer versus an (R,S) — diastereomer
Diastereomer
Diastereomers are stereoisomers that are not enantiomers.Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more of the equivalent stereocenters and are not mirror images of each other.When two diastereoisomers differ from each other at...

s.

The R / S system has no fixed relation to the (+)/(−) system. An R isomer can be either dextrorotatory or levorotatory, depending on its exact substituents.

The R / S system also has no fixed relation to the system. For example, the side-chain one of serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...

 contains a hydroxyl group, -OH. If a thiol group, -SH, were swapped in for it, the labeling would, by its definition, not be affected by the substitution. But this substitution would invert the molecule's R / S labeling, because the CIP priority of CH2OH is lower than that for CO2H but the CIP priority of CH2SH is higher than that for CO2H.

For this reason, the system remains in common use in certain areas of biochemistry, such as amino acid and carbohydrate chemistry, because it is convenient to have the same chiral label for all of the commonly occurring structures of a given type of structure in higher organisms. In the system, they are nearly all consistent - naturally occurring amino acids are all , while naturally occurring carbohydrates are nearly all . In the R / S system, they are mostly S, but there are some common exceptions.

By optical activity: (+)- and (−)-


An enantiomer can be named by the direction in which it rotates the plane of polarized light. If it rotates the light clockwise (as seen by a viewer towards whom the light is traveling), that enantiomer is labeled (+). Its mirror-image is labeled (−). The (+) and (−) isomers have also been termed d- and l-, respectively (for dextrorotatory and levorotatory). Naming with d- and l- is easy to confuse with - and - labeling and is therefore strongly discouraged by IUPAC
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...

.

By configuration: - and -


An optical isomer can be named by the spatial configuration of its atoms. The / system does this by relating the molecule to glyceraldehyde
Glyceraldehyde
Glyceraldehyde is a triose monosaccharide with chemical formula C3H6O3. It is the simplest of all common aldoses. It is a sweet, colorless, crystalline solid that is an intermediate compound in carbohydrate metabolism...

. Glyceraldehyde is chiral itself, and its two isomers are labeled and (typically typeset in
{{redirect|L-form|the bacterial strains|L-form bacteria}}
A chiral molecule ˈ is a type of
molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 that lacks an internal plane of symmetry
Molecular symmetry
Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...

 and thus has a non-superimposable mirror image
Mirror image
A mirror image is a reflected duplication of an object that appears identical but reversed. As an optical effect it results from reflection off of substances such as a mirror or water. It is also a concept in geometry and can be used as a conceptualization process for 3-D structures...

. The feature that is most often the cause of chirality
Chirality
Chirality is a property of asymmetry important in several branches of science. It may refer to:* Chirality , a property of molecules having a non-superimposable mirror image...

 in molecules is the presence of an asymmetric carbon atom
Asymmetric carbon
An asymmetric carbon atom is a carbon atom that is attached to four different types of atom or four different groups of atoms. Knowing the number of asymmetric carbon atoms, one can calculate the maximum possible number of stereoisomers for any given molecule as follows:As an example, malic acid...

.

The term chiral in general is used to describe an object that is non-superimposable on its mirror image.
Achiral (not chiral) objects are objects that are identical to their mirror image. Human hand
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...

s are perhaps the most universally recognized example of chirality: The left hand is a non-superimposable mirror image of the right hand; no matter how the two hands are oriented, it is impossible for all the major features of both hands to coincide. This difference in symmetry becomes obvious if someone attempts to shake the right hand of a person using his left hand, or if a left-handed glove is placed on a right hand. The term chirality is derived from the Greek word for hand, χειρ (cheir). It is a mathematical approach to the concept of "handedness".

In chemistry, chirality usually refers to molecules. Two mirror images of a chiral molecule are called enantiomer
Enantiomer
In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...

s or optical isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

s. Pairs of enantiomers are often designated as "right-" and "left-handed".

Molecular chirality is of interest because of its application to stereochemistry
Stereochemistry
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules....

 in inorganic chemistry
Inorganic chemistry
Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry...

, organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

, physical chemistry
Physical chemistry
Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts...

, biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...

, and supramolecular chemistry
Supramolecular chemistry
Supramolecular chemistry refers to the area of chemistry beyond the molecules and focuses on the chemical systems made up of a discrete number of assembled molecular subunits or components...

.

History


The term optical activity is derived from the interaction of chiral materials with polarized light. A solution of the (−)-form of an optical isomer rotates
Levorotation and dextrorotation
Dextrorotation and levorotation refer, respectively, to the properties of rotating plane polarized light clockwise or counterclockwise , seen by an observer whom the light is approaching...

 the plane of polarization of a beam of polarized light in a counterclockwise direction (levorotatory), vice-versa for the (+) (dextrorotatory) optical isomer. The property was first observed by Jean-Baptiste Biot
Jean-Baptiste Biot
Jean-Baptiste Biot was a French physicist, astronomer, and mathematician who established the reality of meteorites, made an early balloon flight, and studied the polarization of light.- Biography :...

 in 1815, and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals. Louis Pasteur
Louis Pasteur
Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

 deduced in 1848 that this phenomenon has a molecular basis. Artificial composite materials displaying the analog of optical activity but in the microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 region were introduced by J.C. Bose in 1898, and gained considerable attention from the mid-1980s. The term chirality itself was coined by Lord Kelvin in 1873.

The word "racemic
Racemic
In chemistry, a racemic mixture, or racemate , is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was "racemic acid", which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.- Nomenclature :A...

" (mix of both chiralities) is derived from the Latin word "racemus" for "bunch of grapes"; the term having its origins in the work of Louis Pasteur who isolated racemic tartaric acid
Tartaric acid
Tartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds; is commonly combined with baking soda to function as a leavening agent in recipes, and is one of the main acids found in wine. It is added to other foods to...

 from wine.

Symmetry


The symmetry
Symmetry
Symmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...

 of a molecule (or any other object) determines whether it is chiral. A molecule is achiral (not chiral) when an improper rotation
Improper rotation
In 3D geometry, an improper rotation, also called rotoreflection or rotary reflection is, depending on context, a linear transformation or affine transformation which is the combination of a rotation about an axis and a reflection in a plane perpendicular to the axis.Equivalently it is the...

, that is a combination of a rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

 and a reflection in a plane, perpendicular to the axis of rotation, results in the same molecule - see chirality (mathematics)
Chirality (mathematics)
In geometry, a figure is chiral if it is not identical to its mirror image, or, more precisely, if it cannot be mapped to its mirror image by rotations and translations alone. For example, a right shoe is different from a left shoe, and clockwise is different from counterclockwise.A chiral object...

. An equivalent definition is that a chiral molecule lacks a plane of symmetry. For tetrahedral
Tetrahedral molecular geometry
In a tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1 ≈ 109.5° when all four substituents are the same, as in CH4. This molecular geometry is common throughout the first...

 molecules, the molecule is chiral if all four substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

s are different.

A chiral molecule is not necessarily asymmetric (devoid of any symmetry element
Symmetry element
A symmetry element is a point of reference about which symmetry operations can take place. In particular, symmetry elements can be centers of inversion, axes of rotation and mirror planes.-See also:* Symmetry* Group theory* Crystallography...

), as it can have, for example, rotational symmetry
Rotational symmetry
Generally speaking, an object with rotational symmetry is an object that looks the same after a certain amount of rotation. An object may have more than one rotational symmetry; for instance, if reflections or turning it over are not counted, the triskelion appearing on the Isle of Man's flag has...

.

By configuration: R- and S-


For chemists, the R / S system is the most important nomenclature system for denoting enantiomers, which does not involve a reference molecule such as glyceraldehyde
Glyceraldehyde
Glyceraldehyde is a triose monosaccharide with chemical formula C3H6O3. It is the simplest of all common aldoses. It is a sweet, colorless, crystalline solid that is an intermediate compound in carbohydrate metabolism...

. It labels each chiral center R or S according to a system by which its substituents are each assigned a priority, according to the Cahn–Ingold–Prelog priority rules (CIP), based on atomic number. If the center is oriented so that the lowest-priority of the four is pointed away from a viewer, the viewer will then see two possibilities: If the priority of the remaining three substituents decreases in clockwise direction, it is labeled R (for Rectus, Latin for right), if it decreases in counterclockwise direction, it is S (for Sinister, Latin for left).

This system labels each chiral center in a molecule (and also has an extension to chiral molecules not involving chiral centers). Thus, it has greater generality than the {{smallcaps|d/l}} system, and can label, for example, an (R,R) isomer versus an (R,S) — diastereomer
Diastereomer
Diastereomers are stereoisomers that are not enantiomers.Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more of the equivalent stereocenters and are not mirror images of each other.When two diastereoisomers differ from each other at...

s.

The R / S system has no fixed relation to the (+)/(−) system. An R isomer can be either dextrorotatory or levorotatory, depending on its exact substituents.

The R / S system also has no fixed relation to the {{smallcaps|d/l}} system. For example, the side-chain one of serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...

 contains a hydroxyl group, -OH. If a thiol group, -SH, were swapped in for it, the {{smallcaps|d/l}} labeling would, by its definition, not be affected by the substitution. But this substitution would invert the molecule's R / S labeling, because the CIP priority of CH2OH is lower than that for CO2H but the CIP priority of CH2SH is higher than that for CO2H.

For this reason, the {{smallcaps|d/l}} system remains in common use in certain areas of biochemistry, such as amino acid and carbohydrate chemistry, because it is convenient to have the same chiral label for all of the commonly occurring structures of a given type of structure in higher organisms. In the {{smallcaps|d/l}} system, they are nearly all consistent - naturally occurring amino acids are all {{smallcaps|l}}, while naturally occurring carbohydrates are nearly all {{smallcaps|d}}. In the R / S system, they are mostly S, but there are some common exceptions.

By optical activity: (+)- and (−)-


An enantiomer can be named by the direction in which it rotates the plane of polarized light. If it rotates the light clockwise (as seen by a viewer towards whom the light is traveling), that enantiomer is labeled (+). Its mirror-image is labeled (−). The (+) and (−) isomers have also been termed d- and l-, respectively (for dextrorotatory and levorotatory). Naming with d- and l- is easy to confuse with {{smallcaps|d}}- and {{smallcaps|l}}- labeling and is therefore strongly discouraged by IUPAC
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...

.

By configuration: {{smallcaps|d}}- and {{smallcaps|l}}-


An optical isomer can be named by the spatial configuration of its atoms. The {{smallcaps|d}}/{{smallcaps|l}} system does this by relating the molecule to glyceraldehyde
Glyceraldehyde
Glyceraldehyde is a triose monosaccharide with chemical formula C3H6O3. It is the simplest of all common aldoses. It is a sweet, colorless, crystalline solid that is an intermediate compound in carbohydrate metabolism...

. Glyceraldehyde is chiral itself, and its two isomers are labeled {{smallcaps|d}} and {{smallcaps|l}} (typically typeset in
{{redirect|L-form|the bacterial strains|L-form bacteria}}
A chiral molecule ˈ is a type of
molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

 that lacks an internal plane of symmetry
Molecular symmetry
Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...

 and thus has a non-superimposable mirror image
Mirror image
A mirror image is a reflected duplication of an object that appears identical but reversed. As an optical effect it results from reflection off of substances such as a mirror or water. It is also a concept in geometry and can be used as a conceptualization process for 3-D structures...

. The feature that is most often the cause of chirality
Chirality
Chirality is a property of asymmetry important in several branches of science. It may refer to:* Chirality , a property of molecules having a non-superimposable mirror image...

 in molecules is the presence of an asymmetric carbon atom
Asymmetric carbon
An asymmetric carbon atom is a carbon atom that is attached to four different types of atom or four different groups of atoms. Knowing the number of asymmetric carbon atoms, one can calculate the maximum possible number of stereoisomers for any given molecule as follows:As an example, malic acid...

.

The term chiral in general is used to describe an object that is non-superimposable on its mirror image.
Achiral (not chiral) objects are objects that are identical to their mirror image. Human hand
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...

s are perhaps the most universally recognized example of chirality: The left hand is a non-superimposable mirror image of the right hand; no matter how the two hands are oriented, it is impossible for all the major features of both hands to coincide. This difference in symmetry becomes obvious if someone attempts to shake the right hand of a person using his left hand, or if a left-handed glove is placed on a right hand. The term chirality is derived from the Greek word for hand, χειρ (cheir). It is a mathematical approach to the concept of "handedness".

In chemistry, chirality usually refers to molecules. Two mirror images of a chiral molecule are called enantiomer
Enantiomer
In chemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are non-superposable , much as one's left and right hands are the same except for opposite orientation. It can be clearly understood if you try to place your hands one over the other without...

s or optical isomer
Isomer
In chemistry, isomers are compounds with the same molecular formula but different structural formulas. Isomers do not necessarily share similar properties, unless they also have the same functional groups. There are many different classes of isomers, like stereoisomers, enantiomers, geometrical...

s. Pairs of enantiomers are often designated as "right-" and "left-handed".

Molecular chirality is of interest because of its application to stereochemistry
Stereochemistry
Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules....

 in inorganic chemistry
Inorganic chemistry
Inorganic chemistry is the branch of chemistry concerned with the properties and behavior of inorganic compounds. This field covers all chemical compounds except the myriad organic compounds , which are the subjects of organic chemistry...

, organic chemistry
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

, physical chemistry
Physical chemistry
Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of physical laws and concepts...

, biochemistry
Biochemistry
Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...

, and supramolecular chemistry
Supramolecular chemistry
Supramolecular chemistry refers to the area of chemistry beyond the molecules and focuses on the chemical systems made up of a discrete number of assembled molecular subunits or components...

.

History


The term optical activity is derived from the interaction of chiral materials with polarized light. A solution of the (−)-form of an optical isomer rotates
Levorotation and dextrorotation
Dextrorotation and levorotation refer, respectively, to the properties of rotating plane polarized light clockwise or counterclockwise , seen by an observer whom the light is approaching...

 the plane of polarization of a beam of polarized light in a counterclockwise direction (levorotatory), vice-versa for the (+) (dextrorotatory) optical isomer. The property was first observed by Jean-Baptiste Biot
Jean-Baptiste Biot
Jean-Baptiste Biot was a French physicist, astronomer, and mathematician who established the reality of meteorites, made an early balloon flight, and studied the polarization of light.- Biography :...

 in 1815, and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals. Louis Pasteur
Louis Pasteur
Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

 deduced in 1848 that this phenomenon has a molecular basis. Artificial composite materials displaying the analog of optical activity but in the microwave
Microwave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...

 region were introduced by J.C. Bose in 1898, and gained considerable attention from the mid-1980s. The term chirality itself was coined by Lord Kelvin in 1873.

The word "racemic
Racemic
In chemistry, a racemic mixture, or racemate , is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was "racemic acid", which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.- Nomenclature :A...

" (mix of both chiralities) is derived from the Latin word "racemus" for "bunch of grapes"; the term having its origins in the work of Louis Pasteur who isolated racemic tartaric acid
Tartaric acid
Tartaric acid is a white crystalline diprotic organic acid. It occurs naturally in many plants, particularly grapes, bananas, and tamarinds; is commonly combined with baking soda to function as a leavening agent in recipes, and is one of the main acids found in wine. It is added to other foods to...

 from wine.

Symmetry


The symmetry
Symmetry
Symmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...

 of a molecule (or any other object) determines whether it is chiral. A molecule is achiral (not chiral) when an improper rotation
Improper rotation
In 3D geometry, an improper rotation, also called rotoreflection or rotary reflection is, depending on context, a linear transformation or affine transformation which is the combination of a rotation about an axis and a reflection in a plane perpendicular to the axis.Equivalently it is the...

, that is a combination of a rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

 and a reflection in a plane, perpendicular to the axis of rotation, results in the same molecule - see chirality (mathematics)
Chirality (mathematics)
In geometry, a figure is chiral if it is not identical to its mirror image, or, more precisely, if it cannot be mapped to its mirror image by rotations and translations alone. For example, a right shoe is different from a left shoe, and clockwise is different from counterclockwise.A chiral object...

. An equivalent definition is that a chiral molecule lacks a plane of symmetry. For tetrahedral
Tetrahedral molecular geometry
In a tetrahedral molecular geometry a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1 ≈ 109.5° when all four substituents are the same, as in CH4. This molecular geometry is common throughout the first...

 molecules, the molecule is chiral if all four substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...

s are different.

A chiral molecule is not necessarily asymmetric (devoid of any symmetry element
Symmetry element
A symmetry element is a point of reference about which symmetry operations can take place. In particular, symmetry elements can be centers of inversion, axes of rotation and mirror planes.-See also:* Symmetry* Group theory* Crystallography...

), as it can have, for example, rotational symmetry
Rotational symmetry
Generally speaking, an object with rotational symmetry is an object that looks the same after a certain amount of rotation. An object may have more than one rotational symmetry; for instance, if reflections or turning it over are not counted, the triskelion appearing on the Isle of Man's flag has...

.

By configuration: R- and S-


For chemists, the R / S system is the most important nomenclature system for denoting enantiomers, which does not involve a reference molecule such as glyceraldehyde
Glyceraldehyde
Glyceraldehyde is a triose monosaccharide with chemical formula C3H6O3. It is the simplest of all common aldoses. It is a sweet, colorless, crystalline solid that is an intermediate compound in carbohydrate metabolism...

. It labels each chiral center R or S according to a system by which its substituents are each assigned a priority, according to the Cahn–Ingold–Prelog priority rules (CIP), based on atomic number. If the center is oriented so that the lowest-priority of the four is pointed away from a viewer, the viewer will then see two possibilities: If the priority of the remaining three substituents decreases in clockwise direction, it is labeled R (for Rectus, Latin for right), if it decreases in counterclockwise direction, it is S (for Sinister, Latin for left).

This system labels each chiral center in a molecule (and also has an extension to chiral molecules not involving chiral centers). Thus, it has greater generality than the {{smallcaps|d/l}} system, and can label, for example, an (R,R) isomer versus an (R,S) — diastereomer
Diastereomer
Diastereomers are stereoisomers that are not enantiomers.Diastereomerism occurs when two or more stereoisomers of a compound have different configurations at one or more of the equivalent stereocenters and are not mirror images of each other.When two diastereoisomers differ from each other at...

s.

The R / S system has no fixed relation to the (+)/(−) system. An R isomer can be either dextrorotatory or levorotatory, depending on its exact substituents.

The R / S system also has no fixed relation to the {{smallcaps|d/l}} system. For example, the side-chain one of serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...

 contains a hydroxyl group, -OH. If a thiol group, -SH, were swapped in for it, the {{smallcaps|d/l}} labeling would, by its definition, not be affected by the substitution. But this substitution would invert the molecule's R / S labeling, because the CIP priority of CH2OH is lower than that for CO2H but the CIP priority of CH2SH is higher than that for CO2H.

For this reason, the {{smallcaps|d/l}} system remains in common use in certain areas of biochemistry, such as amino acid and carbohydrate chemistry, because it is convenient to have the same chiral label for all of the commonly occurring structures of a given type of structure in higher organisms. In the {{smallcaps|d/l}} system, they are nearly all consistent - naturally occurring amino acids are all {{smallcaps|l}}, while naturally occurring carbohydrates are nearly all {{smallcaps|d}}. In the R / S system, they are mostly S, but there are some common exceptions.

By optical activity: (+)- and (−)-


An enantiomer can be named by the direction in which it rotates the plane of polarized light. If it rotates the light clockwise (as seen by a viewer towards whom the light is traveling), that enantiomer is labeled (+). Its mirror-image is labeled (−). The (+) and (−) isomers have also been termed d- and l-, respectively (for dextrorotatory and levorotatory). Naming with d- and l- is easy to confuse with {{smallcaps|d}}- and {{smallcaps|l}}- labeling and is therefore strongly discouraged by IUPAC
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...

.

By configuration: {{smallcaps|d}}- and {{smallcaps|l}}-


An optical isomer can be named by the spatial configuration of its atoms. The {{smallcaps|d}}/{{smallcaps|l}} system does this by relating the molecule to glyceraldehyde
Glyceraldehyde
Glyceraldehyde is a triose monosaccharide with chemical formula C3H6O3. It is the simplest of all common aldoses. It is a sweet, colorless, crystalline solid that is an intermediate compound in carbohydrate metabolism...

. Glyceraldehyde is chiral itself, and its two isomers are labeled {{smallcaps|d}} and {{smallcaps|l}} (typically typeset in {{smallcaps
Small caps
In typography, small capitals are uppercase characters set at the same height and weight as surrounding lowercase letters or text figures...

 in published work). Certain chemical manipulations can be performed on glyceraldehyde without affecting its configuration, and its historical use for this purpose (possibly combined with its convenience as one of the smallest commonly used chiral molecules) has resulted in its use for nomenclature. In this system, compounds are named by analogy to glyceraldehyde, which, in general, produces unambiguous designations, but is easiest to see in the small biomolecules similar to glyceraldehyde. One example is the amino acid alanine, which has two optical isomers, and they are labeled according to which isomer of glyceraldehyde they come from. On the other hand, glycine, the amino acid derived from glyceraldehyde, has no optical activity, as it is not chiral (achiral). Alanine, however, is chiral.

The {{smallcaps|d}}/{{smallcaps|l}} labeling is unrelated to (+)/(−); it does not indicate which enantiomer is dextrorotatory and which is levorotatory. Rather, it says that the compound's stereochemistry is related to that of the dextrorotatory or levorotatory enantiomer of glyceraldehyde—the dextrorotatory isomer of glyceraldehyde is, in fact, the {{smallcaps|d-}} isomer. Nine of the nineteen {{smallcaps|l}}-amino acids commonly found in proteins are dextrorotatory (at a wavelength of 589 nm), and {{smallcaps|d}}-fructose is also referred to as levulose because it is levorotatory.

A rule of thumb for determining the {{smallcaps|d}}/{{smallcaps|l}} isomeric form of an amino acid is the "CORN" rule. The groups:
COOH, R, NH2 and H (where R is a variant carbon chain)


are arranged around the chiral center carbon atom. Starting with the hydrogen atom away from the viewer, if these groups are arranged clockwise around the carbon atom, then it is the {{smallcaps|d}}-form. If counter-clockwise, it is the {{smallcaps|l}}-form.

Nomenclature

  • Any non-racemic
    Racemic
    In chemistry, a racemic mixture, or racemate , is one that has equal amounts of left- and right-handed enantiomers of a chiral molecule. The first known racemic mixture was "racemic acid", which Louis Pasteur found to be a mixture of the two enantiomeric isomers of tartaric acid.- Nomenclature :A...

     chiral substance is called scalemic.
  • A chiral substance is enantiopure or homochiral when only one of two possible enantiomers is present.
  • A chiral substance is enantioenriched or heterochiral when an excess of one enantiomer is present but not to the exclusion of the other.
  • Enantiomeric excess
    Enantiomeric excess
    The enantiomeric excess of a substance is a measure of how pure it is. In this case, the impurity is the undesired enantiomer .-Definition:...

     or ee is a measure for how much of one enantiomer is present compared to the other. For example, in a sample with 40% ee in R, the remaining 60% is racemic with 30% of R and 30% of S, so that the total amount of R is 70%.

Stereogenic centers


In general, chiral molecules have point chirality at a single stereogenic atom, which has four different substituents. The two enantiomers of such compounds are said to have different absolute configurations at this center. This center is thus stereogenic (i.e., a grouping within a molecular entity that may be considered a focus of stereoisomerism).

Normally when an atom has four different substituents, it is chiral. However in rare cases, two of the ligands differ from each other by being mirror images of each other. When this happens, the mirror image of the molecule is identical to the original, and the molecule is achiral. This is called pseudochirality.

A molecule can have multiple chiral centers without being chiral overall if there is a symmetry between the two (or more) chiral centers themselves. Such a molecule is called a meso compound
Meso compound
A meso compound or meso isomer is a non-optically active member of a set of stereoisomers, at least two of which are optically active. This means that despite containing two or more stereocenters it is not chiral. A meso compound is superimposable on its mirror image, and it does not produce a ""...

.

It is also possible for a molecule to be chiral without having actual point chirality. Common examples include 1,1'-bi-2-naphthol
1,1'-Bi-2-naphthol
1,1'-Bi-2-naphthol is an organic compound that is often used as a ligand for transition-metal catalysed asymmetric synthesis. BINOL has axial chirality and the two enantiomers can be readily separated and are stable toward racemisation. The specific rotation of the two enantiomers is +/- 35.5°...

 (BINOL) and 1,3-dichloro-allene, which have axial chirality
Axial chirality
Axial chirality is a special case of chirality in which a molecule does not possess a stereogenic center but an axis of chirality – an axis about which a set of substituents is held in a spatial arrangement that is not superposable on its mirror image...

, (E)-cyclooctene
Cyclooctene
Cyclooctene is a cycloalkene with an eight-membered ring. It is notable because it is the smallest cycloalkene that can exist as either the cis- or trans-isomer with the cis-isomer more common...

, which has planar chirality
Planar chirality
Planar chirality is the special case of chirality for two dimensions.Most fundamentally, planar chirality is a mathematical term, finding use in chemistry, physics and related physical sciences, for example, in astronomy, optics and metamaterials...

, and certain calixarenes and fullerenes, which have inherent chirality
Inherent chirality
In chemistry, inherent chirality refers to molecules and complexes whose lack of symmetry does not originate from a classic stereogenic element, but is rather the consequence of the presence of a curvature in a structure that would be devoid of symmetry axes in any bidimensional representation.The...

.

It is important to keep in mind that molecules have considerable flexibility and thus, depending on the medium, may adopt a variety of different conformations. These various conformations are themselves almost always chiral. When assessing chirality, a time-averaged structure is considered and for routine compounds, one should refer to the most symmetric possible conformation.

When the optical rotation
Levorotation and dextrorotation
Dextrorotation and levorotation refer, respectively, to the properties of rotating plane polarized light clockwise or counterclockwise , seen by an observer whom the light is approaching...

 for an enantiomer is too low for practical measurement, it is said to exhibit cryptochirality
Cryptochirality
Cryptochirality in stereochemistry is a special case of chirality where due to the electronic properties of the chiral molecule its specific rotation is non-measurable. The term was introduced by K...

.

Even isotopic differences must be considered when examining chirality. Replacing one of the two 1H atoms at the CH2 position of benzyl alcohol
Benzyl alcohol
Benzyl alcohol is an organic compound with the formula C6H5CH2OH. The benzyl group is often abbreviated "Bn", thus benzyl alcohol is denoted as BnOH. Benzyl alcohol is a colorless liquid with a mild pleasant aromatic odor. It is a useful solvent due to its polarity, low toxicity, and low vapor...

 with a deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...

 (2H) makes that carbon a stereocenter. The resulting benzyl-α-d alcohol exists as two distinct enantiomers, which can be assigned by the usual stereochemical naming conventions. The S enantiomer has [α]D = +0.715°.

The identity of the stereogenic atom


The stereogenic atom in chiral molecules is usually carbon, as in many biological molecules. However, it may also be a metal atom (as in many chiral coordination compounds), nitrogen, phosphorus, or sulfur.
The chiral atom Carbon Nitrogen Phosphorus Sulfur Metal (type of metal)
Pseudochiral 1-butene
1-Butene
1-Butene is an organic chemical compound, linear alpha-olefin , and one of the isomers of butene. The formula is .-Stability:1-Butene is stable in itself but polymerizes exothermically. It is highly flammable and readily forms explosive mixtures with air...

 
Methylamine
Methylamine
Methylamine is the organic compound with a formula of CH3NH2. This colourless gas is a derivative of ammonia, but with one H atom replaced by a methyl group. It is the simplest primary amine. It is sold as a solution in methanol, ethanol, THF, and water, or as the anhydrous gas in pressurized...

 
VG
VG (nerve agent)
VG is a "V-series" nerve agent chemically similar to the better-known VX nerve agent. Tetram is the common Russian name for the substance...

, phosphorous acid
Phosphorous acid
Phosphorous acid is the compound described by the formula H3PO3. This acid is diprotic , not triprotic as might be suggested by this formula. Phosphorous acid is as an intermediate in the preparation of other phosphorus compounds.-Nomenclature and tautomerism:H3PO3 is more clearly described with...

 
- N/A
1 stereogenic center Serine
Serine
Serine is an amino acid with the formula HO2CCHCH2OH. It is one of the proteinogenic amino acids. By virtue of the hydroxyl group, serine is classified as a polar amino acid.-Occurrence and biosynthesis:...

, glyceraldehyde
Glyceraldehyde
Glyceraldehyde is a triose monosaccharide with chemical formula C3H6O3. It is the simplest of all common aldoses. It is a sweet, colorless, crystalline solid that is an intermediate compound in carbohydrate metabolism...

 
Sarin
Sarin
Sarin, or GB, is an organophosphorus compound with the formula [2CHO]CH3PF. It is a colorless, odorless liquid, which is used as a chemical weapon. It has been classified as a weapon of mass destruction in UN Resolution 687...

, VX
VX (nerve agent)
VX, IUPAC name O-ethyl S-[2-ethyl] methylphosphonothioate, is an extremely toxic substance whose only application is in chemical warfare as a nerve agent. As a chemical weapon, it is classified as a weapon of mass destruction by the United Nations in UN Resolution 687...

 
Esomeprazole
Esomeprazole
Esomeprazole is a proton pump inhibitor developed and marketed by AstraZeneca which is used in the treatment of dyspepsia, peptic ulcer disease , gastroesophageal reflux disease and Zollinger-Ellison syndrome...

 
Tris(bipyridine)ruthenium(II)
Tris(bipyridine)ruthenium(II) chloride
Trisruthenium dichloride is the coordination compound with the formula [Ru3]Cl2. This red crystalline salt is obtained as the hexahydrate, although all of the properties of interest are in the cation [Ru3]2+, which has received much attention because of its distinctive optical properties...

 (ruthenium), cis-Dichlorobis(ethylenediamine)cobalt(III)
Cis-Dichlorobis(ethylenediamine)cobalt(III) chloride
cis-Dichlorobiscobalt chloride is an organic compound with formula [CoCl22]Cl . It is a violet diamagnetic solid that is soluble in water...

 (cobalt)
2 stereogenic centers Threonine
Threonine
Threonine is an α-amino acid with the chemical formula HO2CCHCHCH3. Its codons are ACU, ACA, ACC, and ACG. This essential amino acid is classified as polar...

, isoleucine
Isoleucine
Isoleucine is an α-amino acid with the chemical formula HO2CCHCHCH2CH3. It is an essential amino acid, which means that humans cannot synthesize it, so it must be ingested. Its codons are AUU, AUC and AUA....

 
Tröger's base
Tröger's base
Tröger's base is an organic compound that exhibits chirality due to the presence of two bridgehead stereogenic nitrogen atoms in its structure. The compound was first synthesised in 1887 from p-toluidine and formaldehyde in acid solution by Julius Tröger. It took until 1935 for the elucidation of...

 
Adenosine triphosphate
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 
-
3 or more stereogenic centers Met-enkephalin
Met-enkephalin
[Met]enkephalin, sometimes referred to as opioid growth factor , is a naturally-occurring, endogenous opioid peptide that has opioid effects of a relatively short duration. It is one of the two forms of enkephalin, the other being [leu]enkephalin...

, leu-enkephalin
Leu-enkephalin
Leu-enkephalin is an endogenous opioid peptide neurotransmitter with the amino acid sequence Tyr-Gly-Gly-Phe-Leu that is found naturally in the brains of many animals, including humans. It is one of the two forms of enkephalin; the other is met-enkephalin. The tyrosine residue at position 1 is...

 
DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 
- Hexol
Hexol
Hexol is a cobalt compound that was first prepared by Alfred Werner in 1914 and represented the first non-carbon-containing chiral compound. The salt with the molecular formula of[Co3]3 was prepared starting from cobalt sulfate....

 (cobalt)

Properties of enantiomers


Normally, the two enantiomers of a molecule behave identically to each other. For example, they will migrate with identical Rf in thin layer chromatography
Thin layer chromatography
Thin layer chromatography is a chromatography technique used to separate mixtures. Thin layer chromatography is performed on a sheet of glass, plastic, or aluminum foil, which is coated with a thin layer of adsorbent material, usually silica gel, aluminium oxide, or cellulose...

 and have identical retention time in HPLC
High-performance liquid chromatography
High-performance liquid chromatography , HPLC, is a chromatographic technique that can separate a mixture of compounds and is used in biochemistry and analytical chemistry to identify, quantify and purify the individual components of the mixture.HPLC typically utilizes different types of stationary...

. Their NMR
NMR
NMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...

 and IR
Infrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...

 spectra are identical. However, enantiomers behave differently in the presence of other chiral molecules or objects. For example, enantiomers do not migrate identically on chiral chromatographic media, such as quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 or standard media that have been chirally modified. The NMR spectra of enantiomers are affected differently by single-enantiomer chiral additives such as Eufod
Eufod
EuFOD is the chemical compound with the formula Eu3, also called Eu3. This coordination compound is used primarily as a shift reagent in NMR spectroscopy...

.

Chiral compounds rotate plane polarized light. Each enantiomer will rotate the light in a different sense, clockwise or counterclockwise. Molecules that do this are said to be optically active.

Characteristically, different enantiomers of chiral compounds often taste and smell differently and have different effects as drugs – see below. These effects reflect the chirality inherent in biological systems.

One chiral 'object' that interacts differently with the two enantiomers of a chiral compound is circularly polarised light: An enantiomer will absorb left- and right-circularly polarised light to differing degrees. This is the basis of circular dichroism
Circular dichroism
Circular dichroism refers to the differential absorption of left and right circularly polarized light. This phenomenon was discovered by Jean-Baptiste Biot, Augustin Fresnel, and Aimé Cotton in the first half of the 19th century. It is exhibited in the absorption bands of optically active chiral...

 (CD) spectroscopy. Usually the difference in absorptivity is relatively small (parts per thousand). CD spectroscopy  is a powerful analytical technique for investigating the secondary structure of proteins and for determining the absolute configurations of chiral compounds, in particular, transition metal complexes. CD spectroscopy is replacing polarimetry
Polarimetry
Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves...

 as a method for characterising chiral compounds, although the latter is still popular with sugar chemists.

In biology


Many biologically active molecules are chiral, including the naturally occurring amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s (the building blocks of protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

s) and sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

s. In biological systems, most of these compounds are of the same chirality: most amino acids are L and sugars are D. Typical naturally occurring proteins, made of L amino acids, are known as left-handed proteins, whereas D amino acids produce right-handed proteins.

The origin of this homochirality
Homochirality
Homochirality is a term used to refer to a group of molecules that possess the same sense of chirality. Molecules involved are not necessarily the same compound, but similar groups are arranged in the same way around a central atom. In biology homochirality is found in the chemical building blocks...

 in biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

 is the subject of much debate. Most scientists believe that Earth life's "choice" of chirality was purely random, and that if carbon-based life forms exist elsewhere in the universe, their chemistry could theoretically have opposite chirality. However, there is some suggestion that early amino acids could have formed in comet dust. In this case, circularly polarised radiation (which makes up 17% of stellar radiation) could have caused the selective destruction of one chirality of amino acids, leading to a selection bias which ultimately resulted in all life on Earth being homochiral.

Enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s, which are chiral, often distinguish between the two enantiomers of a chiral substrate. Imagine an enzyme as having a glove-like cavity that binds a substrate. If this glove is right-handed, then one enantiomer will fit inside and be bound, whereas the other enantiomer will have a poor fit and is unlikely to bind.

D-form amino acids tend to taste sweet, whereas L-forms are usually tasteless{{Citation needed|date=December 2010}}. Spearmint
Spearmint
Mentha spicata syn. M. cordifolia is a species of mint native to much of Europe and southwest Asia, though its exact natural range is uncertain due to extensive early cultivation. It grows in wet soils...

 leaves and caraway
Caraway
Caraway also known as meridian fennel, or Persian cumin is a biennial plant in the family Apiaceae, native to western Asia, Europe and Northern Africa....

 seeds, respectively, contain L-carvone
Carvone
Carvone is a member of a family of chemicals called terpenoids. Carvone is found naturally in many essential oils, but is most abundant in the oils from seeds of caraway and dill.-Stereoisomerism and odor:...

 and D-carvone - enantiomers of carvone. These smell different to most people because our olfactory receptor
Sensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...

s also contain chiral molecules that behave differently in the presence of different enantiomers.

Chirality is important in context of ordered phases as well, for example the addition of a small amount of an optically active molecule to a nematic phase (a phase that has long range orientational order of molecules) transforms that phase to a chiral nematic phase (or cholesteric phase). Chirality in context of such phases in polymeric fluids has also been studied in this context.

The relative abundances of each of the different D-isomers of several amino acids have recently been quantified by collecting experimentally reported data from the proteome across all organisms in the Swiss-Prot database. The D-isomers observed experimentally were found to occur very rarely as shown in the following table in the database of protein sequences containing over 187 million amino acids.
D-amino acid # of Times Experimentally Observed
D-alanine 664
D-serine 114
D-methionine 19
D-phenylalanine 15
D-valine 8
D-tryptophan 7
D-leucine 6
D-asparagine 2
D-threonine 2

Inorganic chemistry


{{main|Complex (chemistry)#Isomerism|l1=Complex (chemistry): Isomerism}}
Many coordination compound
Complex (chemistry)
In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...

s are chiral. At one time, the chirality was associated with organic chemistry, but this misconception was overthrown by the resolution of a purely inorganic compound by Alfred Werner
Alfred Werner
Alfred Werner was a Swiss chemist who was a student at ETH Zurich and a professor at the University of Zurich. He won the Nobel Prize in Chemistry in 1913 for proposing the octahedral configuration of transition metal complexes. Werner developed the basis for modern coordination chemistry...

. Specifically, the cobalt hydroxo complex called hexol
Hexol
Hexol is a cobalt compound that was first prepared by Alfred Werner in 1914 and represented the first non-carbon-containing chiral compound. The salt with the molecular formula of[Co3]3 was prepared starting from cobalt sulfate....

 is significant as being the first compound devoid of carbon to display optical activity. A famous example is tris(bipyridine)ruthenium(II)
Tris(bipyridine)ruthenium(II) chloride
Trisruthenium dichloride is the coordination compound with the formula [Ru3]Cl2. This red crystalline salt is obtained as the hexahydrate, although all of the properties of interest are in the cation [Ru3]2+, which has received much attention because of its distinctive optical properties...

 complex in which the three bipyridine ligands adopt a chiral propeller-like arrangement.


In this case, the Ru atom is the stereogenic center. The two enantiomers of complexes such as [Ru(2,2'-bipyridine)3]2+ may be designated as Λ (left-handed twist of the propeller described by the ligands) and Δ (right-handed twist).

It is now appreciated that chirality is pervasive in inorganic chemistry, an example from the mineral kingdom being quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

.

Chirality of compounds with a stereogenic "lone pair"



When a nonbonding pair of electrons, a lone pair, occupies space, chirality can result. The effect is pervasive in certain amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...

s, phosphines, sulfonium
Sulfonium
A sulfonium ion, also known as sulphonium ion or sulfanium ion, is a positively charged ion featuring three organic substituents attached to sulfur. They have the formula [SR3]+...

 ions, sulfoxide
Sulfoxide
A sulfoxide is a chemical compound containing a sulfinyl functional group attached to two carbon atoms. Sulfoxides can be considered as oxidized sulfides...

s, and even carbanion
Carbanion
A carbanion is an anion in which carbon has an unshared pair of electrons and bears a negative charge usually with three substituents for a total of eight valence electrons. The carbanion exists in a trigonal pyramidal geometry. Formally a carbanion is the conjugate base of a carbon acid.where B...

s. The main requirement is that aside from the lone pair, the other three substituents differ mutually. Chiral phosphine ligands are useful in asymmetric synthesis.

Chiral amine
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...

s (see image) are special in the sense that the enantiomers can rarely be separated. The energy barrier for the inversion
Nitrogen inversion
In chemistry, a nitrogen compound like ammonia in a trigonal pyramid geometry undergoes rapid nitrogen inversion whereby the molecule turns inside out. This interconversion is a room temperature process because the energy barrier is relatively small. Contrast this to phosphine which does not show...

 of the stereocenter is generally about 30 kJ/mol, which means that the two stereoisomers rapidly interconvert at room temperature. As a result, such chiral amines cannot be resolved into individual enantiomers unless some of the substituents are constrained in cyclic structures as in Tröger's base
Tröger's base
Tröger's base is an organic compound that exhibits chirality due to the presence of two bridgehead stereogenic nitrogen atoms in its structure. The compound was first synthesised in 1887 from p-toluidine and formaldehyde in acid solution by Julius Tröger. It took until 1935 for the elucidation of...

.

See also

  • Stereochemistry
    Stereochemistry
    Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. An important branch of stereochemistry is the study of chiral molecules....

     for overview of stereochemistry in general
  • Supramolecular chirality
    Supramolecular chirality
    In chemistry, the term supramolecular chirality is used to describe supramolecular assemblies that are non-superposable on their mirror images.Chirality in supramolecular chemistry implies the non-symmetric arrangement of molecular components in a non-covalent assembly...

  • Chirality (physics)
    Chirality (physics)
    A chiral phenomenon is one that is not identical to its mirror image . The spin of a particle may be used to define a handedness for that particle. A symmetry transformation between the two is called parity...

  • Chirality (mathematics)
    Chirality (mathematics)
    In geometry, a figure is chiral if it is not identical to its mirror image, or, more precisely, if it cannot be mapped to its mirror image by rotations and translations alone. For example, a right shoe is different from a left shoe, and clockwise is different from counterclockwise.A chiral object...

  • Pfeiffer Effect
    Pfeiffer Effect
    The Pfeiffer Effect is an optical phenomenon whereby the presence of an optically active compound influences the optical rotation of a racemic mixture of a second compound....

  • Chemical chirality in popular fiction
    Chemical chirality in popular fiction
    Although little was known about chemical chirality in the time of Lewis Carroll, his work Through the Looking-glass contains a prescient reference to the differing biological activities of enantiomeric drugs: "Perhaps Looking-glass milk isn't good to drink," Alice said to her cat.In the Dorothy L...


External links


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