Stereocenter
Encyclopedia
A stereocenter or stereogenic center is an atom
, bearing groups such that an interchanging of any two groups leads to a stereoisomer.
A chirality center is a stereocenter consisting of an atom holding a set of ligand
s (atoms or groups of atoms) in a spatial arrangement which is not superposable on its mirror image. A chiral center is a generalized extension of an asymmetric carbon atom, which is a carbon
atom bonded to four different entities, such that an interchanging of any two groups gives rise to an enantiomer
.
In organic chemistry
a chirality center usually refers to a carbon
, phosphorus
, or sulfur
atom
, though it is also possible for other atoms to be chirality centers in organic
and inorganic chemistry
.
The term stereocenter was introduced in 1984 by Mislow and Siegel.
Having two chiral centers may give a meso compound
which is achiral. Certain configurations may not exist due to steric
reasons. Cyclic compounds with chiral centers may not exhibit chirality due to the presence of a two-fold rotation axis. Planar chirality
may also provide for chirality without having an actual chiral center present.
atom which is asymmetric. Having a chiral carbon is usually a prerequisite for a molecule to have chirality
, though the presence of a chiral carbon does not necessarily make a molecule chiral (see meso compound
). A chiral carbon is often denoted by C*.
For the carbon to be chiral, it follows that:
Almost any other configuration for the carbon would produce a center of symmetry. For example, an sp or sp2 hybridized molecule would be planar, with a mirror plane. Two identical groups would give a mirror plane bisecting the molecule.
Nitrogen and phosphorus atoms are also tetrahedral. Racemization by Walden inversion
may be restricted (such as ammonium or phosphonium cations), or slow. This allows the presence of chirality.
Metal atoms with tetrahedral or octahedral
geometries may also be chiral due to having different ligands. For the octahedral case, several chiralities are possible. Having three ligands of two types, the ligands may be lined up along the meridian, giving the mer-isomer, or forming a face — the fac isomer. Having three bidentate ligands of only one type gives a propeller-type structure, with two different enantiomers denoted Λ and Δ.
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
, bearing groups such that an interchanging of any two groups leads to a stereoisomer.
A chirality center is a stereocenter consisting of an atom holding a set of ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s (atoms or groups of atoms) in a spatial arrangement which is not superposable on its mirror image. A chiral center is a generalized extension of an asymmetric carbon atom, which is a carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
atom bonded to four different entities, such that an interchanging of any two groups gives rise to an 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...
.
In 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...
a chirality center usually refers to a carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
, phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
, or sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
, though it is also possible for other atoms to be chirality centers in organic
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...
and 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...
.
The term stereocenter was introduced in 1984 by Mislow and Siegel.
Possible number of stereoisomers
A molecule can have multiple stereocenters, giving it many stereoisomers. In compounds whose stereoisomerism is due to tetrahedral stereogenic centers, the total number of hypothetically possible stereoisomers will not exceed 2n, where n is the number of tetrahedral stereocenters. Molecules with symmetry frequently have fewer than the maximum possible number of stereoisomers.Having two chiral centers may give 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 ""...
which is achiral. Certain configurations may not exist due to steric
Steric effects
Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds , and this may affect the molecule's preferred shape and reactivity.-Steric...
reasons. Cyclic compounds with chiral centers may not exhibit chirality due to the presence of a two-fold rotation axis. 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...
may also provide for chirality without having an actual chiral center present.
Chiral carbon
A chiral carbon or asymmetric carbon is a carbonCarbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
atom which is asymmetric. Having a chiral carbon is usually a prerequisite for a molecule to have chirality
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....
, though the presence of a chiral carbon does not necessarily make a molecule chiral (see 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 ""...
). A chiral carbon is often denoted by C*.
For the carbon to be chiral, it follows that:
- the carbon atom is sp3-hybridized
- there are four different groups attached to the carbon atom.
Almost any other configuration for the carbon would produce a center of symmetry. For example, an sp or sp2 hybridized molecule would be planar, with a mirror plane. Two identical groups would give a mirror plane bisecting the molecule.
Other chiral centers
Chirality is not limited to carbon atoms, though carbon atoms are often centers of chirality due to its ubiquity in organic chemistry.Nitrogen and phosphorus atoms are also tetrahedral. Racemization by Walden inversion
Walden inversion
Walden inversion is the inversion of a chiral center in a molecule in a chemical reaction. Since a molecule can form two enantiomers around a chiral center, the Walden inversion converts the configuration of the molecule from one enantiomeric form to the other. For example, in a SN2 reaction,...
may be restricted (such as ammonium or phosphonium cations), or slow. This allows the presence of chirality.
Metal atoms with tetrahedral or octahedral
Octahedral molecular geometry
In chemistry, octahedral molecular geometry describes the shape of compounds where in six atoms or groups of atoms or ligands are symmetrically arranged around a central atom, defining the vertices of an octahedron...
geometries may also be chiral due to having different ligands. For the octahedral case, several chiralities are possible. Having three ligands of two types, the ligands may be lined up along the meridian, giving the mer-isomer, or forming a face — the fac isomer. Having three bidentate ligands of only one type gives a propeller-type structure, with two different enantiomers denoted Λ and Δ.