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A leaving group in chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
 is an ion
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
 or substituent
Substituent

In organic chemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon. The suffix -yl is used when naming organic compounds that contain a substituent....
 with the ability to detach itself from a molecule
Molecule

In chemistry, a molecule is defined as a sufficiently stable, electric charge neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds....
. The remaining molecule or fragment remaining is known as the residual or main part. The term leaving group is dependent on the context of the statement.

The ability for a functional group
Functional group

In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules....
 to leave is called lability
Lability

Lability refers to something that is constantly undergoing change or something that is likely to undergo change....
. Leaving groups affect the intrinsic reactivity
Reaction rate

The reaction rate or rate of reaction for a reactant or product in a particular chemical reaction is intuitively defined as how fast a reaction takes place....
.

The lower the pKa
Acid dissociation constant

An acid dissociation constant, Ka, is a quantitative measure of the strong acid in solution. It is the equilibrium constant for a chemical reaction known as Dissociation in the context of acid-base reactions....
 of the conjugate acid
Conjugate acid

Within the Johannes Nicolaus Br?nsted-Martin Lowry theory of acid-base reaction theories, a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton....
, the better the leaving group.






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A leaving group in chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
 is an ion
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
 or substituent
Substituent

In organic chemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon. The suffix -yl is used when naming organic compounds that contain a substituent....
 with the ability to detach itself from a molecule
Molecule

In chemistry, a molecule is defined as a sufficiently stable, electric charge neutral group of at least two atoms in a definite arrangement held together by very strong chemical bonds....
. The remaining molecule or fragment remaining is known as the residual or main part. The term leaving group is dependent on the context of the statement.

The ability for a functional group
Functional group

In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules....
 to leave is called lability
Lability

Lability refers to something that is constantly undergoing change or something that is likely to undergo change....
. Leaving groups affect the intrinsic reactivity
Reaction rate

The reaction rate or rate of reaction for a reactant or product in a particular chemical reaction is intuitively defined as how fast a reaction takes place....
.

The lower the pKa
Acid dissociation constant

An acid dissociation constant, Ka, is a quantitative measure of the strong acid in solution. It is the equilibrium constant for a chemical reaction known as Dissociation in the context of acid-base reactions....
 of the conjugate acid
Conjugate acid

Within the Johannes Nicolaus Br?nsted-Martin Lowry theory of acid-base reaction theories, a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton....
, the better the leaving group. This is because the lower the pKa of the conjugate acid of the leaving group, the more the leaving group stays in its anion, or leaving group, form (leaving groups may be neutral). This anion stability means that the leaving group is less likely to react with the cation formed by the leaving group's absence (this is the case with bad leaving groups). Without stabilization, a leaving group will become a nucleophile
Nucleophile

In chemistry, a nucleophile is a reagent that forms a chemical bond to its reaction partner by donating both bonding electrons. Because nucleophiles donate electrons, they are by definition Lewis bases ....
 due to its negative charge, and if this happens, the reaction goes in circles. This is why a strong base
Base (chemistry)

In chemistry, a base is most commonly thought of as an aqueous substance that can accept protons. A base is also often referred to as an alkali if OH- ions are involved....
 is a poor leaving group. SN1 reaction
SN1 reaction

The SN1 reaction is a substitution reaction in organic chemistry. "SN" stands for nucleophilic substitution and the "1" represents the fact that the rate-determining step is molecule....
s prefer halide
Halide

A halide is a binary compound, of which one part is a halogen atom and the other part is an chemical element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound....
s, pseudohalides, and non-coordinating ions as leaving groups. Halide salts are particularly useful leaving groups because they can be abstracted by silver ions, to form insoluble silver halide
Silver halide

A silver halide is one of the Chemical compound formed between silver and one of the halogens — silver bromide , silver chloride , silver iodide , and two forms of silver fluorides....
s.

In room temperature
Room temperature

Room temperature is a common term to denote a certain temperature within enclosed space at which humans are accustomed.Room temperature is thus often indicated by general human comfort, with the common range of 10celsius to 23?C , though climate may acclimatize people to higher or lower temperatures....
 water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
, the sequence of lability is:
  • Less lability
  • amine
    Amine

    Amines are organic compounds and functional groups that contain a base nitrogen atom with a lone pair. Amines are derivative s of ammonia, wherein one or more hydrogen atoms are replaced by organic substituents such as alkyl and aryl groups....
     NH2-
  • methoxy
    Methoxy

    In chemistry , methoxy refers to the functional group consisting of a methyl group bound to oxygen. It has the formula:The word is used in organic nomenclature usually to describe an ether....
     CH3O-
  • hydroxyl
    Hydroxyl

    Hydroxyl in chemistry stands for a molecule consisting of an oxygen atom and a hydrogen atom connected by a covalent bond. The neutral form is a hydroxyl Radical and the hydroxyl anion is called a hydroxide....
     HO-
  • carboxylate CH3COO-
  • F-
    Fluoride

    Fluoride is the Redox form of fluorine. Both organic compounds and inorganic compounds containing the chemical element fluorine are considered fluorides....
  • water
    Water

    Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
  • Cl-
    Chloride

    The chloride ion is formed when the chemical element chlorine picks up one electron to form an anion Cl−....
  • Br-
    Bromide

    A bromide ion is a bromine atom with electric charge of -1.Compounds with bromine in formal oxidation state -1 are called bromides, and each individual chemical in this class can be called a bromide, as well....
  • I-
    Iodide

    An iodide ion is an iodine with a −1 electric charge. Compounds with iodine in formal oxidation state −1 are called iodides. This can include ionic compounds such as caesium iodide or covalent compounds such as phosphorus triiodide....
  • azide
    Azide

    Azide is the anion with the formula N3-. It is the conjugate base of hydrazoic acid. N3- is a linear anion that is isoelectronic with carbon dioxide and nitrous oxide....
     N3-
  • thiocyanate
    Thiocyanate

    Thiocyanate is the anion, [SCN]-. Common compounds include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates....
     SCN-
  • nitro
    Nitro

    Nitro may refer to:...
     NO2
  • Greater Lability
    NO3- is also weaker than F-.


A direct application of the difference in leaving group stability is in the carboxylic acid derivatives. The worse the leaving group, the more stable the chemical, which makes these form a hierarchy that can be separated in the lab. Amines, which are the worst leaving group, are the most stable compound, and attacking any carboxylic acid derivative with an amine produces an amide. Esters are the second most stable followed by anhydrides and finally halides.

During the SN2 nucleophilic attack, a partial negative charge forms on the leaving group. During SN1 reactions, the leaving group anion
Ion

An ion is an atom or molecule which has lost or gained one or more electrons, giving it a positive or negative electrical charge. According to the Bohr_model this will be from or in the outer shield 'n'....
izes and leaves. In general, amine, methoxy and hydroxyl groups never act as leaving groups in a substitution reaction.

In the non-mechanistic transformations, leaving group is the actual substituent group present in the substrate
Substrate (biochemistry)

In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalysis chemical reactions involving the substrate. The substrate binds with the enzyme active site, and an enzyme-substrate complex is formed....
 and product.

The structure of the leaving group affects the rates of both SN1 and SN2 reactions. In general, the more stable the leaving group is as a free species--that is after it has left--the faster it will leave. This stability also reflects the basicity of the species: the more stable it is, the weaker base it is.

See also

  • Nucleophile
    Nucleophile

    In chemistry, a nucleophile is a reagent that forms a chemical bond to its reaction partner by donating both bonding electrons. Because nucleophiles donate electrons, they are by definition Lewis bases ....
  • Electrophile
    Electrophile

    In chemistry, an electrophile is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to Chemical bond to a nucleophile....


External links

  • : Bluffton College