Leaving group

In chemistry, the term “ leaving group” , “ leaving group” or “ leaving group ” refers to the area of ​​molecules split off during a chemical reaction. These can be individual atoms or ions, but above all molecules and functional groups .

A nucleofug is a leaving group that takes along the binding electron pair, e.g. B. Cl ^- . An electrofug is a leaving group that does not take the bonding pair of electrons with it, e.g. B. H ⁺ .

Goodness of the leaving group

Whether a leaving group is split off or not depends largely on the stability of the split off molecule ( nucleofugacy ). For example, hydroiodic acid (HI) is a stronger acid than hydrochloric acid (HCl); therefore the iodide ion is normally replaced by chloride ions in a substitution reaction.

Such estimates must always include effects such as B. consider solvation . The Finkelstein reaction, for example, exploits the fact that sodium iodide (NaI) dissolves in acetone , but sodium chloride (NaCl) is insoluble, in order to replace chloride with iodide, contrary to the usual reactivities.

Leaving groups arranged in descending order of quality
RN 2 +	Diazonium salts
R-OR ' 2 +	Oxonium ions
R-OSO 2 C 4 F 9	Nonaflate anion
R-OSO 2 CF 3	Trifluoromethanesulfonate
R-OSO 2 F	Fluorosulfonate
R-OTs, R-OMs etc.	Tosyl group , mesyl group and the like
RI	Iodides
R-Br	Bromides
R-OH 2 +	(Conjugate acid of an alcohol)
R-Cl	Chlorides and carboxylic acid chlorides
R-OHR ' +	Conjugate acid of an ether
R-ONO 2 , R-OPO (OH) 2	Nitrates , phosphates and other inorganic esters
R-SR ' 2 +	Sulfonium compounds
R-NR ' 3 +	Quaternary ammonium compounds
RF	Fluoride
R-OCOR	Esters and acid anhydrides
R-NH 3 +	Ammonium salts
R-OAr	phenol
RAW	Alcohols and carboxylic acids
R-OR	Ethers and esters

Example reactions

The term is used in particular for substitution reactions or eliminations :

Substitutions

In the case of substitutions , the leaving group is replaced by another functional group. One example is Williamson's ether synthesis :

z. B .:

${\displaystyle \mathrm {\ NaOCH_{3}+CH_{3}I\longrightarrow NaI+H_{3}COCH_{3}} }$

The alcoholate attacks the positively polarized carbon of the methyl group, iodide emerges from the molecule as a leaving group and forms the salt NaI .

Eliminations

There are two cases of elimination :

Formation of an unsaturated compound

Another spin-off follows with the formation of a multiple bond:

z. B.

${\displaystyle \mathrm {\ CH_{3}CH_{2}Br\longrightarrow CH_{2}{=}CH_{2}+HBr} }$

Under certain conditions, bromoethane splits off hydrogen bromide and becomes ethene . For the mechanism see elimination reaction .

Addition-Elimination Mechanism

The leaving group leaves the group following a previous addition ; this reaction plays a particularly important role in carbonyl chemistry :

z. B.

${\displaystyle \mathrm {\ CH_{3}COOH+NaOCH_{3}\longrightarrow Na^{+}+(CH_{3}C(OHO^{-}){-}OCH_{3})\longrightarrow NaOH+CH_{3}COOCH_{3}} }$

An ester is formed from carboxylic acid and alcoholate with elimination of hydroxide .

Individual evidence

1. ↑ Entry on leaving group . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.L03493 Version: 2.1.5.
2. ↑ Entry on nucleofuge . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.N04246 Version: 2.1.5.
3. ↑ Entry on electrofuge . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.E01965 Version: 2.1.5.
4. ↑ Michael B. Smith and Jerry March: Advanced Organic Chemistry , 6th Edition (2007), pp. 501-502, ISBN 978-0471720911 .