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 .:
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.
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.
An ester is formed from carboxylic acid and alcoholate with elimination of hydroxide .
Individual evidence
- ↑ Entry on leaving group . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.L03493 Version: 2.1.5.
- ↑ Entry on nucleofuge . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.N04246 Version: 2.1.5.
- ↑ Entry on electrofuge . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.E01965 Version: 2.1.5.
- ↑ Michael B. Smith and Jerry March: Advanced Organic Chemistry , 6th Edition (2007), pp. 501-502, ISBN 978-0471720911 .