A Guidebook to Mechanism in Organic Chemistry

Decarboxylation

O

GJA. " > 0=C=0 + R^9 > R—H

the resultant carbanion R® subsequently acquiring a proton from

solvent or other source. It would thus be expected that this mode of

decarboxylation would be assisted by the presence of electron-

withdrawing groups in R because of the stabilisation they would then

confer on the carbanion intermediate. This is borne out by the

extremely read^Iecomposition of nitroacetate

^O^C-LCH,—N=0 o=c=o+

^CH^N^O

CH 3 N0 2

and by the relative ease with which the decarboxylation of trihajo-

acetates and 2,4,6-trinitrobenzoates may be accomplished.

The decarboxylation of /?-keto acids may also proceed via their

anions and then through stabilised carbanions such as (XLVIII):

O O

CH,—C—Me -> 0=C=0 +

©CH 2 i-C—Me

t I CH 3 —C—Me

I

CHA=C—Me.

(XLVIII)

The overall rate law for the decarboxylation is, however, found to

contain a term referring to [keto acid] itself as well as to the concentra­

tion of its anion; this is believed to be due to incipient transfer of

proton to the keto group by hydrogen bonding: