Organic Chemistry

Section 19.17 Decarboxylation of 3-Oxocarboxylic Acids 819

If, however, the group is bonded to a carbon that is adjacent to a carbonyl
carbon, the group can be removed because the electrons left behind can be delo-
calized onto the carbonyl oxygen. Consequently, 3-oxocarboxylate ions (carboxylate
ions with a keto group at the 3-position) lose when they are heated. Loss of
from a molecule is called decarboxylation.

Notice the similarity between removal of from a 3-oxocarboxylate ion and
removal of a proton from an In both reactions, a substituent— in one
case, in the other—is removed from an and its bonding electrons are
delocalized onto an oxygen.

Decarboxylation is even easier if the reaction is carried out under acidic conditions,
because the reaction is catalyzed by an intramolecular transfer of a proton from the
carboxyl group to the carbonyl oxygen. The enol that is formed immediately tau-
tomerizes to a ketone.

We saw in Section 19.1 that it is harder to remove a proton from an if
the electrons are delocalized onto the carbonyl group of an ester rather than onto the
carbonyl group of a ketone. For the same reason, a higher temperature is required to
decarboxylate a acid such as malonic acid than to decarboxylate a
acid.

malonic acid

C

O

HO CH 3

C

OH

HO CH 2

C C

HO CH 2

135 °C tautomerization

O O

O

+ CO 2

H

b-keto

b-dicarboxylic

a-carbon

C

O

CH 3 CH 3

C

OH

CH 3 CH 2

C C

CH 3 CH 2

tautomerization

3-oxobutanoic acid

acetoacetic acid

a -keto acid

∆

O

H

O

O

+ CO 2

C

O

CH 3 CH 2

C –

O

CH 3 CH 2

C

CH 3 CH 2 H

removing a proton from an -carbon

propanoneacetone H+

O

+

−

H+ a-carbon

a-carbon. CO 2

CO 2

C

O

CH 3 CH 2

C –

O

CH 3 CH 2

C C

CH 3 CH 2

∆

removing CO 2 from an -carbon

3-oxobutanoate ion

acetoacetate ion

O O

O−

+ CO 2

−

CO 2 CO 2

CO 2

CO 2

O

CH 3 CH 2 H CH 3 CH 2 C O−

3-Oxocarboxylic acids decarboxylate

when heated.