6.6 Ester Synthesis

The carboxylic acids can be converted to corresponding esters by treating them with an alcohol in the presence of an acid or a base as a catalyst (Scheme 4). The reaction may be reversible and suitable precautions are to be employed to obtain satisfactory yields.

Scheme 4

6.7 Amide Synthesis

The carboxylic acid chlorides or acyl chlorides can be converted into amides by treating with ammonia. Treatment with an aliphatic or aromatic amine results in formation of corresponding N-substituted amide (Scheme 5).

Scheme 5

6.8 Reactions
6.8.1 Reduction

The reduction of carboxylic acids and its derivatives is an important reaction and can be carried out with variety of reagents. However, the reagents need to be varied according the class of compounds to be reduced. The pathway followed by the reaction remains more or less the same as discussed under reduction of aldehydes and ketones with hydride transfer reagents. As an example, esters can be reduced to alcohols using lithium aluminium hydride as a reducing agent according to the pathway shown in Scheme 6.

Scheme 6

Similarly the reduction of amides is believed to proceed via an imine intermediate (Scheme 7).

Scheme 7

In order to achieve selective reduction or to generate a reducing agent which can induce stereoselectivity to a reduction reaction, the two major hydride sources aluminium hydride and borohydride are suitably modified and their counter cations are also changed. In an example of such type, lithium tri-sec-butylborohydride  (L-selectride) reduces 4-t-butyl cyclohexanone to the corresponding  cis-alcohol while reduction by lithium aluminium hydride gives predominantly trans-alcohol (Scheme 8).

Scheme 8

The selectivity of the reducing agents is given below.