21.6: Introduction to Carboxylic Acid Derivatives
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Many important reactions of carboxylic acids involve attack on the carbon of the carbonyl group by nucleophilic species. These reactions frequently are catalyzed by acids, because addition of a proton or formation of a hydrogen bond to the carbonyl oxygen makes the carbonyl carbon more vulnerable to nucleophilic attack. The following equations illustrate how an acid-catalyzed reaction operates with a neutral nucleophile (H−Nu)(H−Nu):
Subsequent cleavage of a C−OC−O bond and loss of a proton yields a displacement product:
An important example of this type of reaction is the formation of esters, which was discussed previously in connection with the reactions of alcohols in Section 15-4D. Similar addition-elimination mechanisms occur in many reactions at the carbonyl groups of acid derivatives. A less obvious example of addition to carboxyl groups involves hydride ion (H⊖)(H⊖) and takes place in lithium aluminum hydride reduction of carboxylic acids (Sections 16-4E and 18-3C).