# 18.7: Functional Derivatives of Carboxylic Acids


A functional derivative of a carboxylic acid is a substance formed by replacement of the hydroxyl group of the acid by some other group, $$\ce{X}$$, such that it can be hydrolyzed back to the acid in accord with Equation 18-7:

By this definition, an amide, $$\ce{RCONH_2}$$, but not a ketone, $$\ce{RCOCH_3}$$, is a functional derivative of a carboxylic acid. Several derivatives of carboxylic acids are give in Table 18-3, and methods for preparation of these derivatives are summarized in Tables 18-6 and 18-7 at the end of the chapter.

Table 18-3: Functional Derivatives of Carboxylic Acids

The common structural feature of the compounds listed in Table 18-3 is the acyl group $$\ce{RCO}-$$. However, nitriles, $$\ce{RC \equiv N}$$, often are considered to be acid derivatives, even though the acyl group is not present as such, because hydrolysis of nitriles leads to carboxylic acids:

The chemistry of nitriles will be discussed in Section 24-5.

The two main types of reactions of carboxylic acid derivatives with which we now shall be concerned are the replacement of $$\ce{X}$$ by attack of a nucleophile $$\ce{Nu}^\ominus$$ at the carbonyl carbon with subsequent cleavage of the $$\ce{C-X}$$ bond (Equation 18-8), and substitution at the $$\alpha$$ carbon facilitated by the carbonyl group (Equation 18-9):