Carboxylic acids (RCO2H) are one of the most abundant organic compounds in nature. Their common structural feature is that they contain an acyl group bonded to an electronegative atom or substituent that acts as a leaving group in substitution reactions. They consist of acid halides (RCOX), acid anhydrides (RCO2COR'), amides (RCONH2), esters (RCO2R), and nitriles R-C=N), with the active group replacing the -OH group of the acid. Carboxylic acids are weaker than mineral acids (e.g. HCl), but more acidic than alcohols because carboxylate anions are more stable than alkoxide ions. They usually have a pKa around 5. This is because in a carboxylate ion, the negative charge is shared by both oxygen atoms, leaving them stabilized in resonance hybrid structures, whereas in the alkoxide, the negative charge is localized in one oxygen atom. Because acid dissociation is an equilibrium process, the presence of an electron-withdrawing group favors dissociation and increased acidity. For instance chloracetic acid, an acid halide, is stronger than acetic acid by a factor of 75, and dichloracetic acid is stronger by 3000 times.
The chemistry of all carboxylic acid derivatives is similar and there is a single dominant reaction type known as the nucleophilic acyl substitution reaction. This occurs by the addition of as nucelophile to the polar carbonyl group of the acid derivative. This is followed by expulsion of the leaving group. The most important nucleophiles which take part in these reactions are water, alcohols, amines, hydride ions, and Grignard reagents (RMgX). Nitriles are considered as carboxylic acid derivatives because they can undergo nucleophilic additions to the polar CN triple bond in the same way as carbonyl compounds do. The most important reactions of nitriles are their hydrolysis to carboxylic acids, their reduction to primary amines, and their reactions with Grignard reagents to yield ketones. Grignard reagents ad...