mineral acid such as concentrated H SO or HCl gas as a catalyst. . . Reactions Involving Cleavage of C–OH Bond Mechanism of esterification of carboxylic acids: The esterification of carboxylic acids with alcohols is a kind of nucleophilic acyl substitution.
Protonation of the carbonyl oxygen activates the carbonyl group towards nucleophilic addition of the alcohol. Proton transfer in the tetrahedral intermediate converts the hydroxyl group into – + OH group, which, being a better leaving group, is eliminated as neutral water molecule. The protonated ester so formed finally loses a proton to give the ester. .
Reactions with PCl , PCl and SOCl The hydroxyl group of carboxylic acids, behaves like that of alcohols and is easily replaced by chlorine atom on treating with PCl , PCl or SOCl . Thionyl chloride (SOCl ) is preferred because the other two products are gaseous and escape the reaction mixture making the purification of the products easier. . Reaction with ammonia Carboxylic acids react with ammonia to give ammonium salt which on further heating at high temperature give amides.
For example: . . Reactions Involving –COOH Group . Reduction Carboxylic acids are reduced to primary alcohols by lithium aluminium hydride or better with diborane.
Diborane does not easily reduce functional groups such as ester, nitro, halo, etc. Sodium borohydride does not reduce the carboxyl group. . Decarboxylation Carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with sodalime (NaOH and CaO in the ratio of : ).
The reaction is known as decarboxylation. Alkali metal salts of carboxylic acids also undergo decarboxylation on electrolysis of their aqueous solutions and form hydrocarbons having twice the number of carbon atoms present in the alkyl group of the acid. The reaction is known as Kolbe electrolysis (Unit , Class XI) . .
Halogenation Carboxylic acids having an a -hydrogen are halogenated at the a -position on treatment with chlorine or bromine in the presence of small amount of red phosphorus to give a -halocarboxylic acids. The reaction is