Study of the carboxyl group. Acidity and salt formation. Transformation of acid derivatives, which gives rise to introduce a new reaction mechanism: the addition-elimination. Unlike the esters and amides, which are widely distributed in nature, anhydrides are rare and alkanoyl halides are obtained only in the laboratory. There will be a comparative study of their reactivity, there is another fundamental reaction in organic chemistry involving nucleophilic substitution in the acyl group.