Solid-state decomposition of para-substituted salicylic acids

A series of para-substituted salicylic acids, all crystallizing monoclinically and decomposing by decarboxylation, were shown to adhere to Bawn-type kinetics, and the decomposition rate constants were shown to adhere approximately to a Hammett-type relation. The value of the reaction parameter was -8. Both para-substituted salicyclic and benzoic acids occur as dimers in the crystalline state. The mechanism whereby the latter decomposes is speculated to be intermolecular in the sense that the substituent from one dimer interacts with the carboxyl group of a neighboring dimer. In the same sense of the word, the substituted salicyclic acids decompose by an intramolecular mechanism, i.e., within the dimer unit.