THE TERTIARYBUTYLBENZENES: III. THE SYNTHESIS OF 2,4,6-TRI-t-BUTYLBENZOIC ACID AND THE DISSOCIATION OF DI- AND TRI-t-BUTYLBENZOIC ACIDS

Abstract

2,4,6-Tri-t-butylbromobenzene, m.p. 177–177.5°, was prepared by the bromination of 1,3,5-tri-t-butylbenzene in the presence of silver nitrate. The reaction of 2,4,6-tri-t-butylbromobenzene and n-butyllithium gave 2,4,6-tri-t-butylphenyl-lithium, which on carbonation yielded 2,4,6-tri-t-butylbenzoic acid, m.p. 297°. A solution of this acid in fuming sulphuric acid yielded a methyl ester, m.p. 96–98°, when it was poured into absolute methanol. Similar treatment of 1,4-di-t-butylbenzene with bromine gave 2,5-di-t-butylbromobenzene, which on reaction with n-butyllithium and subsequent carbonation gave 2,5-di-t-butylbenzoic acid. The reaction of 1,3,5-tri-t-butylbenzene with acetyl chloride in the presence of aluminum chloride gave a liquid ketone, and hypohalide oxidation of this ketone yielded 3,5-tri-t-butylbenzoic acid. The ultraviolet absorption spectra of the two bromo compounds and the three acids were measured and are discussed. The considerable acid-weakening effect in 2,4,6-tri-t-butylbenzoic acid is attributed to steric hindrance to the formation of the carboxylate anion.