Use of indicators to follow acid-base reactions in benzene

Abstract

To demonstrate the utility of the azoarylamine indicators for studying acid-base neutralization reactions in benzene solution, the reaction of trichloroacetic acid with aliphatic amines in benzene was studied by infra-red spectroscopy and indicator techniques. The amines (and the indicators) form complexes of stoichiometry B . HA, B . 2HA and B . 3HA, and order-of-magnitude estimates for the formation constants of the B . 2HA and B . 3HA complexes are reported. It was confirmed that trichloroacetic acid is essentially monomeric in dilute benzene solution but, owing to the formation of acid-salt complexes, the indicator bases react with the acid in accordance with the expression, log [BH⁺]/[B]∝n log [HA], where n is rarely integral. This, however, proves to be no severe limitation to their use. The hydrogen bond donors, N,N-dimethylacetamide and trioctylphosphine oxide, also were shown by indicator methods to give acid complexes with trichloroacetic acid.