Interaction of 2,4,6-trimethylpyridine with some halogenocarboxylic acids in benzene and dichloromethane. Problem of stoicheiometry

Abstract

The practical molal osmotic coefficients of 2,4,6-trimethylpyridine complexes with dichloroacetic and 2,2-dichloropropionic acid in benzene solutions over the concentration range 0.05–0.5 mol kg^–1 were determined by vapour pressure osmometry (v.p.o.). The non-ideal behaviour of the investigated systems is interpreted on the basis of the stepwise aggregation model and the association constants are derived. ¹H N.m.r. and i.r. spectra are reported for 2,4,6-trimethylpridine with dichloroacetic, 2,2-dichloropropionic, and trifluoroacetic acids with various acid-base ratios (2:1, 1.33:1, and from 1:1 to 1:10). Both the chemical shift of hydrogen-bonded protons and the continuous absorption in i.r. spectra are affected when an excess of base is added to the equimolar mixture of 2,4,6-trimethylpyridine with dichloroacetic and 2,2-dichloropropionic acids. The results can be rationalized in terms of formation of the following complexes: B ⋯ HA, B⁺ H ⋯ A^–, (BHA)_n, and B⁺H(A^–⋯ HA). For trifluoroacetic acid the B⁺H(A^–⋯ HA) complex is formed only when there is an excess of acid, while for dichlorocetic and 2,2-dichloropropionic acids the equilibrium can be completely shifted to the 1:1 complex only when there is a three-fold excess of base.