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.