Kinetics and mechanisms of nucleophilic displacements with heterocycles as leaving groups. Part 20. Aggregation of some quaternary pyridinium salts in chlorobenzene

Abstract

The practical molal osmotic coefficients (φ) and average aggregation numbers (ñ) of five pyridinium salts in chlorobenzene solution over the concentration range (0.5–3.2)×10^–2 mol kg^–1 were determined by vapour pressure osmometry. The non-ideal behaviour of the investigated systems is interpreted in terms of the stepwise aggregation model, and the association constants are derived. For the tetrafluoroborates, the association constants increase with the planarity of the cation. For 1-benzyl-2,4,6-triphenylpyridinium and 1-benzyl-2,4-diphenyl-5,6-dihydrobenzo[h]quinolinium tetrafluoro-borates, H° values are obtained of –7.19 and –9.44 kcal mol^–1, respectively. The structure of the aggregates is discussed. Aggregation is negligible at concentrations ⩽ 2 ×10^–3M, in the range used for kinetic measurements.