Kinetics and Equilibria in the Ionisation of Carbonyl and Nitro Compounds

Abstract

In the first part isotope effects kH/KD for the racemisation of 3-bromaocamphor catalysed by hydroxide ion in 15 to 70% by volume dimethyl sulphoxide (DMSO)-water at 25oC have been measured. The errors involved make interpretation difficult but the results are not inconsistent with a maximum isotope effect in the region of ΔpK = 0. The acid dissociation constant of 3-nitrocamphor has been determined and, with isotope effects kH/kD for its ionisation catalysed by a series of bases in water at 25oC, shows a clear maximum at ΔpK = 0. In addition the Bronsted exponents for these reacations suggest the proton (deuteron) is approximately half transferred in the transition state. In the second part, four series of substituted benzyl compounds [ ethyl (α benzyl) acetoacetate, benzyl acetylacetone, benzoylacetone and benzyl malononitrile ] have been prepared and Bronsted exponents measured by (a) variation of carboxylate anion catalyst and (b) variation of substrate within a series, with various fixed bases. These Bronsted exponents, generated in different ways, have been found to differ and are rationalised in terms of solvation or electrostatic interactions in the transition state.