A POTENTIOMETRIC STUDY OF ACID-BASE EQUILIBRIA OF TRIMETHYLAMINE N-OXIDE SYSTEMS IN NONAQUEOUS MEDIA

The electrometric behaviour of homoconjugating systems containing prot onated and free aliphatic amine N-oxide, i.e. trimethylamine N-oxide, in a number of non-aqueous solvents has been investigated by the use o f a potentiometric titration method. The standard acidity constant K-a (o)(BH+) of protonated trimethylamine N-oxide and standard cationic ho moconjugation constant K-f(o)(BHB+) in systems consisting of this free and protonated N-oxide were determined by e.m.f. measurements in a va riety of polar non-aqueous solvents with diverse polarity and proton-d onor properties, such as (aprotic, protophobic) acetone, acetonitrile, benzonitrile, nitromethane, propylene carbonate (aprotic, protophilic ), N,N-dimethylformamide, and dimethyl sulfoxide, as well as (amphipro tic) methanol. It has been found that trimethylamine N-oxide has signi ficantly higher basicity in non-aqueous media than substituted pyridin e N-oxides (the pK(a)(o) value for protonated trimethylamine N-oxide b eing about 4 to 8.5 powers of 10 greater than those characteristic of pyridine N-oxide). Consequently, the tendency towards homoconjugation in trimethylamine N-oxide systems is also greater than for substituted pyridine N-oxide systems. Furthermore, the lower the basicity of a so lvent, the higher the tendency of N-oxides towards homoconjugation. (C ) 1998 Academic Press Limited.