A potentiometric study of cationic heteroconjugation equilibria in nitromethane and N,N-dimethylformamide

Cationic standard homoconjugation constants were determined in systems containing heterocyclic and aliphatic amine N-oxides, and pyridine, in two polar aprotic solvents: weakly basic protophobic nitromethane (NM); and strongly basic protophilic N,N-dimethylformamide (DMF). The constant values were compared with those previously determined in other polar non-aqueous solvents. In the case of the N-oxides with hydrogen-bonded bridges of the OHO+ type, heteroconjugation equilibria were present in the majority of the systems in nitromethane and in a small number of systems in N,N-dimethylformamide. This helped to place the solvents in the series of nonaqueous media ranked according to declining tendency of the N-oxides towards cationic heteroconjugation. The rank order was consistent with increasing basicity of the polar aprotic solvents. In addition, the cationic standard heteroconjugation constants in these media were found to increase with increasing basicity of the proton acceptor and decreasing basicity of N-oxide conjugated with the proton acceptor. Tn the case of mixed hydrogen-bonded bridges (OHN+, NHO+ type),there were no simple relationships between the tendency towards cationic heteroconjugation and basicities of the proton donor and proton acceptor, as well as the basicity of the solvent. (C) 2000 Academic Press.