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

Cationic standard homoconjugation constants were determined in systems cont aining heterocyclic and aliphatic amine N-oxides, and pyridine, in two pola r aprotic solvents: weakly basic protophobic nitromethane (NM); and strongl y basic protophilic N,N-dimethylformamide (DMF). The constant values were c ompared with those previously determined in other polar non-aqueous solvent s. In the case of the N-oxides with hydrogen-bonded bridges of the OHO+ typ e, 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 heterocon jugation. The rank order was consistent with increasing basicity of the pol ar aprotic solvents. In addition, the cationic standard heteroconjugation c onstants 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+ t ype),there were no simple relationships between the tendency towards cation ic heteroconjugation and basicities of the proton donor and proton acceptor , as well as the basicity of the solvent. (C) 2000 Academic Press.