Proton-transfer equilibria for N-base–trimethyl-N-oxide cation systems in acetonitrile

Abstract

The electrometric behaviour of homo- and hetero-complexes, (Me₃NOHONMe₃)⁺ and (Me₃NOHB)⁺(where B is an N-base), in acetonitrile have been investigated by e.m.f. measurements.

The N-bases, B. studied were: 2-chloropyridine, 3-chloropyridine, pyridine, quinoline, N-methylimidazole, morpholine, N-methylpiperidine, triethylamine and trimethyl-N-oxide.

The formation constant, K_f, and the proton-transfer equilibrium constant, K_PT, for the reactions (Me₃NO + BH⁺) and (B + Me₃NOH⁺) have been evaluated. The sigmoidal titration curve for (Me₃NOH⁺+Me₃NO) and some (Me₃NOH⁺+B) suggest that (—OHO—)⁺ and (—OHN)⁺ hydrogen bridges are very strong. The experimental formation constants of the complexes (Me₃NO)₂H⁺, log K_f= 5.51, and (Me₃NOHB)⁺, log K_f=≈ 4.0, with (NHN)⁺ bridges showing weaker interaction, log K_f≈ 2.0.