Transient nitronic acid formation in the protonation of the carbanion of 2,2′,4,4′-tetranitrodiphenylmethane in acidic methanol

Rates of deprotonation of 2,2′,4,4′-tetranitrodiphenylmethane (1) by a variety of bases B− and of protonation of the resulting carbanion (2) by the conjugated acids BH in methanol have been measured at 20 °C. The Brønsted αBH coefficients for protonation of 2 by phenol and carboxylic acid buffers are equal to 0.58 and 0.43, respectively, as compared with αBH values of 0.59 and 0.52 respectively, for the same reactions in 50% H2O – 50% DMSO. On the other hand, a comparison of the intrinsic rate constants [Formula: see text] (in the Marcus sense) in the two solvents reveals that the effect of going from 50% H2O – 50% DMSO to methanol is to lower [Formula: see text] for the phenol reactions by a factor of 2 but to increase [Formula: see text]for carboxylic acid reactions by a factor of 5. It is shown that the solvent effects on αBH and [Formula: see text] can all be understood in the context of the Principle of Imperfect Synchronisation (PIS) recently proposed by Bernasconi. In addition, a fast equilibrium protonation of the carbanion 2 is found to precede its conversion to 1 at low pH, i.e., pH < 5. The resulting protonated species is suggested to be the nitronic acid 2,H arising from fast protonation of a para NO2 group of 2. The [Formula: see text] value associated with the ionization of 2,H is ∼4.25.