A kinetic study of the deprotonation of arylnitromethanes by alkyl thiolate ions and aryloxide ions in water at 25-degrees-C is reported. Intrinsic rate constants (k(o)) were determined for the reactions of phenylnitromethane and (4-nitrophenyl)nitromethane by extrapolating Bronsted plots of log k1 (k1 is the deprotonation rate constant) vs DELTApK = pK(a)BH - pK(a)CH (BH = RSH or ArOH; CH = ArCH2NO2) to DELTApK = 0. k(o)RS- for the deprotonation of phenylnitromethane was found to be comparable to k(o)ArO- and also to the previously reported k(o)R2NH (deprotonation by secondary alicyclic amines) but larger than the previously reported k(o)RCOO- (deprotonation by carboxylate ions); for the deprotonation of (4-nitrophenyl)nitromethane k(o)RS- was found to exceed k(o)ArO- as well as k(o)R2NH. This is in sharp contrast with the deprotonation of HCN (Bednar and Jencks),4-(4-nitrophenoxy)-2-butanone (Pohl and Hupe), and acetylacetone (Ahrens et al.) where k(o) for thiolate ions is much lower than for oxyanions or amines. These conflicting observations are attributed to a change in the relative importance of hydrogen-bonding stabilization of the transition state (favors reactions with oxyanions and amines over thiolate ions) and the requirement for (partial) desolvation of the base as it enters the transition state (favors reactions with thiolate ions). In reactions where the negative charge in the transition state is substantially localized on the carbon, the hydrogen-bonding factor prevails, whereas for the arylnitromethane reactions where the negative charge is significantly delocalized, the desolvation factor dominates. A Bronsted plot for the deprotonation of seven arylnitromethanes (Z = 2-Me, 3-Me, H, 3-Cl, 3-NO2, 4-NO2, 3,5-(NO2)2) by HOCH2CH2S- yields alpha(CH) = 1.30. In combination with beta(B) = 0.48 (from log k1 vs DELTApK, see above) an imbalance of alpha(CH) - beta(B) = 0.82 is obtained. This imbalance is not significantly different from that reported for the deprotonation of arylnitromethanes by secondary alicyclic amines. The relevance of this result with respect to imbalances in the nucleophilic addition of thiolate ions to nitroolefins is discussed. The point for the 2-methyl derivative deviates positively, the 4-nitro derivative negatively from the Bronsted line. The positive deviation is attributed to a decrease in the resonance stabilization of the carbanion due to steric hindrance of the pi-overlap, the negative deviation to the added resonance stabilization of the carbanion by the 4-nitro substituent.
