Carbanion stabilization by adjacent sulfur: Polarizability, resonance, or negative hyperconjugation? Experimental distinction based on intrinsic rate constants of proton transfer from (Phenylthio)nitromethane and 1-nitro-2-phenylethane

(Phenylthio)nitromethane, PhSCH2NO2, is about as acidic as PhCH2NO2 and about 4 pK(a)-units more acidic than CH3NO2 in water or aqueous DMSO, showing the well-known acidifying effect of thio substituents in the alpha-position of carbon acids. Over the years various interpretations have been offered for the acidifying effect of sulfur groups: d-p pi-resonance, polarizability, and negative hyperconjugation. Assuming that the nature of the factors that potentially stabilize the transition state of the proton transfer from the carbon acid are the same as those that potentially stabilize the carbanion, we show that a distinction between these interpretations can be based on the effect of the phenylthio group on the intrinsic rate constants (k(0)) of proton transfer. Such intrinsic rate constants were determined for the deprotonation of PhSCH2NO2 and PhCH2CH2NO2 by amines in water and 90% DMSO-10% water; in both solvents k(0) for PhSCH2NO2 was found to be substantially higher than for PhCH2CH2NO2 as well as for other nitroalkanes reported previously. Based on a detailed analysis of how various factors such as resonance, inductive effects, polarizability, and positive and negative hyperconjugation affect the intrinsic rate constants for proton transfer, it is concluded that the high k(0) values for PhSCH2NO2 result from a combination of the inductive and polarizability effect of the PhS group and that d-p pi-resonance and negative hyperconjugation play a minor role if any.