Comparison of Brønsted acidities of neutral CH acids in gas phase and dimethyl sulfoxide

Abstract

The Bronsted acidities of a number of neutral CH-acids (substituted toluenes, aryl- and diarylacetonitriles, fluorenes, ethyl esters of phenylcyanoacetic acids, substituted methanes, etc.) were measured in the gas phase (pulsed FT-ICR spectrometry) and in dimethyl sulfoxide (potentiometric titration). Comparison of the Bronsted acidities of the neutral CH-acids in the gas phase and in dimethyl sulfoxide (DMSO) was also carried out. It was shown that, as a rule, substituent effects on the acidity of the studied compounds are significantly attenuated by the transfer of the reaction series of acidic dissociation of neutral acids from the gas phase into DMSO. The weakest attenuation was monitored in the case of aromatic hydrocarbons, which are the conjugate acids of carbanions with very extensive charge delocalization (fluoradene, substituted fluorenes, aryl-substituted cyclopentadienes and indenes, toluene, diphenyl and triphenylmethanes, etc.). The strongest solvent-induced attenuation of the substituent effects is characteristic of meta-substituted phenylacetonitriles and phenylmalononitriles, whose sensitivity towards substituent effects decreases with transfer from the gas phase into DMSO by up to 2.8–3.3 times. At the same time, the reaction series of para and/or ortho-π-acceptor substituted phenylacetonitriles are less sensitive to a change from the gas phase to DMSO. In the series of α-cyanosubstituted toluenes the solvent attenuation of substitution effects in the benzene ring increases with the successive inclusion of cyano groups into the α-position. In the special case of para-acceptor substituted phenylacetonitriles it was demonstrated that the specific solvation induced an increase in the acidity of the para- and/or ortho-acceptor substituted phenylacetonitriles as compared to the behavior of the corresponding meta-substituted phenylacetonitriles by up to 3.6 pKa units.