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

The Bronsted acidities of a number of neutral CH-acids (substituted toluene s, aryl- and diarylacetonitriles, fluorenes, ethyl esters of phenylcyanoace tic acids, substituted methanes, etc.) were measured in the gas phase (puls ed 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 t hat, as a rule, substituent effects on the acidity of the studied compounds are significantly attenuated by the transfer of the reaction series of aci dic 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 solv ent-induced attenuation of the substituent effects is characteristic of met a-substituted phenylacetonitriles and phenylmalononitriles, whose sensitivi ty towards substituent effects decreases with transfer from the gas phase i nto DMSO by up to 2.8-3.3 times. At the same time, the reaction series of p ara and/or ortho-pi-acceptor substituted phenylacetonitriles are less sensi tive to a change from the gas phase to DMSO. In the series of alpha-cyanosubstituted toluenes the solvent attenuation of substitution effects in the benzene ring increases with the successive inc lusion of cyano groups into the alpha-position. In the special case of para-acceptor substituted phenylacetonitriles it was demonstrated that the specific solvation induced an increase in the acidit y of the para- and/or ortho-acceptor substituted phenylacetonitriles as com pared to the behavior of the corresponding meta-substituted phenylacetonitr iles by up to 3.6 pK(a) units.