Acidities of Hydrocarbons and Sulfur-Containing Hydrocarbons in Dimethyl Sulfoxide Solution

Article abstract of DOI:10.1021/ja00283a028

The relative pK_a values in Me2SO solution of 13 hydrocarbons that form carbanions with highly dispersed negative charges on deprotonation were found to differ from literature relative ion-pair acidities in cyclohexylamine (CHA) by only +/-0.7 unit, or less, when the ion-pair pK_a's were anchored to the pK_a of 17.9 for 9-phenylfluorene in Me2SO.The relative pK_a's for phenylacetylene, cyclopentadiene, and indene were found to be 2.4, 2.7, and 0.9 units higher, respectively, in Me2SO than in CHA.Higher relative pK_a's were also observed for PhCH2SO2Ph, PhSO2CH3 and PhSO2CH2Me, by 1.9, 2.7, and 2.8 units, respectively, and for 2-phenyl-1,3-dithiane, 2-(p-phenylphenyl)-1,3-dithiane, 4-methyl-2,6,7-trithiabicyclo<2.2.2>octane, and 1,3-dithiane, by 0.4, 0.65, 0.8, and <*> 1.9 units respectively.The apparent higher relative acidities for these compounds in CHA is probably caused by stronger ion pairing or aggregation of the ions derived therefrom than for the hydrocarbon indicator ions with which they are being compared.These perturbations of the equilibria probably arise because of differences in the extent of charge dispersion in the acid and indicator ions.The pK_a of 1,3-dithiane itself is estimated from an average of three extrapolations to be 39 +/- 2 in Me2SO.Acidities in Me2SO for 1,3-dithianes with electron-withdrawing groups in the 2-position were observed to increase in the following order: 2-C6H5 < 2-p-C6H5phenyl < 2-CONMe2 < 2-E-PhCH=CH < <2-CO2Me < 2-CN.The order of acidities of 3 alkylthio alkyl sulfoxides was found to be MeSCH2S(O)Me < EtCH2S(O)Et < t-BuSCH2S(O)t-Bu.