Correlation analyses of the aqueous-phase acidities of alcohols and gem-diols, and of carbonyl hydration equilibria using electronic and structural parameters

Abstract

Parameters which have been used successfully to quantify gas-phase proton- and hydride-transfer reactions have now been found to be applicable to reactions in aqueous solution. These parameters can be readily derived for all species. Residual electronegativity correlates directly with pK_a values of most polar-substituted alcohols in aqueous solution. A further empirical parameter is necessary to allow alcohols substituted at the methylol carbon by simple non-polar groups, such as methyl or phenyl, to be included. It is believed that this term is related to entropy effects in solution. A few alcohols substituted by CXY₂ groups (X = H, Cl, Br; Y = Cl, Br) also deviate. In this case, the polarisability due to these large groups is believed to be a further means of stabilisation of the negative charge in the solvated anion, just as in the gas phase. gem-Diol pK_a values fit a similar model. The hydration equilibria of ketones and aldehydes can be quantified by the same residual electronegativity parameter along with a hyperconjunction term. These latter two reaction series permit estimation of influences which determine the addition of aqueous OH^– to the CO group, the results of which have been compared with H^– addition to CO in the gas phase.