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

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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 pKa 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 pKa 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.

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