The gas-phase acidities of a significant variety of alcohols, haloalcohols, phenol, carboxylic acids, and mineral acids are reproduced by an empirical theoretical treatment of substituent (X), polarizability (P), field/inductive (F), and π electron delocalization (R, resonance) effects in XOH. The substituent parameters for each of these effects are distinctly noncolinear. A standard deviation of 0.8 kcal/mol has been obtained for acidities covering a range of 50 powers of 10. All three kinds of substituent effects are indicated to be in general of major significance, π Electron donor substituents require separate treatment. The 17.6 powers of 10 gas-phase acidity of phenol greater than that of cyclohexanol is estimated to be primarily due to its 11.7 powers of 10π electron delocalization effect. On the other hand, the 22.5 powers of 10 gas-phase acidity of acetic acid greater than that of ethanol is estimated to be primarily due to the stabilization of the acetate ion by the electrostatic interaction of the acetyl group dipole which contributes 14.0 powers of 10. The treatment supports the ideas that coiling of straight chain alkoxides in the gas phase increases the alcohol acidities by 0.8-3.4 powers of 10 and that negative C-H hyperconjugation in gaseous alkoxides increases acidities by ca. 0.1 kcal/mol/α-C-H. The results of similar analysis of acidities in dimethyl sulfoxide and in aqueous solutions is also presented. Comparisons of both gas phase and solution acidities of the XOH compounds with the correspondingly substituted phenols (p-XC6H4OH) strongly suggests that lone pair-lone pair electronic repulsions in XO- anions decrease the importance of the X substituent π electron acceptor effects relative to the corresponding electrostatic field/inductive effects. © 1990, American Chemical Society. All rights reserved.
