Steric effects of o‐methyl groups on the ionization constants of phenols and anilines

Abstract

AbstractThe values of the ionization constants of the phenols and anilines in water and water‐ethanol mixtures, determined by Gelsema et al.3, have been analysed for the presence of “o‐effects” by the method, devised by McDaniel and Brown7. It appeared that, contrary to the prevailing belief, in the phenol series an “o‐effect” is present, which is attributed to steric hindrance of hydration.The partition coefficients of benzene, toluene, the xylenes and the phenols and anilines derived from these hydrocarbons have been determined for the n‐heptane‐water system at 25°.Assuming that the standard free enthalpy of hydration, referred to the solution in n‐heptane, is in first approximation the sum of contributions of the hydrocarbon moiety and the ‐OH and ‐NH2 groups, respectively, the latter quantities (ΔG° ‐OH and ΔG° ‐NH2) have been calculated. An analysis for “o‐effects” showed them to be definitely present in the phenol series. In the aniline series the effect is so small that its significancy could not be proved.By combining the “o‐effect” on ΔG° ‐OH and ΔG° ‐NH2 (if real) with that on the ionization constants in water, the influence of steric hindrance of hydration by one o‐methyl group on the standard free enthalpy in aqueous solution could be calculated for the species R‐OH, R‐NH2, R‐O⊖ and R‐NH3⊕. The values obtained were 0.4, 0.1, 0.6 and 0.8 kcal/mole, respectively.Heinen's17 recent results on the association of phenols with acetone in carbon tetrachloride solution have also been explained in terms of steric hindrance of solvation by one o‐methyl group.