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

Autor: H. J. H. Kreutzer, C.L. De Ligny, G. F. Visserman
Rok vydání: 2010
Předmět:
Zdroj: Recueil des Travaux Chimiques des Pays-Bas. 85:5-14
ISSN: 0165-0513
DOI: 10.1002/recl.19660850102
Popis: The 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.
Databáze: OpenAIRE