Unexpectedly Small Ortho-Oxygen Substituent Effects on Stabilities of Benzylic Carbocations

Equilibrium constants are reported for the ionization of benzylic alcohols to carbocations stabilized by cyclic or acyclic o-alkyl or o-oxygen substituents. The measurements were stimulated by the observation of small or inverse effects of replacement of an o-CH2 group by O or S in the cyclopentyl ring of indanol (kO/kCH2 = 1.2) or in the cyclohexyl ring of tetralol (kO/kCH2 = 0.6, kS/kCH2 = 0.3) on rates of carbocation formation. Values of pKR (KR = [ROH][H+]/[R+]) have been obtained by combining rate constants, kH, for the acid-catalyzed ionization of the alcohols with kH2O for attack of water on the carbocation measured by the azide clock method. For carbocations derived from the following alcohols, values of pKR are as indicated:  1-indanol, −11.7; 2,3-dihydro-3-hydroxybenzofuran (benzofuran hydrate), −9.3; 1-tetralol, −12.2; 4-chromanol, −12.0; 4-thiochromanol, −12.3; o-methyl-1-phenylethanol, −13.8; o-methoxy-1-phenylethanol, −11.7. The measurements show that, in contrast to its small kinetic effect...