Kinetics and mechanism of acid-catalyzed hydration of 5-hydroxymethyl- and 5-phenoxymethylnorborn-2-enes

Abstract

The disappearance rate constants for exo- and endo-5-hydroxymethylnorborn-2-enes (3 and 4) and exo- and endo-5-phenoxymethylnorborn-2-enes (5 and 6) were measured in aqueous perchloric acid by a capillary GC method at different temperatures and acid concentrations. The rate constants, activation parameters, excess acidity plots and products (for 3 and 4 only) are in agreement with the rate-determining protonation of the double bond (Ad_E2 mechanism). No proof of endo protonation of the double bond via the protonated endo-5-CH₂OH group was obtained. The excess acidity plots were corrected according to the partial protonation of the hydroxylic or ether oxygen atom. In the case of 3 and 4, the slope parameter m^‡, indicative of the transition state, decreases slightly with increasing temperature, the intercept parameter log (k_o/M⁻¹ s⁻¹) depends reasonably on the temperature, and the protonation site parameters of the hydroxymethyl group, m′ and pK_S′H⁺, are temperature-independent. The corresponding parameters for 5 and 6 at 303 K are normal except the peculiar pK_S′H⁺ values, ca. −2.5.