Pure and Applied Chemistry

Abstract: Quinone methides were produced in aqueous solution by photochemical dehydration of o-hydroxybenzyl alcohols (o-HOC₆H₄CHROH; R = H, C₆H₅, 4-CH₃OC₆H₄), and flash photolytic techniques were used to examine their rehydration back to starting substrate as well as their interaction with bromide and thiocyanate ions. These reactions are acid-catalyzed and show inverse isotope effects (k_H+/k_D+ < 1), indicating that they occur through preequilibrium protonation of the quinone methide on its carbonyl carbon atom followed by rate-determining capture of the benzyl carbocations so formed by H₂O, Br^-, or SCN^-. With some quinone methides (R = C₆H₅ and 4-CH₃OC₆H₄) this acid catalysis could be saturated, and analysis of the data obtained in the region of saturation for the example with R = 4-CH₃OC₆H₄ produced both the equilibrium constant for the substrate protonation step and the rate constant for the rate-determining step. Energy relationships comparing the quinone methides with their benzyl alcohol precursors are derived.