Flash photolytic generation and study of p-quinone methide in aqueous solution. An estimate of rate and equilibrium constants for heterolysis of the carbon-bromine bond in p-hydroxybenzyl bromide

Flash photolysis of p-hydroxybenzyl acetate in aqueous perchloric acid solution and formic acid, acetic acid, biphosphate ion, and tris(hydroxymethyl)methylammonium ion buffers produced p-quinone methide as a short-lived species that underwent hydration to p-hydroxybenzyl alcohol in hydronium ion catalyzed (k(H(+)) = 5.28 x 10(4) M(-1) s(-1)) and uncatalyzed (k(uc) = 3.33 s(-1)) processes. The inverse nature of the solvent isotope effect on the hydronium ion-catalyzed reaction, k(H(+))/k(D(+)) = 0.41, indicates that this process occurs by rapid and reversible protonation of the quinone methide on its carbonyl carbon atom, followed by rate-determining capture of the p-hydroxybenzyl carbocation so produced by water, while the magnitude of the rate constant on the uncatalyzed process indicates that this reaction occurs by simple nucleophilic addition of water to the methylene group of the quinone methide. p-Quinone methide also underwent hydronium ion-catalyzed and uncatalyzed nucleophilic addition reactions with chloride ion, bromide ion, thiocyanate ion, and thiourea. The solvent isotope effects on the hydronium ion-catalyzed processes again indicate that these reactions occurred by preequilibrium mechanisms involving a p-hydroxybenzyl carbocation intermediate, and assignment of a diffusion-controlled value to the rate constant for reaction of this cation with thiocyanate ion led to K(SH) = 110 M as the acidity constant of oxygen-protonated p-quinone methide. In a certain perchloric acid concentration range, the bromide ion reaction became biphasic, and least-squares analysis of the kinetic data using a double-exponential function provided k(Br(-)) = 3.8 x 10(8) M(-1) s(-1) as the rate constant for nucleophilic capture of the p-hydroxybenzyl carbocation by bromide ion, k(ionz) = 8.5 x 10(2) s(-1) for ionization of the carbon-bromine bond of p-hydroxybenzyl bromide, and K = 4.5 x 10(5) M(-1) as the equilibrium constant for the carbocation-bromide ion combination reaction, all in aqueous solution at 25 degrees C. Comparisons are made of the reactivity of p-quinone methide with p-quinone alpha,alpha-bis(trifluoromethyl)methide as well as p-quinone methide with o-quinone methide.     

The kinetics of isotope exchange reactions: Use of initial rates to measure isotope effects on carbon acid ionization

Multistep hydrogen isotope exchange reactions, such as the íonization of a carbon acid via a carbanion intermediate in a protic solvent, when conducted using an isotopic tracer to monitor the exchange, have the unusual feature that their rate-determining steps always refer to the transfer of the tracer isotope and never to the isotope present in macroscopic amounts. This property of these reactions is discussed and rationalized using a free energy versus reaction coordinate diagram. It is further shown that this property does not invalidate a commonly used method of measuring kinetic isotope effects on carbon acid ionization in which rates of incorporation of tritium tracers into RH and RD substrates are compared, despite the fact that tritium transfer is rate determining in both exchanges, but it is valid only if initial rate measurements are used. When the comparison is made in a protio solvent, e.g., H₂O, the portion of the initial reaction which may be used depends strongly on the magnitude of the isotope effect. It ranges from less than 1% tritium incorporation for large isotope effects to 10% or more for isotope effects near unity. On the other hand, when a deuterated solvent, e.g., D₂O, is used, the range of validity of the method for large isotope effects is extended dramatically.