Kinetics and Mechanism of Hydrogen Exchange in the 2,4,6-Trihydroxybenzenonium Ion

The n.m.r. line-broadening method was used to measure rates of hydrogen exchange in the 2,4,6-trihydroxybenzenonium ion (carbon-protonated phloroglucinol) in a series of concentrated sulfuric and perchloric acid solutions. Comparison of separately determined rates of exchange of protons in the methylene and vinyl groups of this ion allows a concerted mechanism for the exchange reaction to be excluded and shows that exchange must occur through intermediate formation of completely deprotonated 1,3,5-trihydroxybenzene. This assignment of reaction mechanism allows transformation of the data into deprotonation rate constants, which, together with previously measured positions of equilibrium, permit estimation of specific rates of protonation of 1,3,5-trihydroxybenzene, k + ; the latter are in good agreement with the rate of aromatic hydrogen exchange of 1,3,5-trihydroxybenzene measured by an isotopic tracer method at lower acidities. The kinetic acidity dependence of k + is unusually shallow; this, coupled with the much steeper acidity dependences of other aromatic protonation reactions, serves to reinforce the growing realization that there is no unique relationship between kinetic acidity dependence and reaction mechanism.