STOICHIOMETRY AND MECHANISM OF PROTONATION OF ALKALI-METAL SALTS OF BENZOPHENONE RADICAL-ANIONS BY WEAK PROTON DONORS AND ITS RELEVANCE TO THE BASE-CATALYZED DECOMPOSITION OF BENZOPINACOL

The stoichiometry of the protonation of lithium and potassium salts of benzophenone radical anions and of the lithium salt of the fluorenone radical anion by methanol has been measured and found to be [(Ar2C=O) (.-)]/[MeOH] = 2 : 1. This result, which was obtained by the method of magnetic titration, implies that paramagnetism decays by the reaction between a ketyl anion and a ketyl radical (i.e., a protonated ketyl a nion). The reactivities of alkali metal salts of fluorenone radical an ions in relation to methanol exhibit a pronounced dependence on the na ture of the counterion. No kinetic deuterium isotope effect has been f ound for the protonation of tile lithium salt of the benzophenone radi cal anion in tetrahydrofuran (THF) by tert-pentyl alcohol. The lithium salt of the benzophenone radical anion in N,N,N',N'-tetramethylethyle nediamine (TMEDA) behaves markedly differently. Namely, its protonatio n by methanol exhibits 1 : 1 stoichiometry and it reacts considerably more slowly with sec-butyl alkohol, k(THF)/k(TMEDA) = 2.5. Benzopinaco l undergoes decomposition by an alkoxide base to diphenyl ketyl, which decays into an equimolar mixture of benzophenone and benzhydrol. The reaction follows second-order kinetics and the specific rate constants exhibit an inverse relationship with respect to the initial concentra tion of the alkoxide. With a very strong base benzopinacol decomposes into two diphenyl ketyl anions. On the basis of this information as we ll as on studies of products, relevant mechanisms are proposed for the protonation of ketyl anions and for the decomposition of aromatic pin acols in basic media.