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

Article abstract of DOI:10.1007/BF00696962

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)^*->/ = 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 protonation ketyl anion).The reactivities of alkali metal salts of fluorenone radical anions in relation to methanol exhibit a pronounced dependence on the nature of the counterion.No kinetic deuterium isotope effect has been found for the protonation of the lithium salt of the benzophenone radical anion in tetrahydrofuran (THF) by tert-pentyl alcohol.The lithium salt of the benzophenone radical anion in N,N,N',N-tetramethylethylenediamone (TMEDA) bhaves markedly differently.Namely, its protonation by methanol exhibits 1 : 1 stoichiometry and it reacts considerably more slowly with sec-butyl alcohol, k(THF)/k(TMEDA) = 2.5.Benzopinacol undergoes decomposition by an alkoxide base to diphenyl ketyl, which decays into an equimolar mixture of benzophenone and benzohydrol.The reaction follows second-order kinetics and the specific rate constants exhibit an inverse relationship with respect to the initial concentration of the alkoxide.With a very strong base benzopinacol decomposes into two diphenyl ketyl anions.On the basis of this information as well as on studies of products, relevant mechanisms are proposed for the protonation of ketyl anions and for the decomposition of aromatic pinacols in basic media. - Key words: benzophenone, radical anion; benzopinacol; protonation, stoichiometry, mechanism.