MECHANISM OF THE CANNIZZARO REACTION

An ultimate technique for disqualifying compounds suspected of being intermediates is illustrated by the use of isotope dilution to prove that benzyl benzoate is not an intermediate in the Cannizzaro reaction of 0.5 M benzaldehyde-p-t (tritium labeled) with 0.25 M sodium hydroxide in 74% methanol-26% water at 100°C. The adduct from hydroxide ion and two molecules of benzaldehyde that was thought to rearrange to benzyl benzoate could alternatively rearrange directly to the products, benzoate ion and benzyl alcohol. However, this mechanism also is disproved because methoxide ion acting instead of hydroxide ion should lead to benzyl methyl ether, but less than 1% is found. Two other mechanisms involving a proton transfer concerted with the hydride transfer are disproved by the kD2O/kH2O isotope effect of 1.9. The rate-determining steps can be represented by two hydride transfer reactions to C6H5CHO, from the adduct from HO- + C6H5CHO and from the adduct from CH3O- + C6H5CHO, or. equivalents, by two termolecular reactions, HO- + 2C6H5CHO and CH3O- + 2C6H5CHO. Copyright © 1979, American Chemical Society. All rights reserved.