Half-wave Potentials of Aromatic Ketones

Abstract

THE unsaturated aromatic ketones undergo irreversible reduction at the dropping mercury electrode to give different products in acid and in alkaline media respectively. In the acid medium the reduction product is a dimeric alcohol, the formation of which may proceed through an intermediate free radical by the following mechanism: The half-wave potential (E _1/2) for the process is therefore associated with the addition of an electron and a proton to the oxygen atom, and according to the view advocated by us^1,2 the half-wave potential may be related to the O-localization energy. In order to test this hypothesis the O-localization energies, L(−β), of benzaldehyde, benzophenone and dibenzalacetone have been calculated by the conventional molecular orbital method (taking the Coulomb integral for oxygen as α_c + 2β and for the carbon atom attached to oxygen as α_c + 0.1 β, where α_c is the Coulomb integral for aromatic carbon and β is the resonance integral for the C—C bond) and are given in Table 1 along with the appropriate values for E _1/2. It is evident from Table 1 that as the localization energy increases, the half-wave potential also increases. In the case of acetophenone and benzalacetone, the presence of an electron-donating methyl group at the α-position with respect to the C=O group will increase the electron density at the bond, and consequently the energy necessary to add an electron to the oxygen atom in acetophenone will be greater than in benzaldehyde. In fact, in the acid region, the half-wave potential of acetophenone is higher than that of the benzaldehyde. The localization energy for acetophenone could be calculated by the molecular orbital method taking into consideration the hyperconjugation effect of the methyl group, but the calculation is so involved that it was not considered worth the trouble.