The kinetics of solvolysis and peroxide decomposition of purpurogalloquinone

Abstract

The product of alkaline solvolysis of purpurogalloquinone has been shown by successive reduction, methylation, and permanganate oxidation to 3,4,5,6-tetramethoxyphthalic anhydride to be 2′,3′-dihydroxy-3,4-benzotropolone-1′,4′-quinone. This compound known to mediate in the peroxide oxidation of purpurogalloquinone to tropolone-3,4-anhydride, is formed by attack of the hydrogen peroxide molecule and the hydroperoxide and hydroxide ions on the 4′-position of the quinone dianion. Measurement of the rate of disappearance of this species over the pH range 10·3–12·7 shows the peroxide molecule to be five times as reactive as its conjugate base, which in turn is about five times as reactive as the more strongly basic hydroxide ion. Charge repulsions between the reactant ions are thought to be responsible for this anomalous order of reactivities, and salt-effect studies confirm that these interionic reactions involve the quinone dianion. The highly negative activation entropies associated with the two peroxide reactions are interpreted in terms of the configurations and solvation of the transition states.