Electrochemical reactions with protonations at equilibrium: Part XIII. Experimental study of the homogeneous electron exchange in quinone/dihydroquinone systems

Abstract

The rate of the electron (and proton) exchange reaction ▪, where BQ and BQH 2 are benzoquinones and dihydrobenzoquinone, and XQ and XQH 2 their corresponding monosubstituted forms (substituents CH 3, C 6H 5, or Cl) is studied as a function of pH. A W-shaped curve is obtained for log k f and log k b, whose segments have slopes of ±1 (or 2); the curves parallel that predicted for log k app ( k app being the apparent isotopic rate constant for the system BQ + BQH 2 ⇄ BQH 2 + BQ) on the basis of the theory of the homogeneous nine-member scheme with fast protonations (E. Laviron, J. Electroanal. Chem., 169 (1984) 29) and that of the homogeneous e −– e − exchange (E. Laviron, J. Electroanal. Chem., 148 (1983) 1). The sequence of proton and electron exchange is discussed in detail. It can be concluded in particular that when the pH increases (from H 0 = −2 to pH 7) the global rate for the BQ/BQH 2 system is controlled successively by the reproportionation reaction BQH + + BQH 2 ⇄ BQH + BQH + 2, by the isotopic reactions BQ + BQ − ⇄ BQ − + BQ and BQH 2 + BQH + 2 ⇄ BQH + 2 + BQH 2, and by the reproportionation reactions BQH − + BQ ⇄ BQH + BQ − and BQ 2− + BQ ⇆ 2 BQ − .