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

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Abstract

The rate of the electron (and proton) exchange reaction

, where BQ and BQH2 are benzoquinones and dihydrobenzoquinone, and XQ and XQH2 their corresponding monosubstituted forms (substituents CH3, C6H5, or Cl) is studied as a function of pH. A W-shaped curve is obtained for log kf and log kb, whose segments have slopes of ±1 (or 2); the curves parallel that predicted for log kapp (kapp being the apparent isotopic rate constant for the system BQ + BQH2 ⇄ BQH2 + 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 ee 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 H0 = −2 to pH 7) the global rate for the BQ/BQH2 system is controlled successively by the reproportionation reaction BQH+ + BQH2 ⇄ BQHradical dot + BQHradical dot+2, by the isotopic reactions BQ + BQradical dot ⇄ BQradical dot + BQ and BQH2 + BQHradical dot+2 ⇄ BQHradical dot+2 + BQH2, and by the reproportionation reactions BQH + BQ ⇄ BQHradical dot + BQradical dot and BQ2− + BQ ⇆ 2 BQradical dot.

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