The kinetics of the acid-base equilibrium of 4-carboxybenzophenone ketyl radical. A pulse radiolysis study

Authors:

• Krzysztof Bobrowski,
• Bronisław Marciniak

Abstract

Pulse radiolysis of nitrogen saturated aqueous solutions of 4-carboxybenzophenone below pH 8 has been used to generate the 4-carboxybenzophenone ketyl radical anion to allow the direct observation of the protonation of this radical in neutral and moderately acidic solutions. A transient absorption band with λ_max = 650 nm (assigned to the ketyl radical anion) was observed immediately after the pulse which was subsequently converted to an absorption band with λ_max = 570 nm (assigned to the ketyl radical). The kinetics of this conversion was of pseudo-first-order with a rate constant linearly dependent on the H⁺ concentration. These findings are interpreted in terms of a protonation of the ketyl radical anion by water (CB-^· + H₂O → CBH^· + OH^-) and by protons (CB-· + H⁺ → CBH^·). The respective rate constants are: 2.5 × 10² dm³ mol^-1 s^-1 and 6.8 × 10¹⁰ dm³ mol^-1 s^-1. The rate constants for the reverse reactions are: k(CBH^· + OH^-) = 8.7 × 10⁹ dm³ mol^-1 s^-1 and k(CBH^· → CB-^· + H⁺) = 430 s^-1. © 1994.