Radiation Chemical Studies of the Oxidation of Aniline in Aqueous Solution

Abstract

Oxidation of aniline in aqueous solution by ·OH radicals proceeds predominantly via addition to the ring followed by elimination of OH- on the microsecond time scale to form the aniline radical cation. Direct oxidation by electron transfer accounts for less than 4% of the ·OH reactions. The different hydroxycyclohexadienyl isomers produced by ·OH addition decay at different rates with this decay catalyzed both by protons and phosphate. In basic solution the resultant radical cation deprotonates to form the anilino radical. The p Ka for the equilibrium between the acidic and basic forms of this radical is 7.05 ± 0.05. Secondary radicals such as Br2·- or N3· oxidize aniline directly by electron transfer and allow rapid preparation of the radical cation even in basic solution, as is conclusively demonstrated by observation of the Raman spectrum of the radical cation on the nanosecond time scale at pH 10.4. The deprotonation process can be followed directly by time resolved absorption spectrophotometry in the pH range of 9 - 11 and is shown to occur via reaction of the radical cation with OH- at a rate constant of 2.2 x 1010 M-1 s-1. Reaction of anilino radical with water is relatively slow (k ~ 2 x 103 s-1). Time resolved Raman methods show that benzidine radical cation is an important tertiary transient, indicating that second order reaction between radical cations results to an appreciable extent from coupling at ring positions.