1‐ and 2‐Electron Steps in the Oxidation of Substituted p‐Phenylenediamines with Different Oxidants in Aqueous Solution I. The Reaction with Iodine

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Abstract
Two oxidation paths of paraphenylenediamines (R) forming quinone diimines (T+) are discussed: either two subsequent 1‐electron steps with parallel dismutation of the radical intermediate (S+) or one 2‐electron step with subsequent symproportionation, forming the radical. By means of a stopped flow apparatus with 5 mixing chambers which allows premixing of selected reactants and pH‐jumps it is shown that the (irreversible) oxidation with iodine follows the second mechanism. Iodine is unable to react with the semiquinone diimine. Only unprotonated R and free I2 are the reacting species. The rate constants of 14 p‐phenylenediamines are between 1 · 106 and 2 · 109 l · mol−1 s−1 at 25 °C. The site of attack of iodine is the NR2‐group. Possible intermediates are discussed. The protonation constants of R and the extinction coefficients of the examined compounds at two characteristic wave lengths are given.

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