Kinetics and mechanism of the acid deamination of N-substituted quinone di-imines measured with a multi-mixing, stopped-flow technique

Abstract

A stopped flow apparatus with six syringes and five mixing chambers is described which allows several stoppages at given intervals. Seven partially ring methylated, N-substituted p-phenylenediamines (R) were oxidised by iodine or permanganate. The deamination of the quinone di-imines (T⁺) formed was studied in the pH range 1–10 by means of a colour coupling process after different intervals or from the extinction of the radical intermediate as a function of time. T⁺ is in a fast equilibrium with its protonated form TH²⁺. Deamination is initiated by nucleophilic attack on the most positively charged carbon atom of the ring. Therefore, in acid solution TH²⁺ is deaminated by H₂O giving the substituted quinone monoimine and in alkaline solution T⁺ is deaminated by OH^–, giving the respective unsubstituted quinone monoimine. The equilibria and the rate constants were measured at 25 °C. The stability maximum of T⁺ is at pH 6 whereas the N-substituted quinone monoimines are stable at pH <2. The further deamination of quinone monodimethylimine was studied between pH 1 and 10. Parallel reactions with OH^– and with H₂O were found, giving in both cases p-benzoquinone