Secondary reactions in controlled potential coulometry: Part V. Reversal coulometric study of the reduction of cis- and trans-thioindigo in the presence of carbon dioxide, cinnamonitrile and acrylonitrile

Abstract

Controlled potential reversal coulometry was applied to the following systems: (I) C+ neR; R+C → RC; RC+ ne → products. (II) as in (I) except RC not electroactive. (III) C+ neR; R+X → RX; RX+ ne → products. Equations for these e.c.e. and e.c. mechanisms were obtained and solved numerically. Working curves are presented for the calculation of the rate constants of the homogeneous reactions. This treatment was applied to studies of the reduction of thioindigo (TI) alone and in the presence of several reactants. For TI alone the reaction mechanism involves reaction of the radical anion (TI − ·) with parent molecule followed by a second electron transfer. In the presence of excess dissolved CO 2, acrylonitrile (AN), or cinnamonitrile (CN), reaction of TI − · with these followed by a second electron transfer was proposed. Rate constants for the TI−TI − · coupling reaction were 82 ( cis-thioindigo), and 323 ( trans-thioindigo) l mol −1 s −1; pseudo-first-order reactions of TI − · were 0.016 ( trans-) and 0.047 ( cis-) s −1 with CO 2; 0.023 ( trans-) and 0.033 ( cis-) s −1 with CN; 0.022 ( trans-) and 0.032 ( cis-) s −1 with AN.