Electrochemical reactions. Part XV. Factors which determine the rate of carbon–halogen bond fragmentation in radical anions illustrated by some halogenated derivatives of quinoline, quinoxaline, and phenazine

Abstract

The behaviour of some halogen-derivatives of quinoline, quinoxaline, and phenazine in the potential region of the first reduction waves has been examined by polarography and cyclic voltammetry in dimethylformamide at a mercury cathode. 2-Chloro- and 2-bromo-phenazine, 6-chloroquinoxaline, and 6-fluoroquinoline form radical anions which show no tendency to fragment with loss of halide ion at room temperature on the time scale of cyclic voltammetry. 2-Iodophenazine, 6-bromo- and 6-iodo-quinoxaline, and 6-chloroquinoline form radical anions which fragment to halide ion and, after further reactions, the parent heterocycle. Radical cations from the diprotonated halogeno-quinoxaline and -phenazine salts are stable in perchloric acid. The stability of the carbonhalogen bond in a halogenated radical anion depends upon the strength of this bond and the redox potential of the substrate–radical anion couple. For the isomeric halogeno-derivatives of a given aromatic system we have shown previously that the rate of carbon–halogen bond cleavage is dependent on the free electron density in the radical anion at the carbon terminus of the cleaving bond.