ELECTROCHEMICAL REDUCTION OF PYRETHROID INSECTICIDES BASED ON ESTERS OF ALPHA-CYANO-3-PHENOXYBENZYL ALCOHOL AT GLASSY-CARBON AND MERCURY-ELECTRODES IN ACETONITRILE

The electrochemical reduction of eight commercially important pyrethro id insecticides which are esters of either alpha-cyano-3-phenoxybenzyl alcohol (cycloprothrin, cyphenothrin, cyhalothrin, deltamethrin, esfe nvalerate and cypermethrin) or 4-fluoro-alpha-cyano-3-phenoxybenzyl al cohol (cyfluthrin and flumethrin) has been studied under conditions of voltammetry and bulk electrolysis at both glassy carbon and mercury e lectrodes in acetonitrile. In general, the peak potential of the initi al reduction process observed at very negative potentials at both elec trode surfaces shifts to a more positive value under conditions of con secutive potential cycling. At the hanging mercury drop electrode the reduction occurs at even more negative potentials than at a glassy car bon electrode because a blocking mechanism appears to be operative. De spite this major difference in the primary reduction step, common volt ammetric features are observed at less negative potentials on second a nd subsequent cycles of the electrode potential at either electrode su rface. For example, the initial reduction process always results in th e formation of a species which is reversibly reduced at less negative applied potentials. Furthermore, despite the definition of the voltamm etric response being highly sensitive to the individual pyrethroid str ucture, long time-scale bulk electrolysis experiments at glassy carbon or mercury pool electrodes led to the formation of analogous final pr oducts. The fact that pyrethroids with a widely varying range of acid moieties exhibit similar voltammetric behaviour suggests that the acid moiety is not directly involved in the initial electron transfer proc ess. Controlled potential electrolysis studies at both electrode surfa ces coupled with HPLC and mass spectral identification of products obt ained after ethylation with ethyl iodide showed that the reduction mec hanism on the longer time-scale involves cleavage of the ester with li beration of free cyanide ion. The major reduction product identified w as the anion of either 3-phenoxybenzoic acid or 4-fluoro-3-phenoxybenz oic acid in yields ranging from 31 to 66%. (C) 1997 Elsevier Science S .A.