Dechlorination of chlorinated phenols by zero valent zinc

Abstract

Reductive dechlorination of chlorophenols (Pentachlorophenol, three tetrachlorophenols (TeCPs), six trichlorophenols (TCPs)) with zero valent zinc was examined through batch experiments. Zinc showed much higher reactivity towards PCP than iron and amended iron indicating that zero valent zinc can be a good candidate for reductive dechlorination of chlorinated phenols. Chlorophenols were sequentially dechlorinated and less chlorinated phenols were identified as reduction products. The mass balance was not complete, indicating that by‐products are important and /or that products being measured were lost by unknown pathways. The dechlorination rate of the chlorinated phenols usually followed the order: PCP > TeCPs > TCPs. Among the TeCP and TCP isomers, the reactivity was in the order of 2,3,4,5‐TeCP > 2,3,4,6‐TeCP > 2,3,5,6‐TeCP and 2,3,4‐TCP > 2,3,6‐TCP > 2,3,5‐TCP > 2,4,6‐TCP > 2,4,5‐TCP > 3,4,5‐TCP. The first order reaction rates varied by one order of magnitude or more, depending on the chlorines positions on a phenol ring. A regioselectivity was observed and daughter compound distributions could be rationalized by a mechanism in which radical intermediates were more stabilized by chlorine and hydroxyl groups than by hydrogen; positions a to the radical were found to be the most important in stabilization, followed by β‐positions.