5089

A serious accident occurred in 1976 in a chemical plant in Serviso, Italy when a process for the synthesis of 2,4,5-trichlorophenol from 1,2,4,5-tetrachlorobenzene became overheated. The toxic compound dioxin (2,3,6,7-tetrachlorodibenzo-p-dioxin) was formed and released into the countryside. Suggest a mechanism for the formation of this unwanted product.

5198

The aromatic substitution shown below is unusual because it is a nucleophilic substitution and it is specific for the site bearing a good leaving group. Give a reasonable mechanism for the reaction, and indicate why a nucleophilic substitution is reasonable in this case. Use resonance structures to support your answer.