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I cannot find much guidance in the literature. This is the reaction between free chlorine in water and triclosan. I understand ether cleavage and electrophilic aromatic substitution (maybe) involved. Any ideas as to what the mechanism of this reaction might be? Degradation pathway of triclosan

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Just as ethers can be formed when alcohols are protonated, the reverse reaction, formation of alcohols from ethers can also occur in the presence of acid. In the case at hand, when chlorine is mixed with water hypochlorous acid is one of the compounds formed, and it can serve as the proton source for the following reaction. The nucleophile can be $\ce{OCl^-}$, or any other nucleophile present. In any case this mechanism explains the formation of 2,4-dichlorophenol from TCS, chlorine and water.

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Here is a link to the a study entitled, "Formation of chloroform and chlorinated organics by free-chlorine-mediated oxidation of triclosan". If you click on the "PDF" link in the upper-right corner, you'll get the full text pdf of the article. The article discusses the reaction pathways to the various products and also comments on the formation of 2,4-dichlorophenol from TCS as well as other reaction products, as you've illustrated.

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  • $\begingroup$ Thanks! :) The acidic ether cleavage you illustrated above is an SN2 pathway am I right? And if possible, can you also help to explain the electrophilic substituion of Chlorine from TCS into the Tetraclosans diagramatically as well? Thanks!! $\endgroup$ – Joshua Lee Sep 11 '14 at 17:00
  • $\begingroup$ It's similar, but the aromatic ring blocks the "rear-side approach" normally associated with a traditional $\ce{S_{N}2}$ mechanism. Instead, a $\ce{S_{N}Ar}$ mechanism is probably involved. For example, see en.wikipedia.org/wiki/Nucleophilic_aromatic_substitution $\endgroup$ – ron Sep 11 '14 at 17:16
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I'm not sure if this qualifies as an answer since it's more of a guess. Catechols (1,2-dihydroxybenzene derivatives) can be oxidized to ortho-quinones. Applying that transformation here (chlorine in water is certainly oxidizing conditions) gives the oxonium of the ortho-quinone, which can be hydrolyzed to 2,4-DCP.

proposed mechanism

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