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Wallach rearrangementThe Wallach rearrangement is an organic reaction and a rearrangement reaction converting an aromatic azoxy compound with sulfuric acid to an azo compound with one arene ring substituted by an hydroxyl group in the aromatic para position.[1][2] Additional recommended knowledge60% to 100% sulfuric acid is required. Conceptually related reactions are the Fries rearrangement, the Fischer-Hepp rearrangement, the Bamberger rearrangement, the benzidine rearrangement and the Hofmann-Martius rearrangement. Reaction mechanismThe reaction mechanism for this reaction is not known with great precision despite experimental evidence:
A mechanism not inconsistent with these findings is depicted below: In the first part of the reaction two equivalents of acid tease the oxygen atom away from the azoxy group. The resulting dicationic intermediate 3 with an unusual R-N+:::N+-R motif in this scheme has been observed by proton NMR in a system of fluoroantimonic acid and azoxybenzene at -50°C [3]. In the second part of the reaction the HSO4- anion is a nucleophile in a nucleophilic aromatic substitution to 5 followed by hydrolysis to 6. References
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Wallach_rearrangement". A list of authors is available in Wikipedia. |