A rearrangement reaction occurs when the carbon skeleton of a molecule is rearranged to provide a structural isomer of the original molecule. Frequently, a substituent moves from one atom to another atom in the same molecule. Alongside substitution and addition reactions, rearrangements are of fundamental importance in organic synthesis.
One of the earliest named reactions featuring rearrangement is the Lossen rearrangement. In 1872, the German chemist Wilhelm Lossen discovered that pyrolysis of benzoyl benzohydroxamate, formed by mixing phenylhydroxamic acid with benzoic acid, gave a mixture of phenyl isocyanate and benzoic acid. Ultimately, the conversion of O-acyl hydroxamic acids to their corresponding isocyanates became known as the Lossen rearrangement.
The reaction is still popular today, as although it is closely related to both the Hofmann and Curtius rearrangements, it utilizes much milder reaction conditions and avoids the need to use potentially hazardous azides.
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