In organic chemistry, the Fischer–Hepp rearrangement is a rearrangement reaction in which an aromatic N-nitroso (\sN\dO) or secondary nitrosamine (>N\sN\dO) converts to a carbon nitroso compound: This organic reaction was first described by the German chemist Otto Philipp Fischer (1852–1932) and Eduard Hepp (June 11, 1851 – June 18, 1917) in 1886, and is of importance because para-NO secondary anilines cannot be prepared in a direct reaction. The rearrangement reaction takes place by reacting the nitrosamine precursor with hydrochloric acid. The chemical yield is generally good under these conditions, but often much poorer if a different acid is used. The exact reaction mechanism is unknown but the chloride counterion is likely not relevant, except in a competing decomposition reaction. There is evidence suggesting an intramolecular reaction, similar to that seen in the Bamberger rearrangement. Nitrosation follows the classic patterns of electrophilic aromatic substitution (for example, a meta nitro group inhibits the reaction), although substitution ortho to the amine is virtually unknown. The final step, in which a proton eliminates from the Wheland intermediate, appears to be rate-limiting, and the rearrangement is also suppressed in excessive (e.g. >10M sulfuric) acid.

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