Fischer–Hepp rearrangement

In organic chemistry, the Fischer–Hepp rearrangement is a rearrangement reaction in which an aromatic N-nitroso or secondary nitrosamine converts to a carbon nitroso compound:
This organic reaction was first described by the German chemist Otto Philipp Fischer and
Eduard Hepp 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, 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 acid.