Fischer–Hepp rearrangement

{{Short description|Organic reaction applied to aromatic nitroso and nitrosamine compounds}}

{{Reactionbox

| Name = Fischer-Hepp rearrangement

| Type = Rearrangement reaction

| NamedAfter = Otto Fischer
Eduard Hepp

| Section3 = {{Reactionbox Identifiers

| RSC_ontology_id = 0000095

}}

}}

In organic chemistry, the Fischer–Hepp rearrangement is a rearrangement reaction in which an aromatic N-nitroso ({{chem2|\sN\dO}}) or secondary nitrosamine ({{chem2|>N\sN\dO}}) converts to a carbon nitroso compound:{{Cite journal |last1=Fischer |first1=Otto |last2=Hepp |first2=Eduard |date=July–December 1886 |title=Zur Kenntniss der Nitrosamine |url= https://zenodo.org/record/1425449|journal=Berichte der Deutschen Chemischen Gesellschaft zu Berlin |language=en |volume=19 |issue=2 |pages=2991–2995 |doi=10.1002/cber.188601902297 |issn=0365-9496 |eissn=1099-0682 |s2cid=95280925 |id=Zenodo [https://zenodo.org/record/1425449 1425449]. Gallica [https://gallica.bnf.fr/ark:/12148/bpt6k907075/f473.item ark:/12148/bpt6k907075/f473.item] |hdl-access=free |hdl=2027/njp.32101044028619?urlappend=%3Bseq=1121}}M B Smith, J March. March's Advanced Organic Chemistry (Wiley, 2001) ({{ISBN|0-471-58589-0}}) / {{Cite book |last=Michael B. |first=Smith |url=https://app.knovel.com/hotlink/pdf/id:kt011AYKO1/marchs-advanced-organic/aromatic-s-groups-cleaving |title=March's Advanced Organic Chemistry - Reactions, Mechanisms, and Structure |date=2013 |publisher=John Wiley & Sons, Inc. |isbn=978-0-470-46259-1 |edition=7th |location=Hoboken, New Jersey |page=639 |chapter=11.6.2.2 Groups Cleaving from Nitrogen; Reaction 11-29: Migration of the Nitroso Group: The Fischer–Hepp Rearrangement |lccn=2012027160}}

Image:Fischerhepp.png

This organic reaction was first described by the German chemist Otto Philipp Fischer (1852–1932) and

Eduard Hepp (June 11, 1851 – June 18, 1917) {{Cite book |last1=Pötsch |first1=Winfried R. |title=Lexikon bedeutender Chemiker |last2=Fischer |first2=Annelore |last3=Müller |first3=Wolfgang |date=1988–1989 |publisher=Verlag Harri Deutsch / VEB Bibliographisches Institut Leipzig |others=With the collaboration of Heinz Cassebaum |isbn=3-8171-1055-3 |location=Thun & Frankfurt |pages=148, 197}} 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.{{cite book|pages=678–679|title=March's Organic Chemistry|edition=8th|first1=Michael B.|last1=Smith|publisher=Wiley|year=2020}} 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.{{cite book|pages=115–125|title=Nitrosation|first=D. L. H.|last=Williams|publisher=Cambridge University|location=Cambridge, UK|year=1988|isbn=0-521-26796-X}}