Hoffmann Rearrangement

Hoffmann Rearrangement

Principle :

                  When amide is react with hypohalide it gives primery amine having one carbon atom less than the original amide. In this reaction actual product is isocynate (R-CNO).

R-CONH2 + Br2 + 4KOH ￫ R-NH2 + 2KBr + 2H2O + K2CO3

Mechanism :

● Step - 1 : Formation of N-bromoamide :
                  In the first step amide is react with bromine and give an intermideat i.e, N-bromoamide. In this reaction when bromine molecules come near with the lone pair electrons . polarisation develop in bromine molecule.

● Step - 2 : Reaction between N - bromoamide and KOH :

                  In the second step N - bromoamide contain more acidic hydrogen than that is in original amide , the second proton remove from the N - bromoamide and anion is form.

● Step - 3 : Formation of nitrene and Isocynate ;
                  The anion of N - bromoamide is unstable and remove bromide ion and form unstable nitrene . Nitrene is easily convertade into isocynate by migration of alkyl or aryl group . [ when nitrogene is elecron deficient at that time alkyl or aryl group catch the electrons and it goes with electron deficient nitrogen atom . this group goes from carbon to nitrogen , that is called C →  N migration.

                          


● Step - 4 : Formation of product ;
                   Hydrolysis and decarboxylation of isocynate gives primary amine with one carbon atom less than original amide.

  

↪ The rate of reaction depends upon nature of R - group , if it is more releasing group reaction is more rapid.

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