Hofmann Rearrangement of Amides to Amines
Description: When amides are treated with chlorine and base, a rearrangement occurs, resulting in the loss of carbon dioxide and the formation of an amine.
Notes: Other halogens can be used for this reaction, such as Br2; so can the sources of reactive hydrogen NBS and NCS.
Notes: Note how Cl2 or Br2 can both be used for this reaction. In the third example, the first step makes the amide from the acid chloride. Then, the Hoffmann occurs in the second step.
Mechanism: The first step of this reaction is attack of the lone pair of nitrogen on Br2 (Step 1, arrows A and B) followed by deprotonation (Step 2, arrows C and D). Then, rearrangement (Step 3, arrows E and F) gives a carbocation, whose resonance form (arrow G) is an isocyanate.
Under these reaction conditions the isocyanate is not usually isolated. Instead, hydroxide ion attacks the isocyanate (Step 4, arrows H and I) to give the conjugate base of the amide. Then, protonation (Step 5, arrows J and K) followed by decarboxylation (Step 6, arrows L, M, and N) give the free amine.
Notes: It’s perfectly reasonable to show the deprotonation of the amide occurring before attack on Br2. There are also other reasonable ways of showing the decarboxylation/ protonation in steps 5 and 6; this is just one suggestion.