Hofmann Degradation A Chain-Shortening Reaction

We have met a number of reactions that added one or two carbons to a chain this is the first where one will be removed. The reaction mechanism is quite complex, but it is normally a one-pot process and the outcome straightforward an amide is converted to an amine. 

This is clearly a Hofmann degradation, but something else has happened as well. If we first write out the mechanism of the Hofmann degradation, the last step will become obvious. The loss of CO2 gives a species that is simply the adduct of ammonia and benzaldehyde—and we know from carbonyl chemistry that this is not stable under aqueous conditions  

FIGURE 18.21 Examples of the Baeyer-Villiger reaction. PROBLEM 18.8 

Carbocation rearrangements can occur in substitution, addition, elimination, and Friedel-Crafts reactions. The rearrangements involve conversion of a less stable to a more stable carbocation. 

The migratory aptitude of a group in migrations to electron-deficient atoms (C, O, or N) depends on its abiiity to support a positive charge at the transition state. Stabilization and any irreversible reactions of the product are also important. 

---