Question

Write a short note on Hoffmann elimination.

Answer 1

Hint: We have already studied that when an elimination reaction takes place on an amine , two products are formed. One is the Zaitsev product which is in accordance with the Zaitsev rule. This product is usually the stable and major product. The other type of product is Hoffmann product which follows Hoffmann elimination rule.

Complete answer:
Hoffmann elimination is an elimination reaction of an amine where the least stable or rather least substituted alkene is formed. The elimination reaction is named after its discoverer August Wilhelm von Hoffmann.
The reaction involves the formation of a quaternary ammonium iodide salt by treatment of amine with methyl iodide in excess, also called exhaustive methylation. It is then treated with silver iodide and water leading to the formation of quaternary ammonium hydroxide.
When this salt is heated, the least substituted product or Hoffmann product is formed. This is due to the steric hindrance of the leaving group hence forcing the hydroxide ions to abstract hydrogen atom, from the least substituted carbon atoms.
Hoffmann product is usually the minor product formed and Zaitsev product is major product as the product is most substituted alkene due to lack of steric hindrance.
However, when the leaving group is bulky, steric hindrance plays a crucial role in determining the product. Hence Hoffmann product is the major product when steric hindrance is present in the compound.

Note:
Hoffmann elimination is often mistaken as Hoffmann rearrangement, however they are not the same.
Hoffmann rearrangement is the organic reaction of a primary amide to a primary amine with one carbon atom less than the total number of carbon atoms present in the reactant compound. It is also called Hoffmann degradation reaction.