Hofmann's Rule | Hofmann's Elimination
Hofmann's Rule states that in case of alternative β-hydrogens in the charged substrates (quarternary ammonium and sulphonium salts), the least substituted alkene is predominantly formed.
2-Butylquarternary ammonium salt undergoes Hofmann elimination to give 1-butene as the major product.
It has been observed that the strong electron withdrawing effect of Me3N+ group makes the hydrogens of the β-carbon more acidic for facile abstraction by the base and stabilizes the incipient carbon formation in the transition state on gradual stretching of the β-C-H bond. In this particular compound with alternative β-hydrogen, the β''- hydrogens are less acidic due to +I effect of the adjacent methyl group. Hence, β'-hydrogen which is relatively more acidic is removed to give permanently 1-butene.
Hofmann product increases on increasing branching in the base. Steric effect due to crowding in the leaving group or in the substrate promotes Hofmann elimination.
Examples of Hofmann's Elimination Reaction
