Benzyne Mechanism

Benzyne Mechanism: Elimination-Addition Mechanism

John D. Roberts proposed the benzyne mechanism in 1953. Unactivated aryl halides having atleast one hydrogen at ortho position undergo nucleophlic substitution with a very strong base like KNH₂ or NaNH₂ in liquid ammonia. The reaction also occurs with bases such as Phli and Buli. This reaction proceeds via benzyne (aryne) intermediate and the mechanism is called benzyne(aryne) mechanism. Benzyne mechanism explain the reactivity and intermediate formation of aromatic compounds under certain conditions especially when strong bases or nucleophiles are involved.
The conditions for benzyne mechanism are at least one hydrogen atom must be present at ortho position in the inactivated aryl halide and the incoming group may or may not occupy the position vacated by the leaving group (i.e. cine substitution).

Reaction of chlorobenzene with a strong base, such as sodium amide (NaNH₂), in liquid ammonia is a classic example of the benzyne mechanism.

In unsymmetrical arynes (substituted benzyne is called aryne), at least two products sometimes three can potentially form.

NOTE: When the triple bond is between the ortho and meta carbons, nucleophile attack at the meta position because, the negative charge ends up on the ortho carbon (closer than meta).
When the triple bond is between the meta and para carbons, nucleophile attack at the para position because, the negative charge ends up on the meta carbon (closer than para).

Mechanism of Benzyne Mechanism

This mechanism involves elimination followed by addition, hence it is called elimination-addition mechanism of aromatic nucleophilic substitution.

NEET 2017