18.19 : Nucleophilic Aromatic Substitution: Elimination–Addition

Unactivated halobenzenes typically do not react with nucleophiles. However, the reaction can occur under forced conditions such as using high temperatures and high pressures, or strong bases.

An isotopic labeling experiment indicates that the reaction produces two products in roughly equal amounts.

The reaction involves a benzyne intermediate with two equally reactive carbons at the ends of the triple bond that can be attacked by a nucleophile with equal probability.

The reaction initiates with an amide ion that functions as a strong base and abstracts the proton adjacent to the leaving group to form a carbanion with a localized lone pair in the sp² orbital.

Elimination of the halide leads to an ineffective overlap between the two sp² orbitals

The resulting triple bond in the benzyne species is very strained and highly reactive.

Subsequent addition of the amide ion at either end of the triple bond gives a carbanion, and a final protonation yields the substituted product.