benzyne

Benzyne is one of the strangest creatures in aromatic chemistry: a benzene ring that appears to have a carbon-carbon triple bond squeezed into it. It cannot really hold a normal triple bond — the ring forces the extra bond into a badly bent, strained shape — so benzyne is fantastically reactive and exists only as a fleeting intermediate, never something you can bottle. Yet it explains a reaction that otherwise makes no sense.

It appears in the elimination-addition route of nucleophilic aromatic substitution, used when the ring has no electron-withdrawing groups to enable the normal addition-elimination path. The trick is a very strong base, like sodium amide (NaNH2). The base first removes a ring hydrogen next to a leaving group such as chlorine; then the leaving group falls off, and the two electrons left behind form the extra (strained) bond — that is benzyne. Because the bent extra bond is so reactive and electron-poor, a nucleophile immediately adds across it, and a proton is picked up, giving the substituted product. The full pattern is elimination (to make benzyne) then addition (of the nucleophile).

Benzyne leaves a fingerprint that proves it formed: because the nucleophile can add to either end of the strained bond, the new group can appear on either of two adjacent carbons. So chlorobenzene labeled with a marked carbon gives a mixture of products with the incoming group on that carbon or its neighbor — direct evidence that the reaction did not simply swap one group for another on the same carbon. This 'cine substitution' result, captured in classic isotope-labeling experiments, is the historical proof that the elusive benzyne is real.