Benzyne
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Benzyne is the parent molecule of aryne compounds. It is an unstable intermediate in some reactions involving benzene compounds. More specifically, it is often found as the result of an elimination reaction with substituted aromatic compounds like halogenated or acylated benzene rings.
Elimination of the substituent, often by nucleophilic attack, results in a carbocation. This electronic arrangement, due to its instability, causes spontaneous (or base-assisted in the case of hydrogen) elimination of adjacent substituents resulting in a benzene ring with a triple bond. Another possible mechanism of formation is the abstraction of a group such as hydrogen resulting in a carbanion which causes the elimination of an adjacent group. Both mechanisms result in the formation of a benzyne intermediate.
Common usage of the benzyne product involves a Diels-Alder reaction reacting a diene with benzyne taking the place of the normal alkene resulting in a bicyclic compound, one ring being benzene, the other being the product of the Diels-Alder reaction.
[edit] Instability
Benzyne is an extremely reactive species due to the nature of its triple bond. In normal acetylenic species (such as the simplest, ethyne) the unhybridized p orbitals are parallel to one another above and below the molecular axis. This facilitates maximum orbital overlap. In benzyne, however, the p orbtials are distored to accommodate the triple bond within the ring system, reducing their effective overlap.
