Chemistry - Why is benzyne an intermediate?

Solution 1:

Intermediate is a middle step in the chemical process: reactants -> intermediate(s) -> products. Benzyne is formed from benzene and is highly reactive. Thus, it is never the first or last step of the process. To my knowledge benzyne is rarely observed as a product, because its reacting fast with other molecules because its triple bond.

What makes a molecule an intermediate? It is a highly reactive "middle product" of a reaction. See here and there.

Solution 2:

As Juha says, o-benzyne (to distinguish from e.g. p-benzyne which is generated by the Bergman reaction) is an intermediate, in the sense of o-benzyne itself not being the product of interest; rather it is subsequently reacted with a trapping agent (e.g. a diene for Diels-Alder reactions) within the reaction mixture, and it is the adduct that is isolated.

However, there have been isolation experiments conducted, where o-benzyne is trapped in such a way that it is barred from reacting; for instance, Chapman and coworkers isolated benzyne in an argon gas matrix at 8 K for subsequent spectroscopic study. More recently, Warmuth studied benzyne by trapping it within a so-called hemicarcerand; the benzyne complex, kept at 173 K, is stable enough that NMR experiments can be performed on it.

See this survey article by Wentrup for more references.