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I've always seen benzyne (benzene with a triple bond) classified as an "intermediate". I really don't see why it needs to be an intermediate, though. The possible reasons I can come up with are flimsy:

  • It has an unstable sp2sp2 $\pi$ bent bond: we have bent bonds with more angle strain in cyclopropane, which is not an intermediate. Though those bonds are $\sigma$ bonds.

  • It immediately reacts with itself to form a dimer, biphenylene, in the absence of other reagents: there are many other molecules which spontaneously dimerize (eg $\ce{NO2}$ at certain temperatures), yet they are not classified as "intermediates".

So, what makes benzyne an "intermediate"?

More generally (but possibly too broad), what makes a molecule an "intermediate"?

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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.

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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.

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