I have searched for this answer for a long while but haven't got it. Why is diazobenzene named so and not phenyl diazobenzene or something? The group with two bonded nitrogens is the diazo group. Why does the term diazobenzene refer to a molecule with two benzene rings?

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    $\begingroup$ Does it?$\;\!\!$ $\endgroup$ – Ivan Neretin Sep 13 '18 at 14:15

Diazo group is $\ce{=N2}$.

Example: diazomethane $\ce{CH2=N2}$
(detailed structure: $\ce{H2C=N+=N- <-> H2C^{-}-N+\bond{#}N}$)

$\ce{-N=N-}$ is azo group.

Example: azomethane $\ce{CH3-N=N-CH3}$
However, preferred names are based on diazene $\ce{HN=NH}$, so better name for azomethane is dimethyldiazene.

So, note that “diazobenzene” does not exist. You obviously mean “azobenzene”, $\ce{C6H5-N=N-C6H5}$, whose preferred name is diphenyldiazene.

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    $\begingroup$ Actually my book had written diazobenzene, I think it was a misprint. $\endgroup$ – Hema Sep 13 '18 at 14:37
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    $\begingroup$ @Hema I wonder what the structure was.. there are similar compounds, diazonium salts, like $\ce{C6H5-N_{2}^{+} X^{-}}$ $\endgroup$ – mykhal Sep 13 '18 at 14:39
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    $\begingroup$ the structure in my book was of azobenzene $\endgroup$ – Hema Sep 13 '18 at 14:40

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