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So I was reading benzotriazole's synthesis is o-phenylenediamine with acetic acid and sodium nitrite.

Is this a reversible process?

Not even specifically to o-phenylenediamine but any derivative.

phenylenedaimne's (usually para) and derivatives and generally aromatic amines form various colour dyes on old colour photographic films that I like to experiment with as a hobby.

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Yes, a common synthesis of benzotriazole starts with o-phenylenediamine as shown in the following figure.

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Image Source

The first step in the reaction involves straightforward diazotization of one of the amino groups (exactly like what's done in the first part of the Sandmeyer reaction) to produce the ortho-amino diazonium ion. Instead of eliminating nitrogen as in the Sandmeyer case, the molecule loses a proton and then the diazomium ion internally captures the ortho-imino group to yield the triazole product.

The reaction is not reversible, the benzotriazole is very stable and the intermediate diazonium ion is relatively high energy making the back reaction unlikely under normal conditions. The reaction will also work with substituted o-phenylenediamines as long as the substituent doesn't interfere with the diazotization step. So for example, alkyl- or halo-substituted o-phenylenediamines will produce the corresponding alkyl- or halo-benzotriazoles.

Edit: response to comment

Intramolecular cyclization to produce a triazole will only occur if the amino groups in the starting diamine are ortho to one another. If m- or p-phenylenediamines or non-ortho naphthyl diamines are used, then products resulting from intermolecular azo coupling will occur. See here for some examples.

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  • $\begingroup$ Thanks Ron. Would the reaction yield similar products with meta or para positions as opposed to ortho? $\endgroup$ – Dan Jan 8 '15 at 0:22
  • $\begingroup$ I've edited the answer to cover this point. $\endgroup$ – ron Jan 8 '15 at 0:52

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