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In Q29 of Joint Entrance Exam (JEE) 2016 India, the official answer key mentions that benzoin gives Tollen's test. However, I saw this post which says that it doesn't: Why do α-hydroxy ketones give Tollens' test?

Benzoin:

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I'm very confused now. If benzoin does give the test, how? And what is the mechanism/intermediate involved?

I thought of all possible reactions benzoin may undergo in basic medium:

  • I did its aldol condensation but the product was too crowded and seemed quite unlikely. It didn't have any aldehyde group.
  • Another possibility seemed oxidation to benzil, but I'm not sure. I don't think $\ce{Ag+}$ is strong enough.
  • I was thinking about cleavage of central $\ce{C-C}$ single bond — as the hydroxyl $\ce{O}$ donated its lone pair to the carbon it is attached to, but am not sure on this too.

I couldn't find any related reaction of benzoin or any rearrangement on the internet.

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Of your three "thoughts", you are correct that an aldol reaction is not an option. Not only is the product "crowded", but the reaction is reversible. [BTW, an aldol condensation occurs when water is eliminated from the initial aldol product. In the case of a benzoin aldol product, elimination of water is impossible.]

Your second premise is a good one. Benzoin (1) in the presence of alkaline Tollens' reagent, can be enolized (blue arrows) and the hydroxyl group deprotonated (red arrows). The enediolate 2 is a prime candidate for oxidation by Ag+. This species is the dianion of the enediol 6 tautomer of benzoin. The enediolate 2 is formed in the acyloin condensation of ethyl benzoate (7).

A one-electron oxidation of the enediolate 2 produces resonance stabilized radical anion 3. Protonation of 3 may occur at this point but a second one-electron oxidation produces the diradical 4, which is benzil (5). Under more vigorous alkaline conditions, Liebig's benzylic acid rearrangement of benzil occurs to afford benzilic acid (8), which itself is known to oxidize to benzophenone (9) under a variety of oxidative decarboxylation conditions. Neither of these reactions, i.e., the formation of 8 and 9, is expected to occur under the mild conditions of the Tollens' oxidation. Accordingly, your third idea is unlikely under the reaction conditions.

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  • $\begingroup$ Can you please specify the source of your answer? In particular of the fact that benzil does form?? $\endgroup$ – Kavita Juneja Apr 5 '18 at 12:11
  • $\begingroup$ And.. Not all alpha hydroxy ketones will do this(i.e. get oxidised ).. Am i right on this?? $\endgroup$ – Kavita Juneja Apr 5 '18 at 12:12
  • $\begingroup$ I cannot attest that ALL acyloins react with Tollens to give alpha-diketones. Here is an example: Tetrahedron Letters, (1969), 33, 2823. Acyloins are readily oxidized by Bi2O3, Cr+6, V oxidants, etc. $\endgroup$ – user55119 Apr 5 '18 at 16:07
  • $\begingroup$ here a double bond in conjugation with 2 phenyl rings is being formed, and also a benzyl radical ... i cannot open the Tetrahedron Letters link as i don't have access .. can you send me the paper on email (vishesh.jain331990@gmail.com) ..? @user55119 $\endgroup$ – vishesh jain Jun 18 at 10:03
  • $\begingroup$ have you found any other compounds or acyloins that give a positive test ..? @KavitaJuneja $\endgroup$ – vishesh jain Jun 18 at 10:05

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