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Certain alpha hydroxy ketones which can tautomerise to form an aldehyde will give a positive Tollen's Test. I was pretty clear with this idea until I came across this question and the answer given is A,B,C. I can't figure out why Benzoin gives a positive Tollen's Test as it can't give an aldehyde by tautomerisation.

What exactly does happen to Benzoin over here and why does it give a positive test? Is my idea about tautomerisation of alpha hydroxy ketones to aldehydes wrong?enter image description here

Harish Iniyarajan
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  • As tollen's reagent has base also, we know that the most acidic hydrogen is the alpha hydrogen here. After tautomerising you will get the same product but you may notice that the hydroxy group has now become a ketone, hence being oxidised, it gives tollen's test – vanshita rawat Mar 21 '20 at 15:39

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For benzoin, a similar mechanism of tautomerisation (of alpha hydroxy ketones tautomerising to aldehydes) happens , except for one step, where instead of the hydride ion (H-) migrating to the sp2 hydridized carbon, it is the Benzene carbanion (C6H5- or Ph-) that migrates to the sp2 hybridized carbon, since it is more stable than the hydride ion (an example of the stability of benzene carbanion over the hydride ion can be found in the pinacol-pinacalone rearrangement). Other than that, the rest of the mechanism is exactly the same.

Hope this cleared your doubt.

SacredMechanic
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  • Alpha-hydroxy ketones do not tautomerize via hydride shift; they tautomerize via proton shift as indicated in this answer: https://chemistry.stackexchange.com/questions/73313/why-do-α-hydroxy-ketones-give-tollens-test – Aniruddha Deb Dec 12 '19 at 14:40