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I know that tautomerism causes the $\alpha$-hydroxy ketones to give the all the tests of an aldehyde. But how does it exactly tautomerise and what's the intermediate?

Does benzoin gives Tollens' test? If not, why? I'm really confused with this. I read somewhere that benzoin doesn't give Tollens' test as it does not have an alpha carbon to form enediol intermediate. Please help.

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The Tollens' test is done in base, which mediates tautomerization through the enolate:

a reaction scheme showing the tautomerization of alpha hydroxy acetone to 2-hydroxypropanal in basic solution via the enolate ion

The misconception that you have is that all alpha-hydroxy ketones can give a positive Tollens' test. Aldehydes give a positive test, and so only alpha-hydroxy ketones that can tautomerize to aldehydes will give a positive Tollen's test. My example above is 1-hydroxy-2-propanone, which isomerizes to 2-hydroxypropanol. Most alpha-hydroxy ketones with names that match the pattern 1-hydroxy-2-alkanone will give a positive Tollens' test.

Benzoin will not give a positive test, since it cannot isomerize to an aldehyde. What does benzoin form upon tautomerization?

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  • $\begingroup$ Can you put the answer to your final question in spoiler blockquotes (>)? Or maybe just state it here? Is the answer to "What does benzoin form upon tautomerization?" simply benzoin itself. $\endgroup$ – Gaurang Tandon Mar 15 '18 at 15:27
  • $\begingroup$ @GaurangTandon - Yes. Benzoin tautomerises to benzoin. $\endgroup$ – Ben Norris Mar 16 '18 at 0:52
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    $\begingroup$ This question directly contradicts your final statement that benzoin does not give a positive test. I hope you may want to have a look here. $\endgroup$ – Gaurang Tandon Apr 5 '18 at 1:55

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