I learnt from a certain textbook that aldehydes and formic acid respond positively to Tollen's, Fehling's, Benedict's, Schiff's and HgCl2 tests.

However, in another textbook I read that even alpha-hydroxy ketones respond positively to these tests. What is the reason behind this? I thought ketones are not oxidizable (unless under drastic conditions).

Fructose is an alpha-hydroxy ketone, but all alpha-hydroxy ketones are not fructose. In fructose there is a terminal hydroxy group which facilitates Tollen's test, etc. in basic medium. But why do alpha-hydroxy ketones like benzoin give these tests even though there is no terminal hydroxy group? That part is still unclear to me. Could you please answer that part?

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    $\begingroup$ Related: Why does fructose reduce Tollen's reagent and Fehling's solution? $\endgroup$ – Loong Dec 12 '16 at 19:30
  • $\begingroup$ @Jan (and Klaus) No, it is definitely not a duplicate, not even close. $\endgroup$ – Martin - マーチン Dec 13 '16 at 6:54
  • $\begingroup$ @Doraemon I have reopened the question and inserted your comment, please check and edit yourself if I missed or misinterpreted something. I'm not particularly good with these questions, so I leave that for the experts. $\endgroup$ – Martin - マーチン Dec 13 '16 at 7:03
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    $\begingroup$ I think $\alpha$-hydroxy ketone is being oxidized to a diketone... Still thinking about mechanism of action. $\endgroup$ – Zhe Dec 13 '16 at 13:36
  • $\begingroup$ @Martin-マーチン My brain somehow assumed that the alpha-hydroxy group was primary … I’m sorry. $\endgroup$ – Jan Dec 13 '16 at 23:25

Its because of two tautomerizations happening, the ketone becomes en-ol, and then another tautomerization happens with the original hydroxy group (which is terminal), making it an aldehyde group. All 3 forms exist in equilibrium, so it responds to tollens test.

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    $\begingroup$ Not all alpha hydroxy ketones can undergo double tautomerization to form an aldehyde. $\endgroup$ – Reeshabh Ranjan May 26 '17 at 10:16

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