Why do alpha-hydroxy ketones respond positively to Tollens', Fehling's, Benedict's, Schiff's and HgCl2 tests?

Question

I learnt from a certain textbook that aldehydes and formic acid respond positively to Tollens', 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 Tollens' 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?

Related: Why does fructose reduce Tollen's reagent and Fehling's solution? — , Dec 12 '16 at 19:30
I think $\alpha$-hydroxy ketone is being oxidized to a diketone... Still thinking about mechanism of action. — Zhe, Dec 13 '16 at 13:36
I think you should be able to find your answer here: http://www.acadblock.com/organic-chemistry/tollens-reagent-l4b2/ — electronpusher, Dec 14 '16 at 03:17
There is a nice answer in this link: https://www.quora.com/Why-is-Fehlings-solution-able-to-oxidise-%CE%B1-Hydroxy-ketone-but-not-ketones — , May 09 '17 at 04:15
Duplicate?: https://chemistry.stackexchange.com/questions/94620/in-what-way-could-benzoin-give-tollens-test/94630#94630 — user55119, May 11 '19 at 20:51
I answered a similar question with proposed mechanism : https://chemistry.stackexchange.com/questions/156199/do-all-%ce%b1-hydroxy-ketones-give-tollens-test/156371?noredirect=1#comment321838_156371 — mohamed, Sep 02 '21 at 17:11

score -2 · Answer 1 · edited May 11 '19 at 21:02

-2

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 three forms exist in equilibrium, so it responds to Tollens' test.

edited May 11 '19 at 21:02

answered Apr 01 '17 at 11:07

Steph Curry

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Not all alpha hydroxy ketones can undergo double tautomerization to form an aldehyde. – MrObjectOriented May 26 '17 at 10:16

Why do alpha-hydroxy ketones respond positively to Tollens', Fehling's, Benedict's, Schiff's and HgCl2 tests?

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