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

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?

• – Loong Dec 12 '16 at 19:30
• @Jan (and Klaus) No, it is definitely not a duplicate, not even close. – Martin - マーチン Dec 13 '16 at 6:54
• @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. – Martin - マーチン Dec 13 '16 at 7:03
• I think $\alpha$-hydroxy ketone is being oxidized to a diketone... Still thinking about mechanism of action. – Zhe Dec 13 '16 at 13:36
• @Martin-マーチン My brain somehow assumed that the alpha-hydroxy group was primary … I’m sorry. – Jan Dec 13 '16 at 23:25