The following image depicts the structure of ninhydrin - $\ce{C9H6O4}$

enter image description here

Ninhydrin, as you see has two hydroxyl group attached to same carbon. In general, geminal diol (or gem-diol, for short) are unstable and tend to lose a water molecule with formation of aldehyde or ketone (depending on the carbon position).

But, Ninhydrin isn't unstable. Now, I have the following questions:

  1. If gem-diols are unstable, then why Ninhydrin isn't? Plus, this post depicts an image (marked as D) which shows the keto-form of Ninhydrin.

  2. If there are any more compounds like Ninhydrin which shows this kind of exceptional behaviour, then how one can identify if they are stable or not?

  • $\begingroup$ This may be too broad of a question. I think you should take it as given that gem-diols are unstable and ask why Ninhydrin is an exception. $\endgroup$ – A.K. Sep 28 '18 at 15:02
  • $\begingroup$ @A.K. Thanks for your suggestion. But still, I am curious about the peculiar behaviours. $\endgroup$ – Rahul Verma Sep 28 '18 at 15:19
  • $\begingroup$ chemistry.stackexchange.com/questions/94417/… $\endgroup$ – Mithoron Sep 28 '18 at 15:30
  • $\begingroup$ Maybe chemistry.stackexchange.com/questions/42756/… would be best duplicate. $\endgroup$ – Mithoron Sep 28 '18 at 15:30
  • $\begingroup$ I edited my question, as I got partial answers from the posts. Thanks to @A.K. for the last comment which shows the keto-form of Ninhydrin. $\endgroup$ – Rahul Verma Sep 28 '18 at 17:00