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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?


marked as duplicate by Mithoron, A.K., Jon Custer, a-cyclohexane-molecule, Zhe Sep 28 '18 at 18:19

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ 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. $\endgroup$ – rv7 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. $\endgroup$ – rv7 Sep 28 '18 at 17:00