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Geminal diols are unstable, except for a few exceptions like chloral hydrate.

I wish to ask if a gem-diol alkene is also stable. Their structure:

Generic structure of a 1-alkene-1,1-diol

suggests to me that the intramolecular hydrogen bond should stabilize the gem-diol.

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    $\begingroup$ No enough. This is the enol tautomer of a carboxylic acid... $\endgroup$ – Zhe Jan 3 '18 at 2:21
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    $\begingroup$ @Zhe Oh! Sorry but I didn't know that the carbonyl group on the carboxylic acid can tautomerize. Please post it as an answer. Thank you! $\endgroup$ – Gaurang Tandon Jan 3 '18 at 2:23
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    $\begingroup$ That's a bit short for an answer for me, but you're welcome to answer your own. Nevertheless, the main point stands. Enols are generally not stable. Doesn't really matter if it's a geminal diol. $\endgroup$ – Zhe Jan 3 '18 at 2:48
  • $\begingroup$ I’m sure I answered on a four-membered ring in hydrogen bonds somewhere … chemistry.stackexchange.com/questions/60640/… $\endgroup$ – Jan Jan 3 '18 at 8:10
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    $\begingroup$ Anyway the point isn't really about hydrogen bonding. Even if the stabilisation from the hydrogen bonding were significant it's highly unlikely that an enol of a carboxylic acid would be stable. $\endgroup$ – orthocresol♦ Jan 3 '18 at 20:35
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What you call a "gem-diol alkene" might also be called an "ene-diol" or "endiol" by analogy to the name enol.

Enols are a tautomeric form of aldehydes or ketones. They have one double bond and one C-O single bond. Since "ene-diols" have one double bond and two C-O bonds, the oxidation number of the O-bonded carbon is 2 higher than with enols. This corresponds to the oxidation number of carboxylic acids. Thus, "ene-diols" are (theoretically) tautomers of carboxylic acids.

Ene-diols are not stable because they would rapidly tautomerize the more stable carboxylic acid form.

Hydrogen bonds like the one you have drawn are unlikely to stabilize enols very much, for reasons that Jan noted in a related question here on chem.SE.

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