In the mechanism for Reimer-Tiemann's reaction, I was told that two alcohol substituents on the same carbon atom are unstable, and this is why salicylaldehyde is formed. In fact, this is not the first time I have come across this. But if this is true, why is it that geminal diols exist? Is it because of the intramolecular hydrogen bond? If yes, why does this not apply to Reimer- Tiemann's Reaction? And if not, what's the correct reason? Thanks in advance!

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    $\begingroup$ Geminal diols not only exist, but can be quite stable. Look at formaldehyde. Steric and electronic factors are all at play. $\endgroup$ – Zhe Oct 27 '20 at 12:35
  • $\begingroup$ chemistry.stackexchange.com/questions/94417/… $\endgroup$ – Mithoron Oct 27 '20 at 19:12