Why the carbon with two OH groups in organic compounds is unstable? For example, acetal hydrate is unstable due to two OH groups on one carbon and tends to lose water immediately.
I don't know what level of answer you want. The shortest, is "quantum chemistry of carbon-oxygen bonds." Slightly more useful is "it's unstable because there is a “nearby” lower energy alternative (carbonyl)." It is unstable, because the double bond in C=O is strong, which is part of the reason that burning hydrocarbons releases so much energy. But it's such a basic fact of nature that this is so, for it to be otherwise we would have to do chemistry in a different universe with different basic physical laws. http://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/aldket1.htm See also: http://en.wikipedia.org/wiki/Geminal_diol For a 5% solution of formaldehyde in water, about 80% of it is in the the hydrated state. Other exceptions are: http://en.wikipedia.org/wiki/Hexafluoroacetone where the 6 fluorine substitutions makes the π-bond unstable. And http://en.wikipedia.org/wiki/Decahydroxycyclopentane and http://en.wikipedia.org/wiki/Dodecahydroxycyclohexane where the carbon rings just make it weird
Chloral hydrate below is an example of a stable situation of a carbon with two hydroxyl groups. This is used as a sedative. In this case, the three chlorine atoms are electron withdrawing, making it harder for one of the -OH groups to take elections and leave. Please Register or Log in to view the hidden image!
And another exception, with the same mechanism as Chloral or Hexafluoroacetone is Indane-1,2,3-trione where it's the other carboxyl groups that destabilize the central carboxyl group, allowing formation of Ninhydrin. http://en.wikipedia.org/wiki/Ninhydrin http://chemwiki.ucdavis.edu/?title=...ddition_of_Water_to_form_Hydrates_(Gem-Diols)