View source for Chemistry:Geminal diol

[[File:Ketone-hydrate-2D-skeletal.png|right|thumb|The generic geminal diol. The 'R's represent any groups other than OH.]]
A '''geminal diol''' (or '''gem-diol''' for short) is any [[Chemistry:Organic compound|organic compound]] having two hydroxyl functional groups (-[[Chemistry:Oxygen|O]][[Chemistry:Hydrogen|H]]) [[Chemistry:Covalent bond|bound]] to the same [[Chemistry:Carbon|carbon]] atom. Geminal diols are a subclass of the [[Chemistry:Diol|diol]]s, which in turn are a special class of [[Chemistry:Alcohol|alcohol]]s. Most of the geminal diols are considered unstable.

The simplest geminal diol is [[Chemistry:Methanediol|methanediol]] {{chem|C|H|4|O|2}} or {{chem|H|2|C|(|OH|)|2}}. Other examples are:
*[[Chemistry:Dihydroxymalonic acid|dihydroxymalonic acid]] {{chem|(|HOOC|)|2|C|(|OH|)|2}}
*[[Chemistry:Decahydroxycyclopentane|decahydroxycyclopentane]] {{chem|(|C|(|OH|)|2|)|5}}
*[[Chemistry:Chloral hydrate|chloral hydrate]] {{chem|(|Cl|3|C|)|HC|(|OH|)|2}}.

==Reactions==
===Hydration equilibrium===
Geminal diols can be viewed as [[Chemistry:Ketone|ketone]] (or [[Chemistry:Aldehyde|aldehyde]]) hydrates. The two hydroxyl groups in a geminal diol are easily converted to a carbonyl or keto group C=O by loss of one water molecule.  Conversely, a keto group can combine with water to form the geminal hydroxyl groups.

The equilibrium in water solution may be shifted towards either compound. For example, the [[Chemistry:Equilibrium constant|equilibrium constant]] for the conversion of [[Chemistry:Acetone|acetone]] {{chem|(|H|3|C|)|2|C}}=O to propane-2,2-diol {{chem|(|H|3|C|)|2|C|(|OH|)|2}} is about 10<sup>&minus;3</sup>,<ref>Peter Taylor (2002), [https://books.google.com/books?id=A3PKKplklUUC&pg=PA31 ''Mechanism and synthesis''], Book 10 of ''Molecular world''. Open University, Royal Society of Chemistry; {{ISBN|0-85404-695-X}}. 368 pages.</ref> while that of [[Physics:Formaldehyde|formaldehyde]] {{chem|H|2|C}}=O to methanediol {{chem|H|2|C|(|OH|)|2}} is 10<sup>3</sup>.<ref>Eric V. Anslyn, Dennis A. Dougherty (2006), [https://books.google.com/books?id=gY-Sxijk_tMC&pg=PA544 ''Modern physical organic chemistry'']. University Science Books. {{ISBN|1-891389-31-9}}. 1095 pages</ref> 

For conversion of [[Chemistry:Hexafluoroacetone|hexafluoroacetone]] {{chem|(|F|3|C|)|2|C}}=O to the diol {{chem|(|F|3|C|)|2|C|(|OH|)|2}}, the constant is about 10<sup>+6</sup>, due to the electron withdrawing effect of the [[Chemistry:Trifluoromethyl group|trifluoromethyl group]]s. Similarly, the conversion of [[Chemistry:Chloral|chloral]] {{chem|(|Cl|3|C|)HC}}=O to chloral hydrate is strongly favored by influence of the [[Chemistry:Trichloromethyl group|trichloromethyl group]].

In some cases, such as [[Chemistry:Decahydroxycyclopentane|decahydroxycyclopentane]] and [[Chemistry:Dodecahydroxycyclohexane|dodecahydroxycyclohexane]], the geminal diol is stable while the  corresponding ketone is not. 

Geminal diols can also be viewed as extreme cases of [[Chemistry:Hemiacetal|hemiacetal]]s, formed by reaction of carbonyl compounds with water, instead of with an alcohol.

==See also==
*[[Chemistry:Hemiacetal|Hemiacetal]]
*[[Chemistry:Geminal|Geminal]]
*[[Chemistry:Vicinal|Vicinal]]

==References==
{{Reflist}}

[[Category:Geminal diols| ]]

{{Sourceattribution|Geminal diol}}