geminal diol
Summary
A geminal diol (gem-diol, or hydrate) has two OH groups on the same carbon. It forms when water adds to the C=O of an aldehyde or ketone. For most carbonyls the equilibrium favors the carbonyl, but small aldehydes and carbonyls with electron-withdrawing groups hydrate more.
Detail
Meaning: A geminal diol has the form R₂C(OH)₂, with both hydroxyl groups on one carbon. It forms by nucleophilic addition of water to the electrophilic carbonyl carbon, and acid or base speeds the reaction. Acid protonates the carbonyl oxygen, which makes the carbon more electrophilic. Base supplies hydroxide, a stronger nucleophile than water. The position of equilibrium depends on how electrophilic and how crowded the carbonyl carbon is. Formaldehyde is mostly hydrated in water, most ketones are hydrated very little, and electron-withdrawing groups (as in chloral) favor the hydrate.
Equation: R₂C=O + H₂O ⇌ R₂C(OH)₂, with Khyd = [R₂C(OH)₂] / [R₂C=O]. Khyd is the equilibrium constant for hydration and has no units. Water is left out because it is the solvent. A larger Khyd means more diol.
On the MCAT: You may be asked to rank carbonyls by extent of hydration: formaldehyde > other aldehydes > ketones, and a carbonyl with an electron-withdrawing group such as CF₃ or Cl₃C > an ordinary one. Passages may also show a hydrate as an intermediate, as in oxidation of an aldehyde to a carboxylic acid in aqueous conditions. It is also the water analogue of a hemiacetal, which forms when an alcohol adds instead.
Common confusion: A geminal diol is not a vicinal diol. A vicinal diol has OH groups on adjacent carbons and comes from dihydroxylation of an alkene. Also, most gem-diols cannot be isolated because they revert to the carbonyl when water is removed. Chloral hydrate is a stable exception.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.