diols

Meaning: A diol (glycol) has two –OH groups. In a vicinal diol they sit on adjacent carbons, as in ethylene glycol. In a geminal diol both are on one carbon. It is the hydrate of an aldehyde or ketone and usually favors the carbonyl, though formaldehyde is largely hydrated in water. Extra hydrogen bonding gives diols higher boiling points and greater water solubility than comparable alcohols.

On the MCAT: Vicinal diols are made from alkenes. OsO₄ (or cold, dilute KMnO₄) adds both OH groups to the same face, giving syn addition; cyclohexene gives cis-1,2-cyclohexanediol. A peroxyacid converts the alkene to an epoxide, and aqueous acid opens it by backside attack, giving anti addition. Ethylene glycol also protects ketones and aldehydes as cyclic acetals and joins with diacids to form polyesters.

Common confusion: Students often treat a geminal diol as a stable product like a vicinal diol, or mix up the stereochemistry. A geminal diol is an equilibrium hydrate that usually reverts to the carbonyl. For stereochemistry, OsO₄ reacts through a cyclic osmate ester, which forces syn addition, whereas epoxide opening by water is backside attack and gives anti addition.