vicinal diol

Meaning: "Vicinal" means neighboring, so the two hydroxyl groups sit on adjacent carbons. Ethylene glycol and 1,2-propanediol are examples. Two OH groups give more hydrogen bonding than a simple alcohol, so a diol is more water-soluble and has a higher boiling point than a monoalcohol of similar size.

On the MCAT: Passages ask how to make or break one. Syn dihydroxylation uses OsO₄ (then a reducing workup such as NaHSO₃) or cold, dilute, basic KMnO₄. Both OH groups add to the same face of the alkene through a cyclic intermediate, so a cis cycloalkene gives a cis diol. Anti dihydroxylation takes two steps: a peroxyacid (such as mCPBA) makes an epoxide, then aqueous acid or hydroxide opens it by backside attack, giving the trans diol. You may be asked to predict the stereochemistry, including meso products. Periodic acid (HIO₄) cleaves the C–C bond between the two carbinol carbons and gives aldehydes or ketones. Sugars contain vicinal diols, so this cleavage can appear in carbohydrate questions.

Common confusion: A vicinal diol is not a geminal diol. A geminal diol has both OH groups on the same carbon and is the hydrate of an aldehyde or ketone. It usually favors the carbonyl form. Also, hot or concentrated KMnO₄ goes past the diol and cleaves the C–C bond, so only the cold, dilute, basic conditions stop at the diol.