hydroxyl group

Meaning: A hydroxyl group is an −OH unit covalently bonded to a carbon atom. The O–H bond is polar, so the group can both donate and accept hydrogen bonds. That makes molecules with many hydroxyls, such as sugars, soluble in water, and it gives alcohols higher boiling points than hydrocarbons of similar size. In biochemistry, hydroxyls occur on carbohydrates, on the side chains of serine, threonine and tyrosine, and on the 2′ carbon of ribose. Kinases typically attach phosphate to these side-chain hydroxyls.

On the MCAT: Expect questions that rank solubility or boiling point by the number of hydroxyls, classify serine and threonine as polar and uncharged, or ask why RNA (2′-OH on ribose) is usually less stable than DNA (no 2′-OH). Passages may also ask you to spot phosphorylation sites in a regulatory pathway. Carbohydrate questions use the anomeric hydroxyl: in D-glucose, the α and β anomers differ in the orientation of the −OH on carbon 1.

Common confusion: Hydroxyl (−OH) is a neutral group held by a covalent bond. Hydroxide (OH⁻) is a free ion and a strong base. Alcohols typically do not release OH⁻ in water; they are very weak acids (pKa about 16) and give nearly neutral solutions. The −OH inside a carboxyl group is much more acidic, because the conjugate base is stabilized by resonance.