hydroxyl
Summary
A hydroxyl is an –OH group covalently bonded to carbon, as in alcohols and phenols. It is polar and both donates and accepts hydrogen bonds, which raises boiling point and water solubility. Testable point: –OH is a poor leaving group unless it is first protonated or converted.
Detail
Meaning: The hydroxyl group is an oxygen bonded to a hydrogen and attached by a single bond to a carbon (R–OH). On an sp³ carbon it makes an alcohol (suffix -ol, prefix hydroxy-); on a benzene ring it makes a phenol. Hydroxyls also occur in carboxylic acids, sugars, and the side chains of serine, threonine and tyrosine. Oxygen is electronegative, so the O–H bond is polar, and the group can act as a hydrogen-bond donor and acceptor.
On the MCAT: Expect to rank boiling points and water solubility, where more hydroxyl groups (or a shorter carbon chain) generally mean stronger hydrogen bonding and higher values. Expect oxidation questions: a primary alcohol can be oxidized to an aldehyde or carboxylic acid, a secondary alcohol to a ketone, and a tertiary alcohol resists oxidation because its carbinol carbon has no hydrogen. In substitution and elimination, OH⁻ is a strong base and a poor leaving group, so the group is protonated by acid (giving H₂O as the leaving group) or converted to a tosylate. Phenols are more acidic than alcohols because the phenoxide ion is resonance stabilized.
Common confusion: A hydroxyl group is a covalently attached part of a molecule, whereas hydroxide (OH⁻) is a free ion. Ordinary alcohols do not release OH⁻ in water and are not strong bases.
Sources
- Organic Chemistry (Clayden)
- Organic Chemistry (Wade)
- Organic Chemistry (Klein)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.