alcohol
Summary
An alcohol is an organic compound with a hydroxyl group (–OH) on a saturated sp³ carbon. Hydrogen bonding raises its boiling point. The most testable point is oxidation: primary gives an aldehyde or carboxylic acid, secondary gives a ketone, and tertiary resists oxidation.
Detail
Meaning: An alcohol has a hydroxyl group (–OH) on a saturated sp³ carbon (R–OH), and IUPAC names end in -ol. It is primary, secondary or tertiary by the number of carbons bonded to the carbon bearing the –OH. Hydrogen bonding gives alcohols higher boiling points than alkanes or ethers of similar mass. They are weakly acidic (pKa about 16–18). OH⁻ is a poor leaving group, so substitution or elimination needs the oxygen protonated by acid or converted to a tosylate.
On the MCAT: Expect oxidation questions. A primary alcohol gives an aldehyde with PCC, or a carboxylic acid with a strong oxidant such as CrO₃ in acid or KMnO₄. A secondary alcohol gives a ketone. A tertiary alcohol resists oxidation because its carbinol carbon has no hydrogen to lose. Also tested: ranking boiling points and acid-catalyzed dehydration to an alkene.
Common confusion: A phenol has the –OH on an aromatic ring. Resonance stabilization of the phenoxide ion makes phenol (pKa about 10) much more acidic than an ordinary alcohol. Also, an alcohol's –OH is covalently bonded and does not release hydroxide in water, so alcohols are not bases like NaOH.
Sources
- Organic Chemistry (Clayden, Greeves, Warren)
- 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.