amine

Meaning: An amine has a nitrogen atom with a lone pair bonded to one, two or three carbon groups: primary (RNH₂), secondary (R₂NH) or tertiary (R₃N). The lone pair makes amines Brønsted-Lowry bases, Lewis bases and good nucleophiles. Alkyl groups donate electron density, so simple alkylamines are usually more basic than ammonia, while aromatic amines such as aniline are less basic because the lone pair is delocalized into the ring. Primary and secondary amines hydrogen-bond, so their boiling points fall between those of alkanes and alcohols of similar mass.

Equation: RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻, with Kb = [RNH₃⁺][OH⁻]/[RNH₂], where brackets are molar concentrations (mol/L) and Kb is the base dissociation constant.

On the MCAT: Expect extraction questions. Dilute acid protonates the amine to a water-soluble ammonium salt, and added base regenerates the neutral amine, which returns to the organic layer. Other settings include ranking basicity, amines as nucleophiles in SN2 reactions and amide formation, and the amino group of amino acids.

Common confusion: An amide is not an amine. In an amide the nitrogen is bonded to a carbonyl carbon, and resonance delocalizes the lone pair, so amides are far less basic. A quaternary ammonium ion has no lone pair either, so it cannot act as a base or nucleophile.