amine or ammonia

Meaning: Ammonia is trigonal pyramidal nitrogen with three H and one lone pair. An amine swaps H atoms for carbon groups: primary (one carbon on N), secondary (two), tertiary (three). The lone pair makes amines weak bases and good nucleophiles. Protonation gives an ammonium ion, which is water soluble. Primary and secondary amines can donate and accept hydrogen bonds; tertiary amines can only accept them.

Equation: RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻, with Kb = [RNH₃⁺][OH⁻]/[RNH₂]. Concentrations are in mol/L, and Kb is treated as unitless. For a conjugate pair at 25 °C, pKa of RNH₃⁺ + pKb of RNH₂ = 14.

On the MCAT: Rank basicity. Electron-donating alkyl groups generally make an amine more basic than ammonia in the gas phase, while aniline is weaker because its lone pair delocalizes into the ring. Amides are essentially nonbasic because the lone pair delocalizes onto the carbonyl. Extraction passages: an HCl wash moves an amine into water as its ammonium salt, and NaOH regenerates the neutral amine for the organic layer. Boiling points: primary amines boil higher than alkanes of similar mass but lower than alcohols.

Common confusion: Primary, secondary and tertiary count carbons on nitrogen, not on the attached carbon as with alcohols. So tert-butylamine is a primary amine. Also, an amide nitrogen looks like an amine but is not basic, and ammonium (NH₄⁺) has no lone pair, so it is not nucleophilic.