amine functional group
Summary
An amine is a nitrogen with a lone pair bonded to hydrogen and/or alkyl or aryl groups (RNH₂, R₂NH, R₃N). The lone pair makes it a base and a nucleophile, so below the pKa of its conjugate acid (about 10–11 for simple alkylamines) it exists mainly as the ammonium ion.
Detail
Meaning: An amine is a nitrogen bearing a lone pair and single bonds to hydrogen and/or alkyl or aryl groups. Primary (RNH₂), secondary (R₂NH) and tertiary (R₃N) amines have one, two and three carbon groups on N. The lone pair makes amines basic and nucleophilic. Primary and secondary amines both donate and accept hydrogen bonds; tertiary amines can only accept.
Equation: RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻, with Kb = [RNH₃⁺][OH⁻]/[RNH₂], where concentrations are in mol/L. The conjugate acid RNH₃⁺ has a pKa near 10 to 11 for simple alkylamines.
On the MCAT: Expect basicity rankings. Alkyl groups raise basicity, electron-withdrawing groups lower it, and aniline is a weaker base than alkylamines because its lone pair is delocalized into the ring. Amines are protonated (ammonium) at physiological pH, and an acid wash pulls them into the aqueous layer in an extraction. They also act as nucleophiles in SN2 and acyl substitution reactions.
Common confusion: Amine versus amide. In an amide the nitrogen is bonded to a carbonyl carbon, so its lone pair is delocalized by resonance and the nitrogen is not appreciably basic or nucleophilic.
Sources
- Organic Chemistry (Klein)
- Organic Chemistry (Wade)
- Organic Chemistry (McMurry)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.