Amine Structure
An amine is a nitrogen atom with one or more alkyl or aryl substituents. Classification depends on how many carbons are directly bonded to the nitrogen - a classification scheme DIFFERENT from alcohols (where you count carbons attached to the C-OH carbon).
Classification
- Primary (1°): RNH₂. One C on N; two H’s.
- Secondary (2°): R₂NH. Two C’s on N; one H.
- Tertiary (3°): R₃N. Three C’s on N; zero H’s.
- Quaternary ammonium (4°): R₄N⁺. Four C’s on N; positive charge; no lone pair available.
Geometry and Hybridization
Amine nitrogen is sp³ hybridized with a pyramidal geometry. Three sp³ sigma bonds (to R or H groups) and one sp³ orbital containing the lone pair. The lone pair is essential: it makes amines basic (donor) and nucleophilic (attack electrophiles).
Because of rapid pyramidal inversion (nitrogen flipping through a planar transition state), chiral amines with three different R groups usually do NOT have a fixed configuration - they racemize spontaneously. Quaternary ammoniums with four different R groups CAN be chiral (no lone pair to invert through).
Nomenclature
IUPAC names amines by replacing alkane “-e” with “-amine”:
- CH₃NH₂ = methanamine (common: methylamine).
- (CH₃)₂NH = N-methylmethanamine (common: dimethylamine).
- (CH₃)₃N = N,N-dimethylmethanamine (common: trimethylamine).
The “N-” prefix specifies substituents on the nitrogen. Multiple substituents use N, N’ etc.
For complex molecules, amines usually appear as “-amino-” prefixes when a higher-priority group is present:
- H₂N-CH₂-COOH = 2-aminoethanoic acid (common: glycine).
Common Amine Names
| Structure | Common | IUPAC |
|---|---|---|
| CH₃NH₂ | Methylamine | Methanamine |
| C₆H₅NH₂ | Aniline | Benzenamine |
| (CH₃)₂NH | Dimethylamine | N-methylmethanamine |
| CH₂=CH-CH₂NH₂ | Allylamine | Prop-2-en-1-amine |
| Pyridine (C₅H₅N) | Pyridine | Azabenzene (not used) |
| NH₂-(CH₂)₆-NH₂ | Hexamethylenediamine | Hexane-1,6-diamine |
Aniline and pyridine are retained common names that dominate usage.
Quaternary Ammonium Compounds
Quaternary ammoniums (R₄N⁺) have four R groups and a positive charge. They have no lone pair and are NOT basic. They act as permanent cations (always charged regardless of pH). Examples: choline (HOCH₂CH₂N⁺(CH₃)₃), acetylcholine, benzalkonium chloride (disinfectant). Biologically important in membrane phospholipid head groups.
Amines vs Amides
Amines (RNH₂, R₂NH, R₃N) have sp³ nitrogen with an available lone pair - basic and nucleophilic. Amides (RCONR’₂) have sp² nitrogen with the lone pair delocalized into the carbonyl - NOT basic, NOT nucleophilic at N under physiological conditions.
Students often confuse these. The C=O adjacent to N changes everything: it makes amide N flat, unreactive, and part of protein backbones that resist hydrolysis.