Amines are a group of organic compounds containing a nitrogen atom with a lone pair of electrons, bonded to alkyl or aryl groups. They are considered derivatives of ammonia (NH3) where one or more hydrogen atoms have been replaced by these groups.
Amines are classified based on the number of carbon atoms (from alkyl or aryl groups) directly attached to the nitrogen atom. They are categorized into three main types:
- (a) Primary (1°) amines
- (b) Secondary (2°) amines
- (c) Tertiary (3°) amines
In primary amines, only one alkyl or aryl group is directly bonded to the nitrogen atom. They have the general formula:
R−NH2
Examples of Primary Amines:
| Name | Formula |
|---|
| Methylamine | CH3−NH2 |
| Ethylamine | CH3CH2−NH2 |
| Propylamine | CH3CH2CH2−NH2 |
| Aniline (aryl amine) | C6H5−NH2 |
In secondary amines, two alkyl or aryl groups are directly bonded to the nitrogen atom. They have the general formula:
R−NH−R′
(where R and R' can be the same or different)
Examples of Secondary Amines:
| Name | Formula |
|---|
| Dimethylamine | CH3−NH−CH3 |
| Ethylmethylamine | CH3CH2−NH−CH3 |
| Diethylamine | CH3CH2−NH−CH2CH3 |
In tertiary amines, three alkyl or aryl groups are directly bonded to the nitrogen atom. They have the general formula:
R3N
(where the R groups can be the same or different)
Example of a Tertiary Amine:
| Name | Formula |
|---|
| Trimethylamine | (CH3)3N |
| Triethylamine | (CH3CH2)3N |
Amines are named using the following rules:
- Primary amines: Name the longest carbon chain attached to nitrogen as the parent alkane, replace the terminal '-e' with '-amine'. Example: CH3CH2NH2 → ethanamine.
- Secondary and tertiary amines: The longest chain is the parent; shorter chains on nitrogen are named as N-substituents. Example: CH3−NH−CH2CH3 → N-methylethanamine.
- Multiple identical substituents on N: Use N,N- prefix. Example: CH3−N(CH3)−CH2CH2CH3 → N,N-dimethylpropan-1-amine.
The nitrogen atom in amines is sp3 hybridized:
- Three of the four sp3 hybrid orbitals form σ bonds with carbon or hydrogen atoms.
- The fourth sp3 orbital holds the lone pair of electrons.
- This gives amines a trigonal pyramidal geometry.
- Due to lone pair–bond pair repulsion (VSEPR), bond angles are slightly less than 109.5∘ (approximately 107∘ in trimethylamine).
The presence of a lone pair on the nitrogen atom makes amines basic and nucleophilic.
- Amines act as Brønsted-Lowry bases by accepting a proton (H+) via the lone pair on nitrogen.
- In the gas phase, basicity increases with the number of electron-donating alkyl groups:
3∘>2∘>1∘>NH3
- Alkyl groups exert a positive inductive effect (+I effect), pushing electron density toward nitrogen and making the lone pair more available.
- In aqueous solution, the order changes due to solvation effects:
(CH3)2NH>CH3NH2>(CH3)3N>NH3
- Lower amines (methyl-, ethyl-, trimethylamine) are gases at room temperature with a characteristic fishy/pungent odour.
- Primary and secondary amines can form intermolecular hydrogen bonds (N–H···N) due to the presence of N–H bonds, giving them higher boiling points than tertiary amines of similar molecular mass.
- Tertiary amines lack N–H bonds and cannot hydrogen bond with each other; they have lower boiling points.
- Lower amines are soluble in water (can form hydrogen bonds with water); solubility decreases as the alkyl chain length increases.
For more information on the electronic structure of nitrogen, see Shapes of Orbitals→ and Electronic Configuration→.