Amines: structure, classification and nomenclature
Amines are derivatives of ammonia in which alkyl or aryl groups replace H atoms. The nitrogen is sp³ and pyramidal, with one lone pair.
Structure of amines
Amines are derivatives of ammonia: one, two or all three H atoms of NH₃ are replaced by alkyl or aryl groups. Nitrogen is trivalent and keeps one unshared pair of electrons (a lone pair). It is sp³ hybridised. Three sp³ orbitals overlap with orbitals of H or C, and the fourth holds the lone pair. So amines are pyramidal, like ammonia. The lone pair repels the bonding pairs more strongly than they repel each other. So the bond angle is less than 109.5°: the C-N-C angle in trimethylamine is about 108°. The lone pair also makes amines bases and nucleophiles.
Classification
| Type | General formula | Examples | H atoms on N |
|---|---|---|---|
| Primary (1°) | RNH₂ | CH₃NH₂, C₆H₅NH₂ | Two |
| Secondary (2°) | R₂NH | (CH₃)₂NH, C₆H₅NHCH₃ | One |
| Tertiary (3°) | R₃N | (CH₃)₃N, C₆H₅N(CH₃)₂ | None |
Other ways to classify
- Simple amines have all alkyl or aryl groups the same, as in (CH₃)₃N. Mixed amines have different groups, as in CH₃NHC₂H₅.
- Aliphatic amines have N joined to alkyl groups. Aromatic amines (arylamines) have N joined directly to a benzene ring, as in aniline.
- Benzylamine, C₆H₅CH₂NH₂, is not an arylamine: its N is joined to a CH₂ group, so it behaves like an aliphatic amine.
- The class depends on the number of carbons on N, not on the carbon. (CH₃)₃CNH₂ is a primary amine, although tert-butyl alcohol is a tertiary alcohol.
- R₄N⁺X⁻, with four groups on N, is a quaternary ammonium salt, not an amine.
Common and IUPAC names
| Formula | Common name | IUPAC name |
|---|---|---|
| CH₃NH₂ | Methylamine | Methanamine |
| CH₃CH₂CH₂NH₂ | n-Propylamine | Propan-1-amine |
| CH₃CH(NH₂)CH₃ | Isopropylamine | Propan-2-amine |
| CH₃NHCH₂CH₃ | Ethylmethylamine | N-Methylethanamine |
| (CH₃)₃N | Trimethylamine | N,N-Dimethylmethanamine |
| (C₂H₅)₂NCH₂CH₂CH₂CH₃ | N,N-Diethylbutylamine | N,N-Diethylbutan-1-amine |
| CH₂=CHCH₂NH₂ | Allylamine | Prop-2-en-1-amine |
| H₂N(CH₂)₆NH₂ | Hexamethylenediamine | Hexane-1,6-diamine |
| C₆H₅NH₂ | Aniline | Benzenamine (aniline is accepted) |
| C₆H₅N(CH₃)₂ | N,N-Dimethylaniline | N,N-Dimethylbenzenamine |
| 4-CH₃C₆H₄NH₂ | p-Toluidine | 4-Methylbenzenamine (4-methylaniline) |
| 4-BrC₆H₄NH₂ | p-Bromoaniline | 4-Bromobenzenamine (4-bromoaniline) |
| C₆H₅CH₂NH₂ | Benzylamine | Phenylmethanamine |
How to write the IUPAC name
- 1★Find the longest chain joined to N. Drop the final 'e' of the alkane and add amine.Ethane becomes ethanamine
- 2Number the chain so the carbon bearing N gets the lowest number.CH₃CH(NH₂)CH₃ is propan-2-amine
- 3For secondary and tertiary amines, the largest group gives the parent name. Other groups on N are written as prefixes with N-.CH₃NHC₂H₅ is N-methylethanamine; (CH₃)₃N is N,N-dimethylmethanamine
- 4With two or more NH₂ groups, use di-, tri- with their numbers and keep the 'e' of the alkane.Ethane-1,2-diamine, hexane-1,6-diamine
- 5For arylamines, the parent is benzenamine (aniline). Number the ring from the carbon bearing NH₂ as C1.4-Bromobenzenamine
★ key step
Worked example: name and classify
Give the IUPAC name and the class (1°, 2° or 3°) of: (i) CH₃CH₂CH(NH₂)CH₃ (ii) CH₃CH₂CH₂NHCH₃ (iii) CH₃CH₂C(CH₃)₂NH₂ (iv) CH₃CH₂N(CH₃)CH₂CH₂CH₃
- (i) 4-carbon chain, N on C2. Butan-2-amine; N has two H, so primary.
- (ii) The groups on N are propyl and methyl. Propyl is larger, so the parent is propan-1-amine. N-Methylpropan-1-amine; one H on N, so secondary.
- (iii) 4-carbon chain, numbered from the end nearer N, with a methyl and the NH₂ both on C2. 2-Methylbutan-2-amine; two H on N, so primary.
- (iv) The groups on N are ethyl, methyl and propyl. Propyl gives the parent; ethyl and methyl are N-prefixes in alphabetical order. N-Ethyl-N-methylpropan-1-amine; no H on N, so tertiary.
(i) Butan-2-amine, 1° (ii) N-methylpropan-1-amine, 2° (iii) 2-methylbutan-2-amine, 1° (iv) N-ethyl-N-methylpropan-1-amine, 3°
The smell of fish that is not fresh comes partly from trimethylamine, a tertiary amine. Rubbing fish with lemon juice or tamarind turns the amine into a salt that does not smell.
2 marks: “Write the IUPAC names of (i) CH₃CH₂NHCH(CH₃)₂ (ii) C₆H₅NHCH₂CH₃.” (i) N-Ethylpropan-2-amine. (ii) N-Ethylbenzenamine (N-ethylaniline).
2 marks: “Classify as primary, secondary or tertiary: (CH₃)₂C(NH₂)CH(CH₃)₂, C₆H₅NHCH₂CH₃, CH₃N(C₂H₅)₂, C₆H₅CH₂NH₂.” (CH₃)₂C(NH₂)CH(CH₃)₂: primary (only one carbon on N). C₆H₅NHCH₂CH₃: secondary. CH₃N(C₂H₅)₂: tertiary. C₆H₅CH₂NH₂: primary (an aliphatic amine).
3 marks: “Describe the structure of trimethylamine. Why is its C-N-C angle smaller than the tetrahedral angle?” N is sp³: three sp³ orbitals bond to CH₃ groups and the fourth holds a lone pair, so the shape is pyramidal. Lone pair-bond pair repulsion is stronger than bond pair-bond pair repulsion, so the angle shrinks to about 108°.
Count the H atoms on N, not on C: two H = primary, one H = secondary, none = tertiary.