Physical properties and basic character of amines
Amines are bases because of the lone pair on nitrogen. Aliphatic amines are stronger bases than ammonia; aniline is weaker, as its lone pair is shared with the ring.
Physical properties
| Property | What you see | Why |
|---|---|---|
| State and smell | Lower aliphatic amines are gases with a fishy smell; primary amines with three or more carbons are liquids; higher ones are solids | Larger molecules attract each other more |
| Colour | Aniline and other arylamines are colourless when pure but darken on storage | Slow oxidation by air |
| Solubility in water | Lower aliphatic amines dissolve; solubility falls as molar mass rises; higher amines and aniline hardly dissolve. All dissolve in alcohol, ether and benzene | They form hydrogen bonds with water; a large alkyl or aryl part is hydrophobic |
| Boiling points of isomers | Butan-1-amine 350.8 K > N-ethylethanamine 329.3 K > N,N-dimethylethanamine 310.5 K | Primary amines have two H on N for hydrogen bonding, secondary one, tertiary none |
| Compared with alcohols | Amines boil lower than alcohols of similar molar mass | N is less electronegative than O, so N-H···N hydrogen bonds are weaker than O-H···O bonds |
Amines as bases
- 1★R-NH₂ + HX ⇌ R-NH₃⁺X⁻The lone pair on N accepts H⁺, so amines are Lewis bases. The salts dissolve in water but not in ether
- 2C₆H₅NH₂ + HCl ⇌ C₆H₅NH₃⁺Cl⁻Anilinium chloride
- 3R-NH₃⁺X⁻ + OH⁻ → R-NH₂ + H₂O + X⁻A strong base gives back the amine. This is used to separate amines from non-basic organic compounds that do not dissolve in water
- 4R-NH₂ + H₂O ⇌ R-NH₃⁺ + OH⁻Kb = [RNH₃⁺][OH⁻]/[RNH₂] and pKb = −log Kb. A larger Kb (smaller pKb) means a stronger base
★ key step
pKb values in water
| Amine | pKb |
|---|---|
| Ammonia, NH₃ (for comparison) | 4.75 |
| Methanamine, CH₃NH₂ | 3.38 |
| N-Methylmethanamine, (CH₃)₂NH | 3.27 |
| N,N-Dimethylmethanamine, (CH₃)₃N | 4.22 |
| Ethanamine, C₂H₅NH₂ | 3.29 |
| N-Ethylethanamine, (C₂H₅)₂NH | 3.00 |
| N,N-Diethylethanamine, (C₂H₅)₃N | 3.25 |
| Benzenamine (aniline), C₆H₅NH₂ | 9.38 |
| Phenylmethanamine (benzylamine), C₆H₅CH₂NH₂ | 4.70 |
| N-Methylaniline, C₆H₅NHCH₃ | 9.30 |
| N,N-Dimethylaniline, C₆H₅N(CH₃)₂ | 8.92 |
Explaining the order of basic strength
- Inductive effect: alkyl groups push electrons (+I) towards N and make the lone pair more available. So aliphatic amines are stronger bases than NH₃.
- In the gas phase, only the +I effect acts: 3° > 2° > 1° > NH₃.
- In water, the substituted ammonium ion is also stabilised by hydrogen bonding with water (solvation). More H on N means better solvation, so the 1° ion is best stabilised and the 3° ion least.
- Steric hindrance: bulky groups crowd the N, making it harder to take H⁺ and to be solvated.
- The balance of these three effects gives: (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃, and (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂ > NH₃.
- Arylamines are much weaker bases: in aniline the lone pair is delocalised into the ring by resonance. Aniline has five resonance structures but the anilinium ion only two, so gaining H⁺ costs stability.
- Ring substituents: electron-donating groups (-CH₃, -OCH₃) make aniline more basic; electron-withdrawing groups (-NO₂, -SO₃H, -COOH, -X) make it less basic.
Aliphatic amine vs aniline
Benzylamine (pKb 4.70) behaves like an aliphatic amine because a CH₂ group keeps N away from the ring.
Worked example: arrange in order
Arrange in increasing order of basic strength in water: (i) (C₂H₅)₃N, C₆H₅NHCH₃, NH₃, C₆H₅CH₂NH₂ (ii) aniline, 4-chloroaniline, 4-methoxyaniline.
- (i) In N-methylaniline the lone pair is delocalised into the ring, so it is the weakest (pKb 9.30).
- (i) NH₃ (4.75) and benzylamine (4.70) are close: the CH₂ keeps N off the ring, so benzylamine is a little stronger. Triethylamine (3.25) has three +I ethyl groups and is the strongest here.
- (i) Order: C₆H₅NHCH₃ < NH₃ < C₆H₅CH₂NH₂ < (C₂H₅)₃N.
- (ii) -Cl withdraws electrons from the ring, so 4-chloroaniline is the weakest. -OCH₃ donates electrons into the ring by resonance, so 4-methoxyaniline is the strongest.
- (ii) Order: 4-chloroaniline < aniline < 4-methoxyaniline.
(i) C₆H₅NHCH₃ < NH₃ < C₆H₅CH₂NH₂ < (C₂H₅)₃N (ii) 4-chloroaniline < aniline < 4-methoxyaniline
Fish smells less after a squeeze of lemon or a rub of tamarind: the acid turns smelly amines into salts with no smell. Many medicines that contain amine groups are sold as hydrochloride salts, which dissolve in water.
2 marks: “Why is aniline a much weaker base than benzylamine? (pKb 9.38 and 4.70)” In aniline the N lone pair is delocalised into the benzene ring by resonance, so it is less available for H⁺; the anilinium ion has fewer resonance structures. In benzylamine a CH₂ group separates N from the ring, so the lone pair stays on N.
2 marks: “Arrange in decreasing order of basic strength in water and explain: (CH₃)₃N, CH₃NH₂, (CH₃)₂NH, NH₃.” (CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃. +I effect raises basicity; solvation of the ammonium ion by hydrogen bonding favours fewer alkyl groups; steric crowding lowers the 3° amine.
3 marks: “Account for: (i) butan-1-amine boils higher than N,N-dimethylethanamine, though both are C₄H₁₁N (ii) propan-1-amine mixes with water, but hexan-1-amine is only slightly soluble (iii) aniline dissolves in dilute HCl, though it hardly dissolves in water.” (i) The 1° amine has two N-H bonds for intermolecular hydrogen bonding; the 3° amine has none (350.8 K against 310.5 K). (ii) Both hydrogen-bond with water, but the long hydrophobic chain of hexan-1-amine lowers its solubility. (iii) Aniline is a base and forms anilinium chloride, C₆H₅NH₃⁺Cl⁻, an ionic salt that dissolves in water.
A free lone pair makes a strong base. Alkyl groups push it out (stronger); a ring pulls it in (weaker). In water the 2° amine wins in both series, and aniline is far behind NH₃.