6.4.1Organic Chemistry and Analysis
Amines — OCR A Level Chemistry A H432 Revision Notes & Topical Questions
5 min read 4 parts Lesson 12 of 33
Section 1 of 4Structure and Classification
Amines are derivatives of ammonia in which one or more hydrogen atoms are replaced by alkyl or aryl groups.
Primary
- One hydrogen of NH₃ is replaced, as in ethylamine CH₃CH₂NH₂; the nitrogen is bonded to one carbon group.
Secondary
- Two hydrogens are replaced, as in dimethylamine (CH₃)₂NH, so the nitrogen is bonded to two carbon groups.
Tertiary
- All three hydrogens are replaced, as in trimethylamine (CH₃)₃N, which has no N–H bond.
Aliphatic or aromatic
- If the group on nitrogen is an alkyl chain the amine is aliphatic;
- phenylamine C₆H₅NH₂ is the aromatic example.
Naming
- Amines are named as alkylamines, such as ethylamine, or with the prefix amino- and the position number, such as 1-aminopropane.
| Name | Formula | Type |
|---|---|---|
| Propylamine (propan-1-amine) | CH₃CH₂CH₂NH₂ | Primary aliphatic |
| Phenylamine | C₆H₅NH₂ | Primary aromatic |
| Dimethylamine | (CH₃)₂NH | Secondary aliphatic |
Section 2 of 4Basicity of Amines
The lone pair on nitrogen can accept a proton, so amines behave as Brønsted–Lowry bases.
Basicity of Amines
Reaction with acids
- CH₃CH₂NH₂ + HCl → CH₃CH₂NH₃⁺Cl⁻, forming a soluble ionic salt from the amine.
Reaction with water
- An amine accepts a proton from water, giving RNH₃⁺ and OH⁻, so the solution is weakly alkaline.
Aliphatic amines
- Alkyl groups push electron density onto nitrogen,
- making the lone pair more available,
- so these amines are stronger bases than ammonia.
Phenylamine
- The nitrogen lone pair delocalises into the benzene ring, becoming less available, so phenylamine is a weaker base than ammonia.
Releasing the amine
- Adding NaOH to the salt removes the proton and regenerates the free amine.
Comparison
- The order of base strength is aliphatic amines above ammonia above phenylamine, a common exam comparison.
Section 3 of 4Ranking Base Strength
1
Start with ammonia as the reference base, with one lone pair and three hydrogens on nitrogen.
2
Replace hydrogens with alkyl groups and the electron-donating effect raises electron density on nitrogen.
3
The more available lone pair accepts a proton more readily, so aliphatic amines are stronger bases than ammonia.
4
Attach nitrogen to a benzene ring and the lone pair overlaps with the ring π system.
5
This delocalisation lowers the availability of the lone pair, so phenylamine is the weakest base in the series.
Section 4 of 4Exam Checklist & Tips
6.4.1 – Key Points for the Exam
- Amine An organic compound derived from ammonia by replacing hydrogen with carbon groups.
- Primary amine An amine with one carbon group attached to nitrogen.
- Lone pair A non-bonding pair of electrons on nitrogen that can accept a proton.
- Base A proton acceptor.
Common Mistakes
- Classifying amines like alcohols; the class depends on groups on nitrogen, not on the carbon.
- Saying phenylamine is a stronger base than ammonia because it has a ring.
- Forgetting that alkylammonium salts are ionic and dissolve in water.
- Drawing the proton attached to the nitrogen without using the lone pair.
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