Properties and reactions of amines

CIE A-Level Chemistry· Unit 23: Amines· 25 min read

1. Physical Properties of Amines★★☆☆☆⏱ 8 min

Amines have polar N-H bonds, allowing them to form hydrogen bonds between molecules. This intermolecular force explains their boiling point and solubility trends, which are a common multiple-choice exam question.

📘 Definition

Boiling point trend

For amines of similar molar mass, boiling point decreases from primary to secondary to tertiary. Primary amines have two N-H bonds for hydrogen bonding, secondary have one, and tertiary have no N-H bonds.

Example:

1-aminopropane (b.p. 49°C) > dimethylamine (b.p. 7°C) > trimethylamine (b.p. 3°C)

📐 Worked Example

Explain why ethylamine has a higher boiling point than propane, but lower than ethanol of similar molar mass.

1
Step 1: Compare intermolecular forces of the three compounds
2
Ethylamine has polar N-H bonds that form intermolecular hydrogen bonds. Propane is non-polar and only has weak London dispersion forces.
3
Step 2: Compare strength of hydrogen bonds in ethylamine vs ethanol
4
Oxygen is more electronegative than nitrogen, so O-H bonds are more polar than N-H bonds. This makes hydrogen bonds in ethanol stronger than in ethylamine.
5
Step 3: Draw the conclusion
6
Stronger intermolecular forces require more energy to break, so boiling point order is: ethanol > ethylamine > propane.

Exam tip:

Always link boiling point differences to the number and strength of hydrogen bonds, not just presence or absence of hydrogen bonding.

2. Basicity of Amines★★★☆☆⏱ 10 min

Amines act as Brønsted-Lowry bases because the lone pair of electrons on the nitrogen atom accepts a proton from an acid. Basic strength depends on how available this lone pair is to accept a proton.

📘 Definition

Base dissociation constant

K_b

A measure of base strength: a larger $K_{b}$ (or smaller $p K_{b}$ ) corresponds to a stronger base.

Aliphatic amines are stronger bases than ammonia, due to the positive inductive (+I) effect of alkyl groups donating electron density to nitrogen.
Unsubstituted aromatic amines are weaker bases than ammonia, because the nitrogen lone pair is delocalised into the benzene ring.
In aqueous solution, secondary aliphatic amines are stronger bases than primary, which are stronger than tertiary.

📐 Worked Example

Arrange methylamine, dimethylamine, phenylamine and ammonia in order of increasing basicity.

1
Step 1: Separate aromatic and aliphatic amines
2
Phenylamine is aromatic, so it is the weakest base, as the lone pair is delocalised into the benzene ring.
3
Step 2: Compare aliphatic amines to ammonia
4
All aliphatic amines have +I alkyl groups, so they are all stronger bases than ammonia.
5
Step 3: Order the aliphatic amines
6
Dimethylamine (secondary) has two methyl groups, giving a greater +I effect than methylamine (primary) which has one. So dimethylamine > methylamine > ammonia > phenylamine.
7
In order of increasing basicity: phenylamine < ammonia < methylamine < dimethylamine

Exam tip:

Always mention the availability of the nitrogen lone pair to accept a proton when explaining basicity trends, this is required for full marks.

3. Key Chemical Reactions★★★☆☆⏱ 10 min

Reaction with acids: All amines react with strong acids to form alkylammonium salts.
Nucleophilic substitution with halogenoalkanes: Primary amines react progressively to form secondary, tertiary amines and finally quaternary ammonium salts with excess halogenoalkane.
Nucleophilic addition-elimination with acyl chlorides: Primary and secondary amines form amides; tertiary amines do not form amide products.
Diazotisation: Primary aromatic amines react with nitrous acid (generated in situ) below 10°C to form stable diazonium salts.

📐 Worked Example

Write the equation for the reaction of phenylamine with excess bromomethane, and name the organic product.

1
Step 1: Identify the reaction as nucleophilic substitution, where the amine acts as a nucleophile
2
With excess bromomethane, three sequential substitutions occur, forming a quaternary ammonium salt product.
3
$C_6H_5NH_2 + 3 CH_3Br \rightarrow C_6H_5N^+(CH_3)_3 Br^- + 2 HBr$
4
Product name: phenyltrimethylammonium bromide

4. Identifying Amine Classes★★★★☆⏱ 7 min

The Hinsberg test is the standard method to distinguish between primary, secondary and tertiary amines, and is frequently asked in practical and structured questions.

📘 Definition

Hinsberg Test

A chemical test that uses benzenesulfonyl chloride ( $C_{6} H_{5} S O_{2} C l$ ) to distinguish amine classes, based on differing solubility of products in aqueous alkali.

📐 Worked Example

Describe how to distinguish between unlabelled samples of propylamine (primary), dipropylamine (secondary) and tripropylamine (tertiary) using the Hinsberg test.

1
Step 1: Add benzenesulfonyl chloride and aqueous sodium hydroxide to each sample, then shake.
2
Primary propylamine: Forms a clear solution initially. Acidification of the mixture produces a crystalline precipitate. The sulfonamide product has an acidic proton that dissolves in NaOH.
3
Secondary dipropylamine: Forms an insoluble solid that does not dissolve in NaOH, and does not change when acidified. The product has no acidic proton.
4
Tertiary tripropylamine: No reaction occurs at room temperature. The mixture separates into two layers, with unreacted insoluble tertiary amine forming one layer.

Exam tip:

Remember the pattern: primary = clear solution → precipitate, secondary = insoluble solid, tertiary = no reaction.

5. Common Pitfalls

Wrong move:

Claiming tertiary aliphatic amines are always stronger bases than secondary amines

Why:

This is only true in the gas phase; in aqueous solution (the condition tested by CIE), tertiary amines are less hydrated so overall basicity is lower than secondary

Correct move:

State that in aqueous solution, secondary aliphatic amines are stronger bases than primary and tertiary amines

Wrong move:

Saying tertiary amines do not react with acids

Why:

All amines, including tertiary, have a lone pair on nitrogen that can accept a proton

Correct move:

Recognise that tertiary amines react with acids to form trialkylammonium salts, they just do not form amides in addition-elimination reactions

Wrong move:

Forgetting that diazotisation requires temperatures below 10°C

Why:

Diazonium salts are unstable above 10°C and decompose immediately to phenol and nitrogen gas

Correct move:

Always state the temperature requirement of <10°C when describing diazotisation in exam answers

Wrong move:

Claiming tertiary amines cannot form hydrogen bonds with water

Why:

Even without N-H bonds, the lone pair on nitrogen can form hydrogen bonds with water molecules

Correct move:

Explain that small tertiary amines are soluble in water due to hydrogen bonding with the solvent, but have lower boiling points than primary/secondary amines because they cannot form intermolecular hydrogen bonds

Wrong move:

Generalising that all aromatic amines are weaker than all aliphatic amines without considering substituents

Why:

Aromatic amines with electron-donating substituents (e.g. -CH₃, -OCH₃) can have higher basicity than electron-poor aliphatic amines with electron-withdrawing groups

Correct move:

Follow the general trend only for unsubstituted amines, and adjust for the effect of given substituents

6. Quick Reference Cheatsheet

Property	Primary amine	Secondary amine	Tertiary amine
Boiling point (similar mass)	Highest (2 N-H H-bonds)	Intermediate (1 N-H H-bond)	Lowest (no N-H)
Aliphatic basicity (aqueous)	Weaker than 2°, stronger than 3°	Strongest	Weaker than 2°
Hinsberg test result	Clear solution → precipitate on acid	Insoluble solid, no change	No reaction, 2 layers
Reaction with acyl chloride	Forms N-substituted amide	Forms N,N-disubstituted amide	No amide product
Diazotisation (aromatic)	Forms diazonium salt (<10°C)	No reaction	No reaction

7. Frequently Asked

Why are unsubstituted aromatic amines weaker bases than aliphatic amines?

The lone pair on nitrogen in aromatic amines is delocalised into the benzene ring, making it less available to accept a proton. Aliphatic alkyl groups donate electron density to nitrogen via the +I inductive effect, increasing its proton-accepting ability.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 22
Compare basicity of different amines
2023 · 12
Amine reaction with acyl chloride
2021 · 33
Boiling point comparison of amines

Going deeper

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What's Next

Mastering the properties and reactions of amines is a key foundation for studying other nitrogen-containing organic compounds common in CIE A-Level Chemistry, including amides, amino acids, and proteins. This topic links closely to core concepts like acid-base strength, inductive effects, and nucleophilic reaction mechanisms that appear across all organic chemistry sections of the exam. Questions combining amine basicity trends with other functional groups are very common in both multiple choice and structured questions, so practice applying these trends to unseen molecules to consolidate your understanding.