Primary amines (preparation and reactions)

Chemistry· 12 min read

1. Core Structure and Classification of Primary Amines★☆☆☆☆⏱ 2 min

Primary amines are defined by a single -NH₂ functional group bonded to one other organic group. For aliphatic primary amines, this group is an alkyl chain; for aromatic primary amines such as phenylamine, the nitrogen is directly bonded to a benzene ring.

📘 Definition

Primary amine

R-NH₂ / Ar-NH₂

An amine where the nitrogen atom is covalently bonded to exactly one alkyl/aryl group and two hydrogen atoms, with no additional N-C bonds

Example:

Ethylamine (CH₃CH₂NH₂) and phenylamine (C₆H₅NH₂)

✓ Quick check

Test your understanding of classification:

Which of the following is a primary amine?
- (CH₃)₃N
- CH₃NH₂
- (CH₃)₂NH
- CH₃NHCH₂CH₃
Reveal answer
CH₃NH₂ —
This compound only has one alkyl group bonded to the nitrogen atom, making it a primary amine.

2. Preparation 1: Ammonolysis of Halogenoalkanes★★☆☆☆⏱ 3 min

This SN2 nucleophilic substitution reaction uses concentrated excess ammonia heated under reflux with a halogenoalkane. Excess ammonia minimises side product formation of secondary and tertiary amines by ensuring unreacted ammonia is the dominant nucleophile present.

🔬 Derivation

Goal:

Full balanced equation for 1-bromopropane ammonolysis

Starting from:

CH₃CH₂CH₂Br + NH₃

1
Ammonia attacks the electrophilic carbon bonded to bromine, displacing Br⁻ to form a protonated amine intermediate
2
A second ammonia molecule deprotonates the intermediate to form the free primary amine product

Result:

Final overall equation: $C H_{3} C H_{2} C H_{2} B r + 2 N H_{3} \to C H_{3} C H_{2} C H_{2} N H_{2} + N H_{4} B r$

📐 Worked Example

Write the mechanism for the reaction between excess concentrated ammonia and chloroethane under reflux

1
Draw the chloroethane molecule with a partial positive charge on the carbon bonded to chlorine, and a lone pair on the nitrogen of the ammonia nucleophile
2
Show the lone pair attacking the electrophilic carbon, with the C-Cl bond breaking heterolytically to release Cl⁻
3
Draw the protonated ethylammonium intermediate, then show a second ammonia molecule removing a hydrogen ion from the nitrogen to form ethylamine and NH₄⁺

Exam tip:

Always specify 'excess concentrated ammonia' in your answer, as CIE examiners deduct marks if you omit the excess condition.

3. Preparation 2: Reduction of Nitriles★★☆☆☆⏱ 2 min

This route produces a primary amine with one extra carbon atom compared to the starting halogenoalkane, making it ideal for extending carbon chain length in organic synthesis. Two reagent systems are accepted for CIE assessments.

Lithium aluminium hydride (LiAlH₄) in dry ether at room temperature, followed by dilute acid workup
Catalytic hydrogenation using H₂ gas with a nickel catalyst at 150°C and high pressure

CH_3CN + 4[H] \rightarrow CH_3CH_2NH_2

4. Reaction 1: Primary Amines as Brønsted-Lowry Bases★★★☆☆⏱ 2 min

The lone pair on the nitrogen atom of primary amines accepts a proton, making them weak bases. Aliphatic primary amines are stronger bases than ammonia, as the electron-donating alkyl group increases electron density on the nitrogen atom.

CH_3CH_2NH_2 + H_2O \rightleftharpoons CH_3CH_2NH_3^+ + OH^-

5. Reaction 2: Primary Amines as Nucleophiles★★★☆☆⏱ 3 min

The nitrogen lone pair also allows primary amines to act as nucleophiles in acylation reactions with acyl chlorides, forming substituted amide products. Aromatic primary amines undergo diazotisation at 0-5°C to form stable diazonium salts.

📐 Worked Example

Predict the product and write the balanced equation for the reaction between ethylamine and ethanoyl chloride

1
The ethylamine nucleophile attacks the electrophilic carbonyl carbon of ethanoyl chloride
2
A chloride ion is displaced, and a hydrogen ion is lost from the nitrogen atom
3
The final products are N-ethylethanamide and hydrogen chloride gas: $C H_{3} C H_{2} N H_{2} + C H_{3} C O C l \to C H_{3} C O N H C H_{2} C H_{3} + H C l$

6. Common Pitfalls

Wrong move:

Assuming ammonolysis only produces primary amines

Why:

Without excess ammonia, the primary amine product acts as a nucleophile to form secondary and tertiary amine side products

Correct move:

Always specify concentrated excess ammonia heated under reflux to maximise primary amine yield

Wrong move:

Stating dilute HCl is used for nitrile reduction

Why:

Dilute HCl will hydrolyse nitriles to carboxylic acids, not reduce them

Correct move:

State LiAlH₄ in dry ether, or H₂ with Ni catalyst at high pressure, as the only accepted reduction reagents

Wrong move:

Using a full forward arrow for amine base dissociation in water

Why:

Primary amines are weak bases that only partially ionise in aqueous solution

Correct move:

Use reversible equilibrium arrows for all weak base dissociation equations

Wrong move:

Claiming phenylamine can be made via ammonolysis of chlorobenzene

Why:

The delocalised C-Cl bond in chlorobenzene is too strong for SN2 attack by ammonia

Correct move:

Phenylamine is exclusively prepared via reduction of nitrobenzene with tin and concentrated HCl

Wrong move:

Allowing diazotisation reactions to proceed above 10°C

Why:

Unstable diazonium salts decompose rapidly above 10°C to form phenol and nitrogen gas

Correct move:

Explicitly state 0-5°C ice bath conditions for all diazotisation reactions

7. Quick Reference Cheatsheet

Reaction Type	Reagents	Conditions	Main Product
Ammonolysis of halogenoalkane	Concentrated NH₃	Excess, heated under reflux	Aliphatic primary amine
Reduction of nitrile	LiAlH₄	Dry ether, room temp	Primary amine (+1 C atom)
Amine + strong acid	Dilute HCl	Room temp	Alkylammonium salt
Amine + acyl chloride	RCOCl	Room temp, no water	N-substituted amide
Diazotisation	NaNO₂ + dilute HCl	0-5°C ice bath	Aromatic diazonium salt

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.

2024 · Paper 4
Primary amine preparation reaction
2023 · Paper 3
Amine base titration practical
2022 · Paper 2
Nucleophilic reaction of amines

What's Next

Mastering primary amine preparation and reactions is the foundational step for tackling the full amines unit, which extends to secondary, tertiary and quaternary ammonium compound chemistry. You will next build on your understanding of amine nucleophilicity to explore condensation polymer formation via reaction of diamines with dicarboxylic acid derivatives, a high-weight topic that appears in almost every CIE A2 Paper 4 exam. You will also connect your knowledge of diazotisation to azo dye synthesis, a common practical exam question that tests your ability to predict reaction conditions and identify characteristic product observations. These linked concepts are frequently combined in extended 6-8 mark exam questions, so ensure you have fully memorised the core reaction pathways from this module before progressing.