# Primary amines (preparation and reactions)

> Chemistry · CIE A-Level
> Source: https://www.owlsprep.com/study/cie-9701-u23-primary-amines/

This module walks through standard laboratory preparation methods for aliphatic primary amines, their classification as weak bases, and their key nucleophilic reaction pathways tested in CIE A-Level exams.

**Prerequisites:** [Understanding of nucleophilic substitution mechanisms in halogenoalkanes](https://www.owlsprep.com/study/cie-9701-u16-halogenoalkanes-nucleophilic-substitution/); [Knowledge of functional group priority for organic reaction naming](https://www.owlsprep.com/study/cie-9701-u12-functional-group-nomenclature/)

## Learning objectives

- Describe the two core laboratory preparation routes for aliphatic primary amines
- Explain the nucleophilic substitution mechanism for amine formation from halogenoalkanes
- Recall and predict characteristic reactions of primary amines as bases and nucleophiles
- Construct balanced full and ionic equations for all named reactions of primary amines

## Core Structure and Classification of Primary Amines

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.

**Primary amine** — 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

*Notation:* R-NH₂ / Ar-NH₂

*Example:* Ethylamine (CH₃CH₂NH₂) and phenylamine (C₆H₅NH₂)

**Check your understanding**

Test your understanding of classification:

1. Which of the following is a primary amine?

   - (CH₃)₃N
   - CH₃NH₂
   - (CH₃)₂NH
   - CH₃NHCH₂CH₃

   *Why:* This compound only has one alkyl group bonded to the nitrogen atom, making it a primary amine.

## Preparation 1: Ammonolysis of Halogenoalkanes

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:** 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

*Conclusion:* Final overall equation: $CH_3CH_2CH_2Br + 2NH_3 \rightarrow CH_3CH_2CH_2NH_2 + NH_4Br$

**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.

## Preparation 2: Reduction of Nitriles

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

$$CH_3CN + 4[H] \rightarrow CH_3CH_2NH_2$$

## Reaction 1: Primary Amines as Brønsted-Lowry Bases

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.

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

> **warning**
>
> Never use a full forward arrow for this equilibrium, as primary amines only partially dissociate in water.

## Reaction 2: Primary Amines as Nucleophiles

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: $CH_3CH_2NH_2 + CH_3COCl \rightarrow CH_3CONHCH_2CH_3 + HCl$

## Common pitfalls

- **Wrong:** Assuming ammonolysis only produces primary amines
  - Why it fails: Without excess ammonia, the primary amine product acts as a nucleophile to form secondary and tertiary amine side products
  - Correct: Always specify concentrated excess ammonia heated under reflux to maximise primary amine yield
- **Wrong:** Stating dilute HCl is used for nitrile reduction
  - Why it fails: Dilute HCl will hydrolyse nitriles to carboxylic acids, not reduce them
  - Correct: State LiAlH₄ in dry ether, or H₂ with Ni catalyst at high pressure, as the only accepted reduction reagents
- **Wrong:** Using a full forward arrow for amine base dissociation in water
  - Why it fails: Primary amines are weak bases that only partially ionise in aqueous solution
  - Correct: Use reversible equilibrium arrows for all weak base dissociation equations
- **Wrong:** Claiming phenylamine can be made via ammonolysis of chlorobenzene
  - Why it fails: The delocalised C-Cl bond in chlorobenzene is too strong for SN2 attack by ammonia
  - Correct: Phenylamine is exclusively prepared via reduction of nitrobenzene with tin and concentrated HCl
- **Wrong:** Allowing diazotisation reactions to proceed above 10°C
  - Why it fails: Unstable diazonium salts decompose rapidly above 10°C to form phenol and nitrogen gas
  - Correct: Explicitly state 0-5°C ice bath conditions for all diazotisation reactions

## 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 |

## 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.

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