Preparation of amines

CIE A-Level Chemistry· 12.1.1 Amines· 10 min read

1. Preparation of Aliphatic Amines: Nitrile Reduction★★★☆☆⏱ 4 min

📘 Definition

Nitrile Reduction

Reduction of the nitrile group ( $- C \equiv N$ ) to form a primary amine, adding 4 hydrogen atoms across the triple bond

Example:

Propanenitrile reduces to propan-1-amine

Reduction of nitriles is the preferred method for producing pure primary aliphatic amines, with one more carbon atom than the starting halogenoalkane. Two reagent combinations are accepted in CIE exams: 1) H₂ gas with a nickel catalyst, heated under pressure, or 2) LiAlH₄ in dry ether, followed by dilute acid hydrolysis.

📐 Worked Example

Starting from bromoethane, outline the synthesis of propan-1-amine via nitrile reduction, including all reagents and conditions.

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Step 1: First, bromoethane reacts with KCN in ethanol, heated under reflux, via nucleophilic substitution to form propanenitrile:
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$CH_3CH_2Br + KCN \rightarrow CH_3CH_2CN + KBr$
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Step 2: Propanenitrile is then reduced to propan-1-amine:
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$CH_3CH_2CN + 4[H] \rightarrow CH_3CH_2CH_2NH_2$
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Step 3: If using H₂/Ni, conditions are ~150°C and 5 atm pressure. If using LiAlH₄, reaction occurs in dry ether followed by dilute acid workup.
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The final product is pure primary propan-1-amine, with no secondary or tertiary amine impurities.

2. Preparation of Aliphatic Amines: Ammonia Nucleophilic Substitution★★★☆☆⏱ 3 min

Ammonia acts as a nucleophile to displace the halide leaving group from a halogenoalkane, forming an amine. This method produces a mixture of products, so it is less preferred than nitrile reduction for pure primary amines.

📘 Definition

Excess Ammonia

A large molar excess of ammonia relative to the halogenoalkane, used to favour formation of primary amine over further substitution reactions

Example:

A 10:1 NH₃:halogenoalkane ratio favours primary amine formation

📐 Worked Example

Explain why excess ammonia is required to favour ethylamine formation from bromoethane.

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Step 1: Ammonia first attacks bromoethane to form ethylammonium bromide. A proton is then transferred to excess ammonia to form ethylamine:
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$CH_3CH_2Br + 2NH_3 \rightarrow CH_3CH_2NH_2 + NH_4Br$
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Step 2: Without excess ammonia, the product ethylamine is itself a nucleophile, and attacks another bromoethane molecule to form diethylamine.
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Further substitution continues to form triethylamine and finally a quaternary ammonium salt, so excess NH₃ shifts equilibrium to favour the primary product.

3. Preparation of Aromatic Amines: Nitroarene Reduction★★☆☆☆⏱ 3 min

All primary aromatic amines are prepared by reducing a nitroarene starting material, which is obtained via nitration of the parent arene. The most common example is preparation of phenylamine from nitrobenzene.

📐 Worked Example

Write the preparation of phenylamine from nitrobenzene, including all reagents and conditions.

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Step 1: Nitrobenzene is heated under reflux with tin (Sn) and concentrated hydrochloric acid:
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$C_6H_5NO_2 + 3Sn + 7HCl \rightarrow C_6H_5NH_3^+Cl^- + 3SnCl_2 + 2H_2O$
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Step 2: The intermediate phenylammonium salt is then deprotonated by adding sodium hydroxide solution to release free phenylamine:
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$C_6H_5NH_3^+Cl^- + NaOH \rightarrow C_6H_5NH_2 + NaCl + H_2O$
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CIE also accepts iron (Fe) instead of tin as the metal for this reaction.

4. Common Pitfalls

Wrong move:

Calling NaOH the reducing agent for nitrobenzene

Why:

NaOH only deprotonates the intermediate salt, it does not act as a reducing agent

Correct move:

State Sn/Fe + concentrated HCl, heat under reflux, then add NaOH workup

Wrong move:

Claiming ammonia substitution gives pure primary amine

Why:

Even with excess ammonia, further substitution produces a mixture of products

Correct move:

State nitrile reduction gives pure primary amine, which is why it is the preferred method

Wrong move:

Using NaBH₄ to reduce nitriles or nitro groups

Why:

Sodium borohydride is not a strong enough reducing agent for these groups

Correct move:

Use LiAlH₄ or H₂/Ni for nitriles, Sn/HCl for nitroarenes

Wrong move:

Using aqueous KCN for nitrile formation from halogenoalkanes

Why:

Aqueous KCN causes hydrolysis of the halogenoalkane to an alcohol, not substitution to nitrile

Correct move:

Use KCN dissolved in ethanol, heated under reflux to form the nitrile product

5. Quick Reference Cheatsheet

Amine Type	Method	Reagents & Conditions	Product Purity
Aliphatic Primary	Nitrile Reduction	H₂/Ni (heat/pressure) OR LiAlH₄/dry ether	Pure 1° amine
Aliphatic Primary	Halogenoalkane + NH₃	Excess NH₃, ethanol, heat	Mixture (favours 1°)
Aromatic Primary	Nitroarene Reduction	Sn/Fe + conc HCl, reflux → NaOH workup	Pure 1° aromatic amine

6. Frequently Asked

Do I need different conditions for aliphatic vs aromatic amine preparation?

Yes. Reduction of nitroarenes for aromatic amines uses different reagents and conditions than nitrile reduction for aliphatic amines, as outlined in this guide.

Going deeper

syllabusCIE 9701 Amines SyllabusVerify learning outcome 12.1.1

What's Next

Preparation of amines is a core topic for multi-step organic synthesis questions, which carry high marks in both Paper 2 and Paper 4 of CIE A-Level Chemistry. Mastery of these preparation routes is essential for planning synthetic pathways to target organic molecules, including dyes and pharmaceuticals. Next, you can build on this knowledge by exploring the properties and reactions of amines that are commonly tested in exams.