Primary Amines

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Recall the reactions by which amines can be produced:
    • (a) Reaction of a halogenoalkane with NH₃ in ethanol heated under pressure

Note: Classification of amines will not be tested at AS Level.

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What Are Amines?

Amines are organic compounds (carbon-based molecules) that contain the amine functional group (-NH₂).

A functional group is a specific group of atoms within a molecule that gives the molecule its characteristic chemical properties. Think of it like a building block that determines how the molecule behaves.

The amine functional group consists of:

  • One nitrogen atom (N)
  • Two hydrogen atoms (H)
  • Written as -NH₂

Making Amines from Halogenoalkanes

There is one key method you need to know for producing amines at AS Level:

Reaction of a halogenoalkane with ammonia (NH₃) in ethanol, heated under pressure

Let's break this down step-by-step.

Step 1: Understanding Halogenoalkanes

Halogenoalkanes are organic molecules that contain:

  • A carbon chain (the alkane part)
  • A halogen atom (the halogeno part)

Halogens are elements from Group 7 of the periodic table: fluorine (F), chlorine (Cl), bromine (Br), and iodine (I).

Examples of halogenoalkanes:

  • Bromoethane: CH₃CH₂Br
  • Chloromethane: CH₃Cl
  • Iodopropane: CH₃CH₂CH₂I

Step 2: Why Does This Reaction Happen?

The reaction works because of electronegativity differences and nucleophilic attack.

Electronegativity means how strongly an atom pulls electrons towards itself. Halogen atoms are more electronegative than carbon atoms.

In a halogenoalkane molecule:

  • The halogen atom pulls electrons from the C-X bond (where X = the halogen) towards itself
  • This creates a partial positive charge (δ+) on the carbon atom
  • The halogen gets a partial negative charge (δ-)

Think of it like a tug-of-war with electrons – the halogen is stronger and pulls the electrons towards itself, leaving the carbon slightly electron-poor.

Step 3: The Nucleophilic Attack

Ammonia (NH₃) has a lone pair of electrons on the nitrogen atom. These are two electrons that aren't being used in bonding.

A nucleophile is a species (atom or molecule) that donates a pair of electrons to form a new bond. The word comes from "nucleus-loving" – nucleophiles are attracted to positive charges.

Here's what happens:

  1. The ammonia molecule acts as a nucleophile
  2. The lone pair of electrons on the nitrogen atom is attracted to the partially positive carbon atom in the halogenoalkane
  3. The nitrogen attacks the carbon atom
  4. A new bond forms between the nitrogen and carbon
  5. The C-X bond breaks
  6. The halogen leaves as a halide ion (X⁻)

This type of reaction is called a nucleophilic substitution reaction because:

  • A nucleophile (ammonia) attacks
  • One group (the halogen) is substituted (replaced) by another group (the amine group)

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