Amines

Amines are the nitrogen analogues of alcohols — imagine replacing the O of an alcohol with NH, and you have a primary amine. They are everywhere in biology (amino acids, neurotransmitters, vitamins), pharmaceuticals (more than half of all drugs contain at least one amine group) and industry (dyes, rubber, polymers). They are also mildly basic — a direct consequence of the lone pair on nitrogen.

This lesson covers the OCR A-Level Chemistry A (H432) specification point 6.2.3 (a)–(c): classification, basicity and preparation of amines.

1. Structure and Classification

Amines can be thought of as ammonia (NH₃) with one or more H atoms replaced by carbon groups (alkyl or aryl). The number of carbon substituents determines the class of the amine.

Class	Number of C on N	Example	Formula
Primary (1°)	1	Ethylamine	CH₃CH₂NH₂
Secondary (2°)	2	Dimethylamine	(CH₃)₂NH
Tertiary (3°)	3	Trimethylamine	(CH₃)₃N
Quaternary salt	4	Tetramethylammonium chloride	(CH₃)₄N⁺Cl⁻

Key Definition — Primary amine: An amine in which the nitrogen is bonded to exactly one carbon atom and two hydrogens, i.e. –NH₂ attached to a carbon.

Note the difference from alcohols: an alcohol's classification depends on the carbons around the carbon carrying the OH, whereas an amine's classification depends on the carbons around the nitrogen itself. So (CH₃)₃C–NH₂ is a primary amine (one C on N) even though the carbon bearing the NH₂ is tertiary.

1.1 Quaternary ammonium salts

A quaternary ammonium ion has four carbon groups bonded to N and a permanent positive charge. These salts are completely ionised in water, feel surfactant-like, and are the basis of cationic surfactants, fabric softeners and some antibacterial agents.

graph TD
  A[NH3 ammonia] --> B[NH2R primary amine]
  B --> C[NHR2 secondary amine]
  C --> D[NR3 tertiary amine]
  D --> E[NR4+ quaternary ammonium salt]

2. Nomenclature

For simple cases, amines are named by:

Listing substituents with their positions.
Ending the name with -amine.

Examples:

Structure	Name
CH₃NH₂	Methylamine
CH₃CH₂NH₂	Ethylamine
CH₃CH₂CH₂NH₂	Propylamine
(CH₃)₂NH	Dimethylamine
(CH₃)₃N	Trimethylamine
CH₃CH(NH₂)CH₃	Propan-2-amine (or propan-2-ylamine)
C₆H₅NH₂	Phenylamine (aniline)

For secondary and tertiary amines, OCR also accepts the prefix N- to show a substituent on the nitrogen: e.g. N-methylethylamine for CH₃CH₂–NH–CH₃.

3. Basicity of Amines

Because nitrogen has a lone pair, amines can accept a proton — they are Brønsted-Lowry bases:

$CH_3NH_2 + H_2O \rightleftharpoons CH_3NH_3^+ + OH^-$

The equilibrium constant (Kb or sometimes pKa of the conjugate acid) tells you how basic the amine is.

3.1 Aliphatic vs aromatic — a key contrast

Amine	pKa of conjugate acid	Basicity
Ethylamine CH₃CH₂NH₂	10.75	Strongly basic
Methylamine CH₃NH₂	10.66	Strongly basic
Ammonia NH₃	9.25	Moderately basic
Phenylamine C₆H₅NH₂	4.60	Very weakly basic

The pattern is:

Aliphatic primary amines > ammonia > aromatic amines (as bases).

Why?

Alkyl groups push electrons onto N (positive inductive effect, +I), making the lone pair more available for protonation and stabilising the resulting positively charged R–NH₃⁺ ion. This makes ethylamine a stronger base than ammonia.

In aromatic amines (like phenylamine), the nitrogen lone pair overlaps with the π system of the benzene ring — it is delocalised into the ring. This makes the lone pair much less available to bond to H⁺, so phenylamine is a very weak base. In fact, phenylamine is so weakly basic that it does not turn red litmus blue; you need a strong acid like HCl to protonate it fully.

graph TD
  A[Amine basicity] --> B[Aliphatic: alkyl +I donates, N lone pair more available<br/>Stronger base than NH3]
  A --> C[Aromatic: N lone pair delocalised into ring<br/>Much weaker base than NH3]

3.2 Reaction with acids

Amines react with HCl to form ammonium salts:

$CH_3CH_2NH_2 + HCl \longrightarrow CH_3CH_2NH_3^+Cl^-$

This is how pharmaceutical drugs are often sold — as their water-soluble "hydrochloride" salts. The free base is typically oily and insoluble; the salt is a crystalline solid that dissolves in water for drug delivery.

4. Preparation of Amines

OCR requires you to know two routes to aliphatic amines. Both convert a halide or nitrile into a primary amine.

Lesson 6: Amines