Primary amines
Specification Reference Organic Chemistry, Nitrogen compounds 19.1
Quick Notes – Primary Amines
- Primary amines contain an NH2 functional group
- Can be made by reacting a halogenoalkane with ammonia (NH3).
- The reaction is done in ethanol and heated under pressure.
- It is a nucleophilic substitution reaction where NH3 replaces the halogen and then loses a H+ ion to form NH2.
- A mixture of amines can form if further substitution occurs.
Full Notes – Primary Amines
Amines, their reactions and mechanisms have been outlined in more detail
here.
This page is just what you need to know for CIE A-level Chemistry :)
Preparation of Primary Amines
- Reaction Type: Nucleophilic substitution
- Reagents: NH3 (ammonia) in ethanol
- Conditions: Heat under pressure (sealed tube or pressure vessel)
Example Ethylamine formation
CH3CH2Br + 2NH3 → CH3CH2NH2 + NH4Br
(Ethylamine is formed from bromoethane)
Explanation:
- Ammonia acts as a nucleophile, attacking the δ+ carbon in the halogenoalkane.
- A halide ion is displaced, and the nitrogen bonds to the carbon chain, forming a primary amine.
- A second ammonia molecule removes a proton (H+) from the product to neutralise the positive charge, forming NH4+.
Note on Further Reactions
- The newly formed primary amine can act as a nucleophile and react with more halogenoalkane.
- This leads to the formation of secondary and tertiary amines, and eventually a quaternary ammonium salt.
This is not required for AS level, but explains why excess ammonia is often used — to favour the formation of the primary amine.
Summary
- Primary amines contain an NH2 group.
- Made by nucleophilic substitution of halogenoalkanes with ammonia in ethanol, heated under pressure.
- Ammonia acts as the nucleophile and a second NH3 removes a proton to form NH4+.
- Excess ammonia ensures mainly primary amine product; otherwise mixtures of amines (secondary, tertiary, quaternary) form.