Primary amines
Primary amines
- An amine has an –NH₂ group (nitrogen in place of a hydrogen of ammonia).
- We can make a primary amine from a halogenoalkane.
- It happens by nucleophilic substitution.
Practice
An amine contains the:
An amine has an –NH₂ group, where nitrogen replaces a hydrogen of ammonia.
Making a primary amine
- Heat a halogenoalkane with ammonia dissolved in ethanol, under pressure:
$$\text{C}_2\text{H}_5\text{Br} + \text{NH}_3 \rightarrow \text{C}_2\text{H}_5\text{NH}_2 + \text{HBr}$$
Practice
A primary amine is made by heating a halogenoalkane with:
Ammonia (in ethanol, under pressure) substitutes the halogen to give the amine.
The mechanism
- This is a nucleophilic substitution: the lone pair on ammonia's nitrogen attacks the slightly positive carbon and pushes out the halogen.
- Use an excess of ammonia, or the amine formed reacts again.
Practice
The reaction is a nucleophilic substitution because:
Ammonia is the nucleophile; its nitrogen lone pair attacks the C–X carbon and pushes out the halogen.
Practice
Why is an excess of ammonia used?
The amine is itself a nucleophile and could react again; excess ammonia favours the primary amine.
You've got it
Key idea
- an amine has the –NH₂ group
- made by heating a halogenoalkane with NH₃ in ethanol, under pressure
- it is a nucleophilic substitution (ammonia's lone pair attacks); use excess NH₃