Basicity & Nucleophilicity (A2 Only)
Chemistry: AQA A Level, topic 6.5.2
Basicity and Nucleophilicity
Amines display both basic and nucleophilic behaviour. We also will look at quaternary ammonium salts.
Basicity
- Amines contain a lone pair of electrons. This makes the amine attracted to positive charge and, in particular, to protons.
- This gives them the basic property of being able to accept protons - as shown above.
Nucleophilicty
- The lone pair on the nitrogen also makes amines nucleophilic.
- It is attracted to positive charge.
- They react with halogenoalkanes, acyl chlorides and acid anhydrides in nucleophilic reactions.
Quaternary ammonium salts
- Quaternary ammonium salts can be used as cationic surfactants.
- Surfactants lower the surface tension of a liquid.
- They can act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants.
- The cationic surfactant has a hydrophilic head, which is able to interact with water, and a hydrophobic tail, which interacts with organic matter such as oils or fats.
Strength of Basicity
We can evaluate the strength of an amine's basicity by considering its -R groups.
Strength of basicity
- The basic strength of an amine depends on the availability of electrons.
- If the molecule's electron pair is more available, it will be a stronger base.
- This availability is often affected by whether the -R groups push or pull electron density.
- We can use ammonia (NH3) as a reference and evaluate whether specific -R groups will increase or decrease the electron availability in comparison to ammonia.
Aliphatic amines
- Alkylated amines are more basic than ammonia.
- Any alkyl -R group will increase basicity.
- Alkyl -R groups push electron density towards the nitrogen.
- This increases the electron density on nitrogen, making the lone pair more available.
Aliphatic amines
- So we can say aliphatic amine basicity strength is of the order:
- 3o > 2o > 1o
Aromatic amines
- Aromatic amines are less basic than ammonia.
- An aromatic -R group will decrease basicity.
- Aromatic -R groups pull electron density away from the nitrogen.
- This decreases the electron density on nitrogen, making the lone pair less available.
Test yourself on Basicity & Nucleophilicity (A2 Only)
Which of the following will be most basic amine?
Which of these pairs are less basic?