Base Properties of Amines
Specification Reference Organic chemistry, Amines 3.3.11.2
Quick Notes
- Amines are weak bases
- due to their ability to accept protons (H+) using the lone pair of electrons on the nitrogen atom
- Base strength (how easily a H+ ion is accepted) varies between molecules due to differences in the availability of the lone pair of electrons on the nitrogen atom.
- Primary aliphatic amines are stronger bases than ammonia and ammonia is a stronger base than aromatic amines
- Electron-donating groups (e.g., alkyl groups) increase base strength, while electron-withdrawing groups (e.g., benzene rings) decrease base strength.
Full Notes
Amines as Bases
Amines can act as Bronsted-Lowry bases (proton acceptors).
The lone pair of electrons on the nitrogen atom can accept a H+ ion.
The strength of the base (basicity) depends on how readily nitrogen’s lone pair accepts a proton. This is described as how ‘available’ the lone pair of electrons is.
The more available the lone pair is to accept a H+ ion – stronger base
The less available the lone pair is to accept a H+ ion – weaker base
Comparing the Base Strength of Amines
Ammonia (NH3)
- The nitrogen lone pair is available to accept a H+ ion.
- Moderate base strength.
Primary Aliphatic Amines (RNH2)
- Alkyl groups donate electrons (positive inductive effect), increasing electron density on nitrogen.
- This makes the nitrogen lone pair more available to accept H+.
- Stronger base than ammonia.
Primary Aromatic Amines (e.g., Phenylamine, C6H5NH2)
- Benzene ring withdraws electron density from nitrogen (negative inductive effect) through partial delocalisation into the ring.
- As a result, the nitrogen lone pair is less available to accept H+.
- Weaker base than ammonia.
Summary Table: Base Strength of Amines
| Compound | Base Strength | Explanation |
|---|---|---|
| Primary Aliphatic Amine (CH3NH2) | Strongest | Alkyl group donates electrons, increasing lone pair availability |
| Ammonia (NH3) | Moderate | Lone pair available, but no extra electron donation |
| Primary Aromatic Amine (C6H5NH2) | Weakest | Benzene ring withdraws electrons, reducing lone pair availability |