Basic Strength of Amines in Aqueous Medium
Amines behave as bases because the nitrogen atom contains a lone pair of electrons which can accept a proton (H⁺).
For simple aliphatic amines in an aqueous medium, the commonly accepted order of basic strength is:
For example:
The basicity of amines in water depends mainly on two competing factors:
+
2. Solvation of the Protonated Amine
Alkyl groups donate electron density towards nitrogen and therefore increase its ability to accept H⁺.
However, after protonation, the resulting ammonium ion must be stabilised by interaction with water molecules.
Alkyl groups have an electron-releasing +I (positive inductive) effect. They push electron density towards nitrogen.
↓
Electron density on N ↑
↓
Ability to accept H⁺ ↑
↓
Basicity ↑
Therefore, replacing H atoms by alkyl groups generally increases electron density at nitrogen.
When an amine accepts H⁺, it forms an ammonium ion. This ion is stabilised by hydrogen bonding with water molecules.
The protonated amine is surrounded by water molecules. This process is called solvation.
Primary ammonium ions have more N–H hydrogen atoms available for hydrogen bonding with water than tertiary ammonium ions. Also, bulky alkyl groups in tertiary amines hinder close interaction with water.
Tertiary amines contain three alkyl groups around nitrogen. These groups make the nitrogen environment more crowded.
The bulky alkyl groups can hinder the approach and interaction of water molecules with the protonated amine.
| Feature | 1° Amine | 2° Amine | 3° Amine |
|---|---|---|---|
| General formula | RNH₂ | R₂NH | R₃N |
| Alkyl groups | 1 | 2 | 3 |
| +I effect | Moderate | Higher | Highest |
| Steric hindrance | Low | Moderate | High |
| Solvation of conjugate acid | Good | Very good | Relatively poor |
| Basic strength in water | High | Highest | Lower than 1° and 2° |
Do not apply the simple rule 3° > 2° > 1° to amines in aqueous solution. That order ignores solvation and steric effects.
For simple aliphatic amines in aqueous medium, the commonly taught order is:
The question asks for the increasing order. Therefore, write the weakest base first.
If only primary, secondary and tertiary amines are considered:
Example:
An amine acts as a Brønsted base by accepting a proton from water.
A stronger base has a greater tendency to accept H⁺ and therefore produces a greater concentration of OH⁻.
Q. Arrange primary, secondary and tertiary amines in increasing order of basic strength in aqueous medium.
Answer:
In water, the +I effect of alkyl groups competes with solvation of the protonated amine. Secondary amines have a favourable balance of both effects.
Q. Why is the order of basic strength of amines in aqueous medium not simply 3° > 2° > 1°?
Answer:
- Alkyl groups increase basicity through their +I effect.
- In aqueous medium, the protonated amine is stabilised by solvation and hydrogen bonding with water.
- Tertiary amines have greater steric hindrance and their protonated ions are comparatively less effectively solvated.
Q. Explain the basic strength order of primary, secondary and tertiary amines in aqueous medium.
Answer:
The +I effect of alkyl groups increases electron density on nitrogen and favours protonation. Thus, increasing alkyl substitution tends to increase basicity.
However, in water, the protonated amines are solvated by water molecules. Secondary ammonium ions are effectively solvated and also benefit from the +I effect of two alkyl groups.
In tertiary amines, the three alkyl groups create greater steric hindrance and reduce effective solvation.
Q. Explain in detail the basic strength of primary, secondary and tertiary amines in aqueous medium.
1. +I Effect
Alkyl groups release electron density towards nitrogen. This increases the tendency of nitrogen to accept H⁺.
2. Protonation
The protonated amine is the conjugate acid of the amine. Its stability influences the basicity.
3. Solvation
Water molecules form hydrogen bonds with the protonated ammonium ion. Better solvation stabilises the conjugate acid and favours the protonation equilibrium.
4. Steric Effect
Three alkyl groups around nitrogen in tertiary amines make the environment crowded and reduce effective interaction with water.
5. Overall Order
Therefore, the increasing order is:
3° Amine < 1° Amine < 2° Amine
Q. Discuss all the factors responsible for the basic strength order of primary, secondary and tertiary amines in aqueous medium.
1. Nature of Amines
2° → R₂NH
3° → R₃N
2. +I Effect
Alkyl groups show a +I effect and increase electron density on nitrogen. This favours acceptance of H⁺.
3. Solvation
In aqueous solution, the protonated amines interact with water by hydrogen bonding. The stability of these solvated ions contributes strongly to their observed basic strength.
4. Steric Hindrance
Tertiary ammonium ions are surrounded by three alkyl groups. This crowding makes solvation comparatively less effective.
5. Balance of Effects
↑
Basicity
but
Steric Hindrance
↓
Solvation
The secondary amine provides a particularly favourable balance of electron donation and solvation in aqueous medium.
6. Final Order
2° > 1° > 3° > NH₃
Increasing order:
NH₃ < 3° < 1° < 2°
1. Amines are basic because nitrogen contains:
✅ Answer
B) A lone pair of electrons
2. In aqueous medium, the generally accepted order for simple aliphatic amines is:
✅ Answer
C) 2° > 1° > 3°
3. Increasing order of basic strength is:
✅ Answer
B) 3° < 1° < 2°
4. Alkyl groups increase amine basicity through:
✅ Answer
B) +I effect
5. In water, protonated amines are stabilised by:
✅ Answer
A) Solvation
6. Which amine has three alkyl groups attached to nitrogen?
✅ Answer
C) Tertiary
7. General formula of a primary amine is:
✅ Answer
A) RNH₂
8. General formula of a secondary amine is:
✅ Answer
B) R₂NH
9. General formula of a tertiary amine is:
✅ Answer
C) R₃N
10. Which factor reduces the expected basicity of tertiary amines in water?
✅ Answer
C
11. A stronger Brønsted base has greater tendency to:
✅ Answer
B) Accept H⁺
12. In the reaction RNH₂ + H₂O ⇌ RNH₃⁺ + OH⁻, RNH₂ acts as:
✅ Answer
B) Base
13. The conjugate acid of RNH₂ is:
✅ Answer
B) RNH₃⁺
14. Which has the highest +I effect among the following?
✅ Answer
D) Tertiary amine
15. If only the +I effect were considered, the expected order would be:
✅ Answer
B) 3° > 2° > 1°
16. The observed order in aqueous medium is affected by:
✅ Answer
C
17. Which amine generally has the best balance of +I effect and solvation in water?
✅ Answer
B) Secondary
18. Steric hindrance is greatest around nitrogen in:
✅ Answer
C) Tertiary amines
19. Complete the order: ___ < 1° < 2°.
✅ Answer
A) 3°
20. The correct basic strength order in aqueous medium is:
✅ Answer
C) 2° > 1° > 3° > NH₃
🎯 Quick Revision
Secondary amine: R₂NH
Tertiary amine: R₃N
Alkyl group: +I effect
+I effect: Basicity ↑
Water: Solvation is important
Tertiary amine: Greater steric hindrance
Basic strength: 2° > 1° > 3° > NH₃
Increasing order: NH₃ < 3° < 1° < 2°
