N-butylamine (I), diethylamine (II) and N,N-dimethyl ethylamine(III) have the same molar mass. The increasing order of their boiling point is:

Options

A. `III lt II lt I`
B. `I lt II lt III`
C. `II lt III lt I`
D. `II lt I lt III`

Answer

To determine the increasing order of boiling points for N-butylamine (I), diethylamine (II), and N,N-dimethyl ethylamine (III), we need to analyze the intermolecular hydrogen bonding in each compound.

### Step-by-Step Solution:

1. **Identify the Structures**:
- N-butylamine (I): This compound has a straight-chain structure with one amino group (-NH2) at the end of a four-carbon chain (C4H11N).
- Diethylamine (II): This compound has two ethyl groups (C2H5) attached to a nitrogen atom (N), making it C2H5-N-C2H5.
- N,N-dimethyl ethylamine (III): This compound has one ethyl group and two methyl groups attached to the nitrogen atom, represented as C2H5-N(CH3)2.

2. **Assess Hydrogen Bonding**:
- N-butylamine (I): Contains one -NH2 group, which can participate in hydrogen bonding. It has two hydrogen atoms available for bonding.
- Diethylamine (II): Contains one -NH group with two ethyl groups. It has only one hydrogen atom available for hydrogen bonding.
- N,N-dimethyl ethylamine (III): Contains one -NH group with two methyl groups and one ethyl group. It has no hydrogen atoms available for hydrogen bonding since the nitrogen is bonded to two carbon atoms (methyl groups).

3. **Determine the Extent of Hydrogen Bonding**:
- N-butylamine (I) has the highest capacity for hydrogen bonding due to having two hydrogen atoms attached to nitrogen.
- Diethylamine (II) has moderate hydrogen bonding capacity with one hydrogen atom.
- N,N-dimethyl ethylamine (III) has no hydrogen bonding capacity.

4. **Relate Hydrogen Bonding to Boiling Points**:
- The boiling point of a compound is influenced by the strength and extent of intermolecular forces, including hydrogen bonding. More hydrogen bonding generally leads to higher boiling points.
- Therefore, the order of boiling points based on hydrogen bonding capacity is:
- N-butylamine (I) > Diethylamine (II) > N,N-dimethyl ethylamine (III).

5. **Conclusion**:
- The increasing order of boiling points is:
- N,N-dimethyl ethylamine (III) < Diethylamine (II) < N-butylamine (I).

### Final Answer:
The increasing order of boiling points is:
N,N-dimethyl ethylamine (III) < Diethylamine (II) < N-butylamine (I).