Arrange the following compounds in order of increasing dipole moment .
Toluene `(I)` m-dichlorobenzene `(II)`
o-dichlorobenzene `(III)` . P-dichlorobenzene `(IV)` .

Options

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

Answer

To determine the order of increasing dipole moment for the given compounds (toluene, m-dichlorobenzene, o-dichlorobenzene, and p-dichlorobenzene), we need to analyze the structures and the effects of substituents on the dipole moment.

### Step-by-Step Solution:

1. **Identify the Structures**:
- **Toluene (I)**: This is a benzene ring with a methyl group (–CH₃) attached.
- **m-Dichlorobenzene (II)**: This is a benzene ring with two chlorine atoms at the meta positions (1,3-positions).
- **o-Dichlorobenzene (III)**: This is a benzene ring with two chlorine atoms at the ortho positions (1,2-positions).
- **p-Dichlorobenzene (IV)**: This is a benzene ring with two chlorine atoms at the para positions (1,4-positions).

2. **Consider Electronegativity and Bond Polarity**:
- Chlorine is more electronegative than carbon, which means that the C-Cl bonds are polar, contributing to the dipole moment.
- The dipole moment is directly proportional to the bond polarity and the distance between the charges.

3. **Analyze the Dipole Moment of Each Compound**:
- **Toluene (I)**: Has a small dipole moment due to the presence of the methyl group, which is weakly electron-donating.
- **m-Dichlorobenzene (II)**: The dipole moments of the two C-Cl bonds do not cancel each other out completely, resulting in a moderate dipole moment.
- **o-Dichlorobenzene (III)**: The dipole moments of the two C-Cl bonds add up more effectively due to their close proximity, leading to a higher dipole moment.
- **p-Dichlorobenzene (IV)**: The dipole moments of the two C-Cl bonds cancel each other out to some extent, resulting in a lower dipole moment compared to o-dichlorobenzene.

4. **Rank the Compounds**:
- Based on the analysis:
- **Lowest Dipole Moment**: p-Dichlorobenzene (IV) - due to cancellation of dipole moments.
- **Next**: Toluene (I) - has a small dipole moment.
- **Next**: m-Dichlorobenzene (II) - moderate dipole moment.
- **Highest Dipole Moment**: o-Dichlorobenzene (III) - due to effective addition of dipole moments.

5. **Final Order**:
- The order of increasing dipole moment is:
- (IV) p-Dichlorobenzene < (I) Toluene < (II) m-Dichlorobenzene < (III) o-Dichlorobenzene.

### Conclusion:
The final arrangement of the compounds in order of increasing dipole moment is:
**p-Dichlorobenzene < Toluene < m-Dichlorobenzene < o-Dichlorobenzene**.