Polar covalent molecules exhibit dipole moment. Dipole moment is equal to the product of charge separation , q and the bond length d for the bond. Unit of dipole moment is debye. One debye is equal to `10^(-18)` esu cm.
Dipole moments is a vector quantity. It has both magnitude and direction. Hence, dipole moment of a molecule depends upon the relative orientation of the bond dipoles, but not on the polarity of bonds alone. A symmetrical structure shows zero dipole moment. Thus, dipole moment helps to predict the geometry of a molecules. Dipole moment values can be distinguish between cis- and trans- isomers, ortho, meta and pare-forms of a substance, etc.
Q. Arrange the following compounds in increasing order of dipole moments, toluene (I), o-dichlorobenzene (II), m-dichlorobenzene (III) and p-dichlorobenzene (IV) :

Options

A. `IVltIltIIltIII`
B. `IltIVltIIltIII`
C. `IVltIltIIIltII`
D. `IVltIIltIltIII`

Answer

To arrange the compounds toluene (I), o-dichlorobenzene (II), m-dichlorobenzene (III), and p-dichlorobenzene (IV) in increasing order of dipole moments, we can follow these steps:

### Step 1: Understand the Structures
- **Toluene (I)**: Contains one methyl group (-CH₃) attached to a benzene ring. It has a slight dipole moment due to the presence of the methyl group.
- **o-Dichlorobenzene (II)**: Contains two chlorine atoms on adjacent carbons of the benzene ring. The dipole moments of the two C-Cl bonds add up because they are at an angle (approximately 60°).
- **m-Dichlorobenzene (III)**: Contains two chlorine atoms on carbons that are separated by one carbon. The dipole moments of the two C-Cl bonds are at an angle of approximately 120°.
- **p-Dichlorobenzene (IV)**: Contains two chlorine atoms on opposite sides of the benzene ring. The dipole moments cancel each other out, resulting in a net dipole moment of zero.

### Step 2: Analyze the Dipole Moments
- **p-Dichlorobenzene (IV)**: Since the dipole moments cancel out, the dipole moment is 0.
- **m-Dichlorobenzene (III)**: The angle between the dipole moments is 120°, leading to a moderate dipole moment.
- **o-Dichlorobenzene (II)**: The angle between the dipole moments is 60°, which results in a larger dipole moment than m-dichlorobenzene.
- **Toluene (I)**: Has a small dipole moment due to the presence of the methyl group, but it is less than that of o-dichlorobenzene.

### Step 3: Arrange in Order
Based on the analysis:
1. **p-Dichlorobenzene (IV)**: Dipole moment = 0 (lowest)
2. **Toluene (I)**: Slightly polar, small dipole moment.
3. **m-Dichlorobenzene (III)**: Moderate dipole moment.
4. **o-Dichlorobenzene (II)**: Highest dipole moment due to the angle of 60°.

### Final Order
Thus, the increasing order of dipole moments is:
**p-Dichlorobenzene (IV) < Toluene (I) < m-Dichlorobenzene (III) < o-Dichlorobenzene (II)**

### Answer:
**p-Dichlorobenzene (IV) < Toluene (I) < m-Dichlorobenzene (III) < o-Dichlorobenzene (II)**

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