Which of the following is correct order of dipole moment of 0, m and p-methyl benzonitrile?

Options

A.
B.
C.
D.

Answer

To determine the correct order of dipole moments for ortho, meta, and para-methyl benzonitrile, we need to analyze the effects of the methyl group and the cyano group on the overall dipole moment of the molecules.

### Step-by-Step Solution:

1. **Identify the Groups**:
- The methyl group (-CH₃) is an electron-donating group (EDG).
- The cyano group (-C≡N) is an electron-withdrawing group (EWG).

2. **Understand Dipole Moment**:
- Dipole moment is a vector quantity that depends on the magnitude and direction of the individual bond dipoles in a molecule.
- The overall dipole moment is influenced by the arrangement of these groups in the ortho, meta, and para positions.

3. **Analyze the Ortho Isomer**:
- In ortho-methyl benzonitrile, the methyl group and the cyano group are adjacent to each other.
- The electron-donating effect of the methyl group partially cancels the electron-withdrawing effect of the cyano group.
- The angle between the dipole moments of the two groups is less than 90 degrees, leading to a moderate dipole moment.

4. **Analyze the Meta Isomer**:
- In meta-methyl benzonitrile, the methyl and cyano groups are separated by one carbon atom.
- The electron-donating effect of the methyl group does not effectively counterbalance the electron-withdrawing effect of the cyano group.
- The angle between the dipole moments is also less than 90 degrees, but the overall dipole moment is still lower than that of the ortho isomer.

5. **Analyze the Para Isomer**:
- In para-methyl benzonitrile, the methyl and cyano groups are on opposite ends of the benzene ring.
- The electron-donating effect of the methyl group is maximally effective in this arrangement, leading to a significant dipole moment.
- The dipole moments of the two groups are aligned directly opposite, which adds up to give the highest overall dipole moment.

6. **Conclusion**:
- Based on the analysis, the order of dipole moments from highest to lowest is:
- Para-methyl benzonitrile > Meta-methyl benzonitrile > Ortho-methyl benzonitrile.

### Final Answer:
The correct order of dipole moments is: **p-methyl benzonitrile > m-methyl benzonitrile > o-methyl benzonitrile**.