Which of the following groups is not a meta-directing group in benzene?

Q: Which of the following groups is not a meta-directing group in benzene?

Which of the following groups is not a meta-directing group in benzene?

Question

Accepted Answer

Option 3, $-NHCOCH_{3}$, is the correct answer because it is not a meta-directing group in benzene. Instead, it is an ortho/para-directing group. Here's a detailed explanation:

1. **Explanation for Option 3 ($-NHCOCH_{3}$):**
   - The $-NHCOCH_{3}$ group is an amide group, which contains a nitrogen atom with a lone pair of electrons. This lone pair can participate in resonance with the benzene ring, donating electron density into the ring. This electron donation increases the electron density at the ortho and para positions, making them more reactive towards electrophilic substitution. Therefore, $-NHCOCH_{3}$ is an ortho/para-directing group, not a meta-directing group.

2. **Explanation for why other options are incorrect:**

- **Option 1 (-$SO_3H$):**
     - The sulfonic acid group (-$SO_3H$) is a strong electron-withdrawing group due to the presence of highly electronegative oxygen atoms. It withdraws electron density from the benzene ring through resonance and inductive effects, decreasing electron density at the ortho and para positions. This makes the meta position relatively more reactive, classifying it as a meta-directing group.

- **Option 2 (-COR):**
     - The acyl group (-COR) is also an electron-withdrawing group. It withdraws electron density through resonance and inductive effects, similar to the sulfonic acid group. This electron withdrawal decreases the electron density at the ortho and para positions, making the meta position more favorable for electrophilic substitution. Hence, it is a meta-directing group.

- **Option 4 (-COOR):**
     - The ester group (-COOR) is another electron-withdrawing group. It withdraws electron density from the benzene ring through resonance and inductive effects, reducing electron density at the ortho and para positions. This makes the meta position more reactive, classifying it as a meta-directing group.

In summary, $-NHCOCH_{3}$ is an ortho/para-directing group due to its ability to donate electron density into the benzene ring, while the other groups are meta-directing because they withdraw electron density, making the meta position more reactive.