Which of the following group is meta directing group for incoming eletrophile in benzene ring ?

Question

Accepted Answer

Option 1, -CHO, is the correct answer as a meta-directing group for an incoming electrophile in a benzene ring. Here's why:

1. **Explanation for Option 1 (-CHO):**
   - The -CHO group, also known as the aldehyde group, is an electron-withdrawing group. It decreases the electron density on the benzene ring through a resonance effect, which makes the ortho and para positions less favorable for electrophilic attack. As a result, the meta position becomes relatively more favorable for electrophilic substitution. This is because the positive charge developed during the intermediate carbocation formation is less destabilized at the meta position compared to the ortho and para positions.

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

- **Option 2 ($-NH_2$):**
     - The $-NH_2$ group is an electron-donating group due to its ability to donate electrons through resonance. This increases the electron density at the ortho and para positions, making them more favorable for electrophilic substitution. Therefore, $-NH_2$ is an ortho/para-directing group, not a meta-directing group.

- **Option 3 ($-OH$):**
     - Similar to $-NH_2$, the $-OH$ group is also an electron-donating group. It donates electrons through resonance, enhancing the electron density at the ortho and para positions. This makes $-OH$ an ortho/para-directing group, not a meta-directing group.

- **Option 4 ($-OCH_3$):**
     - The $-OCH_3$ group is another electron-donating group. It donates electrons through resonance, increasing the electron density at the ortho and para positions. Consequently, $-OCH_3$ is an ortho/para-directing group, not a meta-directing group.

In summary, the -CHO group is the only option among the given choices that acts as a meta-directing group due to its electron-withdrawing nature, which makes the meta position more favorable for electrophilic substitution.