It is not always easy to predict the position of attack on multiply substituted benzene. If the benzene ring bears different ortho//para directing group at the `1` and `4` positions, the position of further substitution is not immediately clear. Sometimes steric effects determine the outcome. In other cases, electronic factors determine the outcome, and further reaction will be at the position activated by the more strongly activating group.
Some substituens are so strongly activating that no catalyst is needed, and it is often diffcult to stop substitution after mono substituion. Mild condition are needed to restrict the reaction to mono-substitution.
It is possible to reduce the activity such groups by side chain reaction so that the reaction can be stopped use can sometimes be made of removable blocking groups on the ring.
Which of the following is the correct major product ?

Options

A.
B.
C.
D.

Answer

To determine the major product of the electrophilic substitution reaction on a multiply substituted benzene ring, we need to analyze the substituents present on the benzene ring and their directing effects. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Substituents
In the given problem, we have a benzene ring with different substituents at the 1 and 4 positions. We need to identify whether these substituents are electron-donating groups (EDGs) or electron-withdrawing groups (EWGs).

**Hint:** Look for groups that can either donate or withdraw electron density from the benzene ring. EDGs activate the ring for electrophilic substitution at ortho and para positions, while EWGs deactivate the ring and direct substitution to the meta position.

### Step 2: Analyze the Directing Effects
- **EDGs** (like -OCH3, -CH3) direct substitution to the ortho and para positions.
- **EWGs** (like -NO2, -COOH) direct substitution to the meta position.

In this case, if we have a methoxy group (-OCH3) and an isopropyl group (-C(CH3)2), both are EDGs. The methoxy group has a +R effect (resonance) and a -I effect (inductive), while the isopropyl group primarily exhibits +I effect (inductive) and some +R effect.

**Hint:** Determine which substituent is stronger in terms of activating the benzene ring. The stronger activating group will dictate the major product.

### Step 3: Consider Steric Effects
When bulky groups are present, steric hindrance can influence the position of substitution. For instance, if the electrophile is bulky, it may not be able to approach the ortho position of the ring due to repulsion from the substituents.

**Hint:** Consider the size of the substituents and how they might affect the approach of the electrophile. Bulky groups can hinder substitution at certain positions.

### Step 4: Predict the Major Product
Given the substituents and their effects:
- The methoxy group is a strong ortho/para director due to its resonance effect.
- The isopropyl group is also an ortho/para director but is bulkier, which may hinder substitution at the ortho position.

Thus, when a bulky electrophile approaches the benzene ring, it is more likely to substitute at the para position relative to the methoxy group, as the ortho position may be sterically hindered.

**Hint:** Evaluate the possible positions for substitution and choose the one that is both electronically favorable and sterically accessible.

### Step 5: Conclusion
After analyzing the substituents and considering both electronic and steric factors, the major product of the electrophilic substitution on the benzene ring will be at the para position relative to the methoxy group.

**Final Answer:** The correct major product is the para-substituted product relative to the methoxy group.