Q.12 The major product from the following reaction is

Problem Overview

The given reaction sequence involves three electrophilic aromatic substitution (EAS) steps on tert-butylbenzene:

1. SO₃ / H₂SO₄ → Sulfonation

2. HNO₃ / H₂SO₄ → Nitration

3. H⁺, H₂O, heat → Desulfonation

The substituent on benzene is:
R = tert-butyl (–C(CH₃)₃)

We must identify the major final product.

Key Concepts to Remember

• tert-Butyl group is:

  • Strong electron-donating

  • Ortho/para directing

  • Very bulky, causing steric hindrance at ortho positions

• –SO₃H group:

  • Strong deactivating

  • Meta directing

  • Can be reversibly removed by heating in aqueous acid

Step-by-Step Reaction Analysis

Step 1: Sulfonation (SO₃ / H₂SO₄)

• tert-Butylbenzene undergoes sulfonation.

• Due to steric hindrance, substitution occurs mainly at the para position.

• Product formed:
  para-sulfonated tert-butylbenzene

👉 The –SO₃H group acts as a temporary blocking group.

Step 2: Nitration (HNO₃ / H₂SO₄)

Now the ring has two substituents:

• tert-butyl → ortho/para director

• –SO₃H → meta director

The nitration occurs at a position that:

• Is meta to –SO₃H

• Is least sterically hindered

• Is still activated by tert-butyl

✅ The ortho position relative to tert-butyl (meta to –SO₃H) is favored.

Step 3: Desulfonation (H⁺, H₂O, heat)

• The –SO₃H group is removed

• The nitro group remains intact

Final Major Product

After desulfonation, the ring contains:

• tert-Butyl group

• Nitro group at the ortho position

✅ Correct Answer: (D)

o-Nitro-tert-butylbenzene

Explanation of All Options

Option (A): Nitro + SO₃H present

❌ Incorrect

• –SO₃H should be removed in step 3

• Product still shows sulfonic acid group

Option (B): Different positional isomer with SO₃H

❌ Incorrect

• Again, sulfonic acid group remains

• Desulfonation was ignored

Option (C): Meta-nitro tert-butylbenzene

❌ Incorrect

• tert-Butyl group does not direct meta

• Meta substitution is unlikely without a strong meta director

Option (D): Ortho-nitro tert-butylbenzene

✅ Correct

• Consistent with:

  • Temporary para blocking by –SO₃H

  • Nitration at ortho position

  • Final desulfonation