Question

Identify the product obtained when phenol reacts with concentrated sulphuric acid at $293\text{ K}$ ?

• A. Picric acid
• B. o-Phenol sulphonic acid
• C. p-Phenol sulphonic acid
• D. Salicylic acid

Hint:

Memorize these temperature-dependent phenol reactions:
Cold Sulfonation ($293\text{ K}$) $\rightarrow$ Ortho product (Kinetic)
Hot Sulfonation ($373\text{ K}$) $\rightarrow$ Para product (Thermodynamic)

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
We are asked to identify the major organic product formed by the electrophilic aromatic sulfonation of phenol at a specific, relatively low temperature ($293\text{ K}$, or roughly $20^\circ\text{C}$).

Step 2: Key Formula or Approach:
The hydroxyl group (-OH) on phenol is a strongly activating, ortho/para-directing group.
Sulfonation of phenol is highly temperature-dependent because the reaction is reversible. It yields two different major products based on kinetic vs. thermodynamic control.

Step 3: Detailed Explanation:
When phenol is treated with concentrated $\text{H}_2\text{SO}_4$:
1. At

low temperatures (around $293\text{ K}$ / $20^\circ\text{C}$): The activation energy for ortho-attack is lower, so the reaction forms

o-phenol sulphonic acid faster. This is the kinetically controlled product.
2. At

high temperatures (around $373\text{ K}$ / $100^\circ\text{C}$): The ortho isomer becomes unstable due to steric hindrance and slowly converts into the more stable para isomer.

p-phenol sulphonic acid becomes the major product. This is the thermodynamically controlled product.
Since the given temperature is $293\text{ K}$ (room temperature), the ortho isomer dominates.

Step 4: Final Answer:
The product obtained is o-phenol sulphonic acid, matching option (B).