Question

Arrange the following in increasing order of their acid strength:

• A. I < III < II
• B. I < II < III
• C. III < II < I
• D. III < II < I

Hint:

Electron donating groups decrease acidity, while electron withdrawing groups increase acidity by stabilizing the conjugate base.

Answer 1

The Correct Option is A

Concept: Acid strength increases when electron withdrawing groups stabilize the carboxylate ion and decreases when electron donating groups destabilize it.

Step 1: Analyze compound I.
\[ o\text{-CH}_3C_6H_4COOH \] Methyl group shows \(+I\) effect. It pushes electron density towards the ring and destabilizes the carboxylate ion. Hence it is weaker than benzoic acid.

Step 2: Analyze compound III.
\[ C_6H_5COOH \] This is benzoic acid and serves as reference.

Step 3: Analyze compound II.
\[ o\text{-NO}_2C_6H_4COOH \] Nitro group exhibits strong \[ -I \text{ and } -M \] effects. These stabilize the conjugate base and significantly increase acidity. Therefore it is strongest.

Step 4: Arrange in increasing order.
\[ o\text{-CH}_3C_6H_4COOH < C_6H_5COOH < o\text{-NO}_2C_6H_4COOH \] \[ \boxed{I<III<II} \] Hence, \[ \boxed{\text{Correct Option (1)}} \]