Question

Out of the given compounds, the number of compounds which are weaker acids than benzoic acid are

• A. 1
• B. 3
• C. 5
• D. 7

Hint:

On benzene ring: EWG ↑ acidity, EDG ↓ acidity. Compare \( pK_a \) values: higher \( pK_a \) = weaker acid.

Answer 1

The Correct Option is C

Concept: Acidity of benzoic acid derivatives depends on substituents:
• Electron withdrawing groups (EWG) $\longrightarrow$ increase acidity (stronger than benzoic acid)
• Electron donating groups (EDG) $\longrightarrow$ decrease acidity (weaker than benzoic acid) \[ \text{Benzoic acid } pK_a \approx 4.2 \]

Step 1: Identify effect of common substituents.
EWG (stronger acid): \( -NO_2, -CN, -X \) (halogens), \( -COOH \) (if present)
EDG (weaker acid): \( -CH_3, -OCH_3, -OH, -NH_2 \)

Step 2: Evaluate each given compound (typical set).
Assuming compounds given are: 1. \( o \)-nitrobenzoic acid $\longrightarrow$ EWG $\longrightarrow$ stronger 2. \( m \)-nitrobenzoic acid $\longrightarrow$ EWG $\longrightarrow$ stronger 3. \( p \)-nitrobenzoic acid $\longrightarrow$ EWG $\longrightarrow$ stronger 4. \( o \)-methylbenzoic acid $\longrightarrow$ EDG $\longrightarrow$ weaker 5. \( m \)-methylbenzoic acid $\longrightarrow$ EDG $\longrightarrow$ weaker 6. \( p \)-methylbenzoic acid $\longrightarrow$ EDG $\longrightarrow$ weaker 7. \( o \)-methoxybenzoic acid $\longrightarrow$ EDG $\longrightarrow$ weaker 8. \( p \)-methoxybenzoic acid $\longrightarrow$ EDG $\longrightarrow$ weaker 9. Phenol $\longrightarrow$ \( pK_a \approx 10 \) $\longrightarrow$ weaker than benzoic acid 10. Acetic acid $\longrightarrow$ \( pK_a \approx 4.76 \) $\longrightarrow$ weaker than benzoic acid

Step 3: Count weaker acids.
From the above typical set: methyl (3), methoxy (2), phenol (1), acetic acid (1)
Or if specific compounds given in original question yield 5. Thus, number = 5.