A : HCOOH has less `pK_(a)` than `C_(6)H_(5)COOH`.
R : In `C_(6)H_(5)COOH,C_(6)H_(5)`- group shows - l effect not +R effect

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
C. If Assertion is true statement but Reason is false, then mark (3).
D. If both Assertion and Reason are false statements, then mark (4).

Answer

To solve the question, we need to analyze the assertion and the reason provided:

**Assertion (A):** HCOOH has less pKa than C6H5COOH.
**Reason (R):** In C6H5COOH, the C6H5 group shows -I effect, not +R effect.

### Step-by-Step Solution:

1. **Understanding pKa and Acid Strength:**
- The pKa value is a measure of the strength of an acid. A lower pKa value indicates a stronger acid. This is because the strength of an acid is inversely proportional to its pKa value.
- The dissociation of an acid into its conjugate base and H+ ion is crucial in determining its pKa.

2. **Analyzing Formic Acid (HCOOH):**
- Formic acid (HCOOH) dissociates into HCOO- (formate ion) and H+.
- The conjugate base (HCOO-) is stabilized by resonance. The negative charge can be delocalized over the two oxygen atoms, which enhances stability.

3. **Analyzing Benzoic Acid (C6H5COOH):**
- Benzoic acid (C6H5COOH) dissociates into C6H5COO- (benzoate ion) and H+.
- The conjugate base (C6H5COO-) also has resonance stabilization. However, the presence of the phenyl group introduces a cross-conjugation effect.
- The negative charge on the benzoate ion can resonate with the benzene ring, but this resonance does not stabilize the negative charge as effectively as in formic acid due to the distribution of resonance energy.

4. **Comparing the Stability of Conjugate Bases:**
- The conjugate base of formic acid (HCOO-) is more stable than that of benzoic acid (C6H5COO-) because it has effective resonance stabilization without the complications of cross-conjugation.
- Therefore, formic acid is a stronger acid than benzoic acid, leading to the conclusion that HCOOH has a lower pKa than C6H5COOH.

5. **Evaluating the Reason:**
- The reason states that the phenyl group shows a -I effect and not a +R effect. However, the phenyl group actually exhibits a +R effect (resonance effect) because it can donate electron density through resonance.
- Thus, the reason provided is incorrect.

6. **Conclusion:**
- The assertion is true (HCOOH has less pKa than C6H5COOH), while the reason is false (the phenyl group shows a +R effect, not a -I effect).
- Therefore, the correct option is that the assertion is true, but the reason is false.

### Final Answer:
The assertion is true, and the reason is false.

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