Consider the methyl substituted benzoic acids.
1. `PhCOOH`
2. `o-CH_(3)C_(6)H_(4)COOH`
3. `p - CH_(3)C_(6)H_(4)COOH`
4. `m-CH_(3)C_(6)H_(4)COOH`
The correct sequence of acidity is

Options

A. `1 lt 2 lt 3 lt 4`
B. `2 lt 3 lt 4 lt1`
C. `3 lt 4 lt 1 lt 2`
D. `3 lt 4 lt 2 lt 1`

Answer

To determine the correct sequence of acidity for the methyl substituted benzoic acids, we will analyze the effects of the methyl (−CH₃) substituent on the acidity of benzoic acid. The compounds we are considering are:

1. Benzoic acid (PhCOOH)
2. Ortho-methylbenzoic acid (o-CH₃C₆H₄COOH)
3. Para-methylbenzoic acid (p-CH₃C₆H₄COOH)
4. Meta-methylbenzoic acid (m-CH₃C₆H₄COOH)

### Step 1: Understand the acidity of benzoic acid
Benzoic acid (PhCOOH) has a carboxylic acid group (−COOH) that can donate a proton (H⁺). The strength of an acid is determined by its ability to donate protons and the stability of its conjugate base (the anion formed after losing a proton).

### Step 2: Analyze the substituent effects
- **Ortho (o-CH₃)**: The methyl group is positioned ortho to the carboxylic acid group. The ortho effect can lead to steric hindrance and destabilization of the conjugate base (−COO⁻) due to the proximity of the methyl group. However, it can also enhance the acidity due to resonance effects being disrupted, leading to a more stable conjugate base.

- **Para (p-CH₃)**: The methyl group is positioned para to the carboxylic acid group. In this case, the methyl group can donate electron density through hyperconjugation, which destabilizes the conjugate base (−COO⁻) and decreases the acidity of the acid.

- **Meta (m-CH₃)**: The methyl group is positioned meta to the carboxylic acid group. Here, the methyl group does not significantly affect the resonance stabilization of the conjugate base, leading to a moderate acidic strength.

### Step 3: Compare the acidity
1. **Ortho-methylbenzoic acid (o-CH₃C₆H₄COOH)**: The steric hindrance and disruption of resonance make this acid the strongest.
2. **Benzoic acid (PhCOOH)**: The baseline acidity without any substituents.
3. **Meta-methylbenzoic acid (m-CH₃C₆H₄COOH)**: The methyl group has a weaker effect on acidity compared to the ortho and para positions.
4. **Para-methylbenzoic acid (p-CH₃C₆H₄COOH)**: The electron-donating effect of the methyl group decreases the acidity the most.

### Conclusion
The correct sequence of acidity from strongest to weakest is:
**o-CH₃C₆H₄COOH > PhCOOH > m-CH₃C₆H₄COOH > p-CH₃C₆H₄COOH**

Thus, the final order is:
**2 > 1 > 4 > 3**