The increasing order of acidity among phenol, p-methylphenol, m-nitrophenol and p-nitrophenol is:

Options

A. m-nitrophenol, p-nitrophenol, phenol, p-methylphenol
B. p-methylphenol, m-nitrophenol, phenol, p-nitrophenol
C. p-methylphenol, phenol, m-nitrophenol, p-nitrophenol
D. Phenol, p-methylphenol, p-nitrophenol, m-nitrophenol

Answer

To determine the increasing order of acidity among phenol, p-methylphenol, m-nitrophenol, and p-nitrophenol, we need to analyze the effects of substituents on the acidity of phenolic compounds. Here’s a step-by-step solution:

### Step 1: Understand Acidity in Phenolic Compounds
Acidity in phenolic compounds is influenced by the ability of the compound to donate a proton (H⁺). The presence of electron-withdrawing groups (EWGs) increases acidity, while electron-donating groups (EDGs) decrease acidity.

### Step 2: Identify the Substituents
- **Phenol**: No substituents, has a hydroxyl (-OH) group.
- **p-Methylphenol (para-methylphenol)**: Contains a methyl (-CH₃) group at the para position, which is an electron-donating group (EDG).
- **m-Nitrophenol (meta-nitrophenol)**: Contains a nitro (-NO₂) group at the meta position, which is an electron-withdrawing group (EWG).
- **p-Nitrophenol (para-nitrophenol)**: Contains a nitro (-NO₂) group at the para position, which is also an electron-withdrawing group (EWG).

### Step 3: Analyze the Effects of Substituents on Acidity
- **p-Methylphenol**: The methyl group is an EDG, which decreases the acidity compared to phenol.
- **Phenol**: Acts as a baseline for acidity.
- **m-Nitrophenol**: The nitro group at the meta position has a weaker effect on acidity compared to the para position because it does not stabilize the negative charge on the phenoxide ion as effectively as it would if it were ortho or para.
- **p-Nitrophenol**: The nitro group at the para position strongly withdraws electron density, stabilizing the phenoxide ion and increasing acidity.

### Step 4: Arrange the Compounds in Order of Increasing Acidity
Based on the analysis:
1. **p-Methylphenol** (least acidic due to EDG)
2. **Phenol** (baseline acidity)
3. **m-Nitrophenol** (moderate acidity due to weaker EWG effect)
4. **p-Nitrophenol** (most acidic due to strong EWG effect)

### Final Order of Acidity
The increasing order of acidity is:
**p-Methylphenol < Phenol < m-Nitrophenol < p-Nitrophenol**