Record the following sets of compounds according to increasing `pK_(a) ( = - log Ka)`
Phenol,o-nitrophenol, o-cresol

Answer

To arrange the compounds phenol, o-nitrophenol, and o-cresol according to their increasing pKa values, we need to analyze their acidity based on their structures and the effects of substituents on the phenolic hydroxyl group (-OH).

### Step 1: Understand pKa and Acidity
- pKa is the negative logarithm of the acid dissociation constant (Ka). A lower pKa value indicates a stronger acid, meaning that the compound can donate a proton (H⁺) more easily. Conversely, a higher pKa value indicates a weaker acid.

### Step 2: Analyze the Compounds
1. **Phenol (C₆H₅OH)**:
- The hydroxyl group (-OH) is attached directly to the benzene ring. It has moderate acidity due to resonance stabilization of the phenoxide ion formed after deprotonation.

2. **o-Nitrophenol (C₆H₄(NO₂)OH)**:
- The nitro group (-NO₂) is a strong electron-withdrawing group that stabilizes the phenoxide ion through resonance and inductive effects. This makes o-nitrophenol more acidic than phenol.

3. **o-Cresol (C₆H₄(CH₃)OH)**:
- The methyl group (-CH₃) is an electron-donating group (through +I effect) which destabilizes the phenoxide ion. Therefore, o-cresol is less acidic than phenol.

### Step 3: Compare the Acidity
- Based on the analysis:
- **o-Nitrophenol** is the most acidic (lowest pKa) because the nitro group stabilizes the negative charge on the phenoxide ion.
- **Phenol** is next, as it has moderate acidity.
- **o-Cresol** is the least acidic (highest pKa) because the methyl group destabilizes the phenoxide ion.

### Step 4: Record the Order of Increasing pKa
- The order of increasing pKa values is:
- **o-Nitrophenol < Phenol < o-Cresol**

### Final Answer:
**Increasing order of pKa values:**
o-Nitrophenol < Phenol < o-Cresol

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