Which has highest value of `pK_(a)` ?

Options

A. Phenol
B. Ethanol
C. o-Nitrophenol
D. o-Cresol

Answer

To determine which compound has the highest value of pKa, we need to analyze the acidity of the given compounds. The pKa value is inversely related to the strength of the acid; the weaker the acid, the higher the pKa value.

### Step-by-Step Solution:

1. **Understand pKa and Ka Relationship**:
- pKa is the negative logarithm of the acid dissociation constant (Ka).
- A higher pKa value indicates a weaker acid, while a lower pKa value indicates a stronger acid.

2. **Identify the Compounds**:
- We need to compare the acidity of the following compounds: Phenol, Ethanol, Ortho-nitro-phenol, and another compound (let's assume it is a generic compound for this explanation).

3. **Evaluate the Acidity of Each Compound**:
- **Phenol**: When phenol donates a proton (H+), it forms a phenoxide ion (C6H5O-). The negative charge on the oxygen is stabilized by resonance, making phenol a relatively weak acid.
- **Ethanol**: Ethanol (CH3CH2OH) is a weaker acid compared to phenol because the ethoxide ion (CH3CH2O-) formed after deprotonation is less stable than the phenoxide ion due to lack of resonance stabilization.
- **Ortho-nitro-phenol**: The nitro group is an electron-withdrawing group that stabilizes the negative charge on the phenoxide ion through resonance, making ortho-nitro-phenol a stronger acid than phenol.
- **Generic Compound**: Depending on the structure, if it has electron-donating groups, it may be a weaker acid than ethanol.

4. **Compare the Stability of the Conjugate Bases**:
- The stability of the conjugate base (A-) is crucial in determining the strength of the acid (HA). The more stable the conjugate base, the stronger the acid.
- **Phenoxide ion** (from phenol) is stabilized by resonance.
- **Ethoxide ion** (from ethanol) is less stable due to lack of resonance.
- **Ortho-nitro-phenoxide ion** is more stable than phenoxide due to the electron-withdrawing effect of the nitro group.

5. **Determine the Weakest Acid**:
- Since ethanol has the least stable conjugate base (ethoxide ion), it is the weakest acid among the compounds compared. Therefore, it will have the highest pKa value.

6. **Conclusion**:
- The compound with the highest pKa value is **Ethanol**.

### Final Answer:
Ethanol has the highest value of pKa among the given compounds.