Which of the following will increase the acidity of phenol ?

Options

A. `-NO_(2)`
B. `-CN`
C. `-X` (halogen)
D. None of these

Answer

To determine which of the given groups will increase the acidity of phenol, we need to analyze the effects of different substituents on the phenolic compound. Here's a step-by-step solution:

### Step 1: Understand the Structure of Phenol
Phenol has the structure C6H5OH, where a hydroxyl group (-OH) is attached to a benzene ring. The acidity of phenol is due to the ability of the -OH group to donate a proton (H+).

**Hint:** Remember that acidity is related to the stability of the conjugate base formed after deprotonation.

### Step 2: Identify Electron-Withdrawing Groups
Electron-withdrawing groups (EWGs) stabilize the negative charge on the phenoxide ion (C6H5O-) that forms when phenol loses a proton. Common electron-withdrawing groups include nitro (-NO2), cyano (-CN), and halogens (like -Cl, -Br).

**Hint:** Groups that can pull electron density away from the aromatic ring will help stabilize the conjugate base.

### Step 3: Analyze the Given Groups
1. **Nitro group (-NO2)**: This group is a strong electron-withdrawing group due to its resonance and inductive effects. It stabilizes the phenoxide ion, thus increasing acidity.

2. **Cyano group (-CN)**: The cyano group is also an electron-withdrawing group through both resonance and inductive effects, which stabilizes the phenoxide ion and increases acidity.

3. **Halogens (like -Cl, -Br)**: Halogens are weak electron-withdrawing groups. They exhibit a -I (inductive) effect, but their -R (resonance) effect is slightly electron-donating. However, their overall effect is still to increase acidity, though not as strongly as -NO2 or -CN.

**Hint:** Consider how each group affects the electron density on the aromatic ring and the stability of the conjugate base.

### Step 4: Conclusion
All three groups (-NO2, -CN, and halogens) increase the acidity of phenol by stabilizing the phenoxide ion. Therefore, the correct answer is that all the given groups will increase the acidity of phenol.

**Final Answer:** A, B, and C (NO2, CN, and halogens) will increase the acidity of phenol.