Which of the following is the strongest acid ?

Options

A.
B.
C.
D.

Answer

To determine which of the given compounds is the strongest acid, we need to analyze the acidic strength of each compound based on their structure and the effects of substituents on the phenolic compounds. Here’s a step-by-step breakdown of the solution:

### Step 1: Identify the Compounds
First, we need to identify the compounds that are being compared. Let's assume we have four compounds, with one being phenol and the others having various substituents (like -NO2, -CN, and -Cl).

### Step 2: Understand Acidic Strength
Acidic strength is determined by the ability of a compound to donate a proton (H⁺). The more stable the resulting anion (after losing H⁺), the stronger the acid.

### Step 3: Analyze the Phenol Structure
When phenol loses a proton, it forms the phenoxide ion. The stability of this ion is crucial for determining the acidity of phenol. The negative charge on the phenoxide ion can be stabilized by resonance.

### Step 4: Consider Substituent Effects
Substituents on the phenol ring can either stabilize or destabilize the negative charge on the phenoxide ion:
- **Electron-withdrawing groups (EWGs)** (like -NO2 and -CN) stabilize the negative charge by delocalizing it through resonance or inductive effects.
- **Electron-donating groups (EDGs)** (like -Cl) destabilize the negative charge, making the compound less acidic.

### Step 5: Compare the Effects of Substituents
1. **Cyanide Group (-CN)**: This is an electron-withdrawing group but is less effective than -NO2 in stabilizing the negative charge.
2. **Nitro Group (-NO2)**: This is a strong electron-withdrawing group that can stabilize the negative charge effectively through resonance.
3. **Chlorine (-Cl)**: This group can donate electrons, which would destabilize the negative charge on the phenoxide ion.

### Step 6: Determine the Strongest Acid
Based on the analysis:
- The nitro group (-NO2) is the strongest electron-withdrawing group and will stabilize the phenoxide ion the best, making the compound with this substituent the strongest acid.
- The cyanide group (-CN) is less effective than -NO2.
- The chlorine group (-Cl) destabilizes the negative charge, making it the least acidic.

### Conclusion
The compound with the nitro group (-NO2) is the strongest acid among the given options. Therefore, the correct answer is the compound with the nitro substituent.

### Final Answer
**The strongest acid is the compound with the nitro group (-NO2).**

---