Which of the following has maximum extent of resonance?

Options

A. `R-overset(O)overset(||)(C)-Cl`
B. `R-overset(O)overset(||)(C)-O-overset(O)overset(||)(C)-R`
C. `R-overset(O)overset(||)(C)-OR`
D. `R-overset(O)overset(||)(C)-NH_(2)`

Answer

To determine which of the given compounds has the maximum extent of resonance, we will analyze each option based on the ability of the lone pairs on the atoms involved to participate in resonance structures.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds we are analyzing are:
- Option 1: RCOCl (Acyl chloride)
- Option 2: RCOOR' (Anhydride)
- Option 3: RCOO-R' (Ester)
- Option 4: RCONH2 (Amide)

2. **Analyze Option 1 (RCOCl)**:
- In acyl chloride (RCOCl), the lone pair on chlorine can participate in resonance with the carbonyl group (C=O). However, chlorine is less electronegative than oxygen, which limits the extent of resonance.

3. **Analyze Option 2 (RCOOR')**:
- In an anhydride, the lone pair on the oxygen atom can participate in resonance with both carbonyl groups. This allows for some resonance structures, but the extent is still limited compared to other groups.

4. **Analyze Option 3 (RCOO-R')**:
- In an ester, the lone pair on the oxygen atom is also involved in resonance with the carbonyl group. However, similar to the anhydride, the resonance is limited to one carbonyl group.

5. **Analyze Option 4 (RCONH2)**:
- In an amide, the lone pair on the nitrogen can participate in resonance with the carbonyl group. Nitrogen is less electronegative than oxygen, which means it can donate its lone pair more effectively, allowing for a greater extent of resonance. The resonance structures can involve the nitrogen donating its lone pair to the carbonyl, creating a more stable resonance hybrid.

6. **Comparison of Resonance Extent**:
- When comparing the extent of resonance:
- RCOCl has limited resonance due to chlorine.
- RCOOR' (anhydride) has moderate resonance.
- RCOO-R' (ester) also has moderate resonance.
- RCONH2 (amide) has the highest extent of resonance due to the effective donation of the nitrogen lone pair.

### Conclusion:
Based on the analysis, the compound with the maximum extent of resonance is **Option 4: RCONH2 (Amide)**.

---