Which of the following has longest `C – O` bond:

Options

A.
B.
C.
D.

Answer

To determine which compound has the longest C–O bond, we need to analyze the bond types and resonance structures of the given options. The bond length is inversely related to bond order; that is, the higher the bond order, the shorter the bond length.

### Step-by-Step Solution:

1. **Understand Bond Order and Length**:
- The bond order is the number of bonds between two atoms. A single bond has a bond order of 1, a double bond has a bond order of 2, and a triple bond has a bond order of 3.
- As the bond order increases, the bond length decreases. Therefore, single bonds are longer than double bonds, which are longer than triple bonds.

2. **Identify the Compounds**:
- Assume we have four compounds to analyze (let's denote them as A, B, C, and D). Each compound contains a C–O bond, but they differ in bond type (single, double, or resonance structures).

3. **Analyze Each Compound**:
- **Compound A**: Contains a C=O (double bond). This will have a shorter bond length due to the higher bond order.
- **Compound B**: Contains a C–O (single bond) with resonance. The resonance can give it some partial double bond character, but it will still be longer than a pure double bond.
- **Compound C**: Contains a C–O (single bond) with no resonance. This would have the longest bond length as it has no factors reducing the bond length.
- **Compound D**: Contains a C=O (double bond) with resonance. Similar to Compound A, it will have a shorter bond length than a single bond, but longer than a pure double bond without resonance.

4. **Determine the Longest Bond**:
- Among the analyzed compounds, the one with the C–O single bond and no resonance (Compound C) will have the longest C–O bond length.
- The presence of resonance in Compound B will slightly shorten the bond compared to a pure single bond, but it will still be longer than any double bond present in Compounds A and D.

### Conclusion:
The compound with the longest C–O bond is the one with a C–O single bond and no resonance (Compound C).