Out of benzene , ethylene and acetylene, the carbon-carbon bond length is longest in _______

Answer

To determine which compound has the longest carbon-carbon bond length among benzene, ethylene, and acetylene, we need to analyze the bond order of each compound and how it relates to bond length.

### Step-by-step Solution:

1. **Understanding Bond Length and Bond Order**:
- Bond length is inversely proportional to bond order. This means that as bond order increases, bond length decreases. Conversely, a lower bond order corresponds to a longer bond length.

2. **Identifying the Structures**:
- **Benzene (C6H6)**: Benzene has a cyclic structure with alternating double bonds, which leads to resonance. The effective bond order in benzene is between 1 and 2 due to resonance. The bond length in benzene is approximately 139 picometers.
- **Ethylene (C2H4)**: Ethylene has a double bond between the two carbon atoms. The bond order here is 2. The bond length in ethylene is about 134 picometers.
- **Acetylene (C2H2)**: Acetylene has a triple bond between the two carbon atoms. The bond order is 3. The bond length in acetylene is approximately 120 picometers.

3. **Comparing Bond Lengths**:
- From the bond lengths we have:
- Benzene: 139 picometers
- Ethylene: 134 picometers
- Acetylene: 120 picometers
- The longest bond length is found in benzene.

4. **Conclusion**:
- Therefore, out of benzene, ethylene, and acetylene, the carbon-carbon bond length is longest in **benzene**.

### Final Answer:
The carbon-carbon bond length is longest in **benzene**.