The correct answer is: Option 1 – butane > cyclohexane > pentane > hexane

⛽ What Is the Octane Number?

• The octane number (or octane rating) is a standard measure of a fuel's ability to resist knocking or pinging during combustion, caused by the air-fuel mixture detonating prematurely in the engine. The higher the octane number, the greater the resistance to knocking, making the fuel more suitable for high-compression engines.
• Octane number depends on the structure of the hydrocarbon:
• Branched alkanes have higher octane numbers.
• Straight-chain alkanes have lower octane numbers.
• Cyclic alkanes generally fall in-between, but closer to branched ones.
• Shorter chains typically resist knocking better than longer ones.

🧪 Octane Ratings of the Given Compounds

Let’s examine the relative octane numbers of the four hydrocarbons:

🔁 Correct Order:

Butane > Cyclohexane > Pentane > Hexane

🧬 Why This Order?

• Butane, especially isobutane (methylpropane), has a high octane rating because it’s short and branched, both of which improve anti-knock properties.
• Cyclohexane, being cyclic, is more stable under combustion than straight-chain hydrocarbons of similar size.
• Pentane and hexane are straight-chain alkanes. As the chain length increases (from C₅ to C₆), the octane number decreases due to an increased likelihood of early detonation.

❌ Why Other Options Are Incorrect

✅ Final Conclusion

Based on molecular structure and experimental values, the correct order of octane numbers is:

Butane > Cyclohexane > Pentane > Hexane

Hence, the correct answer is: Option 1.