The correct order of octane number of butane, pentane, hexane and cyclohexane is 

The correct answer is butane > cyclohexane > pentane > hexane.

Key Points

To determine the correct order of octane numbers among butane, pentane, hexane, and cyclohexane, let's consider the general properties and typical octane ratings for each:

• Butane (n-butane) - Typically, butane has a relatively high octane rating. For instance, n-butane has an octane rating of around 94 (Research Octane Number, RON).
• Pentane (n-pentane) - Pentane usually has a lower octane rating compared to butane. n-Pentane, for example, has an octane rating of around 62.
• Hexane (n-hexane) - Hexane's octane rating is generally lower than that of pentane, with n-hexane typically around 25.
• Cyclohexane - Cyclohexane tends to have a lower octane rating than the other listed alkanes. It's typically used as a reference with a very low octane rating, around 82.

Given this information, the correct order of octane numbers from highest to lowest among these compounds is:

• Butane (highest)
• Cyclohexane
• Pentane
• Hexane (lowest)

Thus, the correct answer, based on the typical octane ratings, would be: 3) butane > cyclohexane > pentane > hexane

Additional Information

• The concept of octane rating was developed by chemist Russell Marker in the 1920s. The octane number is a measure of a fuel's ability to resist "knocking" or "pinging" during combustion, caused by the air/fuel mixture detonating prematurely in the engine.
• In the context of engine performance, fuels with higher octane numbers are preferred for high-performance engines to prevent knocking, which can lead to engine damage over time.
• The octane rating of a fuel can be increased by refining processes such as isomerization, which increases the proportion of branched-chain hydrocarbons, or by adding anti-knock agents like lead tetraethyl (although its use has been greatly reduced for environmental reasons).