UPSC CDS I 2024 GK Paper

Concept:Octane number measures a fuel's ability to resist knocking (premature detonation).Smaller, more branched hydrocarbons generally have higher octane numbers.Explanation:Butane ($\mathrm{C}_4\mathrm{H}_{10}$) is the smallest molecule among the four. It burns cleanly and resists knocking well, giving it the highest octane number.Pentane ($\mathrm{C}_5\mathrm{H}_{12}$) is larger than butane. Its straight chain is more prone to knocking, so its octane number is lower than butane's.Hexane ($\mathrm{C}_6\mathrm{H}_{14}$) has an even longer chain. It knocks more easily, so its octane number is lower than pentane's.Cyclohexane ($\mathrm{C}_6\mathrm{H}_{12}$) has a cyclic structure. This structure leads to less efficient combustion and the highest tendency to knock, giving it the lowest octane number among the four.Therefore, the decreasing order of octane number is: butane > pentane > hexane > cyclohexane.Answer:butane > pentane > hexane > cyclohexane (Option A)