Octane number measures the anti-knocking property of fuels. Among straight-chain alkanes, octane number decreases with increasing chain length — so butane > pentane > hexane in octane rating. However, branched and cyclic hydrocarbons have higher octane numbers than their straight-chain counterparts of similar carbon count. Cyclohexane has a higher octane number than n-hexane but less than smaller alkanes like butane and pentane. The correct order is therefore butane > pentane > cyclohexane > hexane.
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