The correct order of octane number of butane, pentane, hexane and cycl

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

[amp_mcq option1=”butane > pentane > cyclohexane > hexane” option2=”butane > pentane > hexane > cyclohexane” option3=”butane > cyclohexane > pentane > hexane” option4=”cyclohexane > butane > pentane > hexane” correct=”option3″]

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UPSC CDS-1 – 2024

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Octane number is a measure of a fuel’s resistance to knocking. Branched hydrocarbons and cyclic hydrocarbons have higher octane numbers than straight-chain hydrocarbons with the same number of carbon atoms. The octane number decreases with increasing chain length for straight-chain alkanes.
Comparing the standard (n-) forms:
n-Butane (C4H10) has a higher octane number than n-pentane (C5H12) and n-hexane (C6H14).
n-Pentane has a higher octane number than n-hexane.
Cyclohexane (C6H12), a cyclic alkane, has a higher octane number than the corresponding straight-chain alkane, n-hexane, and typically higher than n-pentane.
The typical order of RON (Research Octane Number) is approximately: n-Butane (~92) > Cyclohexane (~83) > n-Pentane (~62) > n-Hexane (~25).
Thus, the correct order of decreasing octane number is butane > cyclohexane > pentane > hexane.

Octane number is inversely related to chain length for n-alkanes and higher for branched/cyclic structures compared to straight chains.

Octane numbers are determined relative to two reference hydrocarbons: isooctane (2,2,4-trimethylpentane), assigned an octane number of 100 (highly resistant to knocking), and n-heptane, assigned an octane number of 0 (knocks readily). The octane number of a fuel is the percentage of isooctane in a mixture with n-heptane that has the same knocking characteristics as the fuel.

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