CDS (I) 2024 General Knowledge · Question 34 of 120

Correct — C, butane > cyclohexane > pentane > hexane. Octane number measures resistance to knocking, and it is governed by molecular shape far more than by molecular size. Two rules settle the order. First, among straight-chain alkanes the octane number falls steeply as the chain lengthens: measured research octane numbers run n-butane 94, n-pentane 62 and n-hexane 25, a collapse of nearly seventy points across two added carbons. Second, closing a chain into a ring, like branching it, raises the number sharply — cyclohexane is rated 83, far above the 25 of its straight-chain isomer n-hexane and above n-pentane's 62 as well, though still below n-butane. Put the four in descending order and you get butane, then cyclohexane, then pentane, then hexane.

• (a)butane > pentane > hexane > cyclohexane — Places cyclohexane last, as though a ring behaved like a very long chain. Ring closure raises knock resistance, and cyclohexane at 83 sits above both pentane and hexane.
• (b)butane > pentane > cyclohexane > hexane — Gets cyclohexane above hexane but still below pentane. The measured values are 83 for cyclohexane against 62 for n-pentane, so cyclohexane belongs one place higher.
• (d)cyclohexane > butane > pentane > hexane — Promotes cyclohexane above butane. The short chain still wins — n-butane is rated 94 against cyclohexane's 83.

The octane scale benchmarks a fuel against mixtures of two reference hydrocarbons: 2,2,4-trimethylpentane, called iso-octane, is fixed at 100 and n-heptane at 0. A fuel's octane number is the percentage of iso-octane in the mixture that knocks in the same way. Knocking is premature self-ignition of the fuel-air charge ahead of the flame front, so a high octane number means the molecule resists autoignition. Compact, branched or cyclic molecules resist better than long straight chains, which is why refiners reform straight-run naphtha into aromatics and branched isomers.

You do not need the exact numbers to answer this, but you do need both rules working together. Applying only the chain-length rule ranks butane, pentane, hexane and leaves cyclohexane unplaced — which is precisely the gap options (a) and (b) exploit. The second rule, that ring closure and branching raise octane number, lifts cyclohexane clear of both pentane and hexane while leaving it under the very short butane chain. The published values make this concrete: n-butane 94, cyclohexane 83, n-pentane 62, n-hexane 25. The same structural logic explains why isopentane at about 90 towers over n-pentane at 62, and why toluene at 121 is used as an octane booster.

• Measured research octane numbers: n-butane 94, cyclohexane 83, n-pentane 62, n-hexane 25.
• Iso-octane is defined as 100 on the octane scale and n-heptane as 0.
• Octane number falls steeply as a straight chain lengthens, and rises with branching or ring closure.
• A high octane number means greater resistance to knocking, that is, to premature self-ignition.
• Isopentane at about 90 against n-pentane at 62 shows the same branching effect within one molecular formula.

• Ranking by carbon count alone, which strands cyclohexane at the bottom next to n-hexane.
• Assuming cyclohexane beats every open chain because rings score well; the four-carbon n-butane still outscores it.
• Carrying octane intuitions into diesel — a high octane number means hard to ignite, which is the last thing a diesel fuel wants.

Asked as an ordering item mixing straight chains with one cycloalkane, so both the chain-length rule and the ring effect must be applied together.

• practice — not a real PYQ

  Which one of the following has the highest research octane number?

  • (a)n-heptane
  • (b)n-hexane
  • (c)Isopentane
  • (d)n-pentane

  Answer(c) Isopentane — about 90, against 62 for its straight-chain isomer n-pentane, 25 for n-hexane and 0 for n-heptane.

• practice — not a real PYQ

  On the octane scale, the compound assigned a value of 100 is

  • (a)n-octane
  • (b)2,2,4-trimethylpentane
  • (c)n-hexadecane
  • (d)Toluene

  Answer(b) 2,2,4-trimethylpentane, known as iso-octane — with n-heptane fixed at 0 as the other reference.