CDS (I) 2024 General Knowledge · Question 39 of 120

Correct — A, Hexadecane. Cetane is the trivial name of n-hexadecane, the unbranched sixteen-carbon alkane with the formula C16H34 and the structure CH3(CH2)14CH3. It matters because it anchors the diesel quality scale: cetane 'has a cetane number defined to be 100, while isocetane's measured cetane number is 15', isocetane being the heavily branched 2,3,4,5,6,7,8-heptamethylnonane that serves as the low reference. The scale runs the opposite way to the octane scale — a high cetane number means a fuel ignites readily under compression, which is exactly what a diesel engine needs, whereas a high octane number means a fuel resists igniting on its own.

• (b)Heptadecane — A seventeen-carbon alkane, C17H36. It is a real hydrocarbon but has no role as a reference fuel; the cetane scale is anchored on the sixteen-carbon chain.
• (c)Octadecane — The eighteen-carbon alkane, C18H38. The prefix 'octa' invites confusion with the octane scale, but that scale is anchored on iso-octane, an eight-carbon molecule, not on this one.
• (d)Nonadecane — The nineteen-carbon alkane, C19H40, and again simply the wrong chain length; nothing in fuel testing is benchmarked on it.

Petrol and diesel engines want opposite things from their fuel. A petrol engine ignites the charge with a spark, so the fuel must not self-ignite early — that resistance is measured by the octane number, with iso-octane at 100 and n-heptane at 0. A diesel engine has no spark and relies on the heat of compression, so the fuel must self-ignite promptly — that readiness is measured by the cetane number, with n-hexadecane, called cetane, at 100 and the branched isocetane at 15.

Two things make this item answerable even without recall. First, count carbons: 'hexadeca' is sixteen, and the sixteen-carbon straight chain is the one long enough to autoignite easily but still light enough to be a practical fuel component. Second, notice that the four options differ only in chain length, so the examiner is testing one number and nothing else. The wider point worth carrying away is the inversion between the two scales. A long straight chain like hexadecane has a very low octane number precisely because it self-ignites so readily, and that same property gives it the top cetane rating. Note the historical wrinkle in the cetane scale: isocetane at 15 replaced an earlier low reference, alpha-methylnaphthalene, which had been assigned 0.

• Cetane is n-hexadecane, C16H34, structure CH3(CH2)14CH3, an unbranched saturated alkane.
• Cetane is assigned a cetane number of 100 by definition.
• Isocetane, 2,3,4,5,6,7,8-heptamethylnonane, has a measured cetane number of 15 and is the low reference.
• It replaced alpha-methylnaphthalene, the former low reference, which had been assigned a cetane number of 0.
• High cetane number means easy autoignition under compression — the opposite of what a high octane number signifies.

• Reading 'octadecane' as connected to the octane scale; iso-octane has eight carbons, not eighteen.
• Assuming a high cetane number is desirable in petrol; the two engines want opposite ignition behaviour.
• Forgetting that cetane's value of 100 is a definition, not a measurement.

Asked as a one-line naming item where every option is a straight-chain alkane and only the carbon count separates them.

• practice — not a real PYQ

  A high cetane number in a diesel fuel indicates that the fuel

  • (a)resists ignition under compression
  • (b)ignites readily under compression
  • (c)has a very high boiling point
  • (d)contains a large proportion of aromatics

  Answer(b) ignites readily under compression — the opposite of what a high octane number signifies for petrol.

• practice — not a real PYQ

  On the cetane scale, which compound serves as the low reference with a cetane number of 15?

  • (a)n-heptane
  • (b)Iso-octane
  • (c)Isocetane, that is heptamethylnonane
  • (d)Toluene

  Answer(c) Isocetane, that is heptamethylnonane — it replaced alpha-methylnaphthalene, which had been assigned 0.