Which of the following compounds determines the octane rating of petrol?

Step 1: Analyze the question The question asks which compound is used as the standard for the octane rating (octane number) of petrol.

Step 2: The concept The octane number measures how well a fuel resists "knocking" (premature ignition) in an engine. It is measured on a scale using two reference hydrocarbons:

• n-heptane: knocks badly, so it is given an octane number of 0.
• 2,2,4-trimethylpentane (commonly called iso-octane): resists knocking very well, so it is given an octane number of 100.

A petrol with an octane rating of 90 behaves like a mixture of 90% iso-octane and 10% n-heptane.

Step 3: Check the options

• A. 1,2,3-trimethylpentane: This name is invalid. A methyl group cannot be on carbon 1 of a chain, because it would simply extend the chain.
• B. 2,3,5-trimethyloctane: This has 11 carbons, so it is not the reference compound.
• C. 2,3,5-trimethylpentane: This name is invalid too. A pentane chain has only 5 carbons, so a substituent on carbon 5 would extend the chain.
• D. 2,2,4-trimethylpentane: This is iso-octane, C₈H₁₈. It is the standard reference compound for the octane scale.

Structure of iso-octane:

        CH3   CH3
         |     |
CH3 — C — CH2 — CH — CH3
         |
        CH3

(a pentane chain with two methyl groups on C-2 and one on C-4)

Step 4: Answer The correct option is D. 2,2,4-trimethylpentane.

Exam tip: Remember "Iso-octane = 100, n-heptane = 0". The name "2,2,4" is the one to recognize. The more highly branched a hydrocarbon is, the higher its octane number, because branched chains burn more smoothly and resist knocking.