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

Analyze the question: The octane rating (octane number) of petrol measures how well the fuel resists "knocking" (premature ignition) in an engine. It is defined by comparing the fuel with a standard reference compound.

Concept: The octane scale uses two reference hydrocarbons:

• n-Heptane, which knocks badly and is given an octane number of 0.
• 2,2,4-trimethylpentane (commonly called iso-octane), which resists knocking very well and 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.

Check the structure of 2,2,4-trimethylpentane:

• The parent chain is pentane (5 carbons): C1–C2–C3–C4–C5.
• Two methyl groups are on C2 and one methyl group is on C4.
• Formula: (CH₃)₃C–CH₂–CH(CH₃)₂, which is C₈H₁₈. This is an isomer of octane, hence the name "iso-octane".

Eliminate the other options:

• A. 1,2,3-trimethylpentane is an invalid name, because a methyl group cannot be on C1 (the chain would be longer).
• B. 2,3,5-trimethyloctane has 11 carbons and is not the reference standard.
• C. 2,3,5-trimethylpentane is an invalid name, because C5 would extend the chain (it should be numbered from the other end).
• D. 2,2,4-trimethylpentane is the iso-octane standard.

Answer: D. 2,2,4-trimethylpentane.

Exam tip: Remember "Iso-octane = 100, n-heptane = 0." Whenever you see "octane rating" in a question, look for 2,2,4-trimethylpentane. A highly branched hydrocarbon means a higher octane number and less knocking.