Question

Out of $2,2,3 - {\text{trimethylbutane,2,2,4 - trimethylpentane and 2,2,3,3 - tetramethylbutane}}$ which has the highest octane number. Explain.

Answer 1

Hint : Octane number or octane rating is the standard measure of the performance of an engine or aviation gasoline. Octane rating, also known as anti knock rating is the ability of a fuel to resist knocking when it is ignited in a mixture with air in a cylinder of an internal combustion engine.

Complete Step By Step Answer:
The sharp sound caused in an internal combustion engine due to premature combustion of part of the compressed air fuel mixture in the cylinder is known as knocking. Mathematically, octane number is the percentage by volume of iso-octane in the iso-octane-heptane mixture which matches with the test fuel in a standard test engine. If the octane number is high, then more are the compressions in the fuel which it can withstand before detonating and higher will be the performance. A fuel having zero octane number is considered to be a poor fuel and a fuel with octane number $100$ is considered a good fuel. Iso-octane has an octane number $100$ and it shows minimal knock. Heptanes have octane number $0$ and it shows a bad knock. As the octane number increases as the branching increases, therefore ${\text{2,2,3,3 - tetramethylbutane}}$ which has four branches will have the highest octane number.
Therefore, ${\text{2,2,3,3 - tetramethylbutane}}$ has the highest octane number.

Note :
The octane number is generally calculated by comparing the knock intensity of the given fuel at standard conditions with the blend of two reference fuels: iso-octane, which resists knocking and heptanes, which knock readily.