Question

Which of the following has the highest-octane number?
A. Straight chain aliphatic compound
B. Branched chain aliphatic compound
C. Aromatic compound
D. Cyclic aliphatic compound

Answer 1

Hint: Octane number, is a number which gives the percentage by volume of iso-octane in the mixture of iso-octane and n-heptane which has the same anti-knock qualities as the fuel under consideration. Quality of petrol, gasoline is measured in terms of octane number.

Complete step by step answer:
In a petrol engine, vapour of petrol and air are first compressed to a small volume and then ignited by a spark from the spark plug. For maximum efficiency, the fuel mixture must be highly compressed before ignition. If the quality of petrol is not good, it leads to the pre-ignition of the fuel in the cylinder. It only leads to loss of energy, thus reducing the efficiency of the engine and causes a violent jerk to the piston which damages the engine. This violent jerk gives rise to a metallic sound or rattle known as knocking. The antiknock property of petrol is measured in terms of octane number.
Straight chain hydrocarbons have a very low value of octane number. Octane number goes on decreasing with increase in the length of the chain. Example: octane number of methane is ${\text{122}}$, while octane number of ethane is ${\text{101}}$, of propane ${\text{96}}$ and of butane is ${\text{89}}$.
Branching of chains in hydrocarbon compounds increases the value of octane number. Introduction of double and triple bonds increases the value of octane numbers.
Aromatic hydrocarbons have exceptionally high hydrocarbons
Cycloalkanes have relatively higher value of octane numbers.
Octane number increases in the order: Straight chain alkanes < branched chain alkanes < cycloalkanes < aromatic hydrocarbons.

So, the correct answer is Option C.

Note: Ordinarily gasolines having the octane number of 80 or more are used in motor cars, scooters, and motorcycles. These are known as regular gasolines. Higher the octane number of a fuel, the better is the fuel.