Question

Which of the following has the highest knocking effect in an IC engine?
(A) Branched chain olefins
(B) Olefins
(C) Aromatic hydrocarbons
(D) Straight chain paraffins

Answer 1

Hint: Knocking is the metallic sound produced due to the internal combustion (IC) of the engine. The knocking effect depends upon the structure of olefins.
Complete step by step answer: As internal combustion takes place in the engine, a metallic sound is produced this is termed as knocking effect. To reduce this knocking effect, anti-knocking agents like Tetraethyl lead abbreviated as TEL is used which is known as the anti-knocking agent.
The effect of knocking depends upon the type of olefin used. Olefins are also known as paraffins. The Straight chain paraffin has the highest knocking effect in an IC engine. The least knocking is priced by the aromatic compounds produced due to the presence of cyclic structures. Whereas the straight chain hydrocarbons produce more knocking.
The decreasing order of knocking effect is given below:
\[{\text{Straight chain paraffins > Branched chain paraffins > Olefins > Aromatic hydrocarbons}}{\text{.}}\]

So, the correct option is D.

Additional Information: Basically, engine knock (also known as pinging, detonation and spark knock) occurs when the air/fuel mixture inside a cylinder is incorrect, which makes the fuel burn unevenly. This also creates the common "pinging" noise that is often described when knock is present. The knocking tendency is increased by reducing the engine speed and by advancing the ignition of the engine.

Note: Though to prevent knocking, TEL is used, it leads to the deposition of lead in the engine. To remove the free lead (Pb), ethylene halides are added. They combine with the lead to form volatile halides.