Question

Assertion: A straight chain alkene has higher boiling point than branched chain alkane
with the same number of carbons.
Reason: Branching decreases the surface area, hence Vander wall force of attraction
A) Both assertion and reason are correct and reason is the correct explanation for assertion.
B) Both assertion and reason are correct but reason is the not correct explanation for assertion.
C) Assertion is correct but reason is incorrect.
D) Both assertion and reason are incorrect.

Answer 1

Hint:Straight chain alkane has a higher boiling point than branched chain alkene with the same number of carbons. Because as the branching increases surface area decreases and Vander wall force increases.
Hence both assertion and reason are correct and reason is the correct explanation for assertion.

Complete step by step answer:
-Van der Waals forces are forces of attraction and repulsions between atoms, molecules, and surfaces.
-The difference between Van Der Waals forces and covalent and ionic bonding is that Van Der Waals
forces are caused by the fluctuating polarizations of nearby particles which is a consequence of
quantum dynamics.
-It is the weakest chemical forces, with a strength ranging between 0.4 and 4 kJ/mol, and still can
support structural load.
-Alkene have intermolecular forces i.e. Van Der Waals force that control their boiling point .stronger
These forces will be boiling point.
-Vander Waals force depends upon surface area of two molecular entities and number of electron
surrounding the molecule.
-Straight chain alkane or less branched alkane have a larger surface area while as the branching
increases surface area of molecule goes on decreasing and also Vander Waal force also decreases
and vice versa.
-Branching makes molecules more compact and reduces their surface area.
-Therefore, Straight chain alkane possess high boiling point than branched chain alkane with same
number of carbon atoms. Due to decrease in the Vander Waals force of attraction.
Therefore, option A is the correct answer.

Note:
If we consider the example like n-pentane, iso-pentane and neopentane all contains same number
of carbon atoms but n-pentane has larger surface area while iso-pentane and neopentane .hence
The correct order of boiling point is n-pentane > iso-pentane > neo-pentane.