Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Boiling point of cis isomers is higher than trans isomers.
Reason (R) : The Dipole moment of cis isomers is higher than trans isomers.
In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Rule of thumb for geometric isomers: Cis generally has a higher Boiling Point (due to higher polarity and stronger dipole-dipole forces). Trans generally has a higher Melting Point (because its highly symmetric structure packs much more efficiently into a solid crystal lattice).

Answer 1

The Correct Option is A

Step 1: Concept:
This question tests knowledge of the physical properties (specifically boiling point) of geometric isomers (cis and trans) and how these properties relate to molecular polarity (dipole moment).

Step 2: Key Concept:
Boiling points are directly related to the strength of intermolecular forces in a liquid. Stronger forces require more thermal energy to overcome, leading to higher boiling points. Dipole-dipole interactions are a significant contributor to intermolecular forces.

Step 3: Step-by-step Explanation:

• Evaluating Reason (R): In a cis-isomer, similar functional groups are on the same side of the double bond. Their individual bond dipoles generally point in the same general direction, meaning they add up to give the molecule a significant net dipole moment. In a trans-isomer, similar groups are on opposite sides, causing their bond dipoles to point in opposing directions, mostly or completely canceling each other out. Thus, it is generally true that cis isomers have higher net dipole moments than trans isomers.

• Evaluating Assertion (A): Because cis isomers possess a higher dipole moment, the dipole-dipole attractions between cis molecules in the liquid phase are stronger than those between trans molecules (which rely mostly on weaker London dispersion forces). Because these intermolecular forces are stronger, more energy is required to boil a cis isomer, giving it a higher boiling point than its trans counterpart. Therefore, Assertion (A) is correct.

• Relationship: The increased dipole moment described in (R) directly causes the stronger intermolecular forces that lead to the higher boiling point described in (A). Therefore, (R) correctly explains (A).

Step 4: Final Answer:
Both statements are true, and the Reason provides the physical basis for the Assertion. Option (A) is correct.