Explaining Boiling Point Differences: Cis vs. Trans Isomers

This question asks about the relationship between the boiling points and dipole moments of cis and trans isomers.

Assertion (A) Analysis: Boiling Point of Cis Isomers

The statement asserts that the boiling point of cis isomers is generally higher than that of trans isomers. This is often true because the molecular geometry influences intermolecular forces.

• Intermolecular Forces: Cis isomers frequently possess a net molecular dipole moment. This leads to stronger dipole-dipole interactions between molecules.
• Energy Requirement: Stronger intermolecular forces require more energy to overcome during boiling, resulting in a higher boiling point.

Reason (R) Analysis: Dipole Moment of Cis Isomers

The statement asserts that the dipole moment of cis isomers is higher than that of trans isomers. This is generally correct due to the spatial arrangement of polar groups within the molecule.

• Molecular Geometry: In cis isomers, polar groups are on the same side of the molecule. Their individual bond dipoles add up, creating a larger resultant dipole moment.
• Symmetry Effect: In trans isomers, polar groups are on opposite sides. Their bond dipoles often oppose and partially cancel each other out, leading to a smaller net dipole moment, or even zero in highly symmetrical molecules.

Connecting Reason (R) to Assertion (A)

The higher dipole moment of cis isomers (Reason R) directly leads to stronger dipole-dipole intermolecular forces compared to trans isomers. These stronger forces require more energy to break, thus explaining the higher boiling point of cis isomers (Assertion A).

Conclusion

Both Assertion (A) and Reason (R) are factually correct. Furthermore, the higher dipole moment in cis isomers (R) is the direct cause of the stronger intermolecular forces that lead to their higher boiling points (A). Therefore, Reason (R) is a correct explanation for Assertion (A).