Question

A pair of compounds having the same boiling points are

• A. cis but-2-ene and trans but-2-ene
• B. n-hexane and neo-hexane
• C. Benzene and naphthalene
• D. (+) butan - 2 - ol and (-) butan - 2 - ol

Answer 1

The Correct Option is D

The boiling point of a compound is influenced by factors like molecular weight, intermolecular forces, and symmetry. Let's analyze the given options:

Option A: Benzene and Naphthalene 

This is incorrect. Benzene (C₆H₆) has a boiling point of 80.1°C, while naphthalene (C₁₀H₈) has a boiling point of 218°C. Hence, these two compounds do not have the same boiling points.

Option B: (+)-butan-2-ol and (-)-butan-2-ol

This is correct. Both (+)-butan-2-ol and (-)-butan-2-ol are enantiomers, which are non-superimposable mirror images. These compounds have the same molecular formula and identical intermolecular forces, which results in the same boiling points despite their optical isomerism.

Option C: cis-but-2-ene and trans-but-2-ene

This is incorrect. cis-but-2-ene and trans-but-2-eneare geometric isomers with different physical properties, including different boiling points. cis-but-2-ene has stronger intermolecular forces due to its molecular shape, leading to a higher boiling point than trans-but-2-ene.

Option D: n-hexane and neo-hexane

This is incorrect. n-hexane and neo-hexane are structural isomers, and while their molecular weight is similar, neo-hexane has a more compact structure, resulting in weaker intermolecular forces and a lower boiling point than n-hexane.

Conclusion:

The correct answer is: Option B: (+)-butan-2-ol and (-)-butan-2-ol, as they are enantiomers with the same boiling point.

Answer 2

Compounds with the same molecular formula but different spatial arrangements or chirality can have identical boiling points due to their similar molecular structure and intermolecular forces. The boiling point of enantiomers, such as (+) butan-2-ol and (−) butan-2-ol, can be identical despite their different optical activities. This is because the physical properties like boiling point are largely influenced by the molecular weight and symmetry, which are the same for these enantiomers.

Thus, the correct answer is that (+) butan-2-ol and (−) butan-2-ol have the same boiling points.