The correct answer to the question: "A pair of compounds having the same boiling points are," is Option D: $$(+)$$ butan-2-ol and $$(-)$$ butan-2-ol.

Let's explain why the other options are incorrect:

Option A: cis but-2-ene and trans but-2-ene have different spatial arrangements around the double bond. This difference in geometry leads to different physical properties, especially boiling points. Typically, the trans isomers are less polar and have a lower boiling point compared to the cis isomers.

Option B: n-hexane and neo-hexane (more correctly called 2,2-dimethylbutane) are structural isomers, which means they have different structures and thus different intermolecular forces and boiling points. n-hexane is a straight-chain alkane, whereas neo-hexane is a highly branched alkane. The increased branching in neo-hexane decreases the surface area for intermolecular interactions, leading to a lower boiling point than n-hexane.

Option C: Benzene and naphthalene are aromatic hydrocarbons, but they are not isomeric. Benzene has a single aromatic ring, while naphthalene has two fused aromatic rings. This difference results in naphthalene having a significantly higher boiling point than benzene due to the larger surface area and greater London dispersion forces in naphthalene.

Option D: $$(+)$$ butan-2-ol and $$(-)$$ butan-2-ol are enantiomers, which are stereoisomers that are non-superposable mirror images of each other. Enantiomers have identical physical properties (e.g., boiling point, melting point) in a non-chiral environment because they share the same molecular formula and connectivities. The (+) and (-) refer to the different optical activities of the molecules; that is, they rotate plane-polarized light in different directions, but this does not affect the boiling point of the compound.

Therefore, the only pair of compounds listed with the exact same boiling point would be the R and S enantiomers of butan-2-ol (also known as (+) and (-) butan-2-ol), as noted in Option D.

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Isomerism · Chemistry · KCET

MCQ (Single Correct Answer)