Which of the following pairs of compounds are constitutional isomers of each other?

• A. $C_4{H}_{10}$ (n-butane) and $C_4{H}_{10}$ (isobutane)
• B. $C_6{H}_6$ (benzene) and $C_6{H}_{12}$ (cyclohexane)
• C. $C_5{H}_{12}$ (n-pentane) and $C_5{H}_{12}$ (cyclopentane)
• D. $C_2{H}_6$ (ethane) and $C_3{H}_8$ (propane)

Answer: (A)

Verified step by step guidance

1. Understand that constitutional isomers are compounds that have the same molecular formula but different connectivity of atoms.
2. Compare the molecular formulas of each pair to check if they are identical, since constitutional isomers must have the same molecular formula.
3. For the pair C\_4H\_10 (n-butane) and C\_4H\_10 (isobutane), note that both have the same molecular formula but different structures (straight chain vs branched), so they are constitutional isomers.
4. For the other pairs, observe that their molecular formulas differ (e.g., C\_6H\_6 vs C\_6H\_12 or C\_2H\_6 vs C\_3H\_8), so they cannot be constitutional isomers.
5. Conclude that only the pair with the same molecular formula but different connectivity (n-butane and isobutane) are constitutional isomers.