Which pair of the following compounds are constitutional isomers?

• A. $C_4{H}_{10}$ (n-butane) and $C_4{H}_{10}$ (isobutane)
• B. $C_2{H}_4$ (ethylene) and $C_2{H}_2$ (acetylene)
• C. $C_6{H}_6$ (benzene) and $C_6{H}_{12}$ (cyclohexane)
• D. $C_2{H}_6$ (ethane) and $C_3{H}_8$ (propane)

Answer: (A)

Verified step by step guidance

1. Recall 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 see if they match exactly, since only pairs with identical molecular formulas can be constitutional isomers.
3. For the first pair, both n-butane and isobutane have the formula \(C\\_4H\\_{10}\), so they are candidates for constitutional isomers.
4. For the second pair, ethylene is \(C\\_2H\\_4\) and acetylene is \(C\\_2H\\_2\), so their formulas differ and they cannot be constitutional isomers.
5. For the third pair, benzene is \(C\\_6H\\_6\) and cyclohexane is \(C\\_6H\\_{12}\), so their formulas differ and they are not constitutional isomers; similarly, ethane (\(C\\_2H\\_6\)) and propane (\(C\\_3H\\_8\)) differ in formula and are not isomers.