Which pair of the following molecules are constitutional isomers? (A) $C_4{H}_{10}$ n-butane and $C_4{H}_{10}$ isobutane (B) $C_2{H}_6$ and $C_3{H}_8$ (C) $C_2{H}_4$ and $C_2{H}_6$ (D) $C_3{H}_8$ and $C_3{H}_6$

• A. (B) $C_2{H}_6$ and $C_3{H}_8$
• B. (D) $C_3{H}_8$ and $C_3{H}_6$
• C. (C) $C_2{H}_4$ and $C_2{H}_6$
• D. (A) $C_4{H}_{10}$ n-butane and $C_4{H}_{10}$ isobutane

Answer: (D)

Verified step by step guidance

1. Recall that constitutional isomers are molecules that have the same molecular formula but different connectivity of atoms.
2. Examine each pair of molecules and compare their molecular formulas to see if they match exactly.
3. For pair (A), both n-butane and isobutane have the molecular formula \(C\\_4H\\_{10}\), so they are candidates for constitutional isomers.
4. For pairs (B), (C), and (D), note that the molecular formulas differ between the two molecules in each pair, so they cannot be constitutional isomers.
5. Conclude that the pair with identical molecular formulas but different structures (connectivity) is the correct answer, which is pair (A).