Which of the following pairs of compounds are constitutional isomers?

• A. $C_2{H}_6$ (Ethane) and $C_2{H}_4$ (Ethene)
• B. ${\mathrm{CH}}_3\mathrm{OH}$ (Methanol) and $C_2{H}_5\mathrm{OH}$ (Ethanol)
• C. $C_6{H}_{12}$ (Cyclohexane) and $C_6{H}_6$ (Benzene)
• D. $C_4{H}_{10}$ (Butane) and $C_4{H}_{10}$ (2-methylpropane)

Answer: (D)

Verified step by step guidance

1. Understand the concept of constitutional isomers: Constitutional isomers are compounds that have the same molecular formula but differ in the connectivity of their atoms.
2. Analyze the molecular formula of each pair of compounds: Ethane (C₂H₆) and ethene (C₂H₄) have different molecular formulas, so they cannot be constitutional isomers.
3. Consider methanol (CH₃OH) and ethanol (C₂H₅OH): These compounds have different molecular formulas, so they are not constitutional isomers.
4. Examine cyclohexane (C₆H₁₂) and benzene (C₆H₆): These compounds have different molecular formulas, so they are not constitutional isomers.
5. Evaluate butane (C₄H₁₀) and 2-methylpropane (C₄H₁₀): Both compounds have the same molecular formula, C₄H₁₀, but differ in the connectivity of their atoms, making them constitutional isomers.