Which of the following pairs of compounds are constitutional isomers?

• A. $C_4{H}_{10}$: n-butane and isobutane
• B. $C_5{H}_{12}$: n-pentane and neopentane
• C. $C_2{H}_6$: ethane and ethanol
• D. $C_3{H}_8$: propane and cyclopropane

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the definition of constitutional isomers. Constitutional isomers are compounds that have the same molecular formula but different connectivity of atoms, meaning their atoms are connected in different orders.
2. Step 2: Examine each pair of compounds and compare their molecular formulas. For example, n-butane and isobutane both have the formula \(C\\_4H\\_{10}\), so they are candidates for constitutional isomers.
3. Step 3: Check if the pairs have the same molecular formula but different structures. For instance, n-pentane and neopentane both have the formula \(C\\_5H\\_{12}\) but differ in how the carbon atoms are connected, making them constitutional isomers.
4. Step 4: Identify pairs that do not have the same molecular formula. For example, ethane (\(C\\_2H\\_6\)) and ethanol (\(C\\_2H\\_6O\)) differ in molecular formula due to the presence of oxygen in ethanol, so they are not constitutional isomers.
5. Step 5: For pairs with the same molecular formula but different types of compounds (like propane and cyclopropane, both \(C\\_3H\\_8\)), determine if the difference is in connectivity (cyclopropane is cyclic, propane is acyclic). Since they have the same formula but different connectivity, they are constitutional isomers.