Structural isomers share the same molecular formula but differ in atom connectivity

Steps:

• Define structural isomers as compounds with identical molecular formulas but different bonding arrangements.
• Calculate molecular formulas for each pair to check if they match.
• Verify connectivity: isomers must have the same atoms arranged differently.
• Identify the pair meeting both criteria.

Why B is correct:

• Both CH₃CH₂CH₂CH₃ (n-butane) and CH₃CH(CH₃)CH₃ (isobutane) have formula C₄H₁₀ but differ in branching.

Why the others are wrong:

• A: CH₃CH₂CH₂CH₂OH (C₄H₁₀O) and CH₃CH₂COCH₃ (C₄H₈O) have different formulas.
• C: CH₃CH₂CH(OH)CH₃ and CH₃CH(OH)CH₂CH₃ are the same compound (butan-2-ol), just differently written.
• D: CH₃CH₂OCH₃ (C₃H₈O) and CH₃CH₂CH₂OH (C₃H₈O) have same formula but are functional isomers, not structural (different classes: ether vs. alcohol).

Final answer: B