Structural isomers from hydrogen substitution in n-butane

Steps:

• Represent butane as CH₃-CH₂-CH₂-CH₃, with carbons labeled 1-2-3-4.
• Identify equivalent positions: C1 and C4 (terminal methyl, primary H); C2 and C3 (methylene, secondary H).
• Substitution at C1 (or C4) forms 1-chlorobutane: CH₃-CH₂-CH₂-CH₂Cl.
• Substitution at C2 (or C3) forms 2-chlorobutane: CH₃-CH₂-CHCl-CH₃.

Why B is correct:

• n-Butane has two sets of equivalent hydrogens (primary and secondary), producing two distinct carbon skeletons for chlorobutane per structural isomer definition.

Why the others are wrong:

• A: Ignores positional differences between primary and secondary carbons.
• C: Overcounts by possibly including stereoisomers, which are not structural.
• D: Assumes all four carbons are unique, but C1≡C4 and C2≡C3.

Final answer: B