Equivalent Structural Representations in Isomers

Steps:

• Identify the molecular formula C4H10, which has two isomers: n-butane (straight chain) and isobutane (branched).
• Compare connectivity: structures with identical carbon skeletons and bond arrangements represent the same molecule.
• Examine diagrams: structure 1 shows n-butane, structure 2 and 4 show isobutane with a central carbon bonded to three methyl groups, structure 3 shows n-butane.
• Confirm equivalence: 2 and 4 differ only in drawing orientation, not topology.

Why D is correct:

• Structures 2 and 4 depict the same branched isobutane isomer, as per IUPAC definitions of structural isomers where identical connectivity defines sameness.

Why the others are wrong:

• A: 1 and 2 are different isomers (straight vs. branched chain).
• B: 1 and 3 both show n-butane but are listed separately; question seeks non-identical drawings of the same.
• C: 2 and 3 are different isomers (branched vs. straight).

Final answer: D