Identifying constitutional isomers of butane (C4H10)

Steps:

• Determine butane's molecular formula: C4H10, with straight-chain n-butane as CH3-CH2-CH2-CH3.
• Recall constitutional isomers share the formula but differ in carbon connectivity; butane has two: n-butane and branched 2-methylpropane.
• Examine each option's carbon count and structure to match C4H10 with branching.
• Select the option showing a central carbon bonded to one hydrogen and three methyl groups.

Why C is correct:

• Option C shows 2-methylpropane ((CH3)2CHCH3), the branched constitutional isomer of n-butane, following the same C4H10 formula but with different connectivity.

Why the others are wrong:

• A: Straight-chain n-butane, identical to butane itself, not a distinct isomer.
• B: Contains only three carbons (propane, C3H8), wrong formula.
• D: Five-carbon chain (pentane, C5H12), exceeds butane's formula.

Final answer: C