Which compound has highest boiling point?

Concept: For alkanes, boiling point rises with molecular size — more carbon atoms give a larger electron cloud and stronger van der Waals (London dispersion) forces between molecules. Among isomers that share the same molecular formula, boiling point falls as branching increases, because a more branched, compact shape has less surface area available for intermolecular contact.

Applying this to the four compounds:

• n-Pentane (C5H12) is the straight, unbranched chain isomer — its elongated shape gives adjacent molecules the maximum surface area of contact, producing the strongest van der Waals forces among these four compounds and therefore the highest boiling point.

• Isopentane (2-methylbutane, C5H12) carries one methyl branch, giving it a moderately compact shape. This reduces the surface contact between molecules compared with a fully straight chain, weakening the van der Waals forces and giving it a lower boiling point than the unbranched isomer.

• Neopentane (2,2-dimethylpropane, C5H12) packs four methyl groups around one central carbon, the most branched and most compact of the C5H12 isomers. This near-spherical shape minimises intermolecular contact, so it has the weakest van der Waals attraction and the lowest boiling point of the three C5H12 isomers.

• Isobutane (2-methylpropane, C4H10) has only four carbon atoms, so its molecules are smaller than the five-carbon isomers here. A smaller electron cloud means weaker van der Waals forces overall, giving it a boiling point well below any of the C5H12 isomers.

Cross-check: This matches the known boiling-point order for these compounds — isobutane (-11.7°C) < neopentane (9.5°C) < isopentane (27.8°C) < n-pentane (36.1°C) — confirming that the straight-chain, five-carbon alkane, n-pentane, has the highest boiling point of the four.