Q.Of the following compounds, which has the highest boiling point? a) n-Butyl chloride b) Isobutyl chloride c) t-Butyl chloride d) n-propyl chloride

Step 1. All four options are isomers of C4H9Cl, differing only in branching: n-butyl chloride (straight chain), isobutyl chloride (one branch), t-butyl chloride (most branched), and n-propyl chloride (a shorter, 3-carbon chain, not even isomeric with the other three -- automatically a weaker case).

Step 2. Among isomers of the same molecular formula, boiling point falls as branching increases: a more branched molecule is more compact/spherical, has less surface area for van der Waals contact between molecules, and so has weaker intermolecular attraction and a lower boiling point.

Step 3. n-Butyl chloride is the least branched (straight-chain) of the C4H9Cl isomers, so it has the most surface area and the highest boiling point among them; n-propyl chloride, being a smaller molecule altogether (fewer carbons, less total van der Waals contact), boils lower still.

Rank isomers of the same formula by branching -- least branched (most linear, most surface area) boils highest -- and note the shorter propyl chloride is not even a real competitor.

• Assuming the most branched (t-butyl chloride) boils highest because it 'looks bulkier' -- bulk from branching actually lowers surface contact and boiling point, the opposite of chain length increasing it.