Which of the following compounds has highest boiling point?

The boiling point of alkanes depends on molecular weight and branching. For isomers, molecular weight remains constant, so branching becomes the determining factor.

Increased branching leads to a more spherical molecular shape, reducing surface area available for van der Waals interactions. This results in weaker London dispersion forces and lower boiling points. Less branching increases molecular surface area, strengthening intermolecular forces and raising the boiling point.

Among the options:
$n$-hexane (A) has no branching
$2$-methylpentane (B) has one methyl branch
$3$-methylpentane (C) has one methyl branch
$2,2$-dimethylbutane (D) has two methyl branches

The branching order is: $n$-hexane $< 2$-methylpentane $\approx 3$-methylpentane $< 2,2$-dimethylbutane.

$n$-hexane's linear structure provides maximum surface area for intermolecular interactions, yielding the strongest van der Waals forces and highest boiling point.

Experimental boiling points confirm this: $n$-hexane ($68.7^\circ$C), $2$-methylpentane ($60.3^\circ$C), $3$-methylpentane ($63.3^\circ$C), $2,2$-dimethylbutane ($49.7^\circ$C).