The boiling point of alkanes is primarily determined by molecular mass and branching. (a) Molecular mass : Hexane (C6H14) has a higher molecular mass than the other compounds (C5H12). Therefore, it has the highest boiling point. Order so far : D>(A,B,C) (b) Branching : For the isomers of pentane (C5H12), boiling point decreases with increasing branching due to a reduction in surface area for intermolecular forces. Pentane (C) is a straight chain. 2-Methylbutane (A) has one branch. 2,2-Dimethylpropane ( B ) has two branches and is the most compact. Order for isomers : C>A>B. Combining both factors, the final decreasing order of boiling points is D>C>A>B.