WAEC Chemistry 2019 Paper

Concept:Among alkanes, boiling point increases with longer carbon chains but decreases with more branching.Explanation:Option A is $\mathrm{C_6H_{14}}$, option B is $\mathrm{C_5H_{12}}$, and option C is straight-chain $\mathrm{C_4H_{10}}$ (butane).Option D is $(\mathrm{CH_3})_3\mathrm{CH}$, which is 2-methylpropane, a branched isomer of $\mathrm{C_4H_{10}}$.Options C and D have the same molecular formula, but D is more branched.Branched alkanes have weaker van der Waals forces because their reduced surface area decreases intermolecular contact.Hence, 2-methylpropane boils at a lower temperature than straight-chain butane.For this reason, among the given compounds, option D has the lowest boiling point.Answer:D. $(\mathrm{CH_3})_3\mathrm{CH}$ (2-methylpropane)