Physical Properties

Question

Among chloromethane, chloroethane, 1-chloropropane and 1-chlorobutane, which compound is expected to have the highest boiling point and why?

RankGuru · Accepted Answer

Boiling points of haloalkanes within a homologous series are governed mainly by the strength of van der Waals dispersion forces, which increase with molecular size, mass and surface area. Among the four straight-chain chlorides listed, 1-chlorobutane has the longest carbon chain, the largest surface area and the greatest molecular mass, so it experiences the strongest dispersion attractions and therefore boils highest. The chloromethane option is wrong because, although it is polar, it is the smallest and lightest molecule, giving the weakest overall intermolecular forces. The chloroethane option is incorrect because haloalkanes cannot form hydrogen bonds among themselves, since they lack H directly bonded to a highly electronegative atom. The 1-chloropropane option is wrong as it is both smaller than 1-chlorobutane and not actually branched. This follows the NCERT trend of rising boiling point with chain length. Sanity check: boiling point rises steadily down the series, so the largest member, 1-chlorobutane, must be highest.

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