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Arrange the following compounds in increasing order of their boiling points: (I) C2H5OH (ethanol) (II) C2H5Cl (chloroethane) (III) C2H5CH3 (propane) (IV) C2H5OCH3 (methyl ethyl ether / methoxyethane)

1. (III) < (II) < (I) < (IV)
2. (II) < (III) < (IV) < (I)
3. (IV) < (III) < (I) < (II)
4. (III) < (IV) < (II) < (I)

Correct answer: (III) < (IV) < (II) < (I)

Solution

Propane (III) has only weak London dispersion forces, giving bp -42 deg C. Methoxyethane/ether (IV) has a dipole but cannot H-bond as a donor, bp ~7 deg C. Chloroethane (II) has a larger dipole and higher MW, bp ~12 deg C. Ethanol (I) has strong O-H...O hydrogen bonding, bp ~78 deg C. Increasing order: (III) < (IV) < (II) < (I).

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