Boiling Points of Two Isomers
Two colourless liquids, A and B, are isomers with molecular formula C4H10O. Both are miscible with water to some extent, but A dissolves completely while B forms a separate layer. A reacts with sodium metal to release hydrogen gas; B does not. The boiling point of A is 117 °C and of B is 83 °C. A student claims that because they are isomers, their intermolecular forces must be identical, and that the boiling-point difference is experimental error. When 0.10 mol of each is burned, both release the same amount of energy within 2 %. Infrared spectra show a broad O–H stretch in A but none in B.
1. Identify the functional classes of A and B and explain the boiling-point difference in terms of intermolecular forces.
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A shows a broad O–H stretch and reacts with sodium to give H2, so A is an alcohol; its complete water miscibility is consistent with hydrogen bonding to water. B lacks O–H and does not react with sodium, so B is an ether. The student's premise is wrong: isomers have the same molecular formula but different connectivity, so their intermolecular forces differ. A can donate and accept hydrogen bonds (O–H), giving strong hydrogen bonding between molecules; B can only accept hydrogen bonds weakly via its oxygen lone pairs and cannot donate, so its dominant forces are dipole–dipole and dispersion. Breaking the hydrogen-bond network in A requires more energy, so A boils higher (117 °C) than B (83 °C). The 34 °C gap is far larger than experimental uncertainty and is a real structural effect, not error.
2. The equal combustion energies are offered as evidence that the liquids are the same substance. Evaluate this argument.
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The argument is invalid. Enthalpy of combustion depends chiefly on the number and types of bonds broken and formed, and isomers of the same formula have the same overall C, H and O content, so their combustion enthalpies are expected to be very similar - within a few percent. That similarity therefore confirms the molecular formula, not the identity. Identity is established by properties that depend on connectivity: boiling point, IR O–H stretch, reaction with sodium, and water solubility all differ sharply here. The 2 % agreement in combustion energy is exactly what the isomer hypothesis predicts, so it cannot discriminate between the two structures.