Chemistry / Unit 8 Chem 1385 worked example, 60 to 90 seconds
Predicting the Dominant IMF and Ranking Boiling Points
The idea
Predicting which intermolecular force dominates in a substance means checking, in order, whether hydrogen bonding is possible (H bonded to F, O, or N), then whether the molecule is polar for dipole-dipole attraction, and treating dispersion forces as present regardless — the strongest force found sets the boiling-point ranking against other substances of similar size.
In practice
Comparing dimethyl ether, ethanol, and propane, all near 44-46 g/mol: propane is nonpolar (dispersion only), dimethyl ether is polar (dispersion plus dipole-dipole), and ethanol has an O-H group (dispersion, dipole-dipole, and hydrogen bonding). The predicted order, propane < dimethyl ether < ethanol, matches their measured boiling points of about -42 °C, -25 °C, and 78 °C.
Where people slip
Stopping the check at "polar or nonpolar" without asking whether hydrogen bonding specifically applies misses the single largest jump in this ranking, the one between ordinary dipole-dipole attraction and hydrogen bonding.
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