Dipole–dipole forces Transitional Reasoning Download PDF

Questions

1. 1.

   A student investigates two organic compounds with identical molar masses (44 g/mol): nonpolar propane ($\text{C}_3\text{H}_8$) and polar ethanal ($\text{CH}_3\text{CHO}$). At standard pressure, propane boils at $-42^\circ\text{C}$, whereas ethanal boils at $+20^\circ\text{C}$, even though both molecules possess comparable molecular sizes and total electron counts. Based on molecular structure and intermolecular force theory, which statement best explains the observed difference in their boiling points?

   • A) Ethanal has a higher boiling point because its permanent molecular dipole creates additional dipole–dipole attractions that require more energy to overcome than the dispersion forces in propane.
   • B) Ethanal has a higher boiling point because its carbonyl oxygen atom participates in intermolecular hydrogen bonding with the hydrogen atoms attached to adjacent methyl groups.
   • C) Ethanal has a higher boiling point because boiling the liquid requires breaking the strong intramolecular polar carbon–oxygen double bonds rather than intermolecular interactions.
   • D) Propane has a lower boiling point because nonpolar molecules experience no electrostatic attractive forces whatsoever in the condensed phase.