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CHEMISTRY, M7 2020 HSC 32

The table shows three compounds and their boiling points.
 

An ester does not always have a lower boiling point than both the alcohol and the alkanoic acid from which it is produced.

Using the information in the table, account for this observation.   (4 marks)

Show Answers Only
  • The boiling points of all compounds are a function of the strength of their intermolecular forces.
  • All three compounds are polar and have dispersion forces between molecules and dipole-dipole interactions.
  • Methanol and propanoic acid can also form hydrogen bonds (strongest type of intermolecular force).
  • Methyl propanoate’s larger size gives it stronger dispersion forces than methanol and propanoic acid, but it cannot form hydrogen bonds.
  • Despite having weaker dispersion forces, propanoic acid can form two hydrogen bonds per molecule, which makes up for its weaker dispersion forces and results in stronger overall intermolecular forces than methyl propanoate.
  • Methanol is a polar molecule that can form strong hydrogen bonds due to its hydroxyl group. However, it has the lowest boiling point due to its small molar mass, resulting in weaker dispersion forces.
Show Worked Solution
  • The boiling points of all compounds are a function of the strength of their intermolecular forces.
  • All three compounds are polar and have dispersion forces between molecules and dipole-dipole interactions.
  • Methanol and propanoic acid can also form hydrogen bonds (strongest type of intermolecular force).
  • Methyl propanoate’s larger size gives it stronger dispersion forces than methanol and propanoic acid, but it cannot form hydrogen bonds.
  • Despite having weaker dispersion forces, propanoic acid can form two hydrogen bonds per molecule, which makes up for its weaker dispersion forces and results in stronger overall intermolecular forces than methyl propanoate.
  • Methanol is a polar molecule that can form strong hydrogen bonds due to its hydroxyl group. However, it has the lowest boiling point due to its small molar mass, resulting in weaker dispersion forces.