Which of the following would you expect to have the highest boiling point? Justify your choice. (a) Propane, \(\mathrm{C}_{3} \mathrm{H}_{8}\) (b) Carbon dioxide, \(\mathrm{CO}_{2}\) (c) Ethyl alcohol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (d) Methyl fluoride, \(\mathrm{CH}_{3} \mathrm{~F}\)

First, we must determine if the given molecules are polar or nonpolar. Molecular polarity depends on the electronegativity difference between the atoms involved and the symmetry of the molecule. (a) Propane, \(\mathrm{C}_{3} \mathrm{H}_{8}\): Nonpolar, due to its symmetric hydrocarbon structure with no electronegativity difference. (b) Carbon dioxide, \(\mathrm{CO}_{2}\): Nonpolar, symmetrical linear structure despite the electronegativity difference between the carbon and oxygen atoms. (c) Ethyl alcohol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\): Polar, asymmetric structure with an oxygen atom bonded to a hydrogen atom. (d) Methyl fluoride, \(\mathrm{CH}_{3} \mathrm{~F}\): Polar, asymmetric structure with an electronegativity difference between the carbon and fluorine atoms.

Now that we have determined the molecular polarity, we examine the types of intermolecular forces present in the compounds: (a) Propane: London dispersion forces. (b) Carbon dioxide: London dispersion forces. (c) Ethyl alcohol: London dispersion forces, dipole-dipole interactions, and hydrogen bonding (due to the \(\mathrm{OH}\) group). (d) Methyl fluoride: London dispersion forces and dipole-dipole interactions.

Since we have identified the IMFs in each molecule, we can predict which compound will have the highest boiling point based on the strength of these forces: (a) Propane: low boiling point, due to only experiencing London dispersion forces. (b) Carbon dioxide: low boiling point, due to only experiencing London dispersion forces. (c) Ethyl alcohol: relatively high boiling point, due to the presence of hydrogen bonding along with London dispersion forces and dipole-dipole interactions. (d) Methyl fluoride: relatively low boiling point, due to only experiencing London dispersion forces and dipole-dipole interactions.

Based on the comparison of intermolecular forces, we can determine that ethyl alcohol (c) will have the highest boiling point among these compounds, due to the presence of hydrogen bonding along with London dispersion forces and dipole-dipole interactions.