Question

checkpoint: imf strength 2
you said that hydrogen bonds were the most important intermolecular force.
based on that and the guidelines to the right, which of the following substances would have the highest boiling point?
ch₃oh (32.0 g/mol)
ch₃sh (48.1 g/mol)
which substance has the stronger total intermolecular forces/the higher boiling point?

1. first, look to see if the substance can hydrogen bond. if so, it likely has the strongest intermolecular forces (e.g., hf > hcl).
2. if you are comparing two molecules of very different molar masses, london dispersion forces are more important (e.g., hbr > hcl).
3. if you are comparing two molecules of very similar molar mass, dipole-dipole forces are more important. the molecule with the greater dipole-dipole forces (the greatest electronegativity difference between the atoms within the molecule) will have the greatest boiling point.

Explanation:

Step1: Analyze hydrogen bonding

$CH_{3}OH$ has hydrogen bonding (since it has $O - H$ bond). $CH_{3}SH$ does not have hydrogen bonding (as $S - H$ does not meet the criteria for hydrogen bonding which requires a highly electronegative atom like $O$, $N$ or $F$ bonded to $H$). According to the first guideline, substances with hydrogen bonding tend to have stronger intermolecular forces.

Step2: Consider other forces (not relevant here as hydrogen bonding is decisive)

Even though $CH_{3}SH$ has a higher molar mass, hydrogen bonding in $CH_{3}OH$ is a much stronger intermolecular force than the London dispersion forces (which would be the main additional force in $CH_{3}SH$ besides dipole - dipole if any).

Answer:

$CH_{3}OH$ (32.0 g/mol)