Question

In general, when comparing compounds of similar molecular weight, which of the following would have the highest boiling point?

Answer 1

There are 3 important trends to consider.

1. The relative strength of the four intermolecular forces is: Ionic > Hydrogen bonding > dipole dipole > Van der Waals dispersion forces. The influence of each of these attractive forces will depend on the functional groups present.
2. Boiling points increase as the number of carbons is increased.
3. Branching decreases boiling point. (eventhough having same molecular weight ,n-butane has more boiling point that iso-propane)

The relative strength of the four intermolecular forces .

Compare the different butane alcohol derivatives shown below. Molecules of diethyl ether, C4H10 O, are held together by dipole-dipole interactions which arise due to the polarized C-O bonds. Compare its boiling point of (35 °C)with that of Its isomer butanol (117 °C). The greatly increased boiling point is due to the fact that butanol contains a hydroxyl group, which is capable of hydrogen bonding. Still, the attractive forces in butanol pale in comparison to those of the salt sodium butoxide, which melts at an extremely high temperature (well above 260 °C) and actually decomposes before it can turn into a liquid.