Question

1. Ethane (CH3CH3) and Fluoromethane (CH3F) have the same number of electrons and are essentially the same size. However, ethane has a boiling point of 184.5C and fluoromethane has a boiling point of 194.7C. Which answer best explains this 10 degree difference in boiling point in terms of the van der Waals forces present.

Answer 1

All molecules experience dispersion forces. Dipole-dipole interactions are not an alternative to dispersion forces - they occur in addition to them. Molecules which have permanent dipoles will therefore have boiling points rather higher than molecules which only have temporary fluctuating dipoles.

Surprisingly dipole-dipole attractions are fairly minor compared with dispersion forces, and their effect can only really be seen if you compare two molecules with the same number of electrons and the same size.

For example Ethane and Fluoromethane

Both have identical numbers of electrons, and if you made models you would find that the sizes were similar - as you can see in the diagrams. That means that the dispersion forces in both molecules should be much the same.

The higher boiling point of fluoromethane is due to the large permanent dipole on the molecule because of the high electronegativity of fluorine. However, even given the large permanent polarity of the molecule, the boiling point has only been increased by some 10