Which one is it? – intermolecular forces

Here is a question that should get you thinking …

I have two molecules, chloromethane (CH₃Cl) and iodomethane (CH₃I).

What types of intermolecular forces are found in these molecules.

Well, hopefully you are OK with that – it is London dispersion forces and permanent dipole dipoles.

So, out of these two intermolecular forces, which is the strongest?

Is it the permanent dipole dipole?

Or I it the London dispersion force?

I’m fairly confident that most well taught students would say the dipole – dipole force.

So how could you test this?

How could you work it out?

The answer is simple – you just look up the boiling point of each molecule.

A quick Wikipedia (I know, I know but I don’t have my data book with me as I write this!) tells me that, wait for it:

Chloromethane = -23.8^oC

And iodomethane = 42.4^oC

How can this be – surely, chloromethane has the highest boiling point as it has the strongest type of intermolecular force – or does it?!

What does this data imply?

Well, to me, it implies that the strongest intermolecular force is that in Iodomethane – as the higher boiling point suggests – which means that the London dispersion forces are the strongest.

At this point, you will probably be either scratching your head or racing away to think of the causes of the anomaly – I hope you are doing the latter!

How can I put this, ‘force for force’, permanent dipole – dipole beats London dispersion every time but we don’t have an equal contest here. The number of London dispersion forces in iodomethane is much greater than the permanent dipole – dipole forces in chloromethane.

The clue is in the molecular mass of each molecule – chloromethane is approx. 50 whilst iodomethane is approx. 152 – quite a big difference.

Which means many more electrons in iodomethane, which means that the overall strength of the London dispersion forces is much greater.

Easy?!

This sort of observation explains really basic and straightforward observations like the fact that wax can be a solid at room temperature – even though it is very non polar. It is a big molecule so it’s London dispersion forces will be significantly large enough to keep it solid.

The same pattern is seen in HI (boiling point = -35oC) and HCl (Boiling point = -85oC)

What is also seen in this family of molecule is HF having a boiling point of 20oC. This shows you how much stronger those H-bonds are than the dipole – dipoles.

And this is why I love Chemistry – the simple stuff still causes upsets and problems! Who needs the more difficult concepts?