Organic Chemistry, "Function", Part 4 - Small molecule Properties verses Intermolecular Forces

Greetings,

This post discusses the effects of changing structures of small molecules and their effect on macroscopic properties.

Typically, a course in general chemistry will describe London Dispersion Forces as the weakest type of intermolecular force and then leave students with the impression that those forces, generally, do not significantly affect compound properties.  That scenario is a gross over-simplification, leading students away from the truth of the matter!

It is true that London Dispersion Forces are the weakest intermolecular attractions across single atoms of a molecule pair, but the effect is insignificant only for the smallest of molecules.  In reality, most covalent compounds are made up of molecules with London Dispersion Forces contributing greatly to the overall attractive force between those molecules!  See https://www.blogger.com/blogger.g?blogID=4858578287149958542#editor/target=post;postID=2687633480868585580;onPublishedMenu=posts;onClosedMenu=posts;postNum=3;src=postname

London Dispersion Forces - Short Range Attractions; Long Range Effects

The strength of London Forces reduces as the sixth power of the distance between atoms, so they become more important with greater molecular weight.  London Forces also are more dominant for molecules more spread out (linear) verses less spread out (branched).  Dispersion forces dominate in nonpolar molecules because they are polarizable (able to have electrons shifting to one side of their atoms).  The effect is additive, therefore more pronounced for larger molecules.  This also means that dipoles can be easily induced in atoms of other larger molecules.  The result is a series of very long induced dipole cascades, which can cause a significant overall force of attraction between molecules.

London Dispersion Forces - Always Present; Effects of Other Forces are Added

The strength of London forces should be considered first, because they are always present.  In the case of polar molecules, London force strength is reduced because polar molecules are less polarizable: They are already permanently polarized.  Again, it is only the smallest molecules which have small London Force contributions, compared to other intermolecular forces.  The following chart indicates the effect for small molecules.

 

The effect of London forces is small, only for the smallest molecules, as indicated in the chart.  A future post will show the greater effects of London forces for larger molecules.

As always, thank you for reading!

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