2012: Advent Chemistry: Caffeine

by on December 1, 2012

If we’re looking at molecules I like, then there is really only one place to start.

The molecular structure of caffeine

This, my friends, is caffeine, a molecule of great importance.

Caffeine was discovered in 1819 by Frederick Ferdinand Runge, who named it after its presence in coffee. It is found in tea, coffee, mate, cocoa, and lots of soft drinks. The caffeine they take out of decaff coffee they put into Relentless, and so the cycle of life goes on.
It is, obviously, a stimulant. It’s also a mild diuretic, is toxic to some animals (horses, dogs, parrots… why anyone ever thought a parrot on espresso would be a good idea I cannot guess) and has a surprisingly powerful effect on spiders.

On to the interesting bit: the chemistry!

Caffeine is basically planar – flat. Everything except some of the hydrogens lies in the same plane. The circle inside the six membered ring (on the left) indicates that it’s an “aromatic ring”. “Aromaticity” is an important concept. It means that the atoms in a ring are all putting some of their electrons into a “pi system”, a ring of electrons above and below the atoms. This makes the ring very stable and hard to break.

You can also draw this ring out with alternating single (–) and double (=) bonds, and if you do that you see that the bonds can be shuffled around to include the right-hand ring in the pattern.

The idea of shuffling bonds around, by moving electrons from double bonds onto single bonds, is also an important concept. Chemists call it ‘resonance’, but that is a silly name. We will call it ‘shuffling bonds around’ because that is clearer in meaning. Being able to shuffle the bonds around makes a molecule, or part of a molecule, more stable by strengthening the bonds. The more shuffling you can do, the more stable you get. That ring pattern above is an extreme case of shuffling bonds around. Ring systems do it so well that all the bonds are equally double-ish, and that’s aromaticity.

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