2012: Advent Chemistry: Nicotine

by on December 22, 2012

We’ve covered two popular recreational drugs, ethanol and caffeine, so let’s look at a third. Nicotine is the major active ingredient in tobacco, and is usually ingested by burning tobacco leaves and inhaling the smoke. The nicotine is absorbed through the membranes of the mouth and throat straight into the blood stream, for an immediate effect.

The structure of nicotine, a chiral molecule based on pyridine and pyrrolidine rings.

Nicotine is highly addictive and very hard to shake a dependency to. Unlike caffeine, to which you can form a dependence within a week but lose that dependency just as fast, giving up nicotine takes months of determination.

As drug molecules go, it’s pretty small and simple.  The ring on the left is pyridine, which comes up a lot in organic chemistry. More interesting is the pyrrolidine ring on the right, with the substituted nitrogen – in straight pyrrolidine, there is a hydrogen on the third corner of the nitrogen, while in nicotine that’s replaced by a methyl group.

Nicotine is a nice example of a non-obviously chiral group. The clue is the way that hydrogen is drawn – it’s the only one shown, and it has a wedge-shaped bond to tell us it points outwards from the screen. The other three carbons in the pyrrolidine ring all have two hydrogens attached, one of which points up or down and the other points out, relative to the plane of the ring. To avoid collisions, they have to alternate up and down. So that marked hydrogen sets the pattern for the whole ring, and if it pointed away from us – down, relative to the ring – everything else would have to be the other way around too. Instead of putting a mirror next to the molecule, you have to put one underneath it to find its opposite enantiomer, and that makes it harder to spot that it’s chiral at all.

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