2012: Advent Chemistry: Carvone

by on December 11, 2012

Today, I think we’ll give the bonding issue a rest, and look at something new: isomerism. “Isomers” are molecules that have the same structure, almost. That is, if you add up the numbers of each kind of atom, you get the same totals. But if you look at their structure in detail, the way they’re laid out in space, they’re different.

There are three kinds of isomerism: structural, geometric, and spatial. Structural isomerism is simple and often quite dull. The atoms are joined by a different set of bonds. Very quickly, structural isomers stop being anything like each other. If you’ve got a chain of three carbon atoms, with lots of hydrogens and one oxygen attached, then you can either have proponal (the oxygen on an end carbon) or propanone (the oxygen on the middle carbon). They’re very simple molecules, but they’re fairly obviously different. And most structural isomerism is like that – obvious. You can look at the molecule and see that it’s different, even if all you know is that different letters stand for different things and lines are bonds. Structural isomers react differently, as well as looking different and by this point, having done a chemistry degree, I don’t really think of them as isomers. They’re just different compounds that happen to sound similar.

Geometric isomerism is more properly known as cis/trans isomerism (yes, that’s where the cis/trans terminology in gender comes from). That’s where a double bond prevents something from rotating. A single bond lets the atoms at either end spin around like wheels on an axle, but a double bond holds them fixed, so whatever those atoms are bonded to gets locked into position relative to each other. If two of the same thing (like hydrogen, or chlorine) are on the same side of the double bond, then you’ve got the cis isomer. If they’re on opposite sides, it’s the trans isomer. If you haven’t got matching things, there’s a whole system for working out which term applies.

But that’ s still fairly simple and obvious. To my mind, spatial isomerism is much more interesting, so that’s what the molecules below demonstrate.

The structures of R- and S-carvone with three-dimensional structure shown.

These compounds are spatial isomers of carvone. The one on the left is R-carvone and the one on the right is S-carvone. The initials stand for Right and Sinister, and there are reasons for those labels, but only when you work it out from scratch using the appropriate conventions and that’s an entry all by itself.

The important bit isn’t why they’re called what they are, but why they are different at all.

Look very carefully at those molecules. You should be able to spot that these are the same atoms, connected in the same ways. There’s only one difference. They’re mirror images of each other. In R-carvone, hydrogen points backwards on the right and the carbon group forwards on the left; in S-carvone, hydrogen points backwards on the left and the carbon group forwards on the right.

It looks as though these are the same thing anyway, I know. It still looks that way to me – but my thinking brain has been convinced that they are, in fact, different, so my intuition gets overruled. Unless you’re amazingly good at doing spatial rotations in your head, convincing yourself that S and R carvone aren’t the same thing will take a physical demonstration, something you can turn around in your hands until you’re satisfied.

The difference between these molecules is that they cannot be rotated so that they match. It can’t be done. You always end up with things having swapped places between one side and the other. That property is called “chirality”.

Try this: Rotate your hands so that they match, so that your fingers and thumbs line up with each other and your palms are both facing the same way.

Can’t be done, right? Your hands are mirror images of each other and therefore they aren’t quite the same.

If you want to convince yourself that the same principle works for carvone, try this: Get something round-ish, like an orange or something, and stick four easily distinguishable things (cocktail sticks, teaspoons, pens, whatever) into it so that the points form a tetrahedron, a triangular pyramid. Then make a copy of it, but this time swap around two of the cocktail sticks to make a mirror image of the original. Can you rotate them so that they match?

Done that? Hopefully you now believe that chirality really does make molecules different from each other, but you may still be wondering why it matters. Well, chirality is frequently maintained in reactions, so what you make from S-carvone may be different from what you make from R-carvone. The human body uses enzymes for most of its internal processes, and a lot of enzymes will react differently to mirror-image molecules – to the different “enantiomers” of a compound. One enantiomer of thalidomide reduces nausea, and the other causes birth defects. One enantiomer of glucose is twice as sweet as the other.

And if you still don’t think something so small and hard to see can make much difference, take another look at the pictures up there. The molecule on the right is a major component in caraway and dill oils.

The one on the left tastes of spearmint.

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