2012: Advent Chemistry: Capsaicin

by on December 10, 2012

Capsaicin is a wonderful compound. It appeals to the evil part of me, because a strong solution of capsaicin is essentially liquid pain.

Hey, I never said my evil side was nice.

Capsaicin is the active compound in chilli peppers – the bit that makes them hot – and it looks like this:

A diagram of the structure of capsaicin, drawn in skeletal form

It’s a chemically quite complicated molecule, having five different functional groups. From right to left, those are an alkene (two carbons with a double bond), a ketone (the carbon-oxygen group), a secondary amine (the nitrogen group, which is ‘secondary’ because it is attached to two carbons), an ether at the bottom of the aromatic ring, and an alcohol at the top.

This is a multifunctionalised compound. Isn’t that a wonderful word? Multifunctionalised. Given many functions. Unnecessarily long words provoke glee in me.

The thing is that capsaicin actually only has three functional groups. Some functional groups, when combined, become something different to either alone in the way they react, so we combine them and give them a new name. The carboxylic acid part of the ethanoic acid we looked at could be considered an alcohol and an aldehyde that just happen to be on the same carbon, but it doesn’t behave like an aldehyde plus an alcohol – it behaves like an acid, so that’s what we call it.

The “ketone” and the “secondary amine” in capsaicin are nothing of the sort. When they appear next to each other like that, they’re actually a whole new functional group, called an amide, and an amide reacts differently to either of its components.

The other combination group in capsaicin is the ether and alcohol on the aromatic ring. This isn’t a common group to encounter, and mostly crops up in flavour compounds. It’s called a vanillyl, because it’s the major functional group in vanillin, which is the major flavour compound in vanilla. I bet you’re really surprised by that last bit.

Molecules with several functional groups are hugely important to chemistry. The more functional groups a molecule has, the more ways it can react, and sometime that means in combination, and sometimes that means with itself – one end of the molecule reacting with the other. And sometimes that means with other, also multifunctional compounds, to make proteins and polymers and flavours and dyes. Complicated molecules make life interesting, and the best way to be complicated is to contain more than one thing.

Socially, capsaicin says interesting things about humanity. We deliberately flavour our food with a substance whose only major effect is to cause us pain. Human beings are weird.

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