There’s a Viburnum carlesii bush (sometimes called Koreanspice) near my front door and, right now, it smells amazing. It only flowers for a relatively short time each year and otherwise isn’t that spectacular – especially in the autumn when it drops its leaves all over the doorstop, and I’m constantly brushing them out of the house.

But it’s all worth it for these few weeks in April, when everyone who has any reason to come anywhere near our door says, ‘ooh, what is that smell? It’s gorgeous!’ We also rear butterflies at this time of year, and they love the flowers once they’ve emerged from their chrysalids. (No, of course this isn’t an excuse to include all my butterfly photos in a post. Painted lady, since you ask.)

But let’s talk chemistry – what is in the Viburnum carlesii’s fragrance? Well, it’s a bit complicated. Fragrances, as you might imagine, often are. We detect smells when volatile (things that vaporise easily) compounds find their way to our noses which are, believe it or not, great chemical detectors.

Well, I say great, many animals have far better smell detection: dogs, of course, are particularly known for it. Their noses have some 300 million scent receptors*, while humans “only” have 5-6 million but, and this is the really fantastic part, by some estimates we’re still able to detect a trillion or so smells. We (and other animals) inhale air that contains odour molecules, and those molecules bind to the receptors in our noses, triggering electrical impulses that our brains interpret as smell.

Most scents aren’t just one molecule, but are actually complex mixtures. Our brains learn to recognise combinations and to associate them with certain, familiar things. It’s not that different from recognising patterns of sound as speech, or patterns of light as images, it’s just that we often don’t think of smell in quite the same way.

Viburnum carlesii flowers have a fragrance often described as sweet and spicy.

So my Viburnum bush – and the flowers I’ve cut and put on my desk – is actually pumping out loads of different molecules right now. After a bit of hunting around, I tracked them down to (brace yourself for a list of chemical names) isoeugenol, eugenol, methyleugenol, 4-allylsyringol, vinyl-guaiacol and methyl nicotinate, plus the old favourites methyl salicylate (this stuff turns up everywhere), methyl benzoate (so does this), indole, cinnamic aldehyde and vanillin, and then some isovaleraldehyde, acetoin, hexanal, (Z)-3-hexen-1-ol and methional.

Phew.

Don’t worry, I’m not going to talk about the chemistry of all of those. But just for a moment consider how wondrous it is that our noses and brains work together to detect all of those molecules, in their relevant quantities, and then send the thought to our conscious mind that oh, hey, the Viburnum is flowering! (It’s also pretty astonishing that, in 2021, I can just plug all those names into a search engine and, with only a couple of exceptions, get all sorts of information about them in seconds – back in the old days when I was studying chemistry, you had to use a book index, and half the time the name you wanted wasn’t there. You kids don’t know how good you’ve got it, I’m telling you.)

Anyway, if you glance at those names, you’ll see eugenol popping up quite a bit, so let’s talk about that. It’s a benzene ring with a few other groups attached, and lots of chemicals like this have distinctive smells. In fact, we refer to molecules with these sorts of ring structures as “aromatic” for this exact, historical reason – when early chemists first isolated them, they noticed their distinctive scents.

Eugenol is an aromatic compound, both in terms of chemistry and fragrance (image source)

In fact there are several groups of molecules in chemistry that we tend to think of as particularly fragrant. There are esters (think nail polish and pear drops), linear terpenes (citrus, floral), cyclic terpenes (minty, woody), amines (fishy, rot) and the aromatics I’ve just mentioned.

But back to eugenol: it’s a yellowish, oily liquid that can be extracted from plants such as nutmeg, cloves, cinnamon, basil and bay leaves. This might give you an idea of its scent, which is usually described as “spicy” and “clove-like”.

Not surprisingly, it turns up in perfumes, and also flavourings, since smell and flavour are closely linked. It’s also a local antiseptic and anaesthetic – you may have used some sort of eugenol-based paste, or perhaps just clove oil, if you’ve ever had a tooth extracted.

Plants, of course, don’t go to the trouble and biological expense of making these chemicals just so that humans can walk past and say, “ooh, that smells nice!” No, the benefit for the plant is in attracting insects, which (hopefully) help with pollination. Which explains why my butterflies like the flowers so much. (Another butterfly pic? Oh well, since you insist.) Eugenol, it turns out, is particularly attractive to various species of orchid bee, which use it to synthesise their own pheromones. Nature’s clever, isn’t she?

By the way, notice I mentioned anaesthetics back there? Eugenol turns out to be too toxic to use for this in large quantities, but the study of it did lead to the development of the widely-used drug propofol which, sadly, is pretty important right now – it’s used to sedate mechanically ventilated patients, such as those with severe COVID-19 symptoms. You may have seen some things in the news earlier this year about anaesthetic supply issues, precisely for this reason.

Isoeugenol has the same “backbone” as eugenol, with just a difference to the position of the C=C bond on the right. (image source)

Back in that list of chemical names, you’ll see “eugenol” forming parts of other names, for example isoeugenol. This points back to a time when chemicals tended to be named based on their origins. Eugenol took its name from the tree from which we get oil of cloves, Eugenia, which was in turn named after Prince Eugene of Savoy – a field marshal in the army of the Holy Roman Empire. And then other molecules with the same “backbone” were given the same name with prefixes and suffixes added on to describe their differences. As I said in my last post, this sort of naming system it was eventually replaced with more consistent rules, but a lot of these older substances have held onto their original names.

Still, regardless of what we call the chemicals, the flowers smell delightful. I’m off to replenish the vase on my desk while I still can. Roll on May, vaccines and (hopefully) lockdown easing!

Take care and stay safe.

*it’s even been suggested dogs’ super-powered sense of smell might be able to detect COVID-19 infections.

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