What's the smelliest chemical?

18 June 2019

HOLD-YOUR-NOSE

Kid smelling something bad and holding his nose

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Question

What's the smelliest chemical?

Answer

Ljiljana - Well there are many contenders but no matter which molecule we take, it will definitely contain some thiols. Thiols are sulphur compounds and there are two contenders. One is thioacetone, it's a very small molecule that has a sulphur in its structure and the other one are mercaptans which also have thiols and they're the smelliest compound. The first time when thioacetone was made at the end of the 19th century, people had induced vomiting and they felt nauseated.

Chris - What, when they smelled it?

Ljiljana - Yes, in the range one kilometre around the chemical facility where this was made.

Panel - [groans]

Ljiljana - So it is an extremely potent and smelly compound and skunks, for example, have lots of thiolated compounds...

Chris - They make Methyl mercaptan, don’t they?

Ljiljana - ...Yes

Chris - Because it’s similar to the stuff that asparagus gets metabolised to. When you eat asparagus and have asparagus wee…

Ljiljana - Yes, it’s the same chemical. And the problem with thiols is that they are actually the products of decomposition of proteins. You will find them in cadavers or in all these non-appetising things. And so we are evolutionary primed to feel a little bit of...

Chris - Steer clear because it could be bad?

Ljiljana - Yes! And there are some other compounds which don't have thiols, for example lots of amines…

Chris - Eww a fishy smell.

Ljiljana - Yes, and one of the worst that I worked with was cadaverine, as the name says it a product of cadavers…

Chris - It smells like corpses? 

Ljiljana - Yes. You use it in chemical synthesis because it's very useful to make some amines. And when you use it you can get rid of the smell, so you get out of the lab and people just steer away from you and you know you get used to it already.

Chris - I remember I worked in the lab once where we would add one of these particular sulphur compounds to gels that we were running when we were studying proteins. The lab head would come in and sort of sniff and goes, “it smells like an 'All-Bran' research laboratory in here today”. It does smell like someone's had a very bad bout of flatulence and it's really, really unpleasant!

Ljiljana - Absolutely. And what is really curious, then again, is that we also have lots of foodstuff that is smelly and we like. Just imagine cheeses…

Chris - Yes, french cheeses!

Ljiljana - Or this fruit, durian. I don't know if you ever tried that?

Chris - It's actually banned from taking it indoors in some places like in Singapore. You're not allowed to say that inside because it's a weapon.

Panel - [Laughs]

Chris - Empties a residence fast but it tastes fantastic.

Ljiljana - And some neurologists were actually exploring this, why do things which we are evolutionary primed not to like still tastes so well? And they found something which is called the backwards smelling reflex. That means first you basically smell and you have these nauseous feeling but then you activate some receptors which give you a huge amount of pleasure. So there is a balance.

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