Chemical of the Week 16 – Methanethiol

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As  term continues it is becoming harder to keep up with the posting. However, being in the thick of the classroom definitely gives me new ideas about posts to write. This week’s molecule is one that lingers in every school chemistry lab. And if you can smell it then you know it’s time to check the gas taps. I’ve a Year 8 class this year and one of the first lessons is using a Bunsen burner – who would have guessed learning to light a Bunsen burner could produce such excitement, it really is all down hill after that ! The molecule in question is methanethiol, also referred to as methyl mercaptan, and its put into gas so that we can smell and identify leaks. I read an interesting article that cites the New London gas disaster in the 1930s as the catalyst for gas odorisation.  A School board in Texas were tapping gas from a gas line and an undetected build up of this odourless natural gas caused an explosion killing 295 students and teachers.

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The term mercaptan comes from the Latin to mean ‘captures mercury’ as the thiol group interacts strongly with mercury. This molecule is very similar to methanol with the only difference being a sulfur atom instead of an oxygen atom with the SH group being referred to as the thiol. The difference one atom makes is astounding – the lack of hydrogen bonding between the molecules means that methanethiol is a gas whereas methanol is a liquid and of course there is that smell.

Now if you smell gas but know there is no gas taps around have a look in the fruit bowl ! You might find a Durian which is probably the whiffiest fruit around. It is found in South East Asia and is renowned for its smell and this can be traced to amongst other chemicals the mercaptans. Mercaptans can be found everywhere, in our urine, skunk spray and even in the production of beer and wine. There was a mercaptan leak from a factory in France in 2013. Winds carried the smell over the English Channel and as you can imagine as well as an outcry there were some very witty observations made. Not often would you find a link to a red top on a science blog but this definitely made me giggle ! http://www.mirror.co.uk/news/weird-news/uk-hit-by-french-gas-leak-1549236
Although methyl mercaptan is found in our bodies in high concentrations it is also highly toxic. In November 2014 10 tonnes leaked from DuPont chemical plant in Texas, killing four workers and injuring 15.

So why do thiols smell ? Why does methanethiol have such an distinctive smell compared to its analogue methanol when they only differ by one atom ? The first property that is essential for a molecule to have for us to smell it is to be a gas or volatile liquid but that’s not enough. It seems that we can smell these molecules as there is an interaction between the molecule and olfactory receptors in the nose. This is a complex process and the Nobel prize in medicine was awarded to Linda Buck and Richard Axel in 2004 for work in this field. Smells are so emotive and of course chemistry lies at the centre of this sense – while writing this post it reminded me of a book I read a few years back called Perfume by Patrick Suskind, one to recommend !

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