All over the UK this morning, news organisations are talking about a cloud of sulfurous gas emanating from a factory operated by specialist lubricants and paints firm Lubrizol in Rouen, France. The gas is spreading northwards on the wind, covering vast swathes of southern England, and southwards to the French capital, Paris.

While it has not yet reached the secret Chemistry World bunker, ‘Le pong’ – as some newspaper editors have dubbed it – has already caused considerable disruption and discomfort. A French football match was postponed and lots of people are complaining about the smell.

However, the biggest disruption is caused by the specific nature of the gas and an unfortunate coincidence. Lubrizol has said that the gas is ‘mercaptan’. Chemically speaking, mercaptans are a class of compounds containing an S-H group. They are the sulfur analogues of alcohols, also known as thiols. These compounds, along with related thioethers like dimethylsulfide, are also characterised by their extremely noxious odours (reminiscent of rotting eggs, overcooked cabbage, sweat, diesel fumes and a host of other foul aromas) at anything above the lowest of concentrations.

In this case, it appears that the specific compound involved is methylmercaptan, or methanethiol. Unfortunately, this is also a significant component of the mix of compounds added to the natural gas (methane) supplied to homes all over the UK, to enable us to detect gas leaks more easily. When people say ‘I can smell gas’, they usually mean ‘I can smell thiols’, since methane itself is odourless.

This deep-seated association of the smell of thiols with potentially explosive methane leaks has meant the emergency phonelines at the National Grid, which maintains Britain’s gas infrastructure, have been ringing off the hook. There is also a risk that the smell will mask any actual gas leaks in the affected areas.

On the plus side, one of the reasons methanethiol was chosen as a gas additive in the first place is its low toxicity, coupled with our ability to detect its odour at vanishingly low concentrations. While it may be unpleasant, authorities (including the French minister for ecology, Delphine Batho) have been quick to reassure the public that there is no threat to public health. That said, our noses can pick up the smell of thiols at minuscule concentrations, so ‘Le pong’ is likely to hang around like, well, a bad smell…

But what caused the smell in the first place? Lubrizol has said that the leak was caused by ‘instability with a batch of one of [its] products’. That batch of product is decomposing, releasing the thiol. Operations at the plant have been suspended, but since the problem appears to be with already manufactured product rather than the process or a leaky pipe, the company will need to find a way to either stabilise or safely destroy the offending batch to prevent more gas being produced.

In the lab, waste thiols are often oxidised to eliminate their odours, but clearing up industrial quantities of a degrading speciality chemical product may need to be a bit more subtle than just dumping it in a big bucket of bleach. However, that does appear to be exactly what is happening in the first phase of the clean-up operation, according to the French ministry of the interior.

Meanwhile, if you’re in an affected area, be thankful that your brain has an in-built mechanism for ‘turning off’ its response to bad smells after a certain amount of time. Something I was very thankful for when I worked at a bench next to a big (thankfully ventilated) cupboard full of a variety of stinky, sulfurous reagents.

Phillip Broadwith

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Thiols, mercaptans and the stench from the French, 9.6 out of 10 based on 7 ratings
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