Thioacetone: The World’s Most Pungent Scent?

In 1889, the German city of Freiburg met with a strange catastrophe. Within a half-mile radius of a chemical laboratory, people began to vomit, faint, and erupt into a general panic, overpowered by a suffocating, nauseating odor that had descended suddenly upon them. The culprit was an obscure chemical called thioacetone. Local chemists had produced it by accident, and their mistake proved so disastrous that people evacuated the city, desperate to escape its intolerable smell.

What makes thioacetone so powerful? To find out, we’ll take a dive into the human sense of smell and explore the chemistry and history behind this uniquely repellant substance.

The science of foul smells

Our noses are surprisingly complex organs, containing millions of unique receptor cells. Some of these receptors create positive reactions—which is why we enjoy smelling, say, savory meat or fresh berries. Others tend to be more negative—rotten eggs, for instance, or old socks. Good smell receptors evolved to entice us towards food; bad smell receptors evolved to steer us away from possible dangers. For example, rotten eggs smell bad because of hydrogen sulfide, which is associated with hazardous bacteria.

Durian fruit
The durian is a fruit known for its pungent odor. (Credit: Wikimedia))

But not everything with a nasty smell poses a threat. Based on their chemistry, some substances just happen to set off our negative receptors, sometimes very intensely. Take the durian: this spiky fruit from Southeast Asia has one of the worst smells in the culinary world. It’s described as resembling sewage, rotting onions, or sulfur, with notes of honey, and it is so pungent that some jurisdictions have banned the consumption of durian in public. Scientific analysis has shown that its smell derives from some of the same compounds that are found in soup seasoning, skunk odor, rotten eggs, and soap. Despite all that, durian remains a popular food, eaten for its nutritious qualities and savory taste.

Synthesizing thioacetone

Thioacetone has a formula of (CH3)2CS. In organic chemistry, it is classified as an organosulfur compound—a family which also includes allicin, responsible for the smell of chopped garlic, and sulfur mustard, which was used as poison gas during the First World War. The sulfur in these chemicals makes many of them extremely noticeable to the human sense of smell, especially those with a low molecular weight. When isolated at low temperatures, thioacetone takes the form of a brown liquid, with a melting point of -67 degrees Fahrenheit (-55 degrees Celsius) and a boiling point of 158 degrees Fahrenheit (70 degrees Celsius). At room temperature, however, it rapidly transmutes into other chemicals, producing trithioacetone—a closely related compound—as well as a mix of polymers in the form of a white solid. This makes it highly unstable, though small particles can survive for some time in the air.

Thioacetone does not occur naturally. When German chemists Eugen Baumann and Emil Fromm synthesized it in 1889, in the incident described above, that was the first time anyone had encountered it. They weren’t actually aiming to make thioacetone during their experiment; their goal was actually trithioacetone, and thioacetone occurred only as a trace byproduct. In the event, even minute amounts of the stuff were enough to make life unbearable for people in the surrounding city. The German record gives a terse account, describing “an offensive smell which spread rapidly over a great area of the town.”

Eugen Baumann
German chemist Eugen Baumann, part of the team that first synthesized thioacetone in 1889. (Credit: Wikimedia))

Further experiments in the UK

In 1890, not long after that first, foul-smelling discovery in Freiburg, chemists at the Whitehall Soap Works in Leeds made their own synthesis of thioacetone. They wrote that its odor was “fearful,” and noted that dilution paradoxically seemed to make it smell worse—the less of it there was, the nastier the experience became.

Thioacetone made its next appearance more than half a century later. At another British laboratory, south of Oxford, researchers for Esso (part of ExxonMobil) were studying it as a potential source of commercial polymers. A 1967 report describes how that went:

Recently we found ourselves with an odour problem beyond our worst expectations. During early experiments, a stopper jumped from a bottle of residues, and, although replaced at once, resulted in an immediate complaint of nausea and sickness from colleagues working in a building two hundred yards [180 m] away.

The report goes on to mention that scientists who worked with thioacetone received “hostile stares” from other patrons at a restaurant, with a waitress going so far as to spray deodorant where they were sitting. They noticed the same paradoxical effect that the Leeds researchers had observed in 1890—in the laboratory, large quantities of thioacetone were relatively tolerable, but a faint whiff of it would have people in fits. To prove this, they first dispersed observers up to a quarter of a mile from the lab; next, they placed precisely one drop of thioacetone on a watch glass in a fume cupboard, and within seconds, the observers picked up its overpowering stench. Subsequent work with thioacetone required carefully closed systems to avoid these effects.

Thioacetone diagram
Chemical diagram of thioacetone. The sulfur atom is at the top, connected to three carbon atoms, two of which each connect to three hydrogen atoms. (Credit: Wikimedia))

Does it really smell so bad?

Aside from the initial German discovery in 1889, and the British experiments in 1890 and 1967, few records exist of human interactions with thioacetone. While it isn’t a particularly difficult substance to make, its volatility, lack of practical use, and infamous smell have relegated it to obscurity. In recent years, however, as tales of the chemical’s dreadful potency have circulated on the internet, renewed interest has driven a few modern-day experiments—such as that of the YouTuber LabCoatz, who synthesized thioacetone in a 2022 video. Cracking trithioacetone created a smell he described as “oniony, or similar to a leek.” The strange thing was that there were no fainting or vomiting bystanders this time around; LabCoatz concluded, however, that a number of factors—his own desensitization to thioacetone, his close proximity to the chemical, and the very small quantity he synthesized—combined to spare himself and anyone else the worst effects, creating a bad smell rather than an overwhelmingly horrible one.

Thioacetone may, then, still live up to its reputation as the worst smell in the world. Perhaps another experiment, by someone sufficiently brave and foolhardy, can prove it for sure, and send people panicking in the streets?