1840

Ozone

Christian Friedrich Schönbein (1799–1868), Jacques-Louis Soret (1827–1890), Carl Dietrich Harries (1866–1923), Rudolf Criegee (1902–1975)

While conducting experiments on the electrolysis of water (breaking it down by applying an electric current), German chemist Christian Friedrich Schönbein noticed a distinctive odor in his lab, evidence of a new substance. He named it ozone (after the Greek ozein, to smell) and described it in 1840. More than twenty years later, Swiss chemist Jacques-Louis Soret boldly proposed that this gas was in fact a new form of oxygen, making ozone the first substance to be recognized as an alternate form of a pure element—an allotrope (from the Greek for other way). Regular oxygen has the formula O₂, but ozone, also a gas, is O₃. It can be cooled down to an unpredictably explosive blue liquid, and cooling it even further can turn it into a deep violet–colored solid that very few chemists have ever seen.

Many people have encountered ozone, whether they’ve realized it or not. Produced by lightning bolts, ozone causes the faintly bleach-like “fresh air” smell sometimes noted around thunderstorms. (Its associations with cleansing storms and mountain air gave it a completely undeserved reputation for health—it’s actually quite toxic.) But ozone’s formation in the upper atmosphere forms a layer that absorbs ultraviolet light and protects living creatures below from its damaging effects.

Electric discharge is still the best way to make it in the laboratory. Ozone generators run pure oxygen past a high-voltage electric arc to provide the gas on demand. It has value because its structure lets it react with carbon-carbon double bonds in a 1,3-dipolar cycloaddition, forming a five-membered ring containing three oxygen atoms in a row (a type of compound that can explode very easily!). This rearranges, fortunately, producing a species that can be split apart into two aldehydes, which (overall) gives a chemist a very clean, specific way to break an alkene into two reactive groups that can be used for many other reactions. German chemist Carl Dietrich Harries popularized this reaction in the early twentieth century, but it wasn’t until the 1950s that another German chemist, Rudolf Criegee, worked out the detailed mechanism through isotope labeling studies.

One reason for that fresh mountain air: ozone from lightning bolts.