Acetaldehyde, the chemical your liver makes from a drink
Between a glass of wine and the harmless acetic acid your body finally turns it into sits a middle step: acetaldehyde, a small, volatile molecule that helps cause hangovers. It is also in coffee, bread and ripe fruit, and industry makes hundreds of thousands of tonnes of it every year.
In the liver, an enzyme called alcohol dehydrogenase oxidises ethanol into acetaldehyde, and a second enzyme, acetaldehyde dehydrogenase, converts that into acetic acid. When the second step lags, acetaldehyde piles up. Many East Asian people carry a mutation in the ALDH2 gene that slows it, which produces the alcohol flush reaction: a reddened face and body, nausea and headache after drinking. The drug disulfiram blocks the same enzyme deliberately. In the brain, a different enzyme, catalase, handles most of the conversion. Yeast runs part of the chain in reverse, turning pyruvate into acetaldehyde and then into ethanol during fermentation.
The compound boils near room temperature, so it can exist as either a colourless liquid or a gas. Carl Wilhelm Scheele, the Swedish pharmacist, first noticed it in 1774, and Justus von Liebig named it aldehyde in 1835, with the longer name adopted around mid-century. In 1848 Valentin Hermann Weidenbusch found that acid and cold would join three molecules into a ring called paraldehyde, and was struck that heating it with a trace of the same acid simply undid the reaction.
Factories once made it from ethanol or from acetylene, the latter using mercury salts as a catalyst. Since 1962, ethylene has been the main raw material, oxidised through the Wacker process over palladium and copper. Global output in 2013 was about 438,000 tonnes.
Its biggest traditional job, serving as feedstock for acetic acid, has been fading, because making acetic acid from methanol is cheaper. The overall market is shrinking, even though acetaldehyde still feeds resins, pyridine derivatives and the ingredients of polyvinyl acetate. In 2012 China used almost half the world's supply, with Western Europe second at 20 percent.
Source: Acetaldehyde
