The Daily Molecule

Syn-propanethial-S-oxide ([32157-29-2]) is a volatile gas that triggers tears when an onion is cut. As onions are sliced, cells are broken, allowing enzymes called alliinases to break down amino acid sulfoxides and generate sulfenic acids. Sulfenic acids are unstable and spontaneously rearrange into syn-propanethial-S-oxide. The gas dissipates through the air and eventually reaches the eye, where it reacts with the water to form a dilute solution of sulfuric acid. This acid irritates the nerve endings in the eye, making them sting. Tear glands produce tears to dilute and flush out the irritant.

References

• Scott, Thomas. What is the chemical process that causes my eyes to tear when I peel an onion? Ask the Experts: Chemistry. Scientific American Retrieved on 2007-04-28.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article Syn-propanethial-S-oxide.