Syn-propanethial-S-oxide 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.
Syn-propanethial-S-oxide has a chemical formula of C3H6OS.