Syn-propanethial-S-oxide

Syn-propanethial-S-oxide is a volatile gas that triggers the tears when an onion is cut. As onions are sliced, cells are broken, allowing enzymes called alliinases to break down sulfides and generate sulfenic acids (amino acid sulfoxides). Sulfenic acids are unstable and decompose 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.

Properties
Syn-propanethial-S-oxide has a chemical formula of C3H6OS. It is a slightly flammable gas and can be controlled by burning a candle near a person cutting onions. Leeks do not contain as much of the chemical as onions.