| Section2 = | Section3 = }} 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 irrit… (More on Syn-propanethial-s-oxide)