Syn-propanethial-S-oxide, also sometimes called thiopropanal-S-oxide, isn't actually a medical substance. It's a naturally occurring chemical found in onions that triggers tearing when you cut them. Here's the breakdown:
- Chemical makeup: Syn-propanethial-S-oxide belongs to a group of organosulfur compounds known as thiocarbonyl S-oxides. Its chemical formula is C3H6OS.
- Effect in onions: When you chop an onion, you break its cells. This releases enzymes called alliinases, which break down certain compounds in the onion to produce sulfenic acids. These acids are unstable and quickly rearrange into syn-propanethial-S-oxide, a volatile gas.
- Eye irritation: The gas travels through the air and reaches your eyes. There, it reacts with water to form a weak sulfuric acid solution. This acid irritates the sensory neurons in your eyes, causing a burning, stinging feeling and triggering tears. Tears help dilute and flush out the irritant.
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