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IUPAC name 1-Sulfinylpropane
Other names Thiopropanal S-oxide
CAS number 32157-29-2
PubChem 441491
Molecular formula C3H6OS
Molar mass 90.14 g mol-1
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

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 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.[citation needed] This acid irritates the nerve endings in the eye, making them sting. Tear glands produce tears to dilute and flush out the irritant.[1]


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.[citation needed] Leeks do not contain as much of the chemical as onions.[citation needed]

See also


  1. ^ 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". A list of authors is available in Wikipedia.
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