''syn''-Propanethial ''S''-oxide (or (''Z'')-propanethial ''S''-oxide), a member of a class of

organosulfur Organosulfur chemistry is the study of the properties and synthesis of organosulfur compounds, which are organic compounds that contain sulfur. They are often associated with foul odors, but many of the sweetest compounds known are organosulfur der ...

compounds known as

thiocarbonyl In organic chemistry, thioketones (; also known as thiones or thiocarbonyls) are organosulfur compounds related to conventional ketones in which the oxygen has been replaced by a sulfur. Instead of a structure of , thioketones have the structure ...

''S''-oxides (formerly " sulfines"), is a volatile liquid that acts as a lachrymatory agent (triggers tearing and stinging on contact with the eyes).

Onion release

The chemical is released from

onions An onion (''Allium cepa'' , from Latin ), also known as the bulb onion or common onion, is a vegetable that is the most widely cultivated species of the genus ''Allium''. The shallot is a botanical variety of the onion which was classified ...

, ''Allium cepa'', as they are sliced. The release is due to the breaking open of the onion cells, which releases

enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

called alliinases. Alliinases then break down

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

sulfoxide In organic chemistry, a sulfoxide, also called a sulphoxide, is an organosulfur compound containing a sulfinyl () functional group attached to two carbon atoms. It is a polar functional group. Sulfoxides are oxidized derivatives of sulfides. E ...

s, generating

sulfenic acid In chemistry, a sulfenic acid is an organosulfur compound and oxoacid with the general formula . It is the first member of the family of organosulfur oxoacids, which also include sulfinic acids () and sulfonic acids (), respectively. The base mem ...

s. A specific sulfenic acid,

allicin Allicin is an organosulfur compound obtained from garlic and leeks. When fresh garlic is chopped or crushed, the enzyme alliinase converts alliin into allicin, which is responsible for the aroma of fresh garlic. Allicin is unstable and quickl ...

or 1-propenesulfenic acid, is rapidly rearranged by another enzyme, the lachrymatory factor synthase (LFS) to give ''syn''-propanethial ''S''-oxide. Vapors from this volatile liquid induces tearing.

Related compounds

A structurally related lachrymatory compound, ''syn''-butanethial ''S''-oxide, C₄H₈OS, has been found in another genus ''Allium'' plant, '' Allium siculum''. Propanethial S-oxide forms a stable thio sultone dimer, ''trans''-3,4-diethyl-1,2-dithietane 1,1-dioxide.

References

{{DEFAULTSORT:Propanethial-S-oxide, syn- Organosulfur compounds Lachrymatory agents Pungent flavors Onions Sulfoxides