The common experience of tearing up while chopping onions is triggered not by a traditional carboxylic acid, but by a highly reactive, complex sulfur compound. This phenomenon is a chemical defense mechanism the plant employs against predators. The irritant is a volatile gas created only when the onion’s cellular structure is physically broken, mixing previously separated components. This chemical cascade reveals why the burning sensation is nearly instantaneous when a knife slices through the bulb.
The Lachrymatory Factor
The specific molecule that causes the tear-inducing reaction is formally known as syn-propanethial S-oxide. This compound is the primary lachrymatory factor (LF) in onions, a term for any substance that causes tears. It belongs to a class of organosulfur compounds called thiocarbonyl S-oxides.
The molecule is extremely volatile, quickly turning into a gas and diffusing into the air, reaching the eyes. The syn designation refers to its specific stereochemical structure, accounting for about 95% of the total irritant. This sulfur-containing molecule also plays a part in the characteristic flavor and pungent aroma of onions.
The Enzymatic Reaction That Creates the Irritant
The formation of the lachrymatory factor begins when the onion’s cells are ruptured by cutting or crushing. Within the intact onion cells, sulfur-containing amino acid precursors, specifically S-1-propenyl-L-cysteine sulfoxide, are stored separately from an enzyme. When the cell walls are damaged, these components mix, starting a two-step chemical reaction.
The first step involves the enzyme alliinase, which is released from the ruptured cellular compartments. Alliinase acts upon the amino acid sulfoxides, breaking them down to form an unstable intermediate compound called 1-propenyl sulfenic acid.
The second step requires another enzyme, lachrymatory factor synthase (LFS). This LFS enzyme rapidly converts the unstable 1-propenyl sulfenic acid intermediate into the highly volatile product, syn-propanethial S-oxide.
How the Chemical Affects the Eyes
Once the syn-propanethial S-oxide is formed, its volatile gas quickly rises and spreads through the surrounding air. When this gas makes contact with the eye’s surface, it dissolves into the thin layer of moisture known as the tear film. This reaction creates a mild solution of sulfuric acid, which causes the immediate burning and stinging sensation.
The cornea is equipped with highly sensitive nerve endings belonging to the trigeminal nerve. These sensory neurons detect the chemical irritant and the resulting acidity, sending a signal to the central nervous system. The body interprets this signal as a threat to the eye’s integrity.
In response, the nervous system triggers lachrymation, the protective reflex of producing tears. The lachrymal glands are stimulated to produce a flood of tears to dilute the irritant and physically wash it away from the sensitive corneal surface.