"The grandfather is sitting, dressed in a hundred fur coats, who undresses him sheds tears." The reason for this tearing onion lies in a special substance - lacrimator (Latin lacrima - tear). When the bulb is cut, the lacrimator is released and dissolves in the water and human tears. In this case, sulfuric acid is formed, which irritates the mucous membrane of the eye. If the onion is frozen before peeling, the activity of the lacrimator is sharply reduced. And now it finds its own explanation why the onion is peeled by wetting it or a knife with water - the lacrimator dissolves in water and practically does not escape into the air.
Onions belong to the Alliums group of plants, which includes onions, shallots, garlic, and leeks. Since time immemorial, these species, like many other plants, have been at war with those who try to eat them. Our eyes also fall victim to chemical weapons when we chop onions in the kitchen.
The defensive strategy of many onions is quite complex. Their cells contain enzymes that can break down certain molecules, such as certain types of amino acids. In a normal state, these two substances are stored in different parts of the cell and do not come into contact. But when someone dares to take a bite of onion or garlic, it destroys many cells and intensifies mixing. The same happens when we cut the onion.
The chemical reactions that begin at the same time lead to the appearance of sulfur compounds. Initially, one or more sulfur atoms were already contained in amino acids, but when the cell walls were destroyed, the amino acids reacted with enzymes. The products of the latter are excellent insecticides that repel insects and give the plant an unpleasant taste (we are talking about raw onions), such a protective reaction can sometimes be very effective.
The chemical composition of the "weapon" varies depending on the type of plant. Meanwhile, sulfuric acids are poisonous in ordinary onions, they are unstable and easily react with an enzyme, turning into another substance - thiopropanol-S-oxide. Its molecule has the chemical formula C3H6SO. Thiopropanol-S-oxide is an excellent insect repellent, even better than sulfuric acids - it's not they, but this strong compound that makes cooks cry.
Well, since the substance is very volatile, it easily rises to our eyes, which defend themselves as best they can: they emit tears and wash off a foreign mixture from the shell of the eye. What is most unpleasant is that when in contact with water (including tears), a small part of thiopropanol-S-oxide is converted into sulfuric acid H2SO4. Fortunately, this dose is negligible, but you should not neglect precautions, since we are talking about the strongest acid in existence.