what is the peculiar smell of onion due to? which gas it releases which causes tears in eyes?

Answer 1

The smell comes from a sulphur-based compound found in onions. The compounds create a volatile gas that then causes tears. According to Wikipedia.com:

As onions are sliced, cells are broken open. Onion cells have two sections, one with enzymes called alliinases, the other with sulfides (amino acid sulphoxides). The enzymes break down the sulphides and generate sulphenic acids. Sulphenic acids are unstable and decompose into a volatile gas called syn-propanethial-S-oxide. The gas then dissipates through the air and eventually reaches one's eye, where it will react with the water to form a dilute solution of sulphuric acid. The sulphuric acid irritates the nerve endings in the eyes, making them sting. The tear glands then produce tears to dilute and flush out the irritant.

Different species of onions will release different amounts of sulphenic acids, thus some will cause more tear formation and irritation than others.

Answer 2

[edit] Onions and crying
As onions are sliced, cells are broken open. Onion cells have two sections, one with enzymes called alliinases, the other with sulfides (amino acid sulphoxides). The enzymes break down the sulphides and generate sulphenic acids. Sulphenic acids are unstable and decompose into a volatile gas called syn-propanethial-S-oxide. The gas then dissipates through the air and eventually reaches one's eye, where it will react with the water to form a dilute solution of sulphuric acid. The sulphuric acid irritates the nerve endings in the eyes, making them sting. The tear glands then produce tears to dilute and flush out the irritant.

A firm in Toronto, Canada, attempted to utilize this property of onions in the manufacture of a form of tear gas for civilian use. It was marketed in 1991 but was unsuccessful as it had an effective shelf life of only three months.

Supplying ample water to the reaction prevents the gas from reaching the eyes. Therefore cutting the onions under running tap water or completely under water can help, as can rinsing the onion and leaving it wet while cutting. Chilling the onion will prevent the enzymes from activating, which will limit the amount of gas generated. Some people will freeze their knives to enhance this effect. Finally, using a sharp knife will limit the cell damage, and therefore prevent the release of enzymes.

Different species of onions will release different amounts of sulphenic acids, thus some will cause more tear formation and irritation than others.

The characteristic odor of onions can be removed with lemon.

Answer 3

"Propanethial-S-oxide is a gas that's released when the cell wall of an onion is broken. This gas reacts with the water in your eyes and produces sulphuric acid. Getting acid in your eyes sucks, it also burns."

Answer 4

One thing to add--if your eyes get bothered too much when you cut onions--simply use a bowl, or a clean sink, and chop them underwater.

No more tears!