Why onion on cutting flow tears from our eyes.which chemical copmpound is present in it?

Question

Why onion on cutting flow tears from our eyes.which chemical copmpound is present in it?

Answer 1

Errata to the previous answer.  Propanthial S-oxide is the chief culprit.

Answer 2

"As onions are sliced, cells are broken, allowing enzymes called alliinases to break down amino acid sulphoxides and generate sulphenic acids. Sulphenic acids are unstable and spontaneously rearrange into a volatile gas called propanthial S-oxide ( syn-propanethial-S-oxide, lachrymatory-factor ). The gas diffuses through the air and eventually reaches the eye, where it reacts with the water to form a diluted solution of sulphuric acid. This acid irritates the nerve endings in the eye, making them sting. Tear glands produce tears to dilute and flush out the irritant"