What is it that irritates your eyes and makes you cry when you peel onions?
When you cut an onion, you break cells, releasing their contents. Amino acid sulfoxides form sulfenic acids. Enzymes that were kept separate now are free to mix with the sulfenic acids to produce propanethiol S-oxide, a volatile sulfur compound that wafts upward toward your eyes. This gas reacts with the water in your tears to form sulfuric acid. The sulfuric acid burns, stimulating your eyes to release more tears to wash the irritant away.
More Info:
chemistry.about.com
What are your thoughts on this subject?
73 Comments
Martin Retzolk
The synthase enzyme converts the amino acids sulfoxides of the onion into sulfenic acid, not Sulfuric acid!
56
Dec 14, 2016 10:42PM
Albeck1994
Should be sulfenic
22
Dec 15, 2016 9:01PM
Bill Coe
More of the sites on the Internet quote Sulfenic Acid (RSOH) as the chemical that makes you cry. Your quoted site and your explanation cite Sulfuric Acid (H2SO4). Sulfuric acid would normally be identified by a rotten egg smell which is absent in cutting onions. While I would assume that Sulfenic and Sulfuric acids are closely related, I think this dichotomy needs to be clarified.
21
Dec 10, 2016 6:37PM
Stevie G
Stevie G, Sulfenic ... not Sulfuric.
9
Oct 10, 2018 2:18AM
Richard Freeland
Someone said to peel onions under water but I couldn’t hold my breath long enough to finish one.
9
Mar 17, 2019 12:57AM
Jane Elizabeth
The correct answer is sulfenic acid - not one of the choices!
8
Jan 12, 2019 10:30PM
Gery Tillmanns
Bull. It is not SULPHURIC ACID.
6
Nov 6, 2018 8:27PM
Bob Deardorff
It's Sulfenic acid, not Sulfuric. Damn, that's two in a row.
5
Oct 27, 2016 1:44AM
Paul Swanson
This is an incorrect answer. How did it make it through review. The acid is sulfenic not sulfuric. If it was sulfuric you would burn your hands.
5
Dec 15, 2016 11:38PM
Anthony Broome
Martin Retzolk, which combines with water in the eyes to form a mild sulfuric acid?
0
Sep 6, 2023 10:53PM
Patty K
Knew it was the fumes, but not what type. But I know those fumes are NOT sulfuric acid!
0
Jun 21, 2023 9:36AM
JRW
Even the kookaburra sang SULFENIC up in the old gum tree!
0
Jun 13, 2023 7:55PM
annick thomas
Pas l'acide sulfurique !!!....
0
Jun 9, 2023 10:32AM
Philip Selleck
Yes, wrong answer, it is sulfenic acid
0
Jun 7, 2023 11:46AM
Cheryl Love
Bill Coe, sulfuric acid is odorless, it does not smell like rotten eggs. The sulfur compounds in eggs smell like the characteristic ‘rotten egg’ odor.
1
Apr 16, 2023 9:47PM
Cheryl Love
Wrong, so wrong.
The chemical is syn-Propanethial-S-oxide, aka sulFENIC acid, NOT sulfuric acid. Ruptured onion cells release special enzymes as well as amino acid sulfoxides which combine to form sulfenic acid.
If onions leaked sulfuric acid, we’d all be severely injured, no one could eat them.
How did this question get through?
0
Apr 16, 2023 9:45PM
pabjetster
There is no correct answer presented. The onion produces SULFENIC acid, not sulfuric acid. Does anyone actually check these answers?
0
Feb 9, 2023 9:56AM
Carrie Arnold-Marsh
Wrong answer--AGAIN!
1
Jan 18, 2023 5:37PM
Michael Matheson
The culprit is called the lachrymatory factor, a chemical that irritates the eyes. When the onion is intact, a group of compounds called cysteine sulfoxides are kept separate from an enzyme called alliinase. But when you slice, dice or crush the onion, the barrier separating the compounds and enzyme is broken. The two come together, setting off a reaction: The alliinase causes the cysteine sulfoxides to become sulfenic acid. i want my point because answer is wrong
0
Jan 14, 2023 8:02AM
Chris Brown
OOPS! I accidentally entered my comment before completing it.
Sulfenic acid is present in onion. Sulfenic acids (amino acid sulphoxides) are unstable and decompose into a volatile gas. The gas dissipates through the air and eventually reaches the eye, where it reacts with the water to form a dilute solution of sulphuric acid.
In chemistry, sulfenic acid is an organosulfur compound and oxoacid with the general formula R−S−OH. It is the first member of the family of organosulfur oxoacids, which also include sulfinic acids ( R−S(=O) 2H) and sulfonic acids ( R−S(=O) 2OH), respectively.
0
Dec 14, 2022 5:25AM
Chris Brown
Your correct answer is only partly correct, Brian. I had heard about sulfuric acid but realised there was more to it. Here is some more info for you.
1
Dec 14, 2022 5:15AM
Al Breyette
Another Frank Cowell question where you have to guess which wrong answer he has marked as correct. Its sulfenic acid not sulfuric. Sulfuric acid is a corrosive substance which is destructive to the skin, eyes,teeth and lungs.This is from the Centers for Disease Control and Prevention( THE CDC) and an American government agency.Get with the Program Frank and quit the trick questions or listing an incorrect answer as correct.
1
Dec 13, 2022 10:11AM
Susan Lake-Harris
I think this needs a correction - just saying I agree with all the others who've noted that sulfenic and sulfuric are not the same thing...
1
Dec 6, 2022 12:41AM
timwhitestevens
I found the explanation informative and interesting.
1
Nov 28, 2022 1:03AM
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