I have taken somewhat of an interest in the pungent extracts of spicy/lachrymatory foodstuffs. Capsaicin, piperine, and allyl isothiocyanate are all
extractable to various grades, and have industrial syntheses as well. They are all well documented compounds, with well-known physical properties -
appearance, melting and boiling points, densities, vapor pressures, etc. They have well-documented chemical properties as well. This is all consistent
with most chemical compounds known to man. However, to my amazement, the pungent lachrymator produced by enzymes when onions are cut seems to be an
exception. While it is easy to find reference after reference to the production of the compound and its effects, and its molecular structure and
nomenclature, actual physical and chemical data on it appears to be non-existent.
If I did not know better I would assume that no one has ever tried to isolate this compound (or whatever the active compounds responsible for the
pungency of onions are) by any means such as freeze-drying, vacuum distillation, chemical extraction, etc. I would assume this compound either has
never been isolated or is too unstable to isolate, or perhaps too reactive with air or water, but I cannot even find reference to that, nor any
attempts at isolation or alternative synthesis. No (measured) boiling point, vapor pressure, or any other properties seem to exist. Many sources
describe the compound as a gas, but this contradicts the single (predicted) boiling point and vapor pressure values that I have found, and seems
somewhat unlikely given the molecular structure of this substance. I would presume it to be a volatile liquid at room temperature and standard
pressure. Either way it does not seem to be documented anywhere that I can find.
Does anyone have actual data or experience with this everyday, yet seemingly unknown, compound?bfesser - 16-7-2013 at 04:53
That's exactly what I'm talking about. It should be apparent that I have already looked at that page. It does not contain, nor link to any accessible
documents that contain, any physical or chemical properties of the compound, which is what I am curious about. What's the boiling point at 1atm? Vapor
pressure at any temperature? What are its chemical properties? What's the density of the liquid or vapor?
[Edited on 21-7-2013 by kilowatt]SM2 - 22-7-2013 at 06:34
There is a compound which is unstable, and reacts w/ -0H to produce small amounts of sulphuric acid in your eyes.kilowatt - 24-7-2013 at 22:27
I have read somewhere that it doesn't, that it is a myth that it forms sulfuric acid. I do not recall where unfortunately. As we know, substances are
quite capable of being powerful lachrymators without being actually corrosive or forming any corrosive compounds, and it seems highly likely that that
is the case here especially given the rapid onset and particular sensation of exposure to cut onions. Perhaps someone once guessed or proposed that it
forms sulfuric acid because it's a sulfur compound and that stuck because it's an easy concept for laymen - people know what sulfuric acid is, they do
not know what TRPV1 channels are. On the other hand there is a thread on this site where obtaining sulfuric acid from onions is mentioned, and some
people thought it was possible though impractical. I have not found evidence of any actual test where sulfuric acid was even detected via onions,
though. It would be interesting to see that if anyone knows about it. A lot of people seem to accept it as common knowledge, but laymen also typically
think that peppers burn because they are acidic. It also seems quite possible that even if the compound is unstable and reacts with water to form
sulfuric acid, that this property is not necessarily related to its properties as a lachrymator. The reaction could be slow or in minute quantities. I
would think that if considerable amounts of sulfuric acid were formed, handling onions would be corrosive to the skin, and if a much lower amount was
formed it would not have the potent effects that it does, if the acid was the sole mechanism by which it acted as an irritant. I have been exposed to
mild sulfuric acid mists plenty and it is not nearly that biting. Whatever the case, the fact that Syn-Propanethial-S-Oxide does exist as a distinct
compound would imply that it could be isolated, free of water (by whatever method, if not then synthesized) and would have some physical and chemical
properties.
Of course, there does not seem to be any hard data on any of that. That sort of actual data is what I would be curious about.
[Edited on 25-7-2013 by kilowatt]Dany - 15-11-2014 at 14:28
Kilowatt,
Syn-Propanethial-S-oxide also known as thiopropanal S-oxide can be synthesized according to a procedure described in US Patent (US 3686323 A).
As is mentioned in this synthesis the boiling point of liquid thiopropanal S-oxide is 30°C at 0.1 mmHg. Using a boiling point calculator one find a boiling point of 363°C at 760 mmHg (1 bar pressure) which is pretty high.
Reference:
[1] Thioalkanal-s-oxides (US 3686323 A)
Dany.
Attachment: US3686323.pdf (296kB) This file has been downloaded 789 times
[Edited on 15-11-2014 by Dany]careysub - 15-11-2014 at 18:23
I have read somewhere that it doesn't, that it is a myth that it forms sulfuric acid.
[Edited on 25-7-2013 by kilowatt]
Right.
Early in the 20th Century, when biochemistry was just developing, the noxious effects of famous chemical hazards were often attributed to inorganic
breakdown compounds. Phosgene's toxicity was said to be due to it hydrolyzing to hydrochloric acid in the lungs, mustard gases effect was also
attributed to simple hydrolysis products.
These suppositions were wildly incorrect. Each toxic agent selectively affects specific biological processes directly (which is why they are potent).
"Onion gas" like other potent lachrymators activate a particular sensory receptor called TRPA1.
These incorrect (and completely unsupported) theories though are probably going to be repeated for generations to come in the popular literature.
