h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Good stuff Master B

On a highly y note, the chemical released would be a propanthiOl S-oxide and would form a sulphenic acid (not sulfuric) on contact with water. But you really didn't need to know that did you.

and thanks for referencing my article on antibiotics

Mmmmmm onions.

And not forgetting your acids and bases entry, either.

Anyhoo, Danny put all those links in as Sub. I always forget.

B

Oh, in that case it was rubbish

"On a highly y note, the chemical released would be a propanthiOl S-oxide "

It is not incorrect to say propanethiAL S-oxide. See, for example, this article in a reputable journal:

http://www.liv.ac.uk/~sd21/onion/Crowther_et_al_2005.pdf

i do not know what chemicals but while cutting onions i also keep a potatoe half

>>It is not incorrect to say propanethiAL S-oxide. See, for example, this article in a reputable journal:

http://www.liv.ac.uk/~sd21/onion/Crowther_et_al_2005.pdf
<<

Sorry but yes it is incorrect.

They refer to a "thiopropanal" which is *not* the same as a 'thial' - which doesn't exist (unless they've made a hybrid of thiol and aldehyde which is just plain wrong). All they've done is turn the name around.

The compound contains a three carbon aldehyde (the 'al') and a thiol group (SH). To be honest this makes the name 'propanethial' even more incorrect than I had suggested.


However it doesn't detract from an excellent article.

Incindentally it's nice to see a food science article that's actually interested in the taste of the food. ususally they jsut want to increase yield.

Minor point - I always Vichyssoise was American and not French. Seems to be confirmed at http://members.cox.net/jjschnebel/vichysoi.html

Hmm...interesting point.

Compared to all the chemical nomenclature, certainly.

B

Ref nomenclature:

Well, I know this is not scientific but, if one 'googles' propanethiol S- oxide, one gets 78 'hits'.

If one 'googles' propanethial S- oxide, one gets 394 'hits'. Furthermore, the latter version contains the academic papers, rather thn articles on home baking etc.

Cripes! I'm nearly tempted to sit down and work out its correct nom,enclture from first principles

I meant to say, see

http://www.siena.edu/chemistry/Gillies.asp

(for an 'academic' using thiopropanal S- oxide)

Nomenclature (cont'd)

I came across this in an academic paper on the web:

"The irritant formed in cut onions is syn-propanethial-S-oxide. A recent study
from Japan (Imai et al. 2002) indicates that a previously unknown onion enzyme
is responsible for the formation of syn-propanethial-S-oxide. The enzyme is
lachrymatory-factor synthase. Previously, syn-propanethial-S-oxide was thought
to spontaneously form from propenylsulfenic acid. Inactivating lachrymatory-
factor synthase via genetic engineering could result in tear-free onion
cultivars.

Syn-propanethial-S-oxide is a fairly small molecule. The structure is CH3-
CH=CH-S-O".

From this, one can see why the name is propanethiAL. (However, I think the author has made a different mistake in that the structure he has suggested is actually propenethial S-oxide. The real structure shouldn't have the double bond).

To me, a thiopropanal would be

HS-CH2-CH2-C(H)=O

a propanethial would be

CH3-CH2-CH2C(H)=S

which I'm unaware of the existence of (they might to be fair)

the oxides of these would be

HS(O)-CH2CH2-C(H)=O

A thiopropanal S-Oxide

and
CH3-CH2-CH2-C(H)=S-O <---> CH3-CH=CH-S(H)-O

propanethial S-oxide

If the latter sturucture (shown in two tautomeric forms, the latter of which is the one I'd expect to dominate) is true then I would withdraw my complaint

Well anyway, it's certainly a very confusing (uninformative) name!

Heck and I thought it was just a spelling mistake