Talk:Optical isomerism

Bug[edit]

"For example, one of glyceraldehyde's ligands is a hydroxy group, -OH. If a thiol group, -SH, were swapped in for it, the D/L labeling would, by its definition, not be affected by the substitution. But this substitution would invert the molecule's R/S labeling, due to the fact that sulfur's atomic number is higher than carbon's, whereas oxygen's is lower." Aw(C)=12, Aw(O)=16, Aw(S)=32

Atomic number, not atomic mass, but the article is still wrong. C=6, O=8, S=16

The D/L labeling is unrelated to (+)/(-); but The dextrorotatory isomer of glyceraldehyde is in fact the D isomer.. so does this mean that they are equivalent (in labelling, not in meaning)anyway? If not, someone please add an example.218.102.218.7 09:43, 21 January 2006 (UTC)[reply]

Question: Are optical isomers all optically active (i.e. rotate the polarisation of light)? Is that why they are "optical" isomers? I remember that L- and D- isomers can do this from my A-level chemistry, but I'm not sure if the always do this. -- DrBob

It's a good question, and if I remember from my undergrad courses and my casual encounters with stereochemistry since, you're right--not all compounds containing stereocenters are necessarily optically active.

This is a subtle but crucial point that needs to be drawn out better throughout the pages in which this occurs. "optically active" is used because it is an easily described macroscopically observable phenomenon, but a more abstract description at the atomic level requires discussion in terms of stereochemistry rather than optical activity.

consider, for example, the (still controversial) case of thalidomide, in which it is believed that only one enantiomer is teratogenic. the racemic mixture of thalidomide is not optically active, yet stereochemical concerns still exist for this mixture. Similarly for just about any other racemate--the unresolved, bulk racemate is not optically active in the sense that it doesn't rotate plane polarized light, but the stereocenters still exist.

With that said, in my opinion, we should instead have a page on stereochemistry which includes a discussion on optical activity, instead of the other way round, which we have now. But, I bring this up for discussion instead of just doing it so that we don't have a tug-of-war similar to what is happening with ascorbic acid and vitamin c--I'm not keen to meet the aggravation that such would entail.

Regarding thalidomide, I doubt very much that thalidomide was tested as an enatiomerically pure isomer but then manufactured and administered as the racemate--enantioselective synthesis is almost always more involved and therefore more expensive, and in many cases not possible or when possible, practical. without yet digging up sources, I expect that what happened was that it was developed as a racemate as well, and just not tested for teratogenic effects.

Note, for instance, in this account that the drug was synthesized in 1953, and marketed 1957-1962, but not enantioselectively tested until 1961:

http://www.chem.yale.edu/~chem125/125/thalidomide/thalidomide.html

consider also

http://yarchive.net/env/thalidomide.html

Perhaps thalidomide isn't a great example for optical activity or stereochemistry :-)

maybe the story about Pasteur laboriously separating tartrate salt crystals would be better?

--JoeAnderson

I started the Optical isomerism page and agree that some of the info should be on a stereochemistry page - I don't think there'll be a tug of war. But since I don't actually no anything about stereochemistry compared to Joe above I'm probably not the man to do it.

I deleted the second part of the thalidomide passage as my understanding I think is wrong. Why would each enantiomer need to be tested separately if they had tested both together and found no problems?

The reason this problem was not detected in testing was because only one optical form was involved in trials. In the market version of the drug, it was cheaper to not separate the isomers and the harmful form was also distributed.

--sodium

Sorry, I have just read one of the linked articles and understand :-)

--sodium

-__--_----_--__

A few fine details concerning thalidomide: The stereoisomers are converted to each other in vivo. That is, if a human is given D-thalidomide or L-thalidomide, both isomers can be found in the serum. Hence, administering only one isomer will not prevent the teratogenic effect in humans. Also, animal models failed to predict the human teratogencity because most mammals do not have such toxicity from the drug. Only humans and (I believe) Norwegian rabbits show the teratogenic effect. Eric Chevlen

I'd like to chime in on the side of a more thorough sterochemistry page, maybe even with subtopics for chiral molecules, optically active isomers, enantiomers, racemates and racemic solutions, and a mathmatics section that tells how to predict the optical activity of a solution full of optically active isomers -- some enantiomers of others, some with dextro and some with levo rotation, and all in varying concentrations.

In answer to the first question posted: "Are optical isomers all optically active?" Yes, all optically active isomers are optically active. Hense the name. :-) All optically active isomers are chiral (even if the stereochemical bond is a carbon-carbon double bond) but not all chiral molecules/isomers are optically active.

Let me know if any of you are still paying attention.

Tres (class5 at pacbell.net)

The following was moved from the main article:

The above footnote comes from my Organic Chemistry class notes (exceptionally well taught by Dr. Richard Jagger aka Jag) and as such are copyright by me (Tres) and now licensed under the GNU FDL.

The "footnotes" (*) and (**) are oddly placed. I was confused until I saw them explained in the middle of a section. Mr2001 11:58, 20 Sep 2004 (UTC)

I agree. They need to be cleaned up to make the article less confusing. StuTheSheep 03:17, Mar 10, 2005 (UTC)

I'm satisfied with the cleanup, so I'm removing the notice. StuTheSheep 22:12, May 12, 2005 (UTC)

merging[edit]

I agree a lot of stuff could be combined with stereochemistry. I also don't think we need separate articles on enantiomer and optical isomerism? --User:Chinasaur

I definitely agree with this point. Poppypetty 05:43, 5 December 2005 (UTC)[reply]

I would say it would be better to shorten enantiomers and link people back to either Optical isomerism or Chirality

Chrispounds 6 December 2005

reorg around "systems of naming"[edit]

I started in on a "systems of naming" section because some text without it was confusing and confused, and ended up moving chunks of other text in there because it seems a good way to organize.

Added some discussion of D/L versus R/S.

Removed text that seemed partly redundant, partly far afield: "Since it is rotated a small amount each time it interacts with or passes through the atoms and particles that make up an optically active molecule. This assumes that it will pass through an equal number of dextro and levo molecules. The rules of entropy should assure such a system, and give us equations to calculate the rotation of the polarized light based upon the concentrations of all of the optically active isomers in the solution, and the amount of solution that the polarized light passes through. " Eub 09:15, 1 November 2005 (UTC)[reply]

Enantiomers of Thalidomide[edit]

I'm referring to a statement in this article: It was suspected, and has been widely reported, that one enantiomer was responsible for the teratogenic effects, but this turned out not to be the case.

This seems to contradict what I've read elsewhere: Thalidomide

and in the second link provided by JoeAnderson above (although the statement I quoted seems to agree with what's written in the first link)

It also seems to contradict everything my professors have been saying (that it is one enantiomer that is the source of the teratogenicity, the other one is not teratogenic; although administration of the pure 'good' enantiomer will also cause the problem due to racemisation within the body).

So I'm really confused. Is teratogenicity the effect of one specific enantiomer or do both enantiomers have this effect (even if assuming racemisation in vivo does not occur)? Thanks. --DFLee 22:07, 9 December 2005 (UTC)[reply]

Glycine[edit]

In the main article... "Glyceraldehyde is chiral itself, and its two isomers are labeled D and L. Certain chemical manipulations can be performed on glyceraldehyde without affecting its configuration. For example, it can be converted to the amino acid glycine: glycine has two optical isomers, and they are labeled according to which isomer of glyceraldehyde they come from."

Glycine is NOT chiral, therefore it does not have two optical isomers. Change glycine to alanine and it's fine.

Cave Draco 19:28, 15 December 2005 (UTC)[reply]