Talk:Anomeric effect

From Wikipedia, the free encyclopedia

Maybe Itub should add a paragraph summarizing the experimental data that supports his description of the last edit as "opinion". Vgsbox 20:04, 30 May 2007 (UTC)

A citation supporting the paragraph would suffice to bring it from "opinion" to "published opinion", and therefore make it notable enough to be included. DMacks 20:22, 30 May 2007 (UTC)

Contradictory statements?

This article states that "In the case of dimethoxyethane the trans,trans conformation is 3–5 kcal/mol about lower in energy (more stable) than the gauche,gauche conformations" whereas the article on dimethoxymethane states that "Due to the anomeric effect, dimethoxymetane has a preference toward the gauche conformation around the C–O bonds, instead of the anti conformation."

Can both of these statements be true? Ansuman 13:19, 6 June 2007 (UTC)

No, the statement in this article was wrong and I have corrected it. --Itub 13:25, 6 June 2007 (UTC)

Contents 1 Original research and conflict of interest 2 Chair conformations 3 Compromise proposal regarding the rotamers 4 Yet another idea

[edit] Original research and conflict of interest

User:Vgsbox has been adding his original research and opinion to this article again, which goes against Wikipedia policies. It is also obvious that he has a conflict of interest, because he is citing his own work and giving it undue weight. I'm reverting the latest edit[1] for these reasons.

I'm not saying that the article is perfect, as it could certainly needs expansion. I'm not even saying that the "conventional" explanation for the anomeric effect is The Truth. However, we have to give more weight to the mainstream explanation according to Wikipedia policies. Also, according to the No Original Research policy, preference should be given to secondary and tertiary sources, which means that in this case it is preferable to cite reviews and monographs rather than original research articles. Original articles that have not been cited much in the secondary scientific literature, with theories that have not had enough discussion in the scientific community shouldn't be given as much weight as the reviews and monographs. If Box turns out to be correct, surely future reviews, monographs, and even textbooks will reflect that, and we should certainly update the Wikipedia article accordingly. But that has not happened yet, and Wikipedia is not a crystal ball, so I'm afraid we'll have to wait until that happens. --Itub 12:16, 13 June 2007 (UTC)

[edit] Chair conformations

Six chair conformations for a 6-membered ring? You take one, do a chair-flip to get another, that's all I've ever heard of. Is the article conflating ring conformations with conformations of the side-chains on that ring? Let's please be very clear with terminology here even as there are certainly issues involving side-chain conformations that are based on the same/similar principles as ring-confs themselves. DMacks 14:13, 14 June 2007 (UTC)

I think he is talking about rotation around the exocyclic C-O bond. You have 2 chair conformations and three rotamers around that bond, resulting in a total of 6. However, the argument he added is misleading for reasons that I'll add later when I have time... --Itub 14:30, 14 June 2007 (UTC)

Ok, I'm back. I will call the oxygens "endocyclic" and "exocyclic", which I think is more clear than O1 and O2. The issue is that the exocyclic C-O bond is free to rotate and can adopt three conformations. In two of those, the endocyclic C-O antibonding orbital can interact with a lone pair of the exocyclic oxygen, which is stabilizing according to the hyperconjugation model. But that has nothing to do with which of the two chair conformations is favored. Now, if you look at the endocyclic C-O bond, it has only two (rather than three) conformations available because of the constraint of being in a ring. Of these two conformations, only for the axial one can the exocyclic C-O antibonding orbital interact with a lone pair from the endocyclic oxygen. This is similar to the case of dimethoxymethane in that there are two generalized anomeric effect involved, one per each rotatable bond to an oxygen. But in the case of a sugar or similar molecule, when talking about the non-generalized anomeric effect (dealing only with axial vs equatorial), only one of these two matters. (This prose can be hard to parse, making a drawing or a model certainly helps...) --Itub 15:33, 14 June 2007 (UTC)

Yes, I certainly agree that there are two good chair conformations essentially independent of three exocyclic-bond rotations, and that only the former is relevant to the discussion. There are always lots of effects and many global conformations available obviously. But for the article here and as a teaching example (i.e., not research paper), I think it's best to discuss a single part well rather than only considering the whole picture. DMacks 16:37, 14 June 2007 (UTC)

As I pointed out in the paragraph that I added (tried to add) and that was removed only because it showed obvious flaws in the n-sigma* argument, there are 6 conformations that must be considered, comprised of two chairs (axial, equatorial) and three rotamers about the C-2 - O-2 bond. Of these 6 conformations, three are heavily populated, the axial exoanomeric conformer, and two of the equatorial conformers. ALL of these 3 conformers MUST show n-sigma* interactions involving the O-2 lone pair and the C-2 - O-1 bond. It is specious to argue that a single n-sigma* interaction only occurs in the axial exoanomeric conformer.

Itub's conformational analysis paragraph above is pitiful at best and grossly misleading. Ask one of your organic chemisty friends to comment on it if you don't wish to take my opinion (36 years of teaching organic chemistry) into account.

If you insist on doing this kind of ignorance, then the article will soon be realized for what it is, an attempt to push n-sigma* interactions theory even in the face of other conflicting data. Not all of the people are fools and many will see what you are trying to do. I really didn't think that I would see this level of ignorance in this discussion. Vgsbox

When you have some time, go read the very first page of the paper, published by M.J.S. Dewar, in which SAM1 is introduced. Dewar comments that there are lots of theoretical chemists that have NO knowledge of chemistry. Is this relevant here? Vgsbox 14:27, 20 June 2007 (UTC)

No, what's relevant are the Wikipedia policies that you keep ignoring: WP:NOR and WP:NPA. --Itub 14:54, 20 June 2007 (UTC)

[edit] Compromise proposal regarding the rotamers

Perhaps we've been oversimplifying the issue. Box says that the hyperconjugation model is wrong because it doesn't consider the exo contributions. DMacks and I said that what determines the chair conformation is the endo contribution. In reality, there is a combination of both, that has been recognized for a long time, and when put in less provocative terms might be acceptable to all. I'll quote from Juaristi's review on the topic (p. 5061):

"It may be appreciated that the endo anomeric effect is the souce of the driving force for a polar aglycon to adopt the axial orientation; i.e., 58 [the axial conformer] is the lower energy conformer. In the alpha-anomer 58 the two effects (exo and endo) compete for electron delocalization toward the anomeric carbon. Not surprisingly, structural data suggest that the exo anomeric effect is stronger for the beta-anomer in which such competition is absent.

"In this context, Praly and Lemieux stress that an anomeric effect must be considered as the difference between the sum of the endo- and exo-anomeric effects in the equatorial conformer and the same sum for the axial conformer. However, since the endo anomeric effect is absent in the equatorial isomer, it is stabilized exclusively by exo anomeric interactions.

Anomeric Effect = (exo-AE_eq) - (exo-AE_ax + endo-AE_ax)"

What happens is that in most cases the sum exo-AE_ax + endo-AE_ax is greater than exo-AE_eq alone. The simplification in the discussion above by DMacks and me was that we assumed that exo-AE_ax was equal to exo-AE_eq, or at least that the difference between them was much smaller than endo-AE_ax. However, there are some exceptions. --Itub 15:07, 20 June 2007 (UTC)

Before I begin to reply to Itub, above, please let it be known that in simple six membered ringed compounds, the chair conformer is always the most stable and that has nothing to do with hyperconjugation.

Firstly, by classical conformational theory, the equatorial conformer of any 2-alkoxytetrahydropyran ought to be the lower energied one. That is how the whole issue of the AE got started. Many simple sugars, like glucose derivatives, anomalously show the axial conformer as the lower energied, especially in low polarity solvents.

Secondly, Box did NOT say that "that the hyperconjugation model is wrong because it doesn't consider the exo contributions". The article said that there was only ONE contributor to the hyperconjugation model of the Anoemeric Effect (AE), and I disputed that.

Thirdly, if you ignore the equatorial conformer, how do you explain why glucose does NOT show the AE in water? In fact, many acetals do not show a pronounced AE in very polar solvents. The hyperconjugation model does NOT explain this variation of the size of the AE with solvent polarity.

Maybe we should begin to worry about the true value and academic worth of this article on the Anomeric Effect, if Itub keeps control of the published material. I wonder how my colleagues will react to this discussion? Vgsbox 16:18, 20 June 2007 (UTC)

Nobody controls this article. As you have been repeatly told (and should know from the world of scientific publication in general), gotta get through peer review, and the standard here is WP:RS. Period...there's no cult-of-personality or article-ownership here. We're all waiting/hoping you or your colleagues will add some well-sourced information that supports and clearly explains whatever-you're-interested-in-explaining. Many of us teach chemistry and know how hard it is to explain something based on many inter-related (and often competing) effects. The more clearly each one can be explained as well as how they balance in the "real world" is quite a feat worth doing! DMacks 16:31, 20 June 2007 (UTC)

Glucose does show the anomeric effect in water, as discussed (again) in the review by Juaristi. However, the effect is not strong enough to overcome the preference towards the axial conformer, just to reduce it. Glucose prefers the axial conformation in the gas phase. It is a case of a solvent effect, complicated by all the hydrogen-bonding groups in glucose, which is the reason most model studies use simpler molecules than glucose. I do agree that the dipolar model helps explain the solvent effects better; each model has its strengths and weaknesses, which should explain why we still have two major models instead of only one. If one model were perfect, it would be the only one! --Itub 16:50, 20 June 2007 (UTC)

[edit] Yet another idea

Obviously there are many issues with THPOH, at least in part because there are two anomeric-effect bonds to consider. The ring doesn't seem to simplify anything beyond dimethoxymethane, though it is probably a more common application for many readers. Maybe we should use a 2-haloTHP instead of 2-hydroxy: only one rotatable bond, so we can focus on the anomeric effect itself (dipole/dipole, hyperconjugation, other effects) without also having to consider complications from sidechain conformations, less important H-bonding to solvents, etc.? DMacks 16:46, 20 June 2007 (UTC)