Understanding the Horner-Wadsworth-Emmons Reaction

[LogicX]

Homework Statement

Topic

Homework Equations

http://en.wikipedia.org/wiki/Horner–Wadsworth–Emmons_reaction

This is basically a variation of a wittig reaction.

The Attempt at a Solution

I'm trying to understand this concept, because the chemistry gods have decided that it should be stated in every textbook and online source, but never actually explained why.

So, making a trans product is better thermodynamically. My thinking would be that the opposite of the problem statement would be true. If something is stabilized then maybe it would be able to "get away" with forming a cis product instead of trance (sort of a kinetic vs thermodynamic control argument). But the opposite is true. If you are stabilized, you make the more stable product. If you are not stabilized, you make the less stable product.

Why would this be?

Thanks.

 

[nate808]

Hi there,

I completely understand your confusion about the Horner-Wadsworth-Emmons reaction and the preference for trans products. Let me try to explain it to you.

First, let's review the reaction itself. The Horner-Wadsworth-Emmons reaction is a variation of the Wittig reaction, where an aldehyde or ketone is reacted with a phosphonium ylide to form an alkene. Now, in the Wittig reaction, the trans product is typically formed due to steric hindrance between the bulky phosphorus group and the R group on the carbonyl compound. This is the same in the Horner-Wadsworth-Emmons reaction, where the bulky trialkylphosphine oxide group favors the formation of the trans product.

But why is the trans product more stable? It all comes down to the geometry and bond angles of the alkene formed. The trans product has a geometry that is closer to the ideal 120 degree bond angle for a carbon-carbon double bond, while the cis product has a bond angle of around 90 degrees. This makes the trans product more stable, as it has less strain in its structure.

Now, in terms of your question about stabilization and the preference for trans products, it all has to do with the stability of the intermediate formed during the reaction. The intermediate formed in the Horner-Wadsworth-Emmons reaction is a betaine, which is stabilized by resonance. This stabilization makes it easier for the intermediate to form the more stable trans product, as it has a lower energy barrier to overcome.

I hope this helps to clarify things for you. Let me know if you have any other questions or if there's anything else you would like me to explain.