[ArcHos]

Hello everybody! I have a question about mentioned Alder "ene" reaction.. I didn't quite understand how can sp3 orbital of Carbon be parallel to p orbital of adjacent carbon. I think that they can almost be parallel but not completely. I have watched several videos on youtube and all of them says that they are parallel. And if this turns that they are not fully parallel, does this affect the reaction? Here is what Clayden says:

[Guitarmaniac86]

In order for the reaction to occur the C-H bond has to be parallel to one of the p orbitals on the ene for there to be perfect orbital overlap allowing the new bonds to form.

The frontier orbital explanation is a model to aid in the understanding of how these reactions occur, and as such is presented in the perfect case. In reality, the structure of the ene and eneophile will dictate how well the orbital overlap will be. In cases where there is less than perfect overlap, whether spatial or steric, the reaction may not proceed or may proceed slowly; and this is due to the greater energy of activation required to make and break the new bonds.

If you need more clarification, I will do my best to help.

[ArcHos]

Thank you for your response Guitarmaniac86! Then in order for us to understand this reaction better, such explanation was given? sp3 orbital and p orbital may not be parallel to one another? Throughout chapters about pericyclic reactions in Clayden's Organic chemistry it tells that they are parallel

[Guitarmaniac86]

In ideal systems the orbitals are treated as parallel because that gives the best orbital interations for bond formation. There are molecules where this is the case, where the LUMO on one molecule and the HOMO on another molecule are perfectly aligned.

The orbitals may not be parallel in all cases because of molecular geometry; but the amplitude of the orbitals may be different. The orbital amplitude may compensate for non-ideal overlap (ie, non parallel overlap). If the HOMO has a large amplitude and the LUMO is average, there is the possibility of an interaction. Here, we assume non-parallel overlap. If we increase the size of both the LUMO and HOMO amplitudes, we can get closer to a parallel overlap of orbitals. We can affect the LUMO and HOMO by adding or removing different groups (methyls, nitros etc) or using Lewis acids to increase or decrease orbital amplitude.

Also, do not make the mistake that parallel assumes perpendicular. The orbitals are geometrically aligned in space, so any intermediate that is going to form will have a chair or close to chair form intermdiate it must go through to form bonds.

https://en.wikipedia.org/wiki/Ene_reaction

In this article you can see that although the orbitals are parallel they are not exactly perpendicular. (see section labelled Concerted pathway and transition states). All cases are idealised but in reality, the orbitals may not be in an ideal conformation.

I hope this helps you understand better.