[ch3m_boy]

Hey guy's I’m back

I got a big test coming all about these pericyclic reactions and right at the moment, I don't have a specific question (because I haven't worked a lot of these problems yet) but as I am reading about this stuff, I am getting really overwhelmed....    Are there any kinds of rules that govern like the stereochem of these...?  It kind of freaks me out the way the book does it, which is drawing the HOMO and LUMO out and then deciding whether to rotate conrotatory or disrotatory...  Do I have to do this with everyone one of these problems???  This is nuts, I will not finish my test....  Anyways, I will have more specific questions either tonight or tomorrow but any suggestions now would be great!!!!!!!!!!!

Oh, and I also found out I’m not very good at drawing these homos and lumos...  I just don't see the symmetry and that is REALLY bad... 

[movies]

Electrocyclic reactions are always governed by those rules based on orbital symmetry.  The pattern is regular though, depending on how many electrons are involved.  For example, a 6 pi electrocyclization (hexatriene to cyclohexadiene) will always be disrotatory.  Then it alternates from disrotatory to conrotatoryas you add pairs of electrons.  I remember this as "3D," meaning if there are three electron pairs (6 pi electrons) then it is Disrotatory.  Then it goes on to 4C, 5D, 6C, etc.

For drawing HOMOs and LUMOs, you can use a similar trick.  Take the number of pairs of pi electrons and substract one.  That number will be how many nodes are in the LUMO.  So, for hexatriene, you have 3 electron pairs, 3 - 1 = 2 nodes, which will bisect the sigma bonds between the olefins.

I hope these tricks help.

[ch3m_boy]

Electrocyclic reactions are always governed by those rules based on orbital symmetry.  The pattern is regular though, depending on how many electrons are involved.  For example, a 6 pi electrocyclization (hexatriene to cyclohexadiene) will always be disrotatory.  Then it alternates from disrotatory to conrotatoryas you add pairs of electrons.  I remember this as "3D," meaning if there are three electron pairs (6 pi electrons) then it is Disrotatory.  Then it goes on to 4C, 5D, 6C, etc.

For drawing HOMOs and LUMOs, you can use a similar trick.  Take the number of pairs of pi electrons and substract one.  That number will be how many nodes are in the LUMO.  So, for hexatriene, you have 3 electron pairs, 3 - 1 = 2 nodes, which will bisect the sigma bonds between the olefins.

I hope these tricks help.

Hey those are some neat tricks...  I just learned another cool trick (I don’t know what it is called) but like if you have a multiple of 4n+2  so like 6,10,14,18,etc... So this is a huckel number, then this is called a huckel transition state so stereochem is retained, and it’s also suprafacial (I always just think of this as a diel alder I don’t know if this conrotatory or disrotatory?)  And the other thing is if you have just 4n pi electrons so any multiple of 4, it’s called a mobius number and so a mobius transition state means stereochem is inverted and this is antrafacial...  All these little tricks are amazing, save so much time.  I don't know what all this is called though... my teacher is UPSESSED with MO theory though...

I have also found where my problem is with drawing MOs...  Here is where I always mess up...  There was an example where 5 p A.O.s are apart of a conjugated system, so I know I will make 5 MOs..  And say I wanted to draw the pi 3 MO...  Well this will have 2 nodes, and my teacher puts the nodes on the carbons which cancels them out, when I always think of putting the nodes between C2 and C3 and C3 and C4..  I don't see the symmetry with these things when there is an odd number of p orbitals and its driving me crazy AHHH!!!  What do you suggest about this?   

I will have more questions tomorrow too

[movies]

All of this orbital symmetry stuff falls under the Woodward-Hoffman rules.

I agree that those examples where the nodes intersect a carbon atom are quite confusing.  I always first draw out all of the AOs that make up the MO you are concerned with, and then redraw it with the AOs cancelled out.  It's too hard to see otherwise.

[Dan]

I found this book extremely helpful for pericyclics:

Fleming - Pericyclic Reactions (Oxford Chemistry Primer #67)