[rleung]

Hi,

My book says that the positive charge cannot be delocalized in this molecule since the t-butyl groups prevent the positive chargefrom being in the same plane as the benzene ring.  It says that since electrons can only be delocalized between atoms in the same plane, the positive charge cannot be delocalized, but I am having a hard time understanding how the t-butyl groups actually PREVENT the positive charge from being in the same plane as the benzene ring.  Thanks.

Ryan

[Mitch]

The carbocation is at 90 degrees from the plane of the ring.

[rleung]

Thanks.  But how would you KNOW that it is at 90 degrees from the plane of the ring?  I can't build a totally accurate model of this since the actual structure has all of the electrons delocalized in the benzene ring.  I though that the substituents attached to sp2 hybridized carbons are usually in the same plane as the sp2 hybridized carbon?  Thanks.

Ryan

[Mitch]

They would be if the tertbutyl groups weren't around.

[rleung]

Hmm, I still don't see WHY the t-butyl would cause the carbocation to be in a different plane from the benzene ring.  Thanks.

[Mitch]

Because they big and bulky, and everything would be to crammed if everything was on the same plane.

[Donaldson Tan]

but if the positive charge is allowed to be delocalised, it can move to the 2 carbon atoms in the benzene ring where the t-butyl groups are attached to. The t-butyl groups would definitely help stabilise the positive charge.

Wouldn't this favour the positive charge to move?

[Mitch]

Although the electronics would favor it, the sterics won't allow it.

Everything in organic chemistry can be explained as a stereo-electronic effect.

[movies]

You may want to look into allylic strain arguments.  They will explain the orientation of the isopropyl group.

I can't find a good website that shows this, but it's in a lot of books.

[rleung]

Hmm, I get what you are saying about the t-butyl groups being a steric hindrance, but wouldn't it be possible that the t-butyl groups are NOT in the same plane as the benzene ring, while the positively-charged carbocation would be the group IN the same plane as the benzene ring?  Thanks.

Ryan

[Dude]

That appears to be a dimethylbenzyl cation.  To delocalize, the sp2 cation must have the methyl groups in the plane of the ring (electrons of the ring and empty orbital of the cation 90 degrees to the plane).  If the methyl groups of the dimethylbenzyl cation can not maintain their position in the plane of the ring due to the steric strain of the adjacent t-butyl groups, alignment is disrupted.

[Mitch]

Make a model and prove it to yourself.