benzene ring hybridization confusion?

[omarski]

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I'm having some conflicting answers in regards to the hybridization of a benzene ring.

What I want to clarify is if benzene is sp2 hybridized? I'm going to assume it is since the DAT Bootcamp question said it was, although I've read conflicting things saying that the resonance changes it to more of a "1.5 bonds" per carbon instead of 2.

If the sp2 hybridization of a benzene carbon is correct, then why is it not safe to say alkenes exist in the benzene ring?

 

[Dr. Dan]

Yep you are correct, the hybridization is sp2 for a benzene ring. As far as I'm aware you might be able to say that alkenes exist within the benzene ring, albeit transiently. It's probably more correct to say that the benzene ring is a "pi system". These pi electrons resonate around the ring within this system, and don't necessarily exist as an alkene for any appreciable length in time. To the best of my knowledge, it's usually the "pi system" that takes place in chemical reactions (ex/EAS reactions), but it's difficult to show that with a reaction mechanism.
http://chemwiki.ucdavis.edu/@api/deki/files/4588/image023.png?revision=1

 

[DAT Destroyer]

I'm having some conflicting answers in regards to the hybridization of a benzene ring.

What I want to clarify is if benzene is sp2 hybridized? I'm going to assume it is since the DAT Bootcamp question said it was, although I've read conflicting things saying that the resonance changes it to more of a "1.5 bonds" per carbon instead of 2.

If the sp2 hybridization of a benzene carbon is correct, then why is it not safe to say alkenes exist in the benzene ring?

An alkene is composed of carbons that are sp2 hybridized where the electrons are LOCALIZED.

We therefore can say the bond order is 2. In benzene, the electrons are DELOCALIZED, and we are not dealing with a true double bond. As an approximation, we can say that the bond order is about 1.5. The carbons are also approximated to be sp2.

For the DAT....remember,,,,,all the carbons in benzene are sp2, the molecule is trigonal planar, and all bond lengths equal.

The Wade and Klein book is your best resource to learn the needed fundamentals.

Hope this helps.

Dr. Romano