Benzene

[Asx4Life]

wut, wut, wut, I always thought Benzene had alternating double bonds... =S

[stonecold]

This is resonance as someone mentioned above. It is where the theoretical molecule isn't the same as what we observe. To explain this discrepancy we say that the electron pairs are able to jump around between adjacent atoms to form partial double bonds.

I don't agree with this. Electrons aren't confined to 'bonds'. Classical models only allow single and double bonds. The only way to explain the structure of benzene using classical models is that the bonds are alternating.

In more advanced theory (ones that treat electrons with quantum mechanics), you have delocalisation.

Yes, Mao is correct.  Neither of the resonance structures of Benzene is strictly correct.

Rather, the electrons which are supposed to be involved in the double bonds are delocalised around the whole ring, which is what gives this molecule its stability.

This is why some people prefer the benzene structure with the ring drawn on the inside of the hexagon, however it has limitations, the biggest being it makes it difficult to keep track of electrons.

This is Uni level stuff, so don't worry about it too much.

[Graphite]

This is resonance as someone mentioned above. It is where the theoretical molecule isn't the same as what we observe. To explain this discrepancy we say that the electron pairs are able to jump around between adjacent atoms to form partial double bonds.

I don't agree with this. Electrons aren't confined to 'bonds'. Classical models only allow single and double bonds. The only way to explain the structure of benzene using classical models is that the bonds are alternating.

In more advanced theory (ones that treat electrons with quantum mechanics), you have delocalisation.

Yah but the resonance structure shows all bonds are of equal length and strength, somewhere intermediate between a double and single bond, hence a partial double bond.

[thushan]

This idea about resonance structures - think about benzene as an "average" of each of the resonance structures. When you "average" the resonance structures out, you find that the electrons are delocalised around the whole ring.

[stonecold]

This is resonance as someone mentioned above. It is where the theoretical molecule isn't the same as what we observe. To explain this discrepancy we say that the electron pairs are able to jump around between adjacent atoms to form partial double bonds.

I don't agree with this. Electrons aren't confined to 'bonds'. Classical models only allow single and double bonds. The only way to explain the structure of benzene using classical models is that the bonds are alternating.

In more advanced theory (ones that treat electrons with quantum mechanics), you have delocalisation.

Yah but the resonance structure shows all bonds are of equal length and strength, somewhere intermediate between a double and single bond, hence a partial double bond.

Just watch this:

http://www.youtube.com/watch?v=-EFEfVDUp6M

Edit:

Don't worry about all the stuff about sp2 orbitials, p orbitals, sigma bonds and pi bonds.  just take note that each carbon has 4 electrons.  two which bond with the adjacent carbons electrons to form covelent bonds, another electron which binds with hydrogen to form a covelent bond, and a remaining electron which remains delocalised and contributes to the ring.

[Mao]

This is resonance as someone mentioned above. It is where the theoretical molecule isn't the same as what we observe. To explain this discrepancy we say that the electron pairs are able to jump around between adjacent atoms to form partial double bonds.

I don't agree with this. Electrons aren't confined to 'bonds'. Classical models only allow single and double bonds. The only way to explain the structure of benzene using classical models is that the bonds are alternating.

In more advanced theory (ones that treat electrons with quantum mechanics), you have delocalisation.

Yah but the resonance structure shows all bonds are of equal length and strength, somewhere intermediate between a double and single bond, hence a partial double bond.

I think you are misunderstanding the term 'resonance structure'. A molecule can have several resonance structures, each of them contributing to the overall structure (i.e. the actual structure). Individual resonance structures follow the classical model. The method of 'averaging' these resonance structures is the attempt to explain the '1.5 bond'  phenomenon.

Resonance structures are useful in organic chemistry, though most chemists don't agree with its premise.