Benzene rings! Watch

Srathmore
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I have a question.

How come benzene has a delocalised electron system - even though all the electrons are bonded?
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stratomaster
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Each carbon has a spare p-orbital. These overlap sideways to form pie-bonds (which are rings of delocalised electrons, one above the plane and one below the plane).
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Srathmore
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(Original post by stratomaster)
Each carbon has a spare p-orbital. These overlap sideways to form pie-bonds (which are rings of delocalised electrons, one above the plane and one below the plane).
DO you mean that a p orbital has 2 electrons?
But all the electronsa re bonded :confused:
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Fly.
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(Original post by Srathmore)
I have a question.

How come benzene has a delocalised electron system - even though all the electrons are bonded?

Being delocalised doesn't imply that the electrons are in some way non-bonding. It just means that the bonding molecular orbitals are spread over a larger number of atoms, in this case six. All six delocalised electrons occupy bonding orbitals in benzene.
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sea tea
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each carbon atom forms three bonds (two c-c bonds, one c-h). the spare electrons in the pz orbitals, which each carbon in the ring possesses, are then delocalised across one set of molecular orbitals, which contains 6 electrons, and has numerous lobes, one above and below the ring, and possibly others (i'm not sure...).
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Fly.
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(Original post by sea tea)
each carbon atom forms three bonds (two c-c bonds, one c-h). the spare electrons in the pz orbitals, which each carbon in the ring possesses, are then delocalised across one huge molecular orbital, which contains 6 electrons, and has numerous lobes, one above and below the ring, and possibly others (i'm not sure...).

By the Pauli exclusion principle, an orbital can contain at most two spin unpaired electrons, so there is no 'huge molecular orbital' having six electrons.
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sea tea
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ok, so i phrased it badly. i've edited it. there are six molecular orbitals created by the overlap, the one most commonly seen in textbooks is the one which has a lobe above and below the ring.

three of the molecular orbitals are antibonding, three are bonding. in the lowest energy state, only the threee bonding ones are filled (two spin paired electrons in each).

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flux_mag
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This question isn't worth a new thread in my opinion, but has anyone found out why OH/CH3 etc groups are 2-4 directing? Chem teacher won't take the time to explain at the moment, and normal sources (wiki, chemguide etc) are equally useless.

Anyone?
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charco
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they both increase the electron density at the 2,4 positions (but using different mechanisms - the CH3 is +I and the OH is +M)
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Fly.
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(Original post by flux_mag)
This question isn't worth a new thread in my opinion, but has anyone found out why OH/CH3 etc groups are 2-4 directing? Chem teacher won't take the time to explain at the moment, and normal sources (wiki, chemguide etc) are equally useless.

Anyone?

This can be best understood by considering the resonance structures that can arise by moving electrons around an aromatic ring. Lone electron pairs, such as those of an -OH or Nitro group, increase the electron density in the ortho and para substitution positions and so these sites become more nucleophilic. Groups such as CH3 also direct substitution in the ortho and para positions although for a different reason. In the substitution mechanism one of the carbons adjacent to the substitution site acquires a positive charge which can be stabilised by sigma conjugation from alkyl groups.
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