Aromatic Chemistry

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Benzene/Bonding

Aromatic Compounds are compounds that contain a benzene ring.   A benzene ring has the formula C6H6, It has a cyclic structure, 6 C's joined together in a ring. Ring is planarand hydrogens stick out in the same plane.

Kekule model: Idea that the ring of C atoms with alternating single and double bonds between them. However it has now been proven that all the bonds are simply C-C

Delocalised Model: Bond-Length observations are explained by delocalised model. In this model each carbon donates an electron from its p-orbital. These electrons combine to form a ring of delocalised electrons. All the carbon-carbon bonds are the same. Delocalised electrons are represented by a ring in the centre.

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Delocalisation

All carbon-carbon bonds are the same length. This is good evidence for the delocalisation model.

Even more evidence comes from enthalpy change data !!

Cyclohexene has one double bond. When it's hydrogenated the enthalpy change is -120 kJmol-1. If benzenehas 3 double bonds you would expect to have an enthalpy of hydrogenation of -360 kJmol-1. But the experimental enthalpy of hydrogenationof benzene is -208 kJmol-1  --> far less exothermic that expected.

Energy is put in to break bonds and released when they are made. So more energy must have been put in to break the bonds in benzene than would be needed to break the bonds in a kekule structure. This difference indicates that benzene is more stable than the kekule structure would be. This is thought to be due to the ring of delocalised electrons. In a ring the electron density is shared over more atoms, meaning the energy of the atom is lowered and it becomes more stable.

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Naming aromatic compounds

All contain a benzene ring, they are named in 2 ways :

1... Some cases benzene is the main functional groupand the molecule is named as a substituted benzene ring..... suffix -benzene

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     Chlorobenzene               Nitrobenzene               Methylbenzene               Chloromethybenzene

2..... Benezene not main functional group molecule named after phenyl groupattached. Phenyl-

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       Phenylamine                 Phenol                   Phenylethanone                   Phenylethene

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Electrophilic substitution

Benzene region of high electron density so it attracts electrophiles ---> elcetron pair acceptors (e.g. H+ or NO3^+).    GENERAL MECHANISM:

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Need to know 2 Electrophilic substitution mechanisms: Nitration reaction and Friedel-Crafts acylation..

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Nitration

When you warm benzene with concentrated nitric acid and concentrated sulfuric acid, get nitrobenzene. Overall equation:

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H2SO4 acts as a catalyst --- Helps make nitronium ion, NO2^+ (elecrophile).                                    HNO3  +  H2SO4  ----------> HSO4^-  +  NO2^(+)  +H2O.      Once the nitronium ion is made it can react with benzene to form nitrobenzene. --> electrophilic substitution.....

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If you only want one NO2 group keep temp below 55.                                                                         USES: Nitro compounds can be reduced to form aromatic amines -->used to manufacture dyes + pharmaceuticals. Nitro compounds decompose violently when heated os used as explosives.

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Friedel-Crafts acylation

Sometimes benzene is so stable it is fairly unreactive. Friedel-Crafts acylation reactions are used to add a  acyl group(-C(=O)-R) to the benzene ring. Products --> HCl and phenylketone. Reactants need to be heated under reflux in non-aqueous evironment for reactions to occur:

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Electrophile has to have stong positive charge to attack stable benzene. most arn't but can be made stronger by a halogen carrier. Uses as acyl chlorideto provide electophile and a halogen carrier such as AlCl3 ---> accepts a lone pair from acyl chloride. As lone pairs are pulled away, polarisation of acy chloride increases forms carbocation. Making it a much stronger electrophile, formation of carbocation is first step in mechanism :

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Freidel-Crafts acylation continued....

and second step....

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