Reactions and Mechanisms of Benzene

Bonding Nitrification Halogenation Friedel-Crafts Alkylation

Benzene

Kekule Model

Kekule assumed that benzene had the bonding of cyclohex-1,3,5-triene (pictured right). However there were isues with this: Measurements from X-Ray diffraction showed equal electron density around the ring All the bond lengths were equal, whereas C=C bonds are shorter than C-C bonds are. The enthalpy of hydrogenation was not simply three times that of cyclohexene. Cyclohexene is -120kJ/mol, expecting Kekule's benzene to be -360kJ/mol, but instead it is -208kJ/mol. Therefore, it was proven that bonding in benzene instead has a ring of delocalised electrons, making it more stable than expected.

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Caption: : Kekule's benzene

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Generic Mechanism

Caption: : Electrophilic substitution

Nitration of Benzene

The electrophile in this reaction is (NO2)+, Nitronium.It is generated by mixing conc sulphuric acid and conc nitric acid and heating to 50 degrees Celcius.

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Caption: : Formation of the Nitronium ion

Halogenation of Benzene

Benzene reacts with halogens in the same way it does with nitration - electrophilic substitution, however this time, a catalyst is needed, this is called a "Halogen Carrier".If benzene is reacting with Chlorine, traditionally Aluminium Trichloride (AlCl3) is used, and if with Bromine, Iron Tribromide (FeBr3) is used.The reaction takes place at room temperature.

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Friedal-Crafts Alkylation

This is exactly the same as Halogenation from the previous slide.But instead of a diatomic halogen, there is an alkyl chloride (haloalkane).The entire alkyl group is added on to the benzene ring in the place of the halogen from the slide before.