Reactions and Mechanisms of Benzene

Benzene

• Bonding
• Nitrification
• Halogenation
• Friedel-Crafts Alkylation

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.
• 
• Caption: Kekule's benzene

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.
• 
• 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.
• 

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.