MODULE 6: Aromatic Chemistry
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What are the physical properties of benzene?
Colourless, sweet-smelling, flammable, carcinogenic
What is the Kekule model of the benzene ring?
The theory that aromatic rings are carbon rings with alternating double bonds
List some evidence to disprove Kekule’s model
• Lack of reactivity: does not undergo electrophilic addition, does not decolourise bromine in normal conditions. • Different length of benzene C-C bonds to regular C-C bonds and C=C bonds • Hydrogenation enthalpies: less energy is produced from hydrogenation of benzene that predicted from the Kekule model
Name the features of the delocalised model of the benzene ring
• planar and cyclical • each carbon atom has one electron in a p-orbital at right angles to the plane of the ring • adjacent p-orbitals overlap sideways to form a ring of electron density • this creates a system of pi-bonds which spread over all six carbon atoms • six electrons in the pi-bonds are said to be delocalised
How do you name alkyl, halogen or nitro substituents in benzene?
Using the group prefix before benzene e.g. ethylbenzene
How do you name alkyl chain substitute in benzene?
Using ‘phenyl’ as the prefix for benzene e.g. phenylethanone
How does benzene react?
Electrophilic substitution
How does benzene undergo nitration?
Benzene reacts with nitric acid (catalysed by sulphuric acid) at 50oC to under electrophilic substitution
State the equation to form the electrophile
HNO₃ + H₂SO₄ –> NO₂⁺ + HSO₄⁻ + H₂O Forms a nitronium ion
State the equation to form the nitrobenzene
Benzene + nitronium ion –> intermediate –> nitrobenzene + hydrogen ion
What product is formed in the nitration reaction is raised above 50oC?
Formation of a dinitrobenzene
What happens at the end of nitration?
Reforming of the catalyst
What is required for a halogen to react with benzene?
Halogen carrier (FeBr₃, FeCl₃, AlBr₃, AlCl₃)
State the process for the halogenation of benzene
Step one: formation of the halogen ion X₂ + FeX₃ --> FeX₄⁻ + X⁺ Step two: electrophilic substitution benzene + X⁺ -> intermediate -> halobenzene + hydrogen ion Step three: reformation of catalyst H⁺ + FeX₄⁻ --> FeX₃ + HX
What is alkylation?
When halogen carriers acts as a catalyst to substitute a hydrogen atom with the alkyl group, which is also very similar to the reaction with acyl chloride
What are phenols?
Benzene rings with -OH groups
Compare alcohols, phenols and carboxylic acids
Phenol is more acidic than alcohol, but less acidic than carboxylic acids, hence phenols react with strong bases, carboxylic acids react with weak and strong bases and ethanol does not react with any bases.
How does phenol react with sodium hydroxide?
Forms sodium phenoxide and water
What are the electrophilic substitution reactions of phenols?
Bromination and nitration
Describe the bromination of phenol
Bromine water is added to water to form a white precipitate, and undergoes a 2,4,6 substitution- and does not require a halogen carrier
Describe the nitration of phenol
Phenol reacts with nitric acid at room temperature to form a mixture of 2 and 4 substituted nitrophenol
Compare the reactivity of phenol and benzene
• nitric acid and bromine react more readily with phenol - phenol is more reactive due to lone pair on -OH group, which increases the electron density
How do -NH₂ and -NO₂ effect the position of substitution?
• NH₂ = positions 2 or 4 - NO₂ = position 3