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Substitution reactions of benzene L.O.: Outline the mechanism for mononitration and monohalogenation of benzene.
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Homework Q: 1&3 Extension: Q2
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Recap questions What are the three sources of evidence which led to the delocalised electron model of benzene? Why is benzene less reactive than alkenes?
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Starter activity What is the name for the reaction between bromine and ethene? Draw the mechanism for it Define the term “nucleophile”
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Reactions involving benzene Benzene is more stable than alkenes, therefore less reactive Won’t react with halogens at RTP without a catalyst Will usually undergo electrophilic substitution reactions
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STRUCTURE OF BENZENE
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NITRATION nitrobenzene Conditionsconc HNO 3 conc H 2 SO 4 catalyst 50ºC
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Electrophile curly arrow
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-H + Formation of electrophile HNO 3 + H 2 SO 4 + HSO 4 - + H 2 O NO 2 + Reaction of electrophile with aromatic compound NO 2 + + H nitronium ion NITRATION – ELECTROPHILIC SUBSTITUTION H + + HSO 4 - → H 2 SO 4
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NITRATION Equation (name product) Conditions Mechanism
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NITRATION 2-methylnitrobenzene Conditionsconc HNO 3 conc H 2 SO 4 catalyst 50ºC
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NITRATION Uses of nitro compounds: 1)as explosives 2)to make aromatic amines (used to make dyes) 1,3,5-trinitrotoluene TNT
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Name the mechanism for this reaction. Electrophilic substitution
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Reactions with halogens React in the presence of a catalyst called a halogen carrier Benzene + Chlorine Chlorobenzene + Hydrochloric acid Common halogen carriers are FeCl 3, AlCl 3 and AlBr 3
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Reactions with halogens –mechanism of reaction Electrophilic substitution Br 2 + FeBr 3 → Br + + FeBr 4 - The halogen carrier reacts with the halogen molecule to produce the halide ion which then goes on to react with the benzene
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o OCR past paper Q
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Reactivity of cyclohexene vs benzene (4 marks)
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Comparing benzene with alkenes o benzene is more stable (1) o benzene π electrons are delocalised (1) o benzene has lower electron density (1) o so bromine is less polarised /attracted to it / benzene is less susceptible to electrophiles (1)
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Finishing off Draw the complete mechanism for the nitration of benzene. Include the reaction conditions and catalyst Draw the complete reaction for the chlorination of benzene
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