Substitution reactions of benzene L.O.: Outline the mechanism for mononitration and...
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Transcript of Substitution reactions of benzene L.O.: Outline the mechanism for mononitration and...
Substitution reactions of benzene
L.O.:Outline the mechanism for mononitration
and monohalogenation of benzene.
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?
Starter activity
• What is the name for the reaction between bromine and ethene?
• Draw the mechanism for it• Define the term “nucleophile”
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
-H+
Formation of electrophile
HNO3
+ H2SO4 + HSO4- + H2O
NO2
+
Reaction of electrophile with aromatic compound
NO2
NO2+ NO2
+H
nitronium ion
NITRATION – ELECTROPHILIC SUBSTITUTION
H+ + HSO4- → H2SO4
NITRATION
2-methylnitrobenzene
Conditions conc HNO3
conc H2SO4 catalyst
50ºC
CH3
+ HNO3 + H2O
CH3
NO2
NITRATION
Uses of nitro compounds:
1) as explosives
2) to make aromatic amines (used to make dyes)
CH3
NO2
NO2
O2N
1,3,5-trinitrotolueneTNT
Reactions with halogens
• React in the presence of a catalyst called a halogen carrier
Benzene + Chlorine Chlorobenzene + Hydrochloric acid
Common halogen carriers are FeCl3, AlCl3 and AlBr3
Reactions with halogens –mechanism of reaction
• Electrophilic substitutionBr2 + FeBr3 → Br+ + FeBr4
-
• The halogen carrier reacts with the halogen molecule to produce the halide ion which then goes on to react with the benzene
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)