Substitution reactions of benzene
L.O.:
Outline the mechanism for mononitration
and monohalogenation of benzene.
Homework
Q: 1&3
Extension: Q2
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
STRUCTURE OF BENZENE
H
H
C
H
C
C
C
C
H
C
H
H
STRUCTURE OF BENZENE
STRUCTURE OF BENZENE
NITRATION
NO2
+
+
HNO3
nitrobenzene
Conditions
conc HNO3
conc H2SO4 catalyst
50ºC
H2O
Electrophile curly arrow
NITRATION – ELECTROPHILIC SUBSTITUTION
Formation of electrophile
+
HNO3 + H2SO4
NO2
+ HSO4-
+ H2O
nitronium ion
Reaction of electrophile with aromatic compound
+
NO2
+
NO2
H
+
-H
H+ + HSO4- → H2SO4
NO2
NITRATION
CH3
CH3
NO2
Equation (name product)
Conditions
Mechanism
NITRATION
CH3
CH3
NO2
+
HNO3
+
2-methylnitrobenzene
Conditions
conc HNO3
conc H2SO4 catalyst
50ºC
H2O
NITRATION
Uses of nitro compounds:
1) as explosives
2) to make aromatic amines (used to make dyes)
CH3
O2N
NO2
NO2
1,3,5-trinitrotoluene
TNT
Name the mechanism for this reaction.
Electrophilic substitution
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 substitution
Br2 + 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
o OCR past paper Q
Reactivity of cyclohexene vs benzene (4 marks)
Comparing benzene with alkenes
o
o
o
o
benzene is more stable (1)
benzene π electrons are delocalised (1)
benzene has lower electron density (1)
so bromine is less polarised /attracted to it /
benzene is less susceptible to electrophiles (1)
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