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

C C

C

CC

C HH

H H

H H

STRUCTURE OF BENZENE

STRUCTURE OF BENZENE

STRUCTURE OF BENZENE

NITRATION

+ HNO3 + H2O

NO2

nitrobenzene

Conditions conc HNO3

conc H2SO4 catalyst

50ºC

Electrophile curly arrow

-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

Equation (name product)

Conditions

Mechanism

CH3CH3

NO2

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

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 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

o OCR past paper Q

Reactivity of cyclohexene vs benzene (4 marks)

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)

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