1. 07 December 2018
Reactions of Benzene
C4.2 Aromaticity

2. Starter
What are the 3 anomalies that cannot be explained by Kekule’s structure of benzene? GD

3. Reactions of Benzene
Benzene does not undergo addition reactions like alkenes.
If it did the delocalised ring structure would be disrupted
This would make the product less stable than benzene meaning the reaction would be energetically unfavourable.
For these reasons, benzene undergoes substitution instead of addition. GD

4. Electrophilic substitution
Electrophilic substitution definition: An electron deficient electrophile replaces another atom or group of atoms in a molecule (usually hydrogen).
The mechanism proceeds in 3 steps:
Formation of electrophile through a catalyst
Substitution
Regeneration of catalyst GD

5. Electrophilic Substitution - Nitration
Equation: C6H6 + HNO3  C6H5NO2 + H2O Conditions: Concentrated HNO3 and H2SO4 at 500C Step 1: Formation of electrophile through a catalyst HNO3 + H2SO4  NO2 + HSO4 + H2O
Catalyst
Nitronium ion (electrophile)

6. Electrophilic Substitution - Nitration
Equation: C6H6 + HNO3  C6H5NO2 + H2O Conditions: Concentrated HNO3 and H2SO4 at 500C Step 2: Substitution
NO2 +
NO2 H
NO2 H

7. Electrophilic Substitution - Nitration
Equation: C6H6 + HNO3  C6H5NO2 + H2O Conditions: Concentrated HNO3 and H2SO4 at 500C Step 3: Regeneration of catalyst H + HSO4  H2SO4
Catalyst

8. Electrophilic Substitution - Halogenation
Equation: C6H6 + X2  C6H5X2 + HX Conditions: X2 and anhydrous AlX3/FeX3 at 250C Step 1: Formation of electrophile through a catalyst X2 + AlX3  X + AlX4
Catalyst

9. Electrophilic Substitution - Halogenation
Equation: C6H6 + X2  C6H5X2 + HX Conditions: X2 and anhydrous AlX3/FeX3 at 250C Step 2: Substitution X + H X X H

10. Electrophilic Substitution - Halogenation
Equation: C6H6 + X2  C6H5X2 + HX Conditions: X2 and anhydrous AlX3/FeX3 at 250C Step 3: Regeneration of catalyst AlX4 + H  AlX3 + HX
Catalyst

11. Electrophilic Substitution - Alkylation
Equation: C6H6 + RCl  C6H5R + HCl Conditions: RCl (halogenoalkane) and anhydrous AlCl3 at C (to prevent further substitution) Step 1: Formation of electrophile through a catalyst CH3CH2Cl + AlCl3  CH3CH2 + AlCl4
Catalyst

12. Worked Example: Electrophilic Substitution - Alkylation
Equation: C6H6 + RCl  C6H5R + HCl Conditions: RCl (halogenoalkane) and anhydrous AlCl3 at C (to prevent further substitution) Step 2: Substitution
CH3CH2
CH3CH2 + H
CH3CH2 H

13. Electrophilic Substitution - Alkylation
Equation: C6H6 + RCl  C6H5R + HCl Conditions: RCl (halogenoalkane) and anhydrous AlCl3 at 0-250C (to prevent further substitution) Step 3: Regeneration of catalyst H + AlCl4  AlCl3 + HCl
Catalyst

14. Knowledge Check
Write word and symbol equations for each of the reactions below and then draw a mechanism for each.
benzene with nitric acid in the presence of sulphuric acid
benzene with chloromethane in the presence of aluminium chloride
benzene with bromine in the presence of iron (III) bromide GD

15. Knowledge Check
Benzene reacts by electrophilic substitution. Define the terms electrophile and substitution.
Benzene reacts with chlorine by an electrophilic substitution mechanism. Outline this mechanism including how the electrophile is generated.
Benzene react with 2-chloropropane, in the presence of an aluminium chloride catalyst, by electrophilic substitution. Construct the equation for this reaction.
Deduce the structure of the organic product expected from the reaction of each of the following compounds with benzene in the presence of AlCl3:
(CH3)2CHCOCl
C6H5COCl
Use curly arrows to show the mechanism for the reaction between (CH3)2CHOCl and benzene, clearly showing the role of the catalyst in the reaction. GD

16. Benzene Vs. Halogenoalkanes
The carbon-chlorine bond in chlorobenzene is stronger than that found in a chloroalkane.
This is because there is an interaction between one of the lone pairs on the chlorine atom and the delocalised ring electrons in benzene, and this strengthens the bond. Cl