07 December 2018 Reactions of Benzene C4.2 Aromaticity

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

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

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

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)

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

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

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

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

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

Electrophilic Substitution - Alkylation Equation: C6H6 + RCl  C6H5R + HCl Conditions: RCl (halogenoalkane) and anhydrous AlCl3 at 0- 250C (to prevent further substitution) Step 1: Formation of electrophile through a catalyst CH3CH2Cl + AlCl3  CH3CH2 + AlCl4 Catalyst

Worked Example: Electrophilic Substitution - Alkylation Equation: C6H6 + RCl  C6H5R + HCl Conditions: RCl (halogenoalkane) and anhydrous AlCl3 at 0- 250C (to prevent further substitution) Step 2: Substitution CH3CH2 CH3CH2 + H CH3CH2 H

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

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

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

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