[O Chem] Arenes

Benzene —> Aniline

Electrophilic substitution (nitration)

Conc. H2SO4, conc. HNO3, heat under reflux at 55ºC

In the nitration of benzene, H2SO4 acts as a

Bronsted-Lowry Acid Catalyst (H+ donor)

In the nitration of benzene, HNO3 acts as a

Bronsted-Lowry Base (H+ acceptor)

Formation of stronger electrophile from HNO3

HNO3 + 2H2SO4 → NO2+ + 2HSO4- + H3O+

Benzene —> Chlorobenzene

Electrophilic substitution (halogenation)

Cl2, anhydrous FeCl3/AlCl3/Fe, room temp.

In the chlorination of benzene, FeCl3/AlCl3 acts as a

Lewis acid catalyst (e pair acceptor)

Formation of stronger electrophile from FeCl3

Cl2 + FeCl3 —> Cl+ + FeCl4-

Draw the Lewis structure for FeCl4-

Toluene —> Chlorotoluene

Electrophilic substitution

Cl2, anhydrous FeCl3/AlCl3/Fe

Observations for chlorination and bromination of benzene

• Yellowish-green Cl2/Reddish-brown Br2 decolourises

• White fumes of HCl/HBr produced

Toluene —> Nitrotoluene

Electrophilic substitution

Conc. H2SO4, conc. HNO3, 30ºC

For polysubstitution of nitro group onto toluene, how would the R&Cs change?

heat at temperatures higher than 30ºC

Toluene —> Chloromethylbenzene

Free radical substitution

(limited/excess) Cl2 in UV light

Toluene —> Benzoic acid

Oxidation

• KMnO4 in dilute H2SO4, heat under reflux

• KMnO4 in KOH, room temp.

Distinguish between benzene and toluene

Add KMnO4 in H2SO4, heat in water bath

• Toluene: decolourises purple KMnO4

• Benzene: KMnO4 remains purple

Distinguish between toluene and longer chain alkylbenzene

Add KMnO4 in H2SO4, heat in water bath

• Both decolourise purple KMnO4

• Toluene: no effervescence

• Alkylbenzene: effervescence of CO2 (g) which forms white ppt. with limewater

Observations for strong oxidation of toluene

• Purple KMnO4 decolourises

• White precipitate of benzoic acid observed

Observations for mild oxidation of toluene

• Purple KMnO4 decolourises

• Brown-black precipitate of MnO2 formed

Why does mild oxidation of toluene not give white precipitate?

Benzoic acid is neutralised to form benzoate ions in the alkaline medium

Freidel-Crafts Alkylation of Benzene

Electrophilic substitution

RX, anhydrous FeX3/AlX3, room temp

Observations for Freidel-Crafts Alkylation of Benzene

white fumes of HX produced

Benzene —> cyclohexane

Reduction

H2 gas, Ni cat, high temp and pressure