Topic 12: Arenes (Chemistry)

State the type of reaction, reagents required and conditions for the halogenation of benzene.

• Type: Electrophilic Substitution

• Reagents:

  • Cl2 (Chlorination)/ Br2 (Bromination),

  • anhydrous FeCl3/ anhydrous AlCl3/ Fe (s)

• Conditions: Room Temperature

State the type of reaction, reagents required and conditions for the Friedel-Crafts Alkylation of Benzene.

• Type: Electrophilic Substitution

• Reagents:

  • Any alkyl group bonded to a Cl, for example: CH3Cl

  • Anhydrous FeCl3/ anhydrous AlCl3

  • Room Temperature

For the halogenation of Benzene, state the observations during the chlorination and bromination of benzene respectively.

• For the chlorination of benzene, the yellowish green Cl2 decolourises into white fumes of HCl.

• For the bromination of benzene, reddish brown Br2 decolourises into white fumes of HBr.

For the Friedel-Crafts Alkylation of Benzene, state the observations during the chlorination and bromination of benzene respectively.

White fumes of HCl is evolved.

State the type of reaction, reagents required and conditions for the Nitration of Benzene.

• Type: Electrophilic Substitution

• Reagents: Concentrated H2SO4 and concentrated HNO3

• Conditions: Heat under reflux at 55 degree celsius

For the nitration of Benzene, state the observation.

Yellow nitrobenzene oil is formed.

State what is the Lewis Acid and catalyst in the Halogenation of Benzene, as well as why it is considered a Lewis Acid.

• Lewis Acid and catalyst is AlCl3, because it accepts an electron pair.

State what is the Bronsted Lowry Acid and catalyst in the Nitration of Benzene, as well as why it is considered a Bronsted Lowry acid.

• H2SO4 is the Bronsted Lowry acid and catalyst.

• It is considered so because it donates a proton.

State the type of reaction, reagents required and conditions for the Nitration of METHYLBENZENE.

• Type: Electrophilic Substitution

• Reagents: Concentrated H2SO4, concentrated HNO3

• Conditions: 30 degree celsius

State the difference in the nitration of METHYLBENZENE and BENZENE.

• Nitration of methylbenzene requires temperature of 30 degree celsius, WITHOUT heating under reflux.

• Nitration of benzene requires temperature of 55 degree celsius, while heating under reflux.

State the type of reaction, reagents required and conditions for the Halogenation of METHYLBENZENE.

• Type: Electrophilic Substitution

• Reagents:

  • Chlorination: limited Cl2, anhydrous FeCl3/ anhydrous AlCl3 or Fe (s)

  • Bromination: limited Br2, anhydrous FeCl3/anhydrous AlCl3/ Fe (s)

• Conditions: Absence of UV Light/ in a dark room at Room Temperature.

State the observations from the Halogenation of METHYLBENZENE.

• Chlorination: Yellowish green Cl2 decolourises to give white fumes of HCl.

• Bromination: Reddish brown Br2 decolourises to give white fumes of HCl

State the type of reaction, reagents required and conditions for the Free Radical Substitution of METHYLBENZENE’s SIDECHAIN.

• Type: Free Radical Substitution

• Reagents:

  • Chlorination: limited Cl2 (g)

  • Bromination: limited Br2 (l)

• Conditions: UV Light/ Heat

State the type of reaction, reagents required and conditions for the ACIDIC Oxidation of METHYLBENZENE’s SIDECHAIN.

• Type: oxidation

• Reagents: KMnO4 (aq), H2SO4 (aq)

• Conditions: Heat under reflux

How can methylbenzene and benzene be distinguished from one another?

• Only methylbenzene will undergo oxidation. Hence, purple KMnO4 will decolourise, and white precipitate of Benzoic Acid forms.

• Benzene will not undergo oxidation. Purple KMnO4 does NOT decolourise.

State the type of reaction, reagents required and conditions for the ALKALINE Oxidation of METHYLBENZENE’s SIDECHAIN.

• Type: Oxidation

• Reagents: KMnO4 (aq), NaOH (aq)

• Conditions: Heat under reflux

State the observations from the acidic oxidation of METHYLBENZENE’s sidechain.

• Purple KMnO4 decolourises.

• White precipitate of benzoic acid formed.

State the observations from the alkaline oxidation of METHYLBENZENE’s sidechain.

• Purple KMnO4 decolourises.

• Brown precipitate of MnO2 formed.

How does an electron donating substituent affect the reactivity of benzene towards electrophiles?

• Electron donating substituent increases ELECTRON DENSITY in the Benzene Ring.

• This will make it more susceptible towards an attack from electrophiles because electrophiles are electron deficient.

• Thus, electron donating substituent increases a benzene’s reactivity towards electrophiles.

How does an electron withdrawing substituent affect the reactivity of benzene towards electrophiles?

• Electron withdrawing substituent decreases ELECTRON DENSITY in the Benzene Ring.

• This will make benzene less susceptible towards an attack from electrophiles.

• Thus, electron withdrawing substituent decreases benzene’s reactivity towards electrophiles.

Why do both benzene and alkene undergo reactions with ELECTROPHILES?

• Benzene: The delocalised pi electron cloud gives rise to HIGH electron density in the benzene ring, making it highly susceptible to attack from electrophiles, which are electron deficient and lone pair acceptors.

• Alkene: Electrons in the pi bond of the alkene gives rise to HIGH electron density in the double bond, making it highly susceptible towards electrophilic attack as electrophiles are electron deficient and lone pair acceptors.

Why do benzene undergo substitution reaction and NOT addition reaction?

• The pi electron cloud gives rise to resonance stability.

• Addition reaction would destroy the aromatic ring of delocalised pi electrons, resulting in a product with no resonance stability. This loss of aromatic character is NOT energetically favoured.

• Substitution reaction allows the pi electon cloud to remain intact, thus maintaining resonance stability.