13 Arenes

Question 1

Benzene

Physical Properties

Answer 1

1. Colourless liquid with a characteristic odour
2. Non-polar compound
   a. Insoluble in H2O, soluble in organic solvents
3. Less dense than water
4. Melting point, m.p of 5.5 °C; boiling point, b.p of 80 °C
5. High C: H ratio

Question 2

Reactions of Benzene

Combustion

Answer 2

Reagent(s): Excess O₂
Condition(s): Heat

Product(s): CO₂, H₂O

Characteristic sooty & smoky flame

Question 3

Reactions of Benzene

Reduction, [R]

Answer 3

Reagent(s): H₂, Ni as catalyst
Condition(s): High temperature & pressure

Product(s): Cyclohexane

Question 4

Reactions of Benzene

Nitration, NO₂

Answer 4

Mechanism: Electrophilic Substitution between Bronsted-Lowry acid & base

Reagent(s): Conc. HNO₃, conc. H₂SO₄
Condition(s): 55-60°C

Product(s): Nitrobenzene

Polynitration requires harsher conditions and longer time periods.

Question 5

Bronsted-Lowry Acid & Base

Definition

Answer 5

Acid: H⁺ donor
Base: H⁺ acceptor

Question 6

Reactions of Benzene

Halogenation, X₂

Answer 6

Mechanism: Electrophilic Substitution between Lewis acid catalyst

Reagent(s): X₂, FeX₃/ anhydrous AlX₃/ Fe as catalyst
Condition(s): Room temperature

Product(s): Halobenzene

Question 7

Lewis Acid & Base

Definition

Answer 7

Acid: e^- pair acceptor
Base: e^- pair donor

Question 8

Reactions of Benzene

Friedel-Crafts Alkylation

Answer 8

Mechanism: Electrophilic Substitution with Lewis Acid catalyst

Reagent(s): RCl, anhydrous AlCl₃ as catalyst
Condition(s): Room temperature

Product(s): Alkylbenzene

Question 9

Reactions of Benzene

Friedel-Crafts Acylation

Answer 9

Mechanism: Electrophilic Substitution with Lewis Acid catalyst

Reagent(s): CH3COCL, AlCl3 as catalyst
Condition(s): Room temperature

Product(s): Acylbenzene

Question 10

Determining the relative reactivity of Electrophilic Substitution

Answer 10

1. e^- donating/ withdrawing group?
2. e^- density within the Benzene ring increased/ decreased?
3. dispersal/ intensification of positive charge in the carbocation intermediate?
4. Benzene ring more/ less reactive than benzene alone?
5. Group is activating/ deactivating

Question 11

e^- donating/ withdrawing groups

Factors

Answer 11

1. Inductive Effect
2. Resonance Effect

Question 12

Inductive Effect

Definition

Answer 12

Donation/ withdrawal of electrons through σ bonds due to electronegativity difference between atoms

Question 13

Resonance Effect

Definition

Answer 13

Donation/ withdrawal of electrons through π bonds due to continuous side-on overlap of the substituent group and benzene ring, resulting in delocalisation of electrons towards/ away from the benzene ring

Question 14

Methylbenzene

Physical Properties

Answer 14

1. Colourless liquid
2. Non-polar compound
   a. Insoluble in H₂O but soluble in organic solvents
3. m.p of -90°C, b.p of 111°C
4. Relatively high C:H ratio

Question 15

Reactions of Alkylbenzenes

Halogenation, X₂ of the Alkyl side chain

Answer 15

Mechanism: Free Radical Substitution

Reagent(s): X₂
Condition(s): UV Light/ Heat

Product(s): (Haloalkyl) benzene

Limited X₂ for monosubstitution
Excess X₂ for polysubstitution

Question 16

Reactions of Methylbenzene

Strong Oxidation, [O], In Acidic Medium

Answer 16

Reagent(s): KMnO₄ (aq), H₂SO₄ (aq)
Condition(s): Heat (under reflux)

Product(s): Benzoic acid

K₂Cr₂O₇ (aq), H₂SO₄ (aq) & heat has no reaction at ‘A’ Level

Question 17

Reactions of Methylbenzene

Strong Oxidation, [O], In Alkaline Medium

Answer 17

Reagent(s): KMnO₄ (aq), NaOH (aq)
Condition(s): Heat (under reflux)

Product(s): (Cation) Benzoate & Benzoic Acid upon acidificaton with H₂SO₄ and cooling

Question 18

Reactions of Alkylbenzenes

Oxidation, [O], of longer alkyl side chains

Answer 18

Reagent(s): KMnO₄ (aq), H₂SO₄ (aq)
Condition(s): Heat (under reflux)

Product(s): Benzoic acid, CO₂ and H₂O

Question 19

Reactions of Alkylbenzene

Criteria for oxidation, [O], of side chains

Answer 19

H/ O bonded to the C bonded to the benzene ring i.e benzylic H/ O bonded to benzylic C