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

R&C: Excess O₂, heat

Observations: Sooty & smoky flame

Question 3

Reactions of Benzene

Reduction, [R] to form Cyclohexane

Answer 3

R&C: H₂, Ni as catalyst, high temperature & pressure

Question 4

Reactions of Benzene

Nitration (-NO₂) to form Nitrobenzene

Answer 4

R&C: conc. HNO₃, conc. H₂SO₄, 55-60°C

Mechanism: Electrophilic Substitution (ES) btwn BLA & BLB

where BLA/ B = Bronsted-Lowry acid/ base

Harsher conditions & longer time period for polynitration!

Question 5

Reactions of Benzene

Halogenation (X₂) to form Halobenzene

Answer 5

R&C:
* X₂, FeX₃, room temp.
* X₂, Anhydrous AlX₃, room. temp.
* X₂, Fe, room temp.

Mechanism: ES with LAC

where X = Cl, Br & LAC = Lewis acid catalyst

Question 6

Reactions of Benzene

Friedel-Crafts Alkylation to form Alkylbenzene

Answer 6

R&C: RCl, anhydrous AlCl₃, room temp.

Mechanism: ES with LAC

where LAC = Lewis acid catalyst

Question 7

Reactions of Benzene

Friedel-Crafts Acylation to form Acylbenzene

Answer 7

R&C: RCOCl, AlCl₃, room temp.

Mechanism: ES with LAC

where LAC = Lewis acid catalyst

Question 8

Determining the relative reactivity of ES

Answer 8

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 9

e^--donating groups (EDG)/ e^--withdrawing groups (EWG)

Factors

Answer 9

1. Inductive Effect
2. Resonance Effect

Question 10

Inductive Effect

Definition

Answer 10

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

Question 11

Resonance Effect

Definition

Answer 11

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 12

Methylbenzene

Physical Properties

Answer 12

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 13

Reactions of Methylbenzene

Strong [O] to form Benzoic Acid/ Sodium Benzoate

Answer 13

Acidic Medium
R&C: KMnO_4(aq), H₂SO_4(aq), heat (under reflux)

Alkaline Medium
R&C: KMnO_4(aq), NaOH_(aq), heat (under reflux)

1. For acidic medium, NO rxn at ‘A’ Levels: K₂Cr₂O_7(aq), H₂SO_4(aq), heat
2. For alkaline medium, addition of H⁺ eg. through H₂SO₄, Benzoic Acid is formed.

Question 14

Reactions of Alkylbenzenes

Halogenation to form (RX)Benzene

Answer 14

Mono-sub
R&C: Limited X₂, UV light/ heat
Poly-sub
R&C: Excess X₂, UV light/ heat

Mechanism: FRS

where X = Cl, Br, I

Question 15

Reactions of Alkylbenzenes

Side Chain [O] to form Benzoic Acid &…

Answer 15

R&C(s): KMnO_4(aq), H₂SO_4(aq), heat (under reflux)

Criteria: H/ O is ALSO bonded to C bonded to Benzene ie. benzylic H/ O

CO₂ & H₂O formed as side-products