30 - HYDROCARBON (ARENE)

Question 1

Why arene rings undergo substitution instead of addition?

Answer 1

• addition would disrupt the pi bonding system
• aromaticity will be lost
• product will not be stable

Question 2

State the directing effects of different functional groups?

Answer 2

DONATING —- (3,5 position)

• -NO2
• -CO
• -COOH

Question 3

State the conditions and reagents to convert NO2 –> NH2

Answer 3

• reduction process

- SN, [ ] HCl

Question 4

How to ensure hydrogenation substitution occurs at side chain

Answer 4

• UV light

- reaction: free-radical substitution

Question 5

Draw C=N mechanism substitution to benzene ring(COCH3)

Answer 5

(check sticky note)

• nucleophilic substitution
• occurs to ketones/ aldehyde
• HCN/ NaCN catalyst

Question 6

State the conditions needed to convert

1) ketone –> alcohol
2) alcohol –> alkene

Answer 6

1) reduction process
- LiAlH4
2) elimination process
- Al2O3, heat
- excess [ ] H2SO4, heat at 170 deg

Question 7

Describe hydrogenation of benzene (to ring)

Answer 7

Reagents/ conditions:
- Cl2 , AlCl3 catalyst

Electrophile generated:
AlCl3 + Cl2 –> AlCl4- + Cl+

Draw out mechanism: (check notes)

Question 8

Describe nitration of benzene

Answer 8

Reagents/ conditions:

• [ ] H2SO4, [ ] HNO3
• heat at 55 deg

Electrophile:
2H2SO4 + HNO3 –> 2HSO4- + NO2+ + H3O+

Draw out mechanism: (check notes)

Question 9

Describe friedel- crafts alkylation of benzene

Answer 9

Reagents/ conditions:

• –RX (halogenoalkane) / AlCl3 catalyst
• heat at 55 deg

Electrophile:
AlCl3 + CH3CH2Cl –> AlCl4- + CH3CH2+

Draw out mechanism: (check notes)

Question 10

Describe friedel- crafts acylation of benzene

Answer 10

Reagents/ conditions:

• acyl chloride/ AlCl3 catalyst
• heat at 55 deg

Electrophile:
AlCl3 + CH3CH2COCl –> AlCl4- + CH3CH2CO+

Draw out mechanism: (check notes)

Question 11

Describe oxidation of methylbenzene

Answer 11

Reagents/conditions:

• hot/ alkaline KMnO4
• dil H2SO4

Produce:
benzoic acid/ carboxylic acid

Question 12

Describe reduction process via hydrogenation of benzene

Answer 12

Reagent/ conditions:
- H2 / Ni or Pt catalyst

Produce:
- cyclohexane

Question 13

When drawing mechanism

Answer 13

• remember curly attack for electrophilic attack
  > from ring to electrophile
• draw correct intermediate
• products: write what is lost

Question 14

Why CH3 group in benzene ring directs to 2,4 position instead of 3?

Answer 14

• 2,4 position –> tertiary carbocation
• 3 position –> secondary carbocation
• tertiary is more stable than secondary
• more donating alkyl/methyl groups attached will increase the electron density of the cation hence, increasing its stability