6.1.1 - Aromatics - benzene and phenol

Question 1

Benefit of benzene’s delocalised structure

Answer 1

Very stable compared to other molecules of similar size

Question 2

Define arene

Answer 2

Aromatic compound that contains a benzene ring

Question 3

Evidence for benzene structure

Answer 3

• enthalpy change of hydrogenation for benzene less exothermic than cyclohexene
• all bond lengths between carbon atoms are equal
• resistance to electrophilic addition reactions

Question 4

Why does benzene undergo electrophilic substitution instead of electrophilic addition?

Answer 4

EA would involve breaking the stable delocalised ring of electrons

Question 5

Halogenation of benzene e.g bromination

Answer 5

• ES reaction
• Br2
• FeBr3 catalyst (acts as halogen carrier)
• room temperature

Question 6

Nitration of benzene

Answer 6

• ES reaction
• heat
• concentrated HNO3
• concentrated H2SO4 catalyst
• 55°C for monosub, greater for multiple subs (must be mono for production of aromatic amines)

Question 7

Friedel-Crafts Acylation

Answer 7

• AlCl3 catalyst
• produces phenylketone

Question 8

Reaction of benzene with bromine water

Answer 8

No reaction due to delocalised electron density of pi system - relatively stable so resistant to bromination

Question 9

Properties of phenol

Answer 9

• aromatic alcohol
• weak acid (neutralised with NaOH but will not react with carbonates)

Question 10

Reaction of phenol with bromine water and why

Answer 10

• produces 2,4,6-tribromophenol
• white precipitate
• decolourises bromine water
• why: lone pair on O atom increases electron density of ring

Question 11

Directing groups

Answer 11

2-4 (and 6) directing - electron donating e.g NH2, OH
3 directing - e.g NO2

Question 12

Why is phenol more reactive than benzene (and benzoic acid)?

Answer 12

• lone pair on O delocalised into pi system
• donates electron density into the ring, increasing its electron density = more susceptible to electrophilic attack/attracts electrophiles more strongly
• COOH group is electron withdrawing

Question 13

Bromination of phenol

Answer 13

Br2 (aq), room temp

Question 14

Nitration of phenol

Answer 14

dilute HNO3, room temp
produces a mixture of 2 and 4-nitrophenol

Question 15

Explain the relative resistance to bromination of benzene compared to phenol and cyclohexene

Answer 15

• benzene: pi bonds/electrons are delocalised
• phenol: LP on oxygen delocalised into ring
• cyclohexene: electrons are localised
• benzene has a lower electron density
• benzene cannot polarise Br2

Question 16

Test to distinguish between phenol and carboxylic acid

Answer 16

Na2CO3
- carboxylic acid produces CO2
- phenol does not react

Question 17

Describe similarities and differences between the Kekulé and delocalised model of benzene

Answer 17

Similarities:
- p-orbitals overlap
- pi bonds above and below bonding C atoms

Differences:
- Kekulé has 3 pi bonds whereas delocalised model has pi ring system

Question 18

Suggest ways of processing waste polymers

Answer 18

• landfill
• recycling
• use as an organic feedstock
• combustion for energy production

Question 19

Define electrophile

Answer 19

electron pair acceptor

Question 20

Explain what is meant by ‘delocalised pi bond electrons’

Answer 20

electrons are spread over more than 2 atoms formed by overlap of p-orbitals

Question 21

Describe the bonding and structure of a benzene molecule with a diagram

Answer 21

• p-orbitals overlap above and below ring to form pi bonds
• electrons delocalised
• carbon-carbon bonds all equal lengths
• bond angles = 120°
• sigma bonds between C-C/C-H

Question 22

Compare the different reactivities of benzene and alkenes towards e.g chlorine

Answer 22

• benzene: delocalised ring of electrons and pi bonds
• alkenes: electrons concentrated between 2 carbons
• electrophiles attracted to greater electron density in alkenes so alkenes more reactive