6.1.1: Aromatic compounds

Question 1

Arenes

Answer 1

Aromatic compounds; have a structure based upon C6H6. Simplest arene is benzene.

Question 2

Why are the electrons in benzene delocalised?

Answer 2

• π bonds overlap in ring –> occupy a greater volume than in alkenes
• e- are more spread out, ∴ delocalised

Question 3

Evidence for the delocalisation of benzene

Answer 3

• Hydrogenation energies are less exothermic than expected (should be 2 x -120 but is only -208kJmol-1)
• Benzene’s lack of reactivity: does not decolourise bromine or undergo electrophilic addition reactions
• Carbon-carbon bond lengths are equal in the delocalised system (hence perfect hexagon)

Question 4

Bond angles in benzene

Answer 4

120°

Question 5

Describe how the delocalised system of benzene arises

Answer 5

Side on overlap of atomic p orbitals leads to the formation of a “halo” of electron density above and below the sigma bonds in the plane.

Question 6

Why is a halogen carrier catalyst required in the halogenation of benzene?

Answer 6

• Benzene has delocalised e-
• Alkenes have e- contained in a pi bond
• These pi bond electrons are localised, leading to high electron density –> can induce a dipole without requiring a catalyst.

Question 7

Example of a halogen carrier (Friedel Crafts) catalyst

Answer 7

AlCl₃

Question 8

Ring activator

Answer 8

Donates e- density into the ring

Question 9

Ring activator example

Answer 9

OH in phenol

Question 10

Ring deactivator

Answer 10

Withdraws e- density from the ring

Question 11

Ring deactivator example

Answer 11

NO₂

Question 12

Which groups have a 2 and 4 directing effect?

Answer 12

Ring activators e.g. OH and NH₂

Question 13

Which groups have a 3 directing effect?

Answer 13

Ring deactivators e.g. NO₂

Question 14

Phenol undergoes electrophilic substitution with bromine to give

Answer 14

2,4,6 - tribromophenol

Question 15

Phenol undergoes electrophilic substitution with dilute nitric acid to form

Answer 15

2 - nitrophenol

Question 16

Reagents needed for the nitration of benzene

Answer 16

conc H₂SO₄
conc HNO₃
50°C

Question 17

What is the electrophile formed in the nitration of benzene?

Answer 17

NO₂⁺

Question 18

Equation for electrophile production (nitration of benzene)

Answer 18

HNO₃ + H₂SO₄ ⟶ NO₂⁺ + HSO₄⁻ + H₂O

Question 19

Reagents needed for nitration of phenol

Answer 19

just dilute nitric acid

Question 20

What are the conditions for Friedel-Crafts alkylation?

Answer 20

• Reflux

Question 21

What are the reagents for Friedel-Crafts alkylation?

Answer 21

• Benzene
• Haloalkane
• Halogen carrier catalyst

Question 22

Why is Fridel-Crafts alkylation useful?

Answer 22

for the formation of C-C bonds; can increase the length of the carbon chain

Question 23

What are the reagents needed for Friedel-Crafts acylation?

Answer 23

• Benzene
• Acyl chloride
• Halogen carrier catalyst

Question 24

What are the conditions needed for Friedel-Crafts acylation?

Answer 24

• Reflux

Question 25

Phenol is a weak acid (evidence)

Answer 25

It neutralises NaOH but does not react with carbonates

Question 26

Evidence that phenol is more reactive than benzene

Answer 26

• Bromine and nitric acid react more readily with phenol than benzene
⟶ Phenol is nitrated with dilute nitric acid/Conc nitric and sulphuric acid are required for the nitration of benzene
⟶ No halogen carrier catalyst required for bromination of phenol

Question 27

Reason why phenol is more reactive than benzene

Answer 27

• Increased reactivity of phenol due to lone pair of e- on p orbital of O of OH group
⟶ These e- are donated into the π system of phenol
⟶ Increases e- density of ring in phenol –> better attracts electrophiles than benzene
• E- density of phenol = sufficient to polarise bromine, hence no halogen carrier catalyst required.