18A: Arenes - benzene

Question 1

What are the 2 models that can be used to represent the bonding in benzene?

Answer 1

Delocalised model or Kekulé model:

Question 2

Describe the bonding of benzene in the delocalised model.

Answer 2

The p-orbitals on each carbon overlap to form 2 rings of delocalised π bonds above and below the carbon ring.

Question 3

Explain how data from enthalpy changes of hydrogenation provides evidence for the delocalised model of the bonding in benzene.

Answer 3

The enthalpy change of hydrogenation for benzene is less negative than would be expected for a structure with 3 double bonds,
so benzene must be more stable.

Question 4

Explain why benzene is resistant to bromination, compared with alkenes.

Answer 4

In alkenes, electron density is localised.
In benzene, π-bonds are delocalised,
so it cannot as easily induce a dipole in the bromine or be attacked electrophilically

Question 5

Reaction of benzene with oxygen in air:
Observation.

Answer 5

Smoky flame

Question 6

Reaction of benzene with bromine:
Catalyst,
Product,
Mechanism

Answer 6

FeBr₃ or Fe,
Bromobenzene,
Electrophilic substitution.

Question 7

Describe the reaction:
Benzene -> Bromobenzene

Answer 7

React with bromine and FeBr₃ or Fe catalyst,
Electrophilic substitution.

Question 8

Reaction of benzene with concentrated nitric acid:
Catalyst,
Conditions,
Product,
Mechanism.

Answer 8

Concentrated sulfuric acid,
50°C,
Nitrobenzene,
Electrophilic substitution.

Question 9

Describe the reaction:
Benzene -> Nitrobenzene

Answer 9

React with a mixture of concentrated nitric and sulfuric acids,
Temp = 50°C,
Electrophilic substitution.

Question 10

Reaction of benzene with halogenoalkanes:
Catalyst,
Product,
Mechanism,
(Name of reaction)

Answer 10

Aluminium chloride,
Alkyl benzene,
Electrophilic substitution,
(Friedel-Crafts reaction)

Question 11

Describe the reaction:
Benzene -> Alkyl benzene

Answer 11

(Friedel-Crafts reaction)

React with halogenoalkane and AlCl₃ catalyst,
Electrophilic substitution.

Question 12

Reaction of benzene with acyl chlorides:
Catalyst,
Product,
Mechanism,
(Name of reaction)

Answer 12

Aluminium chloride,
Acyl benzene,
Electrophilic substitution,
(Friedel-Crafts reaction)

Question 13

Describe the reaction:
Benzene -> Acyl benzene

Answer 13

(Friedel-Crafts reaction)

React with acyl chloride and AlCl₃ catalyst,
Electrophilic substitution.

Question 14

State the three steps for the mechanism of the electrophilic substitution reactions of benzene.

Answer 14

Step 1: Generation of the electrophile

Step 2: Electrophilic substitution

Step 3: Regeneration of the catalyst

Question 15

Reaction of phenol with bromine water:
Product,
Mechanism,
Observations.

Answer 15

Tribromophenol,
Electrophilic substitution,
Colour change orange -> colourless,
White precipitate.

Question 16

Describe the reaction:
Phenol -> Tribromophenol

Answer 16

React with bromine water,
Electrophilic substitution,
Colour change orange -> colourless,
White precipitate forms.

Question 17

Explain the relative ease of bromination of phenol, compared to benzene.

Answer 17

The lone pair on the oxygen is partially delocalised in the ring,
so there is more electron density,
so phenol is more susceptible to electrophilic attack.