MODULE 6: Aromatic Chemistry

Question 1

What are the physical properties of benzene?

Answer 1

Colourless, sweet-smelling, flammable, carcinogenic

Question 2

What is the Kekule model of the benzene ring?

Answer 2

The theory that aromatic rings are carbon rings with alternating double bonds

Question 3

List some evidence to disprove Kekule’s model

Answer 3

• Lack of reactivity: does not undergo electrophilic addition, does not decolourise bromine in normal conditions.
• Different length of benzene C-C bonds to regular C-C bonds and C=C bonds
• Hydrogenation enthalpies: less energy is produced from hydrogenation of benzene that predicted from the Kekule model

Question 4

Name the features of the delocalised model of the benzene ring

Answer 4

• planar and cyclical
• each carbon atom has one electron in a p-orbital at right angles to the plane of the ring
• adjacent p-orbitals overlap sideways to form a ring of electron density
• this creates a system of pi-bonds which spread over all six carbon atoms
• six electrons in the pi-bonds are said to be delocalised

Question 5

How do you name alkyl, halogen or nitro substituents in benzene?

Answer 5

Using the group prefix before benzene

e.g. ethylbenzene

Question 6

How do you name alkyl chain substitute in benzene?

Answer 6

Using ‘phenyl’ as the prefix for benzene

e.g. phenylethanone

Question 7

How does benzene react?

Answer 7

Electrophilic substitution

Question 8

How does benzene undergo nitration?

Answer 8

Benzene reacts with nitric acid (catalysed by sulphuric acid) at 50oC to under electrophilic substitution

Question 9

State the equation to form the electrophile

Answer 9

HNO₃ + H₂SO₄ –> NO₂⁺ + HSO₄⁻ + H₂O

Forms a nitronium ion

Question 10

State the equation to form the nitrobenzene

Answer 10

Benzene + nitronium ion –> intermediate –> nitrobenzene + hydrogen ion

Question 11

What product is formed in the nitration reaction is raised above 50oC?

Answer 11

Formation of a dinitrobenzene

Question 12

What happens at the end of nitration?

Answer 12

Reforming of the catalyst

Question 13

What is required for a halogen to react with benzene?

Answer 13

Halogen carrier (FeBr₃, FeCl₃, AlBr₃, AlCl₃)

Question 14

State the process for the halogenation of benzene

Answer 14

Step one: formation of the halogen ion
X₂ + FeX₃ --> FeX₄⁻ + X⁺
Step two: electrophilic substitution
benzene + X⁺ -> intermediate -> halobenzene + hydrogen ion
Step three: reformation of catalyst
H⁺ + FeX₄⁻ --> FeX₃ + HX

Question 15

What is alkylation?

Answer 15

When halogen carriers acts as a catalyst to substitute a hydrogen atom with the alkyl group, which is also very similar to the reaction with acyl chloride

Question 16

What are phenols?

Answer 16

Benzene rings with -OH groups

Question 17

Compare alcohols, phenols and carboxylic acids

Answer 17

Phenol is more acidic than alcohol, but less acidic than carboxylic acids, hence phenols react with strong bases, carboxylic acids react with weak and strong bases and ethanol does not react with any bases.

Question 18

How does phenol react with sodium hydroxide?

Answer 18

Forms sodium phenoxide and water

Question 19

What are the electrophilic substitution reactions of phenols?

Answer 19

Bromination and nitration

Question 20

Describe the bromination of phenol

Answer 20

Bromine water is added to water to form a white precipitate, and undergoes a 2,4,6 substitution- and does not require a halogen carrier

Question 21

Describe the nitration of phenol

Answer 21

Phenol reacts with nitric acid at room temperature to form a mixture of 2 and 4 substituted nitrophenol

Question 22

Compare the reactivity of phenol and benzene

Answer 22

• nitric acid and bromine react more readily with phenol

- phenol is more reactive due to lone pair on -OH group, which increases the electron density

Question 23

How do -NH₂ and -NO₂ effect the position of substitution?

Answer 23

• NH₂ = positions 2 or 4

- NO₂ = position 3