25 Aromatic Compounds

Question 1

What is the Kekule model?

Answer 1

Suggested structure of benzene, based on 6 carbon ring with alternate double and single bonds.

Question 2

What is the evidence used to disprove the Kekule model?

Answer 2

1. Lack of reactivity of benzene e.g. does not decolourise bromine water under normal conditions.
2. All carbon-carbon bonds are the same length.
3. Has a lower hydrogenation enthalpy so it is more stable than predicted.

Question 3

Describe the delocalised model of benzene.

Answer 3

Each carbon has 1 electron in p orbital perpendicular to C and H atoms.

Adjacent orbitals overlap above and below.

Forms system of π bonds in which electrons are delocalised.

Question 4

Exceptions in naming aromatic compounds.

Answer 4

Phenylamine, benzoic acid and benzaldehyde.

Question 5

What are the electrophilic substitution reactions of benzene?

Answer 5

Nitration, halogenation, alkylation and acylation.

Question 6

Alkene vs arene reactivity.

Answer 6

Alkene decolourises bromine by electrophilic addition.

Benzene can only decolourise bromine in presence of halogen carrier, due to insufficient π electron density to polarise bromine molecule.

Question 7

How is the weak acid nature of phenols seen?

Answer 7

Reacts with solutions of strong bases, cannot react with weak bases like carbonates.

Question 8

Why are phenols react more readily with bromine or nitric acid compared to benzene?

Answer 8

Lone pair of electrons from OH- donated to pi system, which increases the electron density.

Question 9

Which directing groups are activating?

Answer 9

2- and 4-

Question 10

Which directing group is deactivating?

Answer 10

3

Question 11

What does it mean when a group is ‘activating’?

Answer 11

Causes aromatic ring to react more readily with electrophiles.