4.1.1- Arenes

Question 0

What are the three pieces of evidence against the Kekulé model for Benzene

Answer 0

1. All bond lengths or equal and intermediate between short C=C and long C-C bond lengths
2. Enthalpy change of hydrogenation is less exothermic than expected compared to 3 times that of cyclohexene
3. Resistance to attack by bromine, only reacts in the presence of a halogen carrier

Question 1

Define aromatic

Answer 1

Any compound with the benzene ring

Question 2

What are the conditions for the nitration of benzene

Answer 2

Concentrated nitric acid, concentrated sulphuric acid, 50°C

Question 3

What are the conditions for the halogenation of benzene

Answer 3

A halogen carrier eg. FeCl3/AlCl3, room temperature

Question 4

Explain the difference in reactivity between benzene and alkenes with electrophiles

Answer 4

Benzene is relatively resistant to reaction with electrophiles, compared with alkenes, because the six π-electrons are delocalised over six carbons in the ring, whereas in alkenes the two π-electrons are localised between two carbons. This means that benzene has a lower electron density, so that it attracts and polarises electrophiles less readily, so that
a catalyst is needed.

Question 5

What is a Phenol

Answer 5

A class of organic compound in which a hydroxyl group (-OH) is attached directly to a benzene ring

Question 6

What is the observation when sodium reacts with Phenol

Answer 6

Effervescence of hydrogen gas

Question 7

What are the conditions for the bromination of Phenol

Answer 7

Room temperature

Question 8

What is the product produced in the bromination of Phenol

Answer 8

2,4,6-tribromophenol

Question 9

What are the observations for the reaction of phenol with bromine

Answer 9

Bromine decolourises and white precipitate of 2,4,6-tribromophenol

Question 10

Why is phenol a weak acid

Answer 10

Phenol forms a weak acid in water by loosing H+ ions from the -OH group.

Question 11

Explain the relative ease of bromination of Phenol compared to benzene

Answer 11

A lone pair of electrons in the p-orbital of the oxygen in the phenol group is drawn into the benzene ring. This increases the electron density of the ring, which is now sufficient enough to polarise bromine molecules and attract them to the ring structure.

Question 12

State three uses of Phenols

Answer 12

1. Production of plastics
2. Antiseptics and Disinfectants
3. Epoxy resins for paints

Question 13

Write example equations for how aluminium chloride can act as a halogen carrier catalyst

Answer 13

AlCl3 + Cl2 -> AlCl4– + Cl+

H+ + AlCl4- -> AlCl3 + HCl