4.2 Aromaticity

Question 1

What does benzene react with and why?

Answer 1

• electrophiles
• it is an electron rich species and very stable

Question 2

Name the 4 types of reactions of benzene

Answer 2

1. Nitration
2. Halogenation
3. Friedel-Crafts alkylation
4. Friedel-Crafts acylation

Question 3

Draw the mechanism for electrophilic substitution of benzene

Question 4

Draw the nitration of benzene and give the conditions needed

Answer 4

• concentrated acids
• 50-60°C

Question 5

Draw the halogenation of benzene and give its catalyst and related equation

Answer 5

AlCl3
Cl2 + AlCl3 —> Cl+ + AlCl4-

Question 6

Draw Friedel-Crafts alkylation of benzene giving the catalyst and related equation

Answer 6

AlCl3
CH3Cl + AlCl3 —> CH3+ + AlCl4-

Question 7

Draw Friedel-Crafts acylation of benzene giving the catalyst and related equation

Answer 7

AlCl3
ethanoyl chloride + AlCl3 —> ethanoyl carbocation + AlCl4-

Question 8

Describe the structure and binding of benzene

Answer 8

• planar molecule
• satisfies laws of valency
• kekule structure
• alternating double and single bonds

Question 9

What are some of the issues with Kekules structure?

Answer 9

• lack of reactivity for an alkene
  —> does not decolourise bromine due to delocalised electrons making it unable to induce dipoles in bromine
• thermal dynamic stability
  —> hydrogenation of benzene is endothermic - more stable than you’d expect due to the delocalised electrons
• bond lengths in benzene
  —> length of single and double bonds differ yet it still forms a perfect hexagon due to the moving bonds

Question 10

Demonstrate the bonding in benzene diagrammatically

Question 11

Draw napthalene

Question 12

Draw anthracene

Question 13

Draw pyridine

Question 14

Draw benzene carboxylic acid and benzaldehyde

Question 15

Draw chlorobenzene, methyl benzene and nitrobenzene

Question 16

Draw phenol, phenylamine and phenylethanone

Question 17

How does the halogen carrier enable electrophilic substitution of benzene to take place?

Answer 17

Induces a dipole on Cl2

Question 18

Compare the different reactivities of benzene and alkenes toward chlorine

Answer 18

• in benzene, pi electrons are delocalised
• in alkenes, pi electrons are localised
• electrophiles are attracted to the greater electron density in alkenes

Question 19

How does the OH- group impact the reactivity of the benzene ring in phenol?

Answer 19

• lone pair from O is delocalised into the ring
• benzene ring in phenol activated
• attracts electrophiles

Question 20

What is meant by the term delocalised pi bond electrons

Answer 20

• electrons already over more than 2 atoms
• formed by overlap of p-orbitals

Question 21

Two types of important material whose manufacture involves nitration of benzene

Answer 21

Dyes & pharmaceuticals

Question 22

State the conditions required for nitration of benzene

Answer 22

Nitric acid and sulphuric acid (both conc)
50-60°C

Question 24

Explain the relative resistance to bromination of benzene compared to phenol and cyclohexene

Answer 24

• benzene electrons are delocalised
• OH group is partially delocalised into the ring
• cyclohexene electrons are localised
• benzene has a lower electron density
• benzene cannot polarise or induce a dipole in Br2 but cyclohexene and phenol can