6.1.1 Aromatic Compounds

Question 1

what is the Kekulé model of benzene?

Answer 1

• 3 double bonds between carbon atoms

Question 2

what is the delocalised model of benzene?

Answer 2

• each carbon bonded to 2 other carbon atoms and one hydrogen atom ( 3 sigma bonds)
• this forms a trigonal planar shape around each carbon (hexagonal planar)
• 4th electron on each carbon above and below the plane of the ring
• these p-orbitals can overlap sideways to form a delocalised pi bonding system
• the 6 pi electrons now spread out above and below the plane of the ring and are delocalised (so less repulsion = more stable benzene)

Question 3

what are the experimental evidence for a delocalised rather than Kekulé model?

Answer 3

Bromine water
- doesn’t react like alkenes and doesn’t decolourise bromine water
Bond lengths
- double bonds are stronger and shorter than single bonds but in benzene all C-C bond lengths are equal
Enthalpy of hydrogenation
- is less exothermic than expected suggesting benzene is more stable than Kekulé’s structure suggests

Question 4

what is the mechanism of benzene reactions?

Answer 4

• electrophilic substitution

Question 5

explain electrophilic substitution of benzene in words

Answer 5

• electrophile is attracted to the electron rich pi system
• electrophile accepts a pair of pi electrons from the ring
• unstable intermediate formed
• H+ leaves, putting 2 electrons back into the ring

Question 6

draw the general mechanism for electrophilic substitution of benzene

Answer 6

see notes

Question 7

what are the conditions for the nitration of benzene?

Answer 7

• conc nitric acid in the presence of conc sulphuric acid catalyst
• reflux at around 50C

Question 8

what happens at higher temperatures?

Answer 8

• further substitution

Question 9

draw the mechanism for the nitration of benzene

Answer 9

see notes

Question 10

what is the equation for the generation of the electrophile for nitration of benzene?

Answer 10

HNO3 + H2SO4 -> HSO4- + NO2+ + H2O

Question 11

what is the equation for the regeneration of the catalyst for the nitration of benzene?

Answer 11

HSO4- + H+ -> H2SO4

Question 12

What are the conditions for the halogenation of benzene?

Answer 12

• halogen in the presence of a halogen carrier catalyst

Question 13

What are examples of halogen carrier catalysts?

Answer 13

• aluminium chloride
• iron chloride
• iron metal

Question 14

Draw the mechanism for the halogenation of benzene

Answer 14

See notes

Question 15

What is the equation for the regeneration of the catalyst?

Answer 15

• AlCl4- + H+ -> AlCl3 + HCl

Question 16

What is the equation for the catalyst generating the electrophile?

Answer 16

• Cl2 + AlCl3 -> AlCl4- + Cl+

Question 17

What are the conditions for the alkylation of benzene? (Friedel-Crafts)

Answer 17

• halogen is replaced by an alkyl group
• a haloalkane in the presence of a halogen carrier catalyst

Question 18

What is the mechanism for the alkylation of benzene?

Answer 18

See notes

Question 19

What are the conditions for acylation of benzene?

Answer 19

• hydrogen is replaced by acyl group
• acyl chloride in the presence of a halogen carrier catalyst
• heating at reflux
• anhydrous conditions

Question 20

What is the mechanism for the acylation of benzene

Answer 20

• see notes

Question 21

What is an acyl group?

Answer 21

R - C = O

Question 22

What are halogen carrier catalysts used for?

Answer 22

• takes a hydrogen atom from the reagent and forms a covalent bond to the halogen
• the H+ ion is then used to regenerate the catalyst
• the catalyst generates an electrophile

Question 23

Why are alkenes more reactive than benzene?

Answer 23

• the electrons are localised in the double bond (benzene has the delocalised π bonding system)
• higher electron density (lower electron density)
• polarises the electron more
• electrophile is more strongly attracted
• alkenes are more reactive
• by reacting with electrophilic substitution benzene retains the delocalised π bonding system

Question 24

Will benzene decolourise bromine?

Answer 24

• no reaction
• needs halogen carrier catalyst
• less reactive than alkenes

Question 25

What is the structure of phenol?

Answer 25

• OH is directly bonded to the benzene ring

Question 26

What are the physical properties of phenol?

Answer 26

• higher bpt than benzene due to hydrogen bonds
• more soluble than benzene in water due to hydrogen bonds than can form between phenol and water molecules

Question 27

How does phenol act as a weak acid?

Answer 27

• phenol partially disassociates to phenoxide, releasing hydrogen ions

Question 28

Draw the reaction of phenol with sodium

Answer 28

See notes

Question 29

How can you identify phenol?

Answer 29

• carboxylic acids can react with NaOH and NaCO3
• phenol can only react with NaOH
• phenol is too weak of an acid to react with carbonates

Question 30

What are the conditions for phenol reacting with bromine?

Answer 30

• bromine water (no catalyst)
• decolourises bromine water and white precipitate forms - visible product

Question 31

Draw the reaction of benzene with phenol

Answer 31

See notes

Question 32

What are the conditions for the reaction of phenol with nitric acid?

Answer 32

• dilute nitric acid (no catalyst)
• further substitution can occur with concentrated nitric acid

Question 33

Draw the reaction of phenol with nitric acid

Answer 33

See notes

Question 34

Why is phenol more reactive than benzene?

Answer 34

• there is a lone pair of electrons in a p orbital on the oxygen atom
• this is able to overlap with the delocalised π bonding system
• the lone pair is drawn into the ring which leads to a higher electron density in the ring
• the electrophile is attracted to phenol and polarised more strongly

Question 35

What are the directing effects of OH, NH2 and NO2?

Answer 35

• OH and NH2 are 2, 4 directing (electron donating groups)
• NO2 is 3 directing (electron withdrawing group)