Aromatic compounds

Question 1

What are the 3 features of benzene that don’t support kekules model

Answer 1

Benzene is resistant to addition reactions

Enthalpy change of hydrogenation if less exothermic than expected

All carbon bond lengths are the same

Question 2

What happens to the 4th electron in the p-orbital of each carbon atom in benzene

Answer 2

It delocalises to form rings of electron density above and below the hexagon, f

Question 3

Describe the current modl of bonding in benzene

Answer 3

Each carbon creates 3 covalent sigma bonds

Remaining electrons is found in a p orbital above and below the structure

P-orbitals sideways overlap evenly create a ring of delocalised pi-electrons above and below the structure

Question 4

Describe in terms of overlap, the similarities and differences between the bonding in the kekule model and the delocalised model

Answer 4

Similarities: Sideways overlap of p-orbitals
Pi ring above and below the molecule

Differences: Kekule has 3 alternating Pi bonds each with 2 delocalised
Delocalised model has a single Pi ring system containing 6 delocalised electrons

Question 5

What type of reaction is the nitration of benzene and what are conditions/reagents needed

Answer 5

Electrophilic substitution
[Nitric acid] + [Sulphuric acid]
50-55 degree c

Question 6

How is No2+ ion formed

Answer 6

H2SO4 + HN03 —-. H2NO3 + HSO4
H2NO3 —> H20 + NO2

Question 7

How is the catalyst reformed in the nitration of benzene

Answer 7

HS04- + H+ —> H2SO4

Question 8

What type of reaction is the halogenation of benzene and what are conditions/reagents needed

Answer 8

Electrophilic substitution
Reflux under heat
Halogen + Halogen carrier

Question 9

How is the halogen carrier formed in the halogenation of benzene

Answer 9

AlX3 + x2 —> AlX4- + x+

Question 10

How is the halogen carrier reformed

Answer 10

AlX4- + H+ —> HX + AlX3

Question 11

What type of reaction is the alkylation of benzene and what are conditions/reagents needed

Answer 11

Electrophilic addition
A halogenalkane and anhydrous AlCl3
RTP. dry inert solvent

Question 12

How is the electrophile formed in the alkylation of benzene

Answer 12

RX + AlX3 —> R+ + AlX4

Question 13

What type of reaction is the acylation of benzene and what are conditions/reagents needed

Answer 13

Electrophilic substitution
Acyl chloride, AlCl3
Reflux, 50 degree c

Question 14

How is the electrophile formed in the alkylation of benzene

Answer 14

RCOCl + AlCl3 —> RCO+ + AlCl4-

Question 15

What is the relative ease of electrophilic substitution of phenol compared with benzene, why?

Answer 15

It is easier for electrophilic addition to occur in phenol because: The OH group are partially delocalised into the ring therefore, increasing electron density of the ring
Making phenol more susceptible to electrophile attacks

Question 16

Why do alkanes have a different reactivity to benzene and phenols

Answer 16

The Pi bond in alkanes has localised electrons which provide a high electron density above and below the molecule
This can induce dipoles in other molecules

Question 17

Why does benzene have a low reactivity compared to alkanes and phenol

Answer 17

the Pi electrons are spread over the ring structure, forming an area of delocalised electrons, lowering electron density to increase stability
Cant induce dipoles in molecules, so requires a halogen carrier