Benzene

Question 1

What contradicts the Kekule structure?

Answer 1

1) Double and single bonds don’t have the same lengths
2) Benzene gives a negative result with bromine water indicating there are no double bonds
3) Hydrogenation of benzene is not 3 times the amount of hydrogenating cyclohexene

Question 2

What are some physical attributes of benzene?

Answer 2

1) Completely delocalised electrons
2) 120 degree bond angle
3) Planar molecule

Question 3

What effect does the delocalisation of electrons have on benzene?

Answer 3

Makes the molecule more stable than expected for a molecule with 3 double bonds

Question 4

What is the evidence that benzene is more stable than another molecule with double bonds?

Answer 4

Hydrogenation of cyclohexene has an enthalpy change of -120KJ/mol. However, hydrogenation of benzene has an enthalpy change of -209KJ/mol not -360KJ/mol which you would assume (3 more double bonds)

Question 5

What reaction does benzene undergo?

Answer 5

Electrophilic substitution
Why?
1) Delocalised electrons provide an electron rich area
2) Delocalised system makes benzene stable and so addition reactions are not favoured

Question 6

What are the 3 types of electrophilic substitution benzene can undergo?

Answer 6

1) Nitration (NO2)
2) Alkylation (Requires a alk-(ane/ene) i.e. chloroalkanes
3) Acylation (whatever a acyl chloride produces)

Question 7

What are the conditions for nitration?

Answer 7

1) 50/50 mixture of conc. H2SO4 and HNO3
2) Under reflux at 50-55degrees
PRODUCT= Nitrobenzene
3) Catalyst H2SO4

Question 8

What are the conditions for alkylation using chlorolkanes?

Answer 8

1) Chloroalkane in the presence of anhydrous aluminium chloride (AlCl3)
2) Under reflux at 50degrees
3) Catalyst AlCl3
PRODUCT= Alkylbenzene

Question 9

What are the conditions for alkylation using alkenes?

Answer 9

1) Alkene in the presence of HCl and AlCl3
2) Reflux is <50 degrees
3) Catalysts HCl and AlCl3
PRODUCT= Alkylbenzene

Question 10

What are the conditions for acylation?

Answer 10

1) Acyl chloride in the presence of anhydrous AlCl3
2) Reflux 50degrees
3) Catalyst AlCl3
PRODUCT= Phenyl Ketone

Question 11

What happens if the temperature of the reaction exceeds 55 degrees?

Answer 11

Further substitution occurs giving products like dinitrobenzene or trimethylbenzene

Question 12

How is phenylamine prepared?

Answer 12

1) Required nitrobenzene which is reduced to phenylamine
2) This requires Tin and Conc. HCl
3) Heated reflux

Question 13

What is the reducing agent used in the formation of phenylamine?

Answer 13

SnCl2

Question 14

What are the uses of aromatic compounds?

Answer 14

1) TNT (trinitrotoluene) as an explosive
2) Dyes- phenylamine can be converted to another substance that can be used as a dye
2) Polystyrene (produced by dehydrogenation with Iron Oxide at high temp then polymerise)