Benzene & Aromatic Compounds

Question 1

Give the formula of benzene

Answer 1

C6H6

Question 2

Which 3 pieces of evidence challenge the kekulé model of benzene?

Answer 2

Resistance to reaction with electrophile
Enthalpy change of hydrogenation
C-C bond length

Question 3

What did kekulé’s model of benzene suggest?

Answer 3

Benzene was an alkene

Question 4

What result occurs when benzene is tested with bromine water?

Answer 4

Nothing happens

Question 5

What are the differences in bond length between all C-C bonds? Compare it to that of benzene C-C bonds

Answer 5

C-C single bond (sigma bond) - LONGEST
C=C double bond (Pi bond) - SHORTEST
C-C bond in benzene - MIDDLE
Benzene carbon-carbon bonds all have the same length

Question 6

Describe the bonding and structure of benzene

Answer 6

P-orbitals overlap above and below the ring to form a Pi bond
The Pi bond electrons are delocalised.
Sigma bonds form between C-C and C-H bonds
Benzene ring is planar with bond angle 120 degrees
C-C bonds are all of equal length

Question 7

Define the term ‘electrophile’

Answer 7

Electron pair acceptor

Question 8

In order for benzene to react with an electrophile, what must happen?

Answer 8

Electrophile must disrupt and remove two electrons from the ring containing 6 Pi electrons

Question 9

What occurs when benzene undergoes electrophilic substitution ?

Answer 9

1: Two of the delocalised electrons in the ring are donated to an electrophile
2: an unstable intermediate containing only 4 Pi bond electrons and a positive charge is formed.
3: intermediate breaks down to reform the delocalised Pi bond containing 6 electrons.

Question 10

Give an example of electrophilic substitution in benzene?

Answer 10

Nitration of benzene

Question 11

Give the catalyst and conditions for nitration of benzene?

Answer 11

Conditions: reflux at 60 degrees with conc HNO3 + conc H2SO4 (catalyst)

Question 12

What is the generally equation for nitration of benzene?

Answer 12

C6H6 + HNO3 –> C6H5NO2 + H2O

Question 13

During nitration of benzene, what is produced if the temperature exceeds 95C?

Answer 13

Nitrobenzene + 1,3dinitrobenzene

Question 14

Which ion is used in nitration of benzene?

Answer 14

Nitronium ion (NO2)

Question 15

Which two compounds contribute to the production of the nitronium ion?

Answer 15

HNO3

H2SO4

Question 16

Give the overall equation for the formation of the nitronium ion

Answer 16

2H2SO4 + HNO3 –> 2HSO4- + H3O- + NO2+

Question 17

What happens during halogenation (bromination) of benzene?

Answer 17

A halogen atom is introduced into the benzene ring using a halogen carrier catalyst

Question 18

What is the halogen carrier for halogenation (bromination) of benzene?

Answer 18

FeBr3
Or
Fe

Question 19

How is the electrophile for halogenation (bromination) formed?

Answer 19

The catalyst and the reagent react together

Br-Br + FeBr3 –> Br+ + FeBr4-

Question 20

Which two halogens react within halogenation?

Answer 20

Bromine

Chlorine

Question 21

Give the halogen carrier for chlorine reacting with benzene?

Answer 21

AlCl3

Question 22

What is the name of catalysts in halogenation?

Answer 22

Halogen carriers

Question 23

Which mechanism is used in halogenation?

Answer 23

Electrophilic substitution

Question 24

Which mechanism is the reaction of cyclohexene with bromine?

Answer 24

Electrophilic addition

Question 25

What can be observed when cyclohexene is reacted with bromine?

Answer 25

Orange -> colourless