4.2 Aromaticity

Question 1

Benzene molecular formula

Answer 1

C6H6

Question 2

Internal C-C-C bond angle of benzene

Answer 2

120 degrees

Question 3

Length of all C-C bond lengths in benzene

Answer 3

All equal
140 pm

Question 4

What was kekulé’s structure

Answer 4

A ring of alternating single and double bonds
An alkene

Question 5

Benzene’s actual structure

Answer 5

•sigma bonds between C and their H
• sp2 hybridised orbitals
• p-orbitals from each C atom overlap and form a delocalised ring system above and below the ring called a delocalised pi-system of electrons

Question 6

Evidence against kekulé’s structure

Answer 6

• all bonds are the same length in reality - single and dbl bond lengths are different
• benzene reacts by substitution reactions - alkenes react by addition
• enthalpy of hydrogenation of benzene is less than calculated if it had alternate single and double bonds - about 150kjmol-1 which means benzene more stable than kekulé’s structure

Question 7

What is attached to benzene if the arene is called chlorobenzene

Answer 7

Chlorine

Question 8

What is an arene

Answer 8

Derivative of benzene

Question 9

What is attached to benzene if the arene is called nitrobenzene

Answer 9

NO2

Question 10

What is attached to benzene if the arene is called methylbenzene

Answer 10

CH3

Question 11

What is attached to benzene if the arene is called phenol

Answer 11

OH

Question 12

What is attached to benzene if the arene is called benzonitrile

Answer 12

C(triplebond)N

Question 13

What is attached to benzene if the arene is called benzaldehyde

Answer 13

O=C-H

Question 14

What is attached to benzene if the arene is called benzoic acid

Answer 14

O=C-O-H

Question 15

What is attached to benzene if the arene is called Benzyl chloride

Answer 15

O=C-Cl

Question 16

Does benzene take part in addition reaction

Answer 16

No

Question 17

What kind of reactions does benzene take part in

Answer 17

Substitution reactions

Question 18

Why doesn’t benzene carry out addition reactions

Answer 18

The stable delocalised electron ring would be disturbed and the pi-cloud lost

Question 19

What does benzene substitute in substitution reactions and why

Answer 19

Substitution of the H atoms
To keep electron ring intact

Question 20

Reactors and conditions for benzene nitration

Answer 20

Conc sulphuric acid
Conc nitric acid
55 degrees celcius

Question 21

Product of benzene nitration

Answer 21

Nitrobenzene
C6H5NO2

Question 22

What is intermediate called in benzene nitration

Answer 22

Wheland intermediate

Question 23

Reactants and conditions of halogenation of benzene

Answer 23

Benzene and halogen(Br or Cl)
Dark- out of direct sunlight
Catalyst such as AlCl3 or FeBr3 (FeCl3)

Question 24

Why does the halogenation of benzene need to be done in the dark

Answer 24

To avoid homolytic fission

Question 25

Product of halogenation of benzene

Answer 25

Bromobenzene
Chlorobenzene
…

Question 26

What is a Friedel-Craft reaction

Answer 26

The alkylation of benzene
It is the substitution of an alkyl group into the benzene ring

Question 27

Commonest catalyst for Friedel-Craft reaction

Answer 27

Aluminium chloride

Question 28

Is the C-Cl bond stronger in chlorobenzene or chloroalkane

Answer 28

Chlorobenzene

Question 29

Why does the C-Cl bond in chlorobenzene require harsher conditions to hydrolise than that in a chloroalkane

Answer 29

It is stronger
Due to overlap between p-electrons on the chlorine and the delocalised pi-system in the ring to give some pi-characteristics to the bond and thus strengthening it