Benzene

Question 0

Explain the overlapping of orbitals in Benzene

Answer 0

Sideways overlap of p-orbitals

Leading to delocalised pi bond above and below plane of molecule

Question 1

Describe the bonding in benzene

Answer 1

Planar molecule with a ring of 6 carbons
Each C is covalently bonded to 2 other carbons and 1 H
Sideways overlap of p-orbitals
Leading to delocalised pi bonds above and below plane of molecule

Question 2

Why is benzene less ready to undergo addition reactions than alkenes?

Answer 2

Alkenes have localised C=C with a greater electron density to attract electrophiles more easily
Benzene has a stable delocalised system that would be disturbed/broken with addition

Question 3

What is meant by delocalisation energy?

Answer 3

The difference between experimental ΔΗ for benzene and ΔΗ reaction according to the Kékule structure
OR
The extra stability in benzene structure due to electron delocalisation

Question 4

The extra stability in Benzene is due to what?

Answer 4

It’s delocalisation energy due to the π system

Question 5

Does Benzene or alkenes have a greater electron density?

Answer 5

Alkenes due to the localised double bond (C=C)

Question 6

What does delocalisation mean?

Answer 6

Electron pairs are shared between three of more carbon atoms- eg in aromatic compounds such as benzene and naphalene

Question 7

How is benzene formed?

Answer 7

For catalytic reforming of crude oil

Question 8

What does electrophilic substitution involve in benzene?

Answer 8

The replacement of ‘ring hydrogen atoms’ by an electron deficient species or group (eg NO2+ or CH3+)

Question 9

What are the conditions and reagents for the nitration of benzene?

Answer 9

Conc HNO3
Conc H2SO4
Reflux at 50-55°C (to avoid thermal decomposition)

Question 10

Give the equation for the nitration of benzene

Answer 10

C6H6 —conc HNO3 conc H2SO4 reflux 50-55°C –> C6H5NO2 + H2O

Question 11

How do you form nitro benzene?

Answer 11

By refluxing benzene with conc HNO3 & conc H2SO4 at 50-55°C

Question 12

How is the nitronium cation (NO2+) formed in the nitration of benzene?

Answer 12

The stronger acid (H2SO4) forces HNO3 to act as a base and accept a proton, resulting in NO2+ and H2SO4- and water

Question 13

What is NO2+?

Answer 13

A nitronium cation

A powerful electrophile

Question 14

What conditions are needed to form bromo benzene?

Answer 14

Br2 FeBr3
In the absence of sunlight
Anhydrous

Question 15

What reaction is FeBr3 involved in and what is its function?

Answer 15

In the halogenation of benzene
As a halogen carrier
Polarises Br-Br bond

Question 16

How do you form chloro benzene?

Answer 16

From benzen with-
AlCl3, Cl2 
No UV
Room temp
Anhydrous

Question 17

What does polarisation mean?

Answer 17

An unequal electron distribution in a covalent bond

Question 18

In what reaction is AlCl3 used?

Answer 18

Chlorination of benzene- as a halogen carrier and catalyst

In the alkylation of Benzene

Question 19

What are the conditions for the alkylation of benzene?

Answer 19

Ch3Cl AlCl3
Anhydrous
Warm

Question 20

Give the equation (including conditions) of the alkylation of benzene

Answer 20

Benzene —CH3Cl AlCl3 anhydrous warm —> C6H5CH3 + HCl

Question 21

Explain the bonding in chloro benzene

Answer 21

P/π orbital overlap

Benzene-Cl bond very strong so not easily substituted

Question 22

What is the only substitution reaction that chloro benzene will undergo?

Answer 22

OH (hydrolysis)

NaOH (aq) 300°C increased pressure

Question 23

How is chloro-nitro benzene formed?

Answer 23

From chloro benzene with the use of H2SO4/HNO3

Question 24

How can chlorophenylmethane be formed from methylbenzene?

Answer 24

Cl2 UV

Free radical substitution