Reactions of benzene

Question 1

Why can’t benzene undergo addition reactions??

Answer 1

Because there are no double bonds in benzene, instead hydrogen atoms are substituted

Question 2

What are the 3 steps of the mechanism of electrophilic substitution of benzene??

Answer 2

1) generation of the electrophile
2) Electrophilic attack of the benzene ring
3) regeneration of the catalyst

Question 3

Nitration of benzene- Step 1 equation

Answer 3

Generation of the electrophile

HNO3 + H2SO4 -> NO2+ (electrophile) + HSO4- + H2O

Question 4

Nitration of Benzene- step 2 draw out the mechanism

Answer 4

Electrophilic attack of benzene

• double arrow pointing from benzene ring circle to NO2+ ion
• Intermediate- electrons from one pi bond on hydrogen are attracted to the positive nucleus of the benzene ring (double headed arrow from H to positive nucleus, 4 electrons left shared across 5 hydrogens, so incomplete circle in ring. NO2 also shown as attached.
• Left with Benzoic acid + H+ ion

Question 5

Nitration of benzene- step 3 equation

Answer 5

Regeneration of the catalyst-

H+ + HSO4- -> H2SO4

Question 6

Halogenation of Benzene- Why can’t benzene and chlorine react at room temperature without a catalyst, whilst an alkene and chlorine can??

Answer 6

The Cl2 molecule can be Polarised by an alkene because the electrons are localised, but can’t be polarised by delocalised electrons above and below benzene ring.

Question 7

Halogenation of benzene- Step 1 general equation for all halogens

Answer 7

X2 + AlX3 -> X+ + AlX4-

X= any halogen

Question 8

Halogenation of benzene- Step 2 draw out the mechanism

Answer 8

Electrophilic attack of benzene

• double arrow pointing from benzene ring circle to Cl+ ion
• Intermediate- electrons from one pi bond on hydrogen are attracted to the positive nucleus of the benzene ring (double headed arrow from H to positive nucleus, 4 electrons left shared across 5 hydrogens, so incomplete circle in ring. Cl also shown as attached.
• Left with chlorobenzene and H+ ion

Question 9

Halogenation of benzene- step 3 equation

Answer 9

H+ + AlCl4 -> AlCl3 + HCl

Question 10

Halogenation of benzene- overall equation

Answer 10

Benzene + Cl2 -AlCl3> Chlorobenzene + HCl

Question 11

Whats the catalysts used in electrophilic substitution and why are they used

Answer 11

Halogen carrier catalysts- central metal atom has space in it’s outer shell to accept pair of electrons

Question 12

Why is benzene relatively more resistant to reactions with bromine compared to alkenes??

Answer 12

In the benzene system, pi bond overlap to create delocalised electron density, whereas electron density is localised in alkenes.

Question 13

What are the conditions needed for nitration of benzene

Answer 13

50 degrees and concentrated sulphuric acid(to make nitrating mixture)

Question 14

What is an electrophile

Answer 14

an electron pair acceptor

Question 15

Exam Q: deduce how many electrons are involved in pi bonding in benzene and an intermediate ring involved in electrophilic addition, and describe how their arrangements are different

Answer 15

• Benzene ring has 6 electrons involved in pi bonding, with 6 p orbitals forming 6 pi bonds when they overlap above and below the benzene ring, each with their own delocalised electron spread across 6 carbons
• Intermediate ring has 4 electrons involved in pi bonding spread over 5 carbons in the overlapping p orbitals above and below the carbon ring

Question 16

What are the conditions needed for the halogenation of benzene

Answer 16

room temperature and halogen carrier catalyst