25.2 Electrophilic Substitution Reactions of Benzene

Question 1

Why can benzene not undergo electrophilic addition reactions?

Answer 1

delocalised π-electrons make benzene stable, since they have a low electron density, and therefore do not attract electrophiles readily.

addition would disrupt delocalised π-electrons

the product would be less stable than the reactants.

so electrophilic addition not thermodynamically viable

Question 2

What types of Benzene Derivatives can be made?

Answer 2

• NITROBENZENE (nitration)
• BROMOBENZENE (bromination)
• CHLOROBENZENE (chlorination)
• (ALKYL)BENZENE (alkylation)
• (ACYL)BENZENE (acylation)

Question 3

Write an equation for the nitration of benzene:

(only 1 substitution)

Answer 3

benzene + nitric acid ⟶ nitrobenzene + water

C₆H₆ + HNO₃ ⟶ C₆H₅NO₂ + H₂O

(50° + H₂SO₄ catalyst)

Question 4

What conditions are required for the nitration of benzene?

Answer 4

50°C

sulfuric acid, H₂SO₄ catalyst

Question 5

Outline the steps occuring in the nitration of benzene, including the mechanism:

Answer 5

1. HNO₃ + H₂SO₄ ⟶ NO₂⁺ + HSO₄^- + H₂O (formation of electrophile)
   
   1. NO₂⁺ accepts electron pair from cloud of delocalised e^-
   2. Forms unstable intermediate. H donates e^- pair
   3. Stable benzene ring reformed with side chain
2. H⁺ + HSO₄^- ⟶ H₂SO₄ (regeneration of catalyst)

Question 6

If increased above 50°C, what could occur during the nitration of benzene?

Answer 6

further substitution, forming products such as dinitrobenzene or trinitrobenzene

each substitution releases 1 water molecule

Question 7

From what parts of the reactants is a water molecule released from the nitration of benzene?

Answer 7

H from the benzene. (which is replaced by the NO₂)

HO from the HNO₃. (leaves NO₂ which replaces H on benzene)

Question 8

Write an equation of the bromination of benzene:

Answer 8

benzene + bromine ⟶ bromobenzene + hydrogen bromide

C₆H₆ + Br₂ ⟶ C₆H₅Br + HBr

(FeBr₃/AlBr₃ catalyst)

Question 9

What conditions are required for the bromination of benzene?

Answer 9

FeBr₃/AlBr₃ catalyst

Question 10

Outline the steps occurring in the bromination of benzene, including the mechanism:

Answer 10

1. Br₂ + FeBr₃ ⟶ FeBr₄^- + Br⁺ (formation of electrophile)
   
   1. Br⁺ accepts electron pair from cloud of delocalised e^-
   2. Forms unstable intermediate. H donates e^- pair
   3. Stable benzene ring reformed with side chain
2. H⁺ + FeBr₄^- ⟶ FeBr₃ + HBr (regeneration of catalyst)

Question 11

From what parts of the reactants is a HBr molecule released from the bromination of benzene?

Answer 11

H from the benzene (which is replaced by the Br)

1 Br from the Br₂ molecule (the atom which is not substituting the H)

Question 12

What is the role of molecules such as AlCl₃, FeBr₃, e.t.c in the halogenation of benzene?

Answer 12

halogen carriers

act as catalyst

by reacting with reactant to form electrophile.

are regenerated at the end of reaction

Question 13

Write an equation of the chlorination of benzene:

Answer 13

benzene + chlorine ⟶ chlorobenzene + hydrogen chloride

C₆H₆ + Cl₂ ⟶ C₆H₅Cl + HCl

(FeCl₃/AlCl₃ catalyst, room temperature + pressure)

Question 14

What conditions are required for the chlorination of benzene?

Answer 14

FeCl₃/AlCl₃ catalyst

room temperature + pressure

Question 15

Outline the steps occurring in the chlorination of benzene, including the mechanism:

Answer 15

1. Cl₂ + FeCl₃ ⟶ FeCl₄^- + Cl⁺ (formation of electrophile)
   
   1. Cl⁺ accepts electron pair from cloud of delocalised e^-
   2. Forms unstable intermediate. H donates e^- pair
   3. Stable benzene ring reformed with side chain
2. H⁺ + FeCl₄^- ⟶ FeCl₃ + HCl (regeneration of catalyst)

Question 16

From what parts of the reactants is a HCl molecule released from the chlorination of benzene?

Answer 16

H from the benzene (which is being replaced by the Cl)

1 Cl from the Cl₂ molecule (the atom which is not substituting the H)

Question 17

Write an equation for the alkylation of benzene:

Answer 17

Benzene + Chloroalkane ⟶ (alkyl)benzene + Hydrochloric Acid

(AlCl₃ catalyst)

Question 18

What conditions are required for the alkylation of benzene?

Answer 18

FeCl₃ catalyst

Question 19

Outline the steps occuring in the alkylation of benzene:

Answer 19

(using chloroethane as an example)

1. C₂H₅Cl + AlCl₃ ⟶ C₂H₅⁺ + AlCl₄^- (formation of electrophile)
2. H replaced with C₂H₅, forming ethylbenzene
3. H⁺ + AlCl₄^- ⟶ AlCl₃ + HCl (regeneration of catalyst)

Question 20

From what parts of the reactants is a HCl molecule released from the alkylation of benzene?

Answer 20

H from the benzene (which is replaced by the alkyl group on the chloroalkane)

Cl from the chloroalkane

Question 21

Write an equation for the acylation of benzene using ethanoyl chloride:

Answer 21

Benzene + Ethanoyl Chloride ⟶ Phenylethanone + Hydrochloric Acid

C₆H₆ + CH₃COCl ⟶ C₆H₅CH₃ + HCl

(AlCl₃ catalyst)

Question 22

What conditions are required for the acylation of benzene?

Answer 22

AlCl₃ catalyst

Question 23

Outline the steps occuring in the acylation of benzene:

Answer 23

(using ethanoyl chloride as an example)

1. CH₃COCl + AlCl₃ ⟶ CH₃CO⁺ + AlCl₄^- (formation of electrophile)
2. H replaced with CH₃CO, forming phenylethanone
3. H⁺ + AlCl₄^- ⟶ AlCl₃ + HCl (regeneration of catalyst)

Question 24

From what parts of the reactants is a HCl molecule released from the acylation of benzene?

Answer 24

H from the benzene (which is being replaced by most of the acyl chloride molecule)

Cl from the acyl chloride

Question 25

Compare the reactivity of alkenes & arenes with bromine:

Answer 25

Alkenes more reactive than arenes

• due to higher electron density of localised pi electrons
• can polarise bromine molecules
• forming electrophile

Arenes less reactive than alkenes

• delocalised pi electrons have low electron density
• cannot form electrophile by polarising Br₂
• requires halogen carrier catalyst to form electrophile