Chapter 11 Hydrocarbons

Question 1

Arenes are very

Answer 1

stable compounds due to the delocalisation of π electrons in the ring

• This is because the negative charge is spread out over the molecule instead of being confined to a small area

Question 2

Arenes undergo a series of reactions including:

Answer 2

• Substitution
• Nitration
• Friedel-Crafts alkylation
• Friedel-Crafts acylation
• Complete Oxidation
• Hydrogenation

Question 3

Substitution (Halogenation)

Answer 3

• Halogenation reactions are examples of electrophilic substitution reactions
• Arenes undergo substitution reactions with chlorine (Cl₂) and bromine (Br₂) in the presence of anhydrous AlCl₃ or AlBr₃ catalyst respectively to form halogenoarenes (aryl halides)
  
  • The chlorine or bromine act as an electrophile and replaces a hydrogen atom on the benzene ring
  • The catalyst is required for the reaction to take place, due to the stability of the benzene structure
  • Multiple substitutions occur when excess halogen is used

Question 4

Substitution of Alkylarenes

Answer 4

• such as methylbenzene undergo halogenation on the 2 or 4 positions
• This is due to the electron-donating alkyl groups which activate these positions
  
  • Phenol (C₆H₅OH) and phenylamine (C₆H₅NH₂) are also activated in the 2 and 4 positions
• The halogenation of alkylarenes therefore result in the formation of two products

Question 5

In the presence of excess halogen, multiple substitutions occur

Answer 5

Question 6

Nitration

Answer 6

• Another example of a substitution reaction is the nitration of arenes
• In these reactions, a nitro (-NO₂) group replaces a hydrogen atom on the arene
• The benzene is reacted with a mixture of concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄) at a temperature between 25 and 60 ^oC
• Again, due to the electron-donating alkyl groups in alkylarenes, nitration of methylbenzene will occur on the 2 and 4 position

Question 7

Nitration of benzene

Answer 7

Question 8

Nitration of alkylarenes

Answer 8

• Again, due to the electron-donating alkyl groups in alkylarenes, nitration of methylbenzene will occur on the 2 and 4 position

Question 9

Friedel-Crafts reactions

Answer 9

• Friedel-Crafts reactions are also electrophilic substitution reactions
• Due to the aromatic stabilisation in arenes, they are often unreactive
• To use arenes as starting materials for the synthesis of other organic compounds, their structure, therefore, needs to be changed to turn them into more reactive compounds
• Friedel-Crafts reactions can be used to substitute a hydrogen atom in the benzene ring for an alkyl group (Friedel-Crafts alkylation) or an acyl group (Friedel-Crafts acylation)

Question 10

Like any other electrophilic substitution reaction, the Friedel-Crafts reactions consist of three steps:

Answer 10

• Generating the electrophile
• Electrophilic attack on the benzene ring
• Regenerating aromaticity of the benzene ring

Question 11

Friedel-Crafts alkylation

Answer 11

• In this type of Friedel-Crafts reaction, an alkyl chain is substituted into the benzene ring
• The benzene ring is reacted with a chloroalkane in the presence of an AlCl₃ catalyst
• An example of an alkylation reaction is the reaction of benzene with chloropropane (CH₃CH₂CH₂Cl) to form propylbenzene

Question 12

Friedel-Crafts acylation

Answer 12

• In the Friedel-Crafts acylation reaction, an acyl group is substituted into the benzene ring
  
  • An acyl group is an alkyl group containing a carbonyl, C=O group
• The benzene ring is reacted with an acyl chloride in the presence of an AlCl₃ catalyst
• An example of an acylation reaction is the reaction of methylbenzene with propanoyl chloride to form an acyl benzene
  
  • Note that the acyl group is on the 4 position due to the -CH₃ group on the benzene

Question 13

Example of a Friedel-Crafts acylation reaction step 1 and 2

Answer 13

Question 14

Example of a Friedel-Crafts acylation reaction step 3

Answer 14

Question 15

Complete oxidation

Answer 15

• Normally, alkanes are not oxidised by oxidising agents such as potassium manganate(VII) (KMnO₄)
• However, the presence of the benzene ring in alkyl arenes affect the properties of the alkyl side-chain
• The alkyl side-chains in alkyl arenes are oxidised to carboxylic acids when refluxed with alkaline potassium manganate(VII) and then acidified with dilute sulfuric acid (H₂SO₄)
  
  • For example, the complete oxidation of ethylbenzene forms benzoic acid

Question 16

Hydrogenation

Answer 16

• The hydrogenation of benzene is an addition reaction
• Benzene is heated with hydrogen gas and a nickel or platinum catalyst to form cyclohexane
• The same reaction occurs when ethylbenzene undergoes hydrogenation to form cycloethylbenzene

Question 17

Summary of reactions of arenes table

Answer 17

Question 18

The electrophilic substitution reaction in arenes consists of three steps

Answer 18

• Generation of an electrophile
• Electrophilic attack
• Regenerating aromaticity

Question 19

The halogenation and nitration of arenes are both examples

Answer 19

of electrophilic substitution reactions

Question 20

Mechanism of electrophilic substitution (first Step_

Answer 20

• • For the halogenation reaction, this is achieved by reacting the halogen with a halogen carrier
    
    • The halogen molecules form a dative bond with the halogen carrier by donating a lone pair of electrons from one of its halogen atoms into an empty 3p orbital of the halogen carrier
    • In the nitration reaction, the electrophile NO₂⁺ ion is generated by reacting it with concentrated nitric acid (HNO₃) and concentrated sulfuric acid (H₂SO₄)
• Step 1 of the halogenation reaction of arenes
• Step 1 of the nitration reaction of arenes

Question 21

Mechanism of electrophilic substitution (Step 2)

Answer 21

• Once the electrophile has been generated, it will carry out an electrophilic attack on the benzene ring
  
  • The nitrating mixture of HNO₃ and H₂SO₄ is refluxed with the arene at 25 – 60 ^oC
• A pair of electrons from the benzene ring is donated to the electrophile to form a covalent bond
  
  • This disrupts the aromaticity in the ring as there are now only four π electrons and there is a positive charge spread over the five carbon atoms
• Step 2 of the halogenation reaction of arenes
• Step 2 of the nitration reaction of arenes

Question 22

Mechanism of electrophilic substitution (final Step)

Answer 22

• In the final step of the reaction, this aromaticity is restored by heterolytic cleavage of the C-H bond so that bond electrons in this bond go into the benzene π bonding system
• Step 3 of the halogenation reaction of arenes
• Step 3 of the nitration reaction of arenes

Question 23

Addition reactions of arenes

Answer 23

• The delocalisation of electrons (also called aromatic stabilisation) in arenes is the main reason why arenes predominantly undergo substitution reactions over addition reactions
• In substitution reactions, the aromaticity is restored by heterolytic cleavage of the C-H bond
• In addition reactions, on the other hand, the aromaticity is not restored and is in some cases completely lost
  
  • The hydrogenation of arenes is an example of an addition reaction during which the aromatic stabilisation of the arene is completely lost
  • The cyclohexane formed is energetically less stable than the benzene

Question 24

Arenes will undergo substitution reactions with halogens to form

Answer 24

• aryl halides
  
  • This reaction is also called a halogenation reaction

Question 25

Depending on the reaction conditions, halogenation can occur:

Answer 25

• In the aromatic ring
• In the side chain

Question 26

Halogenation in the aromatic ring

Answer 26

• Halogenation of alkylarenes in the aromatic ring will occur when a halogen and anhydrous halogen carrier catalyst (such as AlBr₃ or AlCl₃) is used
• Aryl halides are less reactive than halogenoalkanes as the carbon-halogen bond in aryl halides is stronger
• This is due to the partial overlap of the lone pairs on the halogen atom with the π system in the benzene ring
• The carbon-halogen bond, therefore, has a partial double bond character

Question 27

Halogenation of alkylarenes in the aromatic ring

Answer 27

Question 28

Aryl halides are unreactive due to the partial double bond character of the carbon-halogen bond

Answer 28

Question 29

Halogenation in the side chain

Answer 29

• Halogenation of alkylarenes in the side chain will occur when the halogen is passed into boiling alkylarene in the presence of ultraviolet (UV) light
  
  • This is a free-radical substitution reaction
  • If excess halogen is used, all hydrogen atoms on the alkyl side-chain will be substituted by the halogen atoms
  • Note that no substitution into the benzene ring occurs under these conditions

Question 30

Halogenation of alkylarenes in the side chain is an example of a free-radical substitution reaction

Answer 30

Question 31

In excess halogen, all hydrogen atoms on the alkyl side-chain will be replaced

Answer 31

Question 32

Arenes readily undergo

Answer 32

• electrophilic substitution of one of their hydrogen atoms with another species
• Substituents that are already present on the arenes can affect where the substitution of the hydrogen atom on the arene takes place
  
  • These groups are said to direct substitution reactions to different ring positions

Question 33

Electron-withdrawing & -donating groups

Answer 33

• The substituents on the arenes can either be electron-withdrawing or electron-donating groups
• Electron-withdrawing substituents remove electron density from the π system in the benzene ring making it less reactive
  
  • These groups deactivate attack by electrophiles and direct the incoming electrophile to attack the 3 and/or 5 positions
• Electron-donating substituents donate electron density into the π system of the benzene ring making it more reactive
• These groups activate attack by electrophiles and direct the incoming electrophile to attack the 2, 4 and/or 6 positions

Question 34

For example, the nitro group in nitrobenzene is an electron-withdrawing group

Answer 34

• Upon bromination of nitrobenzene, the bromine electrophile will be directed to the 3 and/or 5 position
• The products are 3-chloronitrobenzene and 5-chloronitrobenzene

Question 35

For example, the methyl group in methylbenzene is an electron-donating group

Answer 35

• Upon bromination of methylbenzene, the bromine electrophile will be directed to the 2 and/or 4 position
• The products are 2-chloromethylbenzene and 4-chloromethylbenzene

Question 36

Electron-withdrawing & -donating substituents table

Answer 36