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Chemistry A Level CIE > Chapter 11 Hydrocarbons > Flashcards

Chapter 11 Hydrocarbons Flashcards

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1
Q

Arenes are very

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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
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2
Q

Arenes undergo a series of reactions including:

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  • Substitution
  • Nitration
  • Friedel-Crafts alkylation
  • Friedel-Crafts acylation
  • Complete Oxidation
  • Hydrogenation
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3
Q

Substitution (Halogenation)

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  • 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
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4
Q

Substitution of Alkylarenes

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  • 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
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5
Q

In the presence of excess halogen, multiple substitutions occur

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6
Q

Nitration

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  • 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
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7
Q

Nitration of benzene

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8
Q

Nitration of alkylarenes

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  • Again, due to the electron-donating alkyl groups in alkylarenes, nitration of methylbenzene will occur on the 2 and 4 position
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9
Q

Friedel-Crafts reactions

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  • 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)
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10
Q

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

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  • Generating the electrophile
  • Electrophilic attack on the benzene ring
  • Regenerating aromaticity of the benzene ring
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11
Q

Friedel-Crafts alkylation

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  • 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
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12
Q

Friedel-Crafts acylation

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  • 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
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13
Q

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

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14
Q

Example of a Friedel-Crafts acylation reaction step 3

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15
Q

Complete oxidation

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  • 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
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16
Q

Hydrogenation

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  • 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
17
Q

Summary of reactions of arenes table

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18
Q

The electrophilic substitution reaction in arenes consists of three steps

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  • Generation of an electrophile
  • Electrophilic attack
  • Regenerating aromaticity
19
Q

The halogenation and nitration of arenes are both examples

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of electrophilic substitution reactions

20
Q

Mechanism of electrophilic substitution (first Step_

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    • 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
21
Q

Mechanism of electrophilic substitution (Step 2)

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  • 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
22
Q

Mechanism of electrophilic substitution (final Step)

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  • 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
23
Q

Addition reactions of arenes

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  • 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
24
Q

Arenes will undergo substitution reactions with halogens to form

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  • aryl halides
    • This reaction is also called a halogenation reaction
25
Q

Depending on the reaction conditions, halogenation can occur:

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  • In the aromatic ring
  • In the side chain
26
Q

Halogenation in the aromatic ring

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  • 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
27
Q

Halogenation of alkylarenes in the aromatic ring

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28
Q

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

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29
Q

Halogenation in the side chain

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  • 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
30
Q

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

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31
Q

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

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32
Q

Arenes readily undergo

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  • 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
33
Q

Electron-withdrawing & -donating groups

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  • 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
34
Q

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

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  • 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
35
Q

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

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  • 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
36
Q

Electron-withdrawing & -donating substituents table

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Chemistry A Level CIE (18 decks)

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