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AQA A Level Organic Chemistry > 10. Aromatic Chemistry > Flashcards

10. Aromatic Chemistry Flashcards

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1
Q

What structure are arenes based on?

A

A benzene ring structure

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2
Q

How many carbons and hydrogens does benzene contain?

A

6 of each

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3
Q

How was the bonding in a benzene ring originally thought to be?

A

A ring of six carbon atoms containing alternate single and double bonds

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4
Q

What is the IUPAC name of a cyclic structure with alternate single and double bonds?

A

Cyclohexa - 1, 3, 5 - triene

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5
Q

What is the molecular formula of cyclohexa - 1, 3, 5 - triene?

A

C6H6

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6
Q

What is the Kekule structure?

A

What the structure of benzene was originally thought to be; six carbon atoms with alternate single and double bonds

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7
Q

Do double and single bonds have the same length?

A

No

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8
Q

Why are double bonds shorter than single bonds?

A

The extra overlap of the π orbitals brings the atoms closer together and the bond is shorter

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9
Q

What was the evidence that benzene could not have the Kekule structure?

A

Analysis of benzene shows all bonds have the same length; but single and double bonds are different lengths

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10
Q

What are the length of the bonds in benzene?

A

0.142 nm - intermediate between single and double

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11
Q

What is the length of a C-C bond?

A

0.154 nm

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12
Q

What is the length of a C=C bond?

A

0.134 nm

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13
Q

What was the alternate suggestion to the Kekule structure for benzene?

A

Contained delocalised electrons in six overlapping p orbitals at right angles to the plane of atoms

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14
Q

Describe the structure of benzene

A
  • delocalised electrons in six overlapping p orbitals at right angles to the plane of atoms
  • p-orbitals overlap sideways with both neighbours and the six electrons are delocalised above and below the ring
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15
Q

How can the delocalised electrons in benzene be represented?

A

As a circle within the hexagonal ring

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16
Q

What effect do the p-electrons in benzene have on stability?

A

They have a stabilising effect, meaning it is hard for an addition reaction to take place

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17
Q

What are arenes?

A

Hydrocarbons based on benzene

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18
Q

Why is it hard for an addition reaction to take place with benzene?

A

The 6 delocalised p electrons have a stabilising effect

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19
Q

What shape is benzene?

A

Planar

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20
Q

What are all the bond angles in benzene?

A

120°

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21
Q

What happens when benzene is added to water?

A

It doesn’t decolourise

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22
Q

Why doesn’t benzene decolourise when added to water?

A

There are no double bonds so it is difficult for an addition reaction to take place

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23
Q

What is more stable: benzene or cyclohexa - 1, 3, 5 - triene?

A

Benzene due to delocalised electrons

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24
Q

What are the two ways aromatic compounds can be named?

A
  • based on benzene

* based on phenyl

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25
Q

What do aromatic compounds all contain?

A

Benzene

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26
Q

What does it mean, if aromatic compounds are named based on benzene?

A

The benzene ring is classed as the main functional group

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27
Q

How are aromatic compounds named if they are named based on benzene?

A

With the suffix -benzene

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28
Q

How would a benzene ring with one chlorine atom instead of a hydrogen be named?

A

Chlorobenzene (C6H5Cl)

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29
Q

How would a benzene ring with one NO2 group instead of a hydrogen be named?

A

Nitrobenzene (C6H5NO2)

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30
Q

How would a benzene ring with one CH3 group instead of a hydrogen be named?

A

Methylbenzene (C6H5CH3)

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31
Q

How would a benzene ring with one CH2Cl group instead of a hydrogen be named?

A

Chloromethylbenzene (C6H5CH2Cl)

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32
Q

What does it mean, if aromatic compounds are named based on phenyl?

A

When the benzene is not the main functional group

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33
Q

How are aromatic compounds named if they are named based on phenyl?

A

The benzene is named as a prefix using -phenyl, with the suffix coming from the other functional group

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34
Q

How would a benzene ring with one NH2 group instead of a hydrogen be named?

A

Phenylamine (C6H5NH2)

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35
Q

How would a benzene ring with one COCH3 group instead of a hydrogen be named?

A

Phenylethanone (C6H5COCH3)

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36
Q

How would a benzene ring with one CH=CH2 group instead of a hydrogen be named?

A

Phenylethene (C6H5CH2CH2)

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37
Q

What are common examples of aromatic compounds which do not use benzyl or phenyl in full in the name?

A
  • benzoic acid (C6H5COOH)

* phenol (C6H5OH)

38
Q

Structural formula for benzoic acid?

A

C6H5COOH

39
Q

Structural formula for phenol?

A

C6H5OH

40
Q

How are you supposed to know when you should name a compound based on phenyl or benzene?

A

There is no easy way to know - just learn common examples and use these to base the name of other molecules on

41
Q

When should numbers be used in the names of aromatic compounds?

A

When there is more than one functional group bonded to the benzene ring

42
Q

How can the unexpected stability of benzene be illustrated?

A

By considering thermochemical data from hydrogenation of benzene in comparison to that of cyclohexene

43
Q

How does the enthalpy change for cyclohexene and benzene show that benzene has delocalised electrons?

A
  • cyclohexene = - 120 kJ mol-1 (breaking 1 double bond in process)
  • enthalpy change for benzene should be 3 x -120 = -360 kJmol-1 (but it is only -208kJmol-1)
  • benzene reaction is less exothermic than expected illustrating benzene is more stable than that of the Kekule structure
44
Q

What type of reactions does benzene not favour?

A

Those which would involve breaking up the delocalised system - so benzene won’t readily undergo addition reactions

45
Q

What do the delocalised electrons in benzene constitute?

A

An electron rich area

46
Q

What does it mean, that delocalised electrons constitute an electron rich area in benzene?

A

They are susceptible to attack by electrophiles

47
Q

Why does benzene undergo electrophilic substitution?

A

The electron rich area of delocalised electrons makes them susceptible to attack by electrophiles

48
Q

What do the delocalised electrons prevent benzene reacting as?

A

An alkene

49
Q

Why is electrophilic addition unlikely to occur in benzene?

A

The energy required for the ring to break in an electrophilic addition reaction is too high

50
Q

What reactions do benzene undergo?

A

Electrophilic substitution

51
Q

What does benzene undergo, as opposed to electrophilic addition?

A

Electrophilic substitution

52
Q

Why does electrophilic substitution occur in benzene?

A
  • presence of delocalised electrons means the benzene ring is electron rich
  • therefore an electrophile that is able to accept an electron pair donated by the ring is the most likely type of reagent to react with benzene
53
Q

In electrophilic substitution of benzene, why is the positive intermediate species more stable than a normal cation?

A

Due to the delocalised electrons

54
Q

In electrophilic substitution of benzene, how is the stability of the ring restored?

A

A proton is eliminated

55
Q

What type of electrophile is needed for electrophilic substitution with benzene? Why is this?

A

A ‘powerful’ one (a polar molecule or induced dipole isn’t enough), in order to react with such a stable molecule

56
Q

Can polar molecules be electrophiles in the electrophilic substitution of benzene?

A

No - because benzene is too stable so the electrophile needs to be more ‘powerful’

57
Q

What generates the electrophile in the electrophilic substitution of benzene?

A

A catalyst

58
Q

What is nitration?

A

The electrophilic substitution of benzene where the electrophile is a nitronium ion

59
Q

What is the formula for a nitronium ion?

A

NO₂⁺

60
Q

In nitration, how is the nitronium ion generated?

A

By the reaction between nitric and sulphuric acid

61
Q

What is the process of generating nitronium ions?

A
  1. Nitric acid (the weaker acid) acts as a base and accepts a proton

H₂SO₄ + HNO₃ → H₂NO₃⁺ + HSO₄⁻

  1. Water is lost

H₂NO₃⁺ → H₂O + NO₂⁺

62
Q

What are the two equations and overall equation for how nitronium ions are formed?

A

H₂SO₄ + HNO₃ → H₂NO₃⁺ + HSO₄⁻

H₂NO₃⁺ → H₂O + NO₂⁺

overall: H₂SO₄ + HNO₃ → NO₂⁺ + HSO₄⁻ + H₂O

63
Q

What happens with benzene, once a nitronium ion is generated?

A

Electrophilic substitution takes place

64
Q

What is the overall equation for benzene and nitric acid?

A

C6H6 + HNO3 → C6H5NO2 + H2O

65
Q

What mechanism is nitration?

A

Electrophilic substitution

66
Q

What are the reagents for nitration?

A

Concentrated nitric acid and concentrated sulphuric acid

67
Q

What are the conditions for nitration?

A

Warm to 50°C

68
Q

In nitration, what must be ensured about the temperature?

A

That it doesn’t rise above 55°C

69
Q

Why in nitration is the temperature ‘warm to 50°C’?

A

Otherwise further substitution will occur

70
Q

What does nitration produce?

A

Compounds which can be turned into other molecules ie they are useful in the synthesis of other organic compounds

71
Q

What are the uses of nitration?

A
  • synthesis of amines and dyes

* manufacture of explosives

72
Q

How is nitrobenzene used in the synthesis of amines and dyes?

A

It can be reduced to phenyl amine by treating it with tin and concentrated HCl and heating under reflux

73
Q

How can nitrobenzene be reduced to phenyl amine?

A

By treating it with tin and concentrated HCl and heating under reflux

74
Q

Why must nitrobenzene be heated under reflux to form phenylamine?

A

Otherwise it is impossible to introduce NH2 into a benzene ring

75
Q

Why should LiAlH₄ and NaBH₄ not be used in the reduction of nitrobenzene to phenyl amine?

A

Since they give other products

76
Q

Equation for the reduction of nitrobenzene to form phenyl amine?

A

C6H5NO2 + 6[H] → C6H5NH2 + 2H2O

77
Q

What can be used as a reducing agent, as opposed to tin and concentrated HCl, for the reduction of nitrobenzene to phenyl amine?

A

Hydrogen gas in the presence of a suitable catalyst

78
Q

What type of reaction is that of nitrobenzene to form phenyl amine?

A

Reduction

79
Q

What are phenyl amines used to manufacture?

A

Dyes

80
Q

What happens to nitro compounds when heated?

A

Decompose violently

81
Q

Why are nitro compounds used as explosives?

A

They decompose violently when heated

82
Q

What is the IUPAC name for TNT?

A

2, 4, 6 - trinitromethylbenzene

83
Q

How is TNT made?

A

By introducing 3 nitro groups into methyl benzene by reacting with concentrated nitric acid and sulphuric acids

84
Q

What are Friedel-Crafts acylation reactions?

A

Electrophilic substitution where a hydrogen atom on the benzene ring is substituted for an acylium ion

85
Q

What is an acylium ion?

A

R - C = O⁻

86
Q

In Friedel-Crafts acylation reactions, how is the acylium ion generated?

A

Using an acyl chloride and an anhydrous AlCl₃ catalyst

87
Q

What are the main steps in Friedel-Crafts acylation reactions?

A
  • acylium ion generated
  • RCO⁺ acts as electrophile that attacks benzene ring
  • AlCl₃ is then regenerated
88
Q

What is the equation for the formation of an acylium ion using an acyl chloride and an anhydrous AlCl₃ catalyst?

A

R-COCl + AlCl₃ → R-CO⁺ + AlCl₄⁻

89
Q

Why is the formation of an acylium ion possible?

A

As the Al atom can accept a lone pair from the chloride atom in the acyl chloride forming a coordinate bond

90
Q

What is the equation for the regeneration of the AlCl₃ catalyst in the Friedel-Crafts acylation reactions?

A

AlCl₄⁻ + H⁺ → AlCl₃ + HCl

91
Q

What 3 reactions are covered in the aromatic chemistry topic?

A
  1. nitration (benzene → nitrobenzene)
  2. acylation (benzene → phenylketone)
  3. reduction (nitrobenzene → phenyl amine)

AQA A Level Organic Chemistry (16 decks)

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