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A Level Chemistry > Module 6: Chapter 25 - Aromatic Chemistry > Flashcards

Module 6: Chapter 25 - Aromatic Chemistry Flashcards

1
Q

What does the kekulé structure of benzene look like?

A
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2
Q

What is the molecular formula of benzene?

A

C₆H₆

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3
Q

What is the relative formula mass of benzene?

A

78

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4
Q

What is an arene?

A

An aromatic hydrocarbon, a hydrocarbon containing one or more benzene rings

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5
Q

What are the risks associated with benzene?

A
  • It is highly toxic and carcinogenic
  • It is colourless and highly volatile
  • It can be absorbed through the skin
  • It is highly flammable
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6
Q

What are the 3 problems associated with the kekulé structure?

A
  1. The lack of reactivity of Benzene
  2. The length of carbon-carbon bonds
  3. Hydrogenation enthalpies
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7
Q

What are the 2 models of benzene?

A
  • Kekulé structure
  • Delocalised Model
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8
Q

How is the lack of reactivity of benzene evidence disproving the kekule model?

A

If benzene contains double bonds it should decolourise bromine water in an electrophilic addition reaction. However benzene does not undergo any electrophilic addition reactions and it does not decolourise bromine water under normal conditions

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9
Q

How is the lengths of carbon-carbon bonds evidence disproving the kekule model?

A

Double bonds are slightly shorter than single bonds, this should make the structure of benzene an irregular hexagon. However, benzene is a regular hexagon with each bond being of equal length

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10
Q

How are hydrogenation enthalpies evidence disproving the kekule model?

A

If benzene did have the kekule structure, it would be named cyclohexa-1,3,5-triene. Therefore, it would be expected to have an enthalpy change of hydrogenation of 3 times that of cyclohexene (as it contains 3 times as many double bonds). However, the actual enthalpy change is much lower (in magnitude) than that expected

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11
Q

Why was the delocalised model of Benzene produced?

A

Due to evidence disproving the kekule structure

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12
Q

Describe the structure of the delocalised model of benzene

A
  • Benzene is planar, cyclic, hexagonal hydrocarbon containing 6 carbon atoms and 6 hydrogen atoms
  • Each carbon atom uses 3 of its available electrons in bonding to 2 other carbon atoms and 1 hydrogen atom
  • Each carbon atom has one electron in the p orbital at right angles to the plane of the bonded carbon and hydrogen atoms
  • Adjacent p orbital electrons overlap sideways in both directions above and below the plance of the carbon atoms to form a ring of electron density
  • This overlapping of the p-orbitals creates a system of π-bonds which spread over all six of the carbon atoms in the ring structure
  • The 6 electrons occupying this system of π-bonds are said to be delocalised
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13
Q

Which groups are substituent to benzene?

A
  • Short alkyl chains (6 carbon atoms or less)
  • Halogens
  • Nitro groups
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14
Q

How do you name an aromatic compound when it has a substituent group attatched to it?

A

Add the prefix of the substituent group onto benzene

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15
Q

When is benzene considered to be the substituent?

A
  • When it is connected to an alkyl chain with a functional group
  • When it is connected to an alkyl chain 7 or more carbon atoms long
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16
Q

How do you name an aromatic compound when benzene is the substituent?

A

Instead of using “benzene”, the prefix “phenyl” is used

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17
Q

Name this molecule

A

ethylbenzene

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18
Q

Name this molecule

A

Nitrobenzene

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19
Q

Name this molecule

A

Phenylethanone

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20
Q

Name this molecule

A

2-phenyloctane

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21
Q

What are 4 common compounds that are special cases in the aromatic nomenclature scheme?

A
  • Benzoic acid / benzenecarboxylic acid (rather than phenyl methanoic acid)
  • Phenylamine
  • Benzaldehye (rather than phenyl methanal)
  • Phenol (rather than hydroxybenzene)
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22
Q

Name this molecule

A

Benzoic acid

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23
Q

Name this molecule

A

Phenylamine

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24
Q

Name this molecule

A

Benzaldehyde

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25
Q

How do you name an aromatic compound if it has more than 1 substituent on a benzene chain?

A
  • The carbon atoms on the ring are numbered 1-6
  • Number the carbon atoms so that you use the smallest numbers possible
  • List the substituent groups in alphabetical order
    i.e: 2-bromomethylbenzene, 1,4-dichlorobenzene, 3-chloromethylbenzene
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26
Q

What is toluene?

A

Methylbenzene

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27
Q

What is phenol?

A

Hydroxybenzene

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28
Q

What is important when drawing aromatic compounds?

A

Sigma bonds coming off the benzene ring must be the same length as the bonds within the benzene ring

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29
Q

Name this molecule

A

2,4,6-trinitromethylbenzene

(2,4,6-trinitrotoluene aka TNT!)

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30
Q

Name this molecule

A

2-nitrophenol

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30
Q

Name this molecule

A

3-hydroxybenzoic acid

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31
Q

Name this molecule

A

2-ethylbenzoic acid

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32
Q

Name this molecule

A

benzene-1,2-diol

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33
Q

name this molecule

A

Phenylpropan-1-one

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34
Q

If the atom directly bonded to the carbon ring is also connected to a less electronegative atom, the substituent is:

IMPORTANT

A
  • Electron-donating
  • Activating
  • Ortho-para directing
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35
Q

If the atom directly bonded to the carbon ring is also connected to a more electronegative atom, the substituent is:

A
  • Electron-withdrawing
  • Deactivating
  • Meta directing
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36
Q

Name this molecule

A

2-phenylphenol

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37
Q

Name this molecule

A

2-benzyl-4-chlorophenol

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38
Q

What is the benzyl group?

A

Benzyl is a substituent group which consists of a benzene ring connected to a CH2 group connected to another group. C₆H₅CH₂-
The groups highlighted in blue are the benzyl group:

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39
Q

What is a Phenyl group?

A

A benzene ring as a substituent

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40
Q

What type of reaction mechanism does benzene undergo?

A

Electrophilic Aromatic Substitution

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41
Q

What are Activating groups?

A

Electron donating groups

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42
Q

What are deactivating groups?

A

Electron withdrawing groups

43
Q

What direction do deactivating groups direct towards?

A

Meta (With the exception of halogens)

44
Q

What direction do activating groups direct towards?

A

ortho-para

45
Q

What direction do halogens direct towards?

A

ortho-para
Despite being a deactivating group, halogens are 2-, 4- (ortho-, para-) directing. This is an exception to the rule as all other deactivating groups are meta directors

46
Q

If there are multiple groups bonded to the benzene, what determines the direction of the incoming group?

A

If there are multiple groups bonded to the benzene, then the most activating or the least deactivating group is what directs the incoming group

47
Q

Is benzene stable or reactive?

A

Stable

48
Q

What is the typical electrophilic substitution reaction for benzene?

A

An electrophile is substituted in for a hydrogen

49
Q

What are the 2 mechanisms you have to know for benzene reactions?

A
  • Nitration of Benzene
  • Halogenation of Benzene
50
Q

What is the equation for the Nitration of benzene?

A

Benzene + Nitric acid -> Nitrobenzene + Water
Benzene reacts slowly with nitric acid to form nitrobenzene and water. Therefore, a sulfuric acid catalyst is used and it is heated to 50 degrees celcius

51
Q

What are the conditions for the nitration of benzene?

A
  • Sulfuric acid catalyst
  • 50 degrees celcius
52
Q

Why must the temperature be maintained at 50 degrees celcius in the nitration of benzene?

A
  • It must be raised to 50 from room temperature in order to increase the rate of reaction
  • However, any higher and further substitution may occur, leading to the production of dinitrobenzene
53
Q

What is the mechanism for the nitration of benzene?

IMPORTANT

A
54
Q

What are the uses of nitrobenzene?

A

It is an important starting material in the preparation of:
* Dyes
* Pharmaceuticals (such as paracetamol)
* Pesticides

55
Q

What is the nitronium ion?

A

NO₂⁺

56
Q

What is the bromonium ion?

A

Br⁺

57
Q

What is the reaction for the halogenation of benzene (use bromination as an example)

A

Benzene reacts with halogen with a halogen carrier catalyst to form halobenzene and hydrogen halide

58
Q

What are the conditions of halogenation of benzene?

A
  • Halogen carrier catalyst
59
Q

What is a halogen carrier?

A

a catalyst which consists of a metal bonded to halogens

60
Q

What are common halogen carrier catalysts?

A
  • FeBr₃
  • AlBr₃
61
Q

What is the mechanism for halogenation of benzene (use bromination as an example)

A
62
Q

Why does the halogenation of benzene require a halogen carrier as a catalyst?

A

Benzene is too stable to directly react with a non-polar halogen molecule, therefore it must react with the positive ion

63
Q

What is a Friedel-crafts alkylation reaction?

A

The substitution of a hydrogen atom in the benzene ring by an alkyl group

64
Q

What is the reaction for a Friedel-crafts alkylation reaction?

A

Benzene + haloalkane -> alkylbenzene + hydrogen halide

65
Q

What is the catalyst in a Friedel-crafts alkylation reaction?

A

AlCl₃
Aluminium Chloride

66
Q

What is an acylation benzene reaction?

A

When benzene reacts with an acyl chloride in the presence of aluminium chloride (AlCl₃) catalyst, an aromatic ketone is formed

67
Q

What is the reaction for the acylation of benzene?

A

Benzene + acyl chloride -> phenylketone + hydrogen halide

68
Q

What are the conditions for the acylation of benzene?

A

AlCl₃ catalst

69
Q

Why are alkenes more reactive than arenes?

A

Alkenes contain an area of localised electron density (within the pi bond), whereas arenes do no have this localised charge as the electrons are delocalised in the benzene ring. This makes alkenes more reactive than arenes

70
Q

Why can alkenes discolour bromine water but arenes cannot?

A

The pi bond in the alkene contains localised electrons above and below the plane of the 2 carbon atoms. This produces an area of high electron density. This area of high electron density induces a dipole in the non-polar bromine molecule allowing it to act as an electrophile and allowing an electrophilic addition reaction to occur. However, unlike alkenes benzene does not contain this localised charge (as the pi electrons are delocalised in the ring structure) and therefore cannot induce a dipole in the benzene ring

71
Q

How can you determine if a group is activating/electron donating or deactivating/electron withdrawing?

A

Determine if the group will push electron density into the ring or draw it out of the ring by considering the electronegativity of all neighbouring atoms and any lone pairs present

72
Q

Where is the ortho position?

A

carbon atoms 2 and 6

73
Q

Where is the meta position?

A

Carbon atoms 3 and 5

74
Q

Where is the para position?

A

Carbon atom 4

75
Q

How can directing effects be used in organic synthesis?

A

The directing effect of substituent groups can be used when planning an organic synthesis, When carrying out more than one electrophilic substitution reaction on an aromatic compound, you may have to consider the order in which the reactions are carried out to ensure the correct substitution pattern and that the required product is prepared

76
Q
A
77
Q

Are alcohols or phenols more soluble in water and why?

A

Alcohols are more soluble than phenols. This is because phenols have the presence of the non-polar benzene ring causing it to be less soluble in the polar solvent water

78
Q

Is phenol or alcohols more acidic?

A

Phenol is more reactive than alcohols as it partially dissociates in solution. It is classified as a weak acid

79
Q

Why is phenol classified as a weak acid?

A

Phenol partially dissociates in solution to form a proton and the phenoxide ion

80
Q

What is the equation for phenol partially dissociating?

A

Phenol ⇌ Phenoxide ion + Proton

81
Q

What is the phenoxide ion?

A
82
Q

How can you distinguish between alcohols, phenols, and carboxylic acids?

A

Phenols are more acidic than alcohols but less acidic than carboxylic acids, this can be used to distinguish them:
* Alcohols do not react with strong bases (sodium hydroxide) or weak bases (sodium carbonate)
* Phenols react with strong bases (sodium hydroxide) but do not react with weak bases (sodium carbonate)
* Carboxylic acids react with strong bases (sodium hydroxide) and weak bases (sodium carbonate)

83
Q

What is an example of a strong base?

A

Sodium hydroxide

84
Q

What is an example of a weak base?

A

Sodium carbonate

85
Q

What is the reaction of phenol with sodium hydroxide?

A

Phenol + Sodium Hydroxide -> Sodium Phenoxide + Water

86
Q

What is the reaction for the bromination of phenol?

A

Phenol + 3Br₂ -> 2,4,6-tribromophenol + 3 HBr

87
Q

What are the observations of the addition of phenol to bromine water?

A
  • Bromine water is decolourised
  • A white precipitate is formed
88
Q

What are the conditions for the bromination of phenol compared to benzene?

A

It reacts at room temperature and does not require a halogen carrier catalyst

89
Q

What is the reaction for the nitration of phenol?

A

Phenol reacts readily with nitric acid at room temperature to form a mixture of 2-nitrophenol and 4-nitrophenol

90
Q

What are the conditions for the nitration of phenol compared to benzene?

A

Phenol reacts with much milder conditions, it takes place at room temperature and does not require a sulfuric acid catalyst

91
Q

Compare and explain the difference in reactivity of phenol and benzene

A

Phenol reacts much more readily with bromine and nitric acid than benzene does. This increased reactivity is caused by the lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the pi-system. therefore it is acting as an activating group causing the benzene ring to be more reactive

92
Q

Why does an activating group make the benzene ring more reactive?

A

The activating group pushes electron density into the benzene ring. This increased electron density attracts electrophiles more readily and it is therefore more susceptible to attack from electrophiles.

93
Q

How do activating and deactivating groups affect the reactivity of the benzene ring?

A
  • Activating groups - increase reactivity
  • Deactivating groups - decrease reactivity
94
Q

Why can phenol react with bromine without a catalyst?

A

The oxygen atom in the hydroxy group pushes electron density into the benzene ring. This increased electron density allows the phenol ring to polarise bromine molecules and therefore no halogen carrier catalyst is required

95
Q

Describe the direction and type of “-COOH” as a directing group

A
  • Deactivating (electron withdrawing group)
  • 3 (meta) directing
96
Q

Describe the direction and type of “-CN” as a directing group

A
  • Deactivating (electron withdrawing group)
  • 3 (meta) directing
97
Q

Describe the direction and type of “-OR” as a directing group

A
  • Activating (Electron Donating Group)
  • 2,4 (ortho, para directing)
98
Q

Describe the direction and type of “-NHCOR” as a directing group

A
  • Activating (Electron Donating Group)
  • 2,4 (ortho, para directing)
99
Q

Describe the direction and type of “-NH₂” as a directing group

A
  • Activating (Electron Donating Group)
  • 2,4 (ortho, para directing)
100
Q

Describe the direction and type of “-NO₂” as a directing group

A
  • Deactivating (electron withdrawing group)
  • 3 (meta) directing
101
Q

Explain the Electrophilic Aromatic substitution mechanism with benzene

A
102
Q

What is the difference between “Phenol” and “Phenols”?

A
  • Phenol is a specific molecule, it consists of a single hydroxyl group bonded directly to a benzene ring and no other substituents
  • Phenols are a family of molecules, they are any molecules which contain a hydroxyl group bonded directly to the benzene ring
103
Q

Which of these molecules is not a phenol?

A

The fourth molecule is not a phenol as the hydroxyl group is not bonded directly to the benzene ring. It is instead an aromatic alcohol

104
Q

What role does sulfuric acid play in the nitration of benzene?

A

It protonates the nitric acid, which leads to the loss of water and the formation of a nitronium ion

105
Q

Name this molecule:

A

3-methylcyclohexene

Methyl group is on carbon 3

106
Q
A

A Level Chemistry (31 decks)

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