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Chemistry > Chapter 25 - Aromatic compounds > Flashcards

Chapter 25 - Aromatic compounds Flashcards

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1
Q

Describe benzene

A

A colourless, sweet smelling, highly flammable liquid

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2
Q

Where is benzene found ?

A
  • It is found naturally in crude oil

- Found in cigarette smoke

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3
Q

What is benzene classified as ?

A

A carcinogen

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4
Q

What is a carcinogen ?

A

Something that can cause cancer

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5
Q

What is benzene also classed as ?

A

An arene

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6
Q

What is the reactivity of benzene like ?

A

Even though it has three double bonds it is fairly unreactive

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7
Q

What does the reactivity of benzene disprove ?

A

It disproves the kekule structure of benzene

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8
Q

What did Kekule suggest the structure of benzene was ?

A

A six membered ring of carbon atoms joined by alternate single and double bonds

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9
Q

What are the three main evidences that disprove kekules structure ?

A
  • The lack of reactivity of benzene
  • The length of the C-C bonds in benzene
  • The hydrogenation enthalpies of benzene
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10
Q

Explain the lack of reactivity of benzene

A
  • If benzene contained C=C bonds it should decolourise bromine in an electrophilic addition reaction
    • However benzene does not undergo electrophilic addition reactions
    • It does not decolourise bromine under normal conditions
  • This means that it does not have C=C bonds
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11
Q

How can the length of bonds be measured in chemistry ?

A

Using the X - ray diffraction method

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12
Q

Explain the length of the C-C bonds in benzene

A
  • Bond length in benzene is 0.139 nm

- This is in between the bond length of a single and double bond

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13
Q

Explain the hydrogenation enthalpies of benzene

A
  • If benzene had the kekule structure then it would be expected to have an enthalpy change of hydrogenation that is three times that of cyclohexene
  • But it doesnt
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14
Q

According to Kekule what should the hydrogenation enthalpy of benzene be ?

A

-360 KJ/mol

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15
Q

What is the actual hydrogenation enthalpy of benzene and what does it mean ?

A
  • 208 KJ/mol

- This means that the actual structure is more stable than the kekule structure

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16
Q

What are the main features of the delocalised model of benzene ?

A
  • It is a planar, cyclic hexagonal hydrocarbon
  • Each carbon atom uses three of its available four electrons in bonding
  • Each carbon has one electron in a p orbital at right angles to the plane of the bonded carbon and hydrogen atoms
  • Adjacent p orbital electrons overlap sideways, above and below the plane of the carbon atoms to form a ring of electron density
  • This overlapping of the p orbitals creates a system of pi bonds which spread over all six of the carbon atoms in the ring structure
  • These electrons are said to be delocalised
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17
Q

Define monosubstituted

A

A compound that has one substituent group

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18
Q

In an aromatic compound, what is the benzene ring ?

A

The parent chain

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19
Q

When the benzene ring is attached to a functional group or a carbon chain longer than 7, what is the benzene ring ?

A

It is the substituent group

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20
Q

What kind of reactions does benzene undergo ?

A

Electrophilic substitution

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21
Q

What typically reacts with benzene ?

A
  • Electrophiles

- Positively charged

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22
Q

What type of reaction is the nitration of benzene ?

A

Electrophilic substitution

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23
Q

What are the conditions for the nitration of benzene ?

A
  • Sulphuric acid catalyst (H2SO4)
  • 50 degrees
  • Water bath is used
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24
Q

What does the nitration of benzene form ?

A

Nitrobenzene and water

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25
Q

What happens if the temperature rises above 50 degrees ?

A

Further substitution may occur

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26
Q

What does further substitution of the nitration of benzene form ?

A

It forms dinitrobenzene

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27
Q

Where is nitrobenzene important ?

A
  • Important starting material in the preparation of dyes, pharmaceuticals and pesticides
  • Starting material in the preparation of paracetamol
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28
Q

When do halogens react with benzene ?

A

In the presence of a halogen carrier catalyst

29
Q

Give examples of halogen carriers

A
  • AlCl3
  • AlBr3
  • FeCl3
  • FeBr3
30
Q

How are halogen carriers formed ?

A

In situ from the metal and the halogen

31
Q

What type of reaction is the bromination of benzene ?

A

Electrophilic substitution

32
Q

What are the conditions of the bromination of benzene ?

A
  • Room temperature and pressure

- Presence of a halogen carrier

33
Q

What is an alkylation reaction ?

A

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

34
Q

What are the conditions for an alkylation reaction ?

A

Benzene and a haloalkane in the presence of AlCl3

35
Q

How does alkylation affect the number of carbon atoms in a compound ?

A

Alkylation increases the number of carbon atoms in a compound by forming carbon carbon bonds

36
Q

What is an acylation reaction ?

A

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

37
Q

What are the conditions of an acylation reaction ?

A

AlCl3 catalyst

38
Q

What is the first member of the acyl chloride homologous series ?

A

Ethanoyl chloride

39
Q

Does benzene react with bromine ?

A

Only when a halogen carrier catalyst is present

40
Q

Why does benzene not react with bromine ?

A

This is because benzene has delocalised pi-electrons spread above and below the plane of the carbon atoms in the ring structure

41
Q

Compare the electron density in benzene and an alkene

A

The electron density around any two carbon atoms in the benzene ring is less than that in a C=C double bond in an alkene

42
Q

Why is a reaction prevented when a non polar molecule approaches benzene ?

A

There is insufficient pi-electron density around any two carbon atoms to polarise the non polar molecule

43
Q

What is a phenol ?

A

A type of organic chemical containing a hydroxyl functional group directly bonded to an aromatic ring

44
Q

What is the simplest member of the phenol group ?

A
  • Phenol

- C6H5OH

45
Q

How are phenols classified ?

A
  • It is a phenol if the benzene ring is directly bonded to an OH
  • If there is a carbon side chain and then an OH group, it is an alcohol rather than a phenol
46
Q

Compare the solubility of phenol and alcohol

A

Phenol is less soluble in water than alcohols

47
Q

Why is phenol less soluble in water than alcohols ?

A

This is due to the presence of the non polar benzene ring

48
Q

When dissolved in water what does phenol form ?

A

A phenoxide ion and a proton

49
Q

Why is phenol classified as a weak acid ?

A

Because of its ability to partially dissociate to produce protons

50
Q

Compare the acidity of phenol with alcohols and carboxylic acids

A

Phenols are more acidic than alcohols but less acidic than carboxylic acids

51
Q

Why are alcohols the least acidic ?

A

They do no react with either weak or strong bases

52
Q

Why are carboxylic acids the most acidic ?

A

They are strong enough to react with weak bases e.g. Na2CO3

53
Q

How can you tell the difference between a phenol and a carboxylic acid ?

A
  • A reaction with sodium carbonate can be used to distinguish between a phenol and a carboxylic acid
    • The carboxylic acid reacts with the sodium carbonate to produce CO2
54
Q

How does phenol react with NaOH ?

A

Forms the salt sodium phenoxide and water in a neutralisation reaction

55
Q

What kind of reactions do phenols undergo ?

A

Electrophilic substitutions

56
Q

How do phenols undergo electrophilic substitution reactions compared to benzene ?

A
  • Under milder conditions than benzene

- More readily than benzene

57
Q

How does phenol react with bromine water ?

A
  • Forms a white precipitate of 2,4,6-tribromophenol
  • The reaction decolourises the bromine water
    • Orange to colourless
58
Q

Conditions of the bromination of phenol

A
  • No halogen carrier catalyst

- Room temperature

59
Q

Why does the bromination of phenol not require a halogen carrier catalyst ?

A

The electron density in the phenol ring is sufficient to polarise the bromine molecules and so no halogen carrier catalyst is required

60
Q

How does phenol react with dilute nitric acid ?

A
  • Forms a mixture of :
    • 2-nitrophenol
    • 4-nitrophenol
61
Q

Compare the reactivity of phenol and benzene

A
  • Bromine and nitric acid react more readily with phenol than they do with benzene
  • Phenol is nitrated with dilute nitric acid rather than concentrated nitric acid as with benzene
62
Q

Why is there an increased reactivity with phenol ?

A
  • It is caused by a lone pair of electrons from the oxygen p orbital of the OH group being donated into the pi system of phenol
  • The electron density of the benzene ring in phenol is increased
  • The increased electron density attracts electrophiles more strongly than with benzene
63
Q

Define activation

A

An activating group increases the rate of electrophilic aromatic substitution compared to that of benzene

64
Q

Define deactivation

A

A deactivating group decreases the rate of electrophilic aromatic substitution compared to that of benzene

65
Q

Outline differences between phenol and nitrobenzene reacting with bromine

A
  • Phenol is rapid reaction, nitro is slower
  • Nitro needs halogen carrier
  • Phenol at room temp, nitro needs high temp
  • Phenol can have multiple substitutions, nitro can have only one
66
Q

What do activating groups do with their electrons in benzene?

A

Donate electrons

67
Q

What do deactivating groups do with their electrons in benzene?

A

Withdraw electrons

68
Q

Where do the NH2 and OH groups direct on benzene?

A

2,4 and 6 positions

69
Q

Where does the NO2 group direct on benzene?

A

3 and 5 positions

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