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Chemistry - Module 6 > 25-Aromatic Compounds > Flashcards

25-Aromatic Compounds Flashcards

1
Q

Benzene properties

A

• Colourless with a sweet smell
• Found naturally in crude oil
• Carcinogenic

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2
Q

Kekule model

A
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3
Q

Evidence to disprove kekule

A
  • lack of reactivity
  • length of c-c bonds
  • hydrogenation enthalpies
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4
Q

• Lack of reactivity

A

o Double bonds should decolourise bromine water
o Benzene does not discolourise bromine water
o Therefore doesn’t undergo electrophilic addition reactions
o Suggests benzene cant have any double bonds

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5
Q

• Length of C-C bonds

A

o Bond lengths can be measured using x-ray diffraction
o The bond length of a single bond = 0.153 nm
o Bond length of double bond = 0.134 nm
o In benzene, all the bonds were the same length (0.139 nm)

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6
Q

• Hydrogenation enthalpies

A

o The hydrogenation enthalpy of benzene is less exothermic than expected.
o The structure is more stable than the Kekulé model of benzene suggests.

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7
Q

Shape of benzene

A

trigonal planar

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8
Q

How is benzene formed

A

• The p-orbitals are above and below the plane of the benzene ring

• Sideways overlap of p-orbitals creates a delocalised ring of electron density above and below the plane of the benzene ring

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9
Q

Features of new benzene model

A
  1. Planar, cyclic, hexagonal hydrocarbon with 6 Cs and 6 Hs.
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10
Q

Main difference between kekules and new model

A

• In kekule or any double bond // electrons in pi bond are localized + in benzene they are delocalized

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11
Q

When is benzene a parent chain

A

Prefixes to benzene:

o Alkyl groups (any hydrocarbon chain with less than 7 C’s) – E.g. CH3, C2H5
o Halogens - F, Cl, Br
o Nitro groups - NO2

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12
Q

Name

A

propylbenzene

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13
Q

Name

A

bromobenzene

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14
Q

When is benzene considered a substituent chain

A

• An alkyl chain with a functional group
• An alkyl chain with seven or more carbon atoms

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15
Q

What to do when Benzene is a substituent chain

A

• In these cases, the prefix phenyl- is used

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16
Q

Name

A

2-phenyldecane

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17
Q

Name

A

3-phenylpropanoic acid

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18
Q

What to do with Compounds with more then 1 substituent on con benzene ring

A

• Need to number the carbon ring
• Numbering begins with one of the substituent groups
• Working out where the number starts – look to see if it is based on methylbenzene, chlorobenzene or nitrobenzene etc.
• The substituent groups are listed in alphabetical order using the smallest numbers possible

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19
Q

Practise

A
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20
Q

Explain the difference in reactivity between arenes and alkenes

A
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21
Q

Electrophilic substitution

A

• When benzene reacts with an electrophile, a hydrogen atom is replaced by another atom or group of atoms.

• The electrophile is attracted to the electron rich benzene ring.

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22
Q

What is the electrophile attracted to

A

electron rich benzene ring.

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23
Q

Electrophilic substitution of benzene and chloride ion

A
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24
Q

Electrophilic substitution with benzene - four types

A
  1. Nitration of Benzene
  2. Halogenation of Benzene (bromination and chlorination)
  3. Alkylation Reactions
  4. Acylation Reactions
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25
Q

Nitration of benzene

A

• reacts slowly with nitric acid to form nitrobenzene.

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26
Q

Conditions of Nitration of benzene

A

sulfuric acid catalyst and heat to 50°C (water bath)

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27
Q

Overall reaction Nitration of benzene

A

• One hydrogen atom is replaced by a nitro (-NO2) group

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28
Q

What happens at higher temps - Nitration of benzene

A

benzene will react with the excess nitric acid to form 1,3-dinitrobenzene.

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29
Q

Mechanism of nitration of benzene

A
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30
Q

Step 1 - Mechanism of nitration of benzene

A

formation of the electrophile = nitronium ion (NO2+)

• Formed by reacting concentrated nitric acid with concentrated sulfuric acid

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31
Q

Step 2 - Nitration of benzene

A

Electrophile (NO2+) reacts with benzene

• Electrophile accepts a pair of electrons from the benzene ring to form a dative covalent bond.

• Forms an unstable intermediate, which breaks down to form nitrobenzene and H+ ion

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32
Q

Horseshoe in intermediate rules

A
  • mist take up more then half of the ring
  • must be open at the position where substitution occurs
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33
Q

Don’t forget….. (nitration of benzene)

A

Ion in last drawing in step 2 (H+)

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34
Q

Step 3 - nitration of benzene

A

• H+ ion reacts with HSO4- generated in step 1 to reform the sulfuric acid catalyst.

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35
Q

• Halogens will ONLY react with benzene if…

A

there is a halogen carrier catalyst

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36
Q

Examples of halogen carrier catalysts

A

AlCl3, FeCl3, AlBr3, FeBr3

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37
Q

How are halogen carrier catalysts generated

A

in situ from the metal and the halogen

38
Q

Halogenation of benzene - conditions

A

halogen carrier catalyst appropriate one), room temp + pressure.

39
Q

Bromination - overall reaction

A
40
Q

Why is the bromonium ion = Br+ made

A

• Benzene too stable to react with bromine on own

41
Q

Bromination of benzene mechanism

A
42
Q

Chlorination of benzene mechanism

A
43
Q

Alkylation of benzene

A

• H atom replaced with an alkyl group - increases the number of carbon-carbon atoms in a compound

44
Q

Alkylation of benzene conditions

A

• React benzene with a haloalkane in the presence of AlCl3 which acts as a halogen carrier catalyst.

45
Q

Reaction of benzene and chloroethane:

A
46
Q

Acylation of benzene + conditions

A

• React benzene with an acyl chloride in the presence of halogen carrier catalyst

47
Q

What does acylation of benzene form

A

aromatic ketone

48
Q

Draw acyl chloride

A
49
Q

Acylation of benzene - mechanism

A
50
Q

Name

A
51
Q

What is a phenol

A

• -OH bonded directly to an aromatic ring (in this case benzene)

52
Q

What happens if -OH on side chain

A

Alcohol

53
Q

Formula for phenol

A

• C6H50H

54
Q

Name

A
55
Q

Why are alcohols and phenols different

A

due to the closeness of the delocalised ring affecting the –OH group in phenol

56
Q

What is more soluble in water - alcohol or phenol

A

Alcohol o

57
Q

Why is phenol less soluble

A

• Due to the presence of the non-polar (insoluble) benzene ring

• Phenol partially dissociates in water, releasing protons and phenoxide ions.
o Phenol is a weak acid

58
Q

What does this show

A

Phenol is a weak acid

59
Q

The higher the Ka,

A

The higher the acidity

60
Q

Acidity order

A

Alcohols < Phenols < Carboxylic Acids

61
Q

How to distinguish between Alcohols Phenols Carboxylic Acids

A

react with a weak base – sodium carbonate – and a strong base – sodium hydroxide

62
Q

Weak base

A

Sodium carbonate

63
Q

Strong base

A

Sodium hydroxide

64
Q

Results for distinguishing between Alcohols Phenols Carboxylic Acids

A
65
Q

Reaction of phenol with NaOH

A

• Neutralisation reaction / electrophilic substitution

66
Q

Reaction of phenol with NaOH - what does it form

A

• Forms salt – sodium phenoxide

67
Q

Overall reaction - phenol with NaOH

A
68
Q

What’s weird

A

No line between O- and Na +

69
Q

Why is there No line between O- and Na +

A

not a covalent bond

70
Q

Bromination of phenol - conditions

A

• Phenol reacts with bromine water: room temperature, no halogen carrier catalyst

71
Q

Observations in Bromination of phenol

A

• Bromine water will decolourise + white precipitate will form (2,4,6-tribromophenol)

72
Q

What forms when Bromination of phenol happens

A

2,4,6-tribromophenol

73
Q

Overall reaction Bromination of phenol

A
74
Q

Nitration of phenol + conditions

A

• reacts with dilute nitric acid at room temperature, no catalyst required

75
Q

Nitration of phenol - products

A

mixture of products: 2-nitrophenol, 4-nitrophenol

76
Q

Overall reaction - Nitration of phenol

A
77
Q

What reacts more readily phenol or benzene

A

Phenol

78
Q

Example of phenol reacting more readily then benzene

A
79
Q

What happens in phenol that makes it more readily react

A
80
Q

Why is no catalyst required for Bromination of phenol

A

Electron density of phenol ring is sufficient enough to polarise the bromine molecules

81
Q

Directing groups depend on…

A

functional group ALREADY present – not one being added on

82
Q

what does electron withdrawing functional groups mean

A

functional groups that withdraw electrons from the delocalised ring in benzene

83
Q

electron withdrawing groups also known as

A

deactivating groups

84
Q

what positions do electron withdrawing direct substitution at

A

3- position

(5- position too but thats technically the 3 position when flipped)

85
Q

example of electron withdrawing functional groups

A

NO2

86
Q

what does electron donating functional groups mean

A

functional groups that donate electrons from the delocalised ring in benzene

87
Q

electron donating groups also known as

A

activating groups

88
Q

what positions do electron donating direct substitution at

A

2, 4, 6

89
Q

examples of electron donating groups

A

OH

NH2

90
Q

is phenol electron donating / withdrawing + what position

A

OH

donating

2,4,6

91
Q

is NO2 electron donating / withdrawing + what position

A

withdrawing

3 position

92
Q

is NH2 electron donating / withdrawing + what position

A

donating

2,4,6

Chemistry - Module 6 (5 decks)

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