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Chemistry Y13 > Aromatic chemistry > Flashcards

Aromatic chemistry Flashcards

1
Q

BENZENE

A
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2
Q

what is aromatic chemistry?

A

the study of organic compounds containing benzene rings

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3
Q

Why is the name ‘aromatic’ used?

A

because a lot of these compounds have a distinct smell (an aroma)

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4
Q

what are aromatics important in the synthesis of?

A

aromatics are important in the synthesis of substances such as dyes, drugs, explosives and some plastics

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5
Q

what is the molecular formula for benzene?

A

C6H6

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6
Q

What is benzene said to be the ‘parent’ of?

A

benzene is said to be the ‘parent’ of a group of cyclic unsaturated compounds called the ARENES

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7
Q

what is the empirical formula for benzene?

A

CH

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8
Q

what is the molecular mass for benzene?

A

78

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9
Q

is benzene or alkenes more reactive?

A

benzene does not react like toher unstaurated compounds.
Benzene is fairly unreactive compared to the alkenes

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10
Q

How is benzene hydrogenated?

A

In the presence of Hydrogen gas + Nickel catalyst

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11
Q

Give the symbol equation to show the hydrogenation of benzene?

A

C6H6 +3H2 —–> C6H12
when benzene is hydrogenated, (h2 gas/Ni catalyst), 3 moles of H2 react with each mole of the benzene to form cyclohexane

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12
Q

what is formed when benzene is hydrogenated?

A

cyclohexane

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13
Q

What did Kekulè propose about the structure of benzene?

A

Kekulè proposed that benzene had a cyclic structure with alternating single and double carbon-carbon bonds

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14
Q

Draw the two proposed structures of benzene

A
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15
Q

what was benzene considered to be, but does not actually exist?

A

benzene is often considered to be a resonance hybrid of the 2 Kekulè structures, neither of which actually exists

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16
Q

what are the 4 main problems with kekulè structures?

A
  1. if benzene was unsaturated it should undergo electrophilic addition readily but it does not
  2. the reaction with benzene with chlorine, does in fact not produce 2 products as would be expected, instead only one product is formed (1,2-dichlorobenzene)
  3. X-ray diffraction studies have shown that the benzene molecule is planar and a regular hexagon. all single C-C bond lengths are equal (all 0.139nm). However, in the kekulè structure, the bond lengths would be different because single C-C bonds are longer than double C=C bonds. So c-c bond lengths would be (0.154nm) and c=c bonds to be (0.134nm)
  4. the enthalpy of hydrogenation value for benzene provides further evidence that the kekulè structures are incorrect
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17
Q

explain bond lengths in a real benzene molecule?

A
  • the c-c bonds are all of equal length and equal strength (intermediate between single and double)
  • Sigle c-c bonds are long and weak
  • benzene c-c bonds are intermediate length and strength
  • double c=c bonds are short and strong
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18
Q

enthalpies of hydrogenation

A
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19
Q

what evidence shows that benzene is more stable than kekulè proposed structure?

A

thermochemical evidence - enthalpies of hydrogenation

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20
Q

write out the symbol equation for the hydrogenation of cyclohexene

A
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21
Q

How much energy is released when cyclohexene is hydrogenated?

A

120KJ/mol-1

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22
Q

so when we hydrogenate kekulè structure (cyclohexa-1,3,5triene) we expect how much energy to be released?

A

3x more energy to be released than cyclohexene (has an extra 2 double bonds)

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23
Q

draw ou the symbol equation of kekulè structure being hydrogenated

A
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24
Q

what is the actual observed value for the hydrogenation of benzene?

A

208KJ/Mol-1

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25
Q

what do these results tell us?

A
  • this is 152KJ/Mol-1 less than expected
  • these results suggest that more energy is needed to break the ‘double bonds’ in benzene than to break 3 double bonds in an alkene
  • benzene is therefore more stable than expected and less energy is released when benzene molecules are hydrogenated
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26
Q

Delocalisation stability

A
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27
Q

Why is benzene more stable?

A

the explanation for the extra stability is that π electrons in the carbon- carbon double bonds are delocalised around the benzene ring. the delocalisation of the electrons bring more stability to the benzene molecule

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28
Q

draw the skeletal and structural formula of benzene

A
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29
Q

what does te circle in benzene represent?

A

the circle represents the 6 delocalised electrons and all the carbon atoms bonded identically

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30
Q

describe the structure and bonding in benzene?

A
  • the 6 carbon atoms in benzene are planar and arranged as a regular hexagon (all bond angles 120°)
  • all bonds in benzene are identical
  • each carbon atom is covalently bonded to 2 other carbon atoms and 1 hydrogen atom
  • therefore, each carbon atom has 1 ‘unused’ p-electron
  • delocalisation of the 6 ‘unused’ p-electrons produces electron density above and below the plane of the ring (pi electrons)
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31
Q

because the delocalisation satbilises the benzene molecule, this means that benzene is able to undergo?

A

overall addition reactions with difficulty

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32
Q

what is the delocalised energy/ resonance energy?

A

the increase in stability associated with electron delocalisation

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33
Q

Benzene - Electrophilic Substitution

A
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34
Q

Why is the benzene molecule attacked by electrophiles?

A
  • benzene has a region of high density above and below the plane of the molecule. so the molecule is attacked by positively charged species (electrophiles)
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35
Q

what reaction can benzene undergo but with difficulty?

A

Benzene can undergo addition (e.g hydrogenation) but only with difficulty (delocalisation energy is lost)

36
Q

Electrophilic substitution of benzene mechanism. What is step 1?

A

The formation of the electrophile (E+) - a positively charged species

37
Q

Which two main electrophiles are regenerated?

A

e.g +NO2 nitronium (for nitration), R-C(+)=O acylium cation (for Friedel-crafts reaction)

38
Q

describe what happens in step 2 of the reaction mechanism?

A
  1. Addition of the electrophile forms a..
  2. highly unstable cation intermediate which readily loses (eliminates) a proton (H+) to reform the stable delocalised ring
  3. left with substituted product
39
Q

why does benzene undergo substitution reactions and not really any other?

A

because these reactions retain the extra stability given by the delocalised electrons

40
Q

What is Nitration?
what is the electrophile for this reaction?

A

a nitro group is joined onto the benzene ring
+NO2 ( Nitronium ion)

41
Q

What is acylation?
what is the electrophile for this reaction?

A

a acyl group is joined onto a benzene ring (a Friedel-Crafts reaction)
acylium carbon R-C(+)=O

42
Q

what is the overall equation for Nitration?

A

Benzene + HNO3 —-conc H2SO4 + 50—–> Nitrobenzene + H2O

43
Q

what are the reagents for nitration?

A

concentrated HNO3 and concentrated H2SO4

44
Q

which reagent acts as a catalyst?

A

concentrated H2SO4 (homogenous)

45
Q

Draw and explain the electrophilic substitution of benzene via Nitration. Include both step 1 the summary of step 1 and full mechanism of step 2

A

Step 1: making the electrophile (+NO2)
HNO3 + 2H2SO4 —> +NO2 + H3O+ + 2HSO4-

summary:
HNO3 +H2SO4 —> [H2NO3]+ + HSO4-
[H2NO3]+ –> H20 + +NO2
H2O + H2SO4 –> H3O+ + HSO4-

Step 2: nucleophilic substitution of Nitronium ion

Electrophilic substitution of the benzene ring by the positive electrophile (E+)

highly unstable cation intermediate readily loses (eliminates) a proton (H+) to reform the stable delocalised ring

left with substituted product AND
H+ reformed

46
Q

what are nitrated benzene derivatives used as?

A

Explosives (e.g TNT)

47
Q

what does TNT stand for?

A

TriNitroTolulene or methyl-2,4,6-trinitrobenzene

48
Q

how is TNT formed?

A

when methylbenzene is nitrated

49
Q

what makes methylbenzene attractive to electrophiles? this means there are more..?

A

the methyl group is electron-releasing and so makes the benzene ring even more attractive to electrophiles. therefore more nitro groups present in the molecules

50
Q

the nitro group can be reduced to make..?

A

aromatic amines

51
Q

Which two processes can nitro groups be reduced by?

A
  • catalytic hydrogenation (H2/Ni)
  • using metal/acid combinations (Sn/HCl)
52
Q

the nitro benzene ring is then converted into..?

A

primary amines

53
Q

uses of primary aromatic amines?

A

very useful intermediates in organic synthesis

54
Q

what are primary aromatic amines used to make?

A

Synthetic dyes - (Azo dyes)

55
Q

Friedel-Crafts Acylation

A
56
Q

why is Friedel-crafts important in synthesis?

A

leads to the formation of the C-C bond

57
Q

what is the electrophile?

A

acylium cation

58
Q

what does benzene react with during electrophilic substitution of benzene via Friedel-Crafts mechanism?

A

reacts with acyl chlorides or acid anhydrides in the presence of an aluminium chloride catalyst

59
Q

what is formed?

A

A aromatic ketone

60
Q

what is the overall equation?

A

benzene + ethanoyl chloride –> phenylethanone + HCL

61
Q

what are the reagents for this reaction?

A

ethanoyl chloride and aluminium chloride catalyst

62
Q

Draw and explain the electrophilic substitution mechanism vias Friedel-crafts acylation.

A

Step 1; making the electrophile
CH3COCL + AlCl3 —> CH3-C(+)=O

Step 2: electrophilic substitution
draw

regeneration of the catalyst:
H+ + AlCl4- –> HCL + AlCl3

63
Q

what does the aluminum chloride catalyst act as?

A

a Lewis acid

64
Q

what is a Lewis acid?

A

Lewis acids act as electron pair acceptors

65
Q

how does the aluminium chloride catalyst help to form an electrophile?

A

the aluminium atom has a vacant orbital and can accept (attract0 a pair of electrons to activate a molecule to make an electrophile

66
Q

show how this reaction occurs in a symbol equation?

A
67
Q

What bond does AlCl4- contain?

A

contains a dative or coordinate bond (1 chlorine donates both electrons to make the covalent bond)

68
Q

when an acid anhydride is used instead of an acyl chloride, what does the regeneration of the catalyst look like?

A

(RCO)2O + AlCl3 –> R-C(+)=O + RCOOAlCl3-

69
Q

explain why multiple substitutions do not occur with acylation?

A

the acyl group is electron withdrawing and makes the ring less attractive to electrophiles (deactivates the ring)

70
Q

Naming Aromatics

A
71
Q

when are the four instances where the root of the name ‘benzene’ is used?

A
  1. an alkyl group (methyl, ethyl)
  2. a halogen (Br, Cl, F)
  3. a nitro group (NO2)
  4. a carboxylic acid group (COOH)
72
Q

how to name aromatics with one substituent?

A

include the substituent first then add benzene

73
Q

how to name aromatics with more than 1 substituent?

A

lowest possible numbers for ring positions and listed alphabetically (together with appropriate number)

74
Q

Which group takes priority over another group?

A

nitro group takes priority over methyl group when naming
(also carboxylic acid group takes priority over alcohol group)

75
Q

how to name aromatics when an alkyl group is attached to a benzene ring?

A

the carbon atom attached to the alkyl group linked to the ring is always allocated the prefix 1- e.g (1-methylpropyl) benzene

76
Q

what else can benzene ring be considered as?

A

a constituent

77
Q

when the benzene ring is regarded as a constituent, what is the benzene group known as?

A

phenyl group

78
Q

draw 2-phenylbutane. why is this not a straight chain with a butyl group?

A

the phenyl group is a substituent on carbon number 2

79
Q

what is hydroxybenzene also commonly known as?

A

Phenol - this can be used in exam questions - get used to this

80
Q

Draw out the structure of Phenylethanote and methylbenzenecarboxylate. why are they named like this?

A

phenylethanoate - benzene is on the –O- section of the ester functional group, so comes first as phenyl group

Methylbenzenecarboxylate - benzene is on the –C=O section of the ester func. group. the methyl group comes first, and then the ester is formed from benzenecarboxylic acid so has second name ‘benzenecarboxylate.

81
Q

Making Polystyrene

A
82
Q

what are the uses of Polystyrene/ (poly(phenylethene))?

A

is an extremely useful polymer used in packaging, insulation and cups

83
Q

draw the two structures of polystyrene

A
84
Q

how is a polymer of polystyrene made?

A

by the addition polymerisation of the monomer phenylethene

85
Q

draw the structure of phenylethene

A
86
Q

what is a possible synthetic route to making polystyrene from benzene?

Draw each route

A

step 1- Friedel-Crafts acylation (CH3COCl +AlCl3)
Step 2 - Reduction (NaBH4)
Step 3 - Dehydration using Conc. H3PO4
Step 4 - addition polymerisation

87
Q

FINISH

A

Chemistry Y13 (13 decks)

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