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a-level oxford chem > 3.3.10 Aromatic Chemistry > Flashcards

3.3.10 Aromatic Chemistry Flashcards

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1
Q

What is the molecular formula of benzene?

A

C6H6

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2
Q

State the structure of benzene

A

planar cyclic

(6 carbons are joined together in a flat ring)

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3
Q

Draw the skeletal formula for benzene

A
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4
Q

Describe the structure of benzene

A
  • Each C atom forms single covalent bonds to C on either side and to 1 H
  • Final unpaired electron on each C atom is located in p-orbital that sticks out above and below the plane of ring
  • P-orbitals on each C atom combine to form ring of delocalised electrons
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5
Q

Describe the carbon-carbon bonds in benzene

A

All carbon-carbon bonds in ring are same = so same length (140 pm)

Lies between the length of a single C-C bond (154 pm) and a double C=C bond (135 pm)

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6
Q

Why is benzene very stable?

A

∵ delocalised ring of electrons

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7
Q

State how you can prove that benzene is far more stable than theoretical compound cyclohexa-1,3,5-triene (where ring would be made up of alternating single and double bonds)

A

By comparing enthalpy change of hydrogenation for benzenes with enthalpy change of hydrogenation for cyclohexene

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8
Q

Cyclohexene has 1 double bond and when it’s hydrogenated, the enthalpy change is -120 kJ mol-1. State what the theoretical enthalpy of hydrogenation would be if benzene had 3 double bonds.

A

-360 kJ mol-1

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9
Q

Describe how the experimental enthalpy of hydrogenation of benzene is different to the theoretical value of -360 kJ mol-1

A
  • Amount of energy is less
    • far less exothermic than expected
  • (Experimental enthalpy of hydrogenation of benzene is -208 kJ mol-1)
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10
Q

Explain the difference between the theoretical enthalpy of hydrogenation of benzene and the experimental value.

A
  1. Energy is put in to break bonds and released when bonds are made
  2. More energy must have been put in to break bonds in benzene than would be needed to break bonds in a theoretical cyclohexa-1,3,5-triene molecule
  3. Difference indicates benzene is more stable than cyclohexa-1,3,5-triene would be
    • Thought to be due to delocalised ring of electrons
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11
Q

What are aromatic compounds or arenes?

A

Compounds containing a benzene ring

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12
Q

Name the compound

A
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13
Q

Name the compound

A
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14
Q

Name the compound

A
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15
Q

Name the compound

A
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16
Q

Name the compound

A

In other molecules the benzene ring can be regarded as a substituent side group on another molecule, like alkyl groups are. The C6H5 - group is known as the phenyl group

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17
Q

Explain why arenes attract electrophiles

A

Benzene ring is a region of high electron density so it attracts electrophiles

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18
Q

Explain why arenes undergo electrophilic substitution opposed to electrophilic addition

A
  • As benzene ring’s so stable, doesn’t undergo electrophilic addition reactions, which would destroy delocalised ring of electrons
  • Instead undergoes electrophilic substitution reactions where one of H atoms (or another functional group) is substituted for electrophile
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19
Q

Give 2 examples of useful chemicals that contain benzene rings

A

Dyes and pharmaceuticals

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20
Q

Why is it tricky to make chemicals that contain benzene

A

∵ benzene is so stable = fairly unreactive

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21
Q

What does Friedel-Crafts acylation do to benzene rings?

A
  • Adds acyl group (RCO-) to benzene ring
  • Once acyl group has been added, side chains can be modified using further reactions to make useful products
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22
Q

Why can’t most electrophiles attack a benzene ring?

A

Most aren’t polarised enough

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23
Q

How can electrophiles be made into stronger electrophiles?

A

By using a catalyst called a halogen carrier

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24
Q

State what electrophile Friedel-Crafts acylation uses

A

Acyl chloride

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25
Q

Give an example of halogen carrier that Friedel-Crafts acylation could use

A

AlCl3

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26
Q

Describe how AlCl3 makes the acyl chloride electrophile stronger

A
  • AlCl3 accepts lone pair of electrons from acyl chloride
  • As lone pair of electrons is pulled away, polarisation in acyl chloride increases and it forms a carbocation
  • Makes it stronger electrophile and gives it a strong enough charge to react with benzene ring
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27
Q

Draw a diagram showing how AlCl3 makes an acyl chloride electrophile stronger

A
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28
Q

Draw the mechanism for Friedel-Crafts acylation (electrophilic substitution)

A
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29
Q

State the conditions for Friedel-Crafts acylation to occur

A

Reactants need to heated under reflux in non-aqueous solvent (like dry ether) for reaction to occur

30
Q

Describe nitration briefly

A

When you warm benzene with concentrated nitric and sulfuric acids = nitrobenzene

31
Q

State the catalyst used in nitration

A

Sulfuric acid

32
Q

Nitration

Why is sulfuric acid used as a catalyst?

A

Helps to make nitronium ion NO2+ = electrophile

33
Q

Nitration

Write equations showing how the nitronium ion, electrophile, is made

A
34
Q

Draw the electrophilic substitution mechanism for nitration of benzene

A
35
Q

Nitration

State what you can do, if you want only 1 NO2 group (mononitration) substituted?

A

Need to keep temperature below 55°C

Above this temperature = get lots of substitutions

36
Q

Name 2 uses of nitration reactions

A
  1. Nitro compounds can be reduced to form aromatic amines
    • Use to manufacture dyes and pharmaceuticals
  2. Some nitro compounds can be used as explosives
    • e.g. 2,4,6-trinitromethylbenzene (TNT)
37
Q

What happens when methylbenzene is nitrated with a mixture of nitric and sulfuric acid?

A

Produces mixture of 1-methyl-2-nitrobenzene and 1-methyl-4-nitrobenzene

(More vigorous conditions + more nitro-groups substituted)

38
Q

How can a mixture of 1-methyl-2-nitrobenzene and 1-methyl-4-nitrobenzene be separated?

A

By thin layer chromatography

39
Q

Sulfonation is an important step in…

A

synthesis including the manufacture of surfactant and sulfonamides

40
Q

State the electrophile in sulfonation

A

SO3

41
Q

Why is SO3 polarised?

A

∵ oxygen is more electronegative than sulfur

42
Q

Where is SO3 present? i.e. in what acid

A
  • Present in sulfuric acid
    • H2SO4 ⇌ SO3 + H2O
  • And in fuming sulfuric acid (H2S2O7)
    • Solution of SO3 in concentrated H2SO4
43
Q

Draw the electrophile SO3

A
44
Q

Draw the mechanism (electrophilic substitution) for sulfonation of benzene

A
45
Q

State the product when benzene is sulfonated

A

benzenesulfonic acid

46
Q

Write an equation showing when benzene is sulfonated

A
47
Q

Describe how alkylation (variant of Friedel Crafts acylation reaction) generates an electrophile

A

Uses chloroalkane as reagent with catalyst of aluminium chloride to generate electrophile

48
Q

Alkylation

Write an equation showing how the electrophile is generated from aluminium chloride and a chloroalkane (e.g. CH3Cl)

A

CH3Cl + AlCl3 → CH3+ + AlCl4-

49
Q

What products do alkylation produce?

A

alkyl benzene

e.g. benzene reacts to form methylbenzene

50
Q

State the mechanism used in alkylation

A

electrophilic substitution

51
Q

Write an equation showing how the alkylation benzene yields methylbenzene e.g. with electrophile = CH3+

A
52
Q

Describe how methylbenzene reacts compared to benzene

A

Reacts similarly to benzene but is more reactive than benzene towards electrophilic substitution

53
Q

Explain why methylbenzene is more reactive than benzene towards electrophilic substitution

A

∵ methyl group releases electrons onto benzene ring = electrophile attracted more

54
Q

What does a sulfonation of methylbenzene produce?

A

Mixture of 2- and 4- substituted products

55
Q

What does free-radical attack by chlorine result in for benzene?

A

Ring addition to benzene

56
Q

What does free-radical attack by chlorine result in for methylbenzene?

A

Side chain substitution in methylbenzene

57
Q

Reaction of benzene with chlorine when exposed to UV is an _____ reaction

A

addition

58
Q

Free Radical Reactions

What is the final product when benzene reacts with chlorine that has been exposed to UV?

A

1,2,3,4,5,6-hexachlorocylohexane

(Exists as number of isomers)

59
Q

Free Radical Reactions

Draw an equation showing how benzene reacts with chlorine that has been exposed to UV

A
60
Q

Free Radical Reactions

Describe what products are produced when methylbenzene is boiled in presence of chlorine exposed to UV

A

Methylbenzene produces substitution products of methyl group

Behaves like an alkane rather than arene

61
Q

Free Radical Reactions

Draw a diagram showing how methylbenzene reacts when boiled in presence of chlorine exposed to UV

A
  • H atoms of methyl group are substituted by chlorine in a series of steps
  • Each step produces a molecule of HCl
62
Q

Reactions of Halogen-Substituted Arenes

Alkylbenzenes (e.g. methylbenzene) have halogen substituents on ring or alkyl group. The reactivities of these compounds differ according to…

A

where halogen atom is

63
Q

Describe how Cl is substituted in phenylchloromethane

A
  • Reacts similar way to chloromethane
  • Cl substituted by OH group by heating with alkaline solution containing -OH ions
    • Nucleophilic substitution
64
Q

Describe how Cl is substituted in 1-chloromethylbenzene (isomer of phenylchloromethane)

A
  • Cl substituted under very vigorous conditions
  • Molecule behaves like arene
  • Doesn’t easily react with nucleophiles ∵ of high electron density on benzene ring
65
Q

Explain why it’s harder to substitute the chlorine atom in 1-chloromethylbenzene than in phenylchloromethane

A
  • C-Cl bond in 1-chloromethylbenzene is stronger than in phenylchloromethane
  • ∵ electrons in filled p-orbitals on Cl atom interact with delocalised electrons on benzene ring
66
Q

How do substituents affect the reactivity of benzene ring towards electrophiles?

A

By releasing electrons onto ring or by withdrawing them

67
Q

Outline the electrophilic substitution mechanisms of alkylation using CH3+ as an electrophile

A
68
Q

Friedel-Crafts Acylation

Write an equation to show the role of aluminium chloride as a catalyst in this reaction (with ethanoyl chloride)

A

CH3COCl + AlCl3 → CH3CO+ + AlCl4-

69
Q

Friedel-Crafts Acylation

State the product formed

A

Phenylketone

70
Q

Explain why methylbenzene does not react easily with nucleophiles such as ammonia (2)

A
  • Delocalised ring is electron rich
  • Repels nucleophiles
71
Q

The formula of the ester is (CH3)C6H4COOCH2CH2CH3. State why this is not a structural formula. (1)

A

the CH3 group can be in different positions on the (aromatic) ring

a-level oxford chem (34 decks)

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