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Aromatic compounds Flashcards

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1
Q

What is the empirical formula of benzene?

A

CH

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2
Q

How was the empirical formula of benzene calculated?

A

Faraday conducted a combustion analysis on benzene

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3
Q

What is Kekule’s proposed structure of benzene?

A

A circle of 6 carbon atoms with alternating single and double bonds, and a single hydrogen atom on each carbon molecule.

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4
Q

What is the systematic name for Kekule’s benzene?

A

cyclohexa-1,3,5-triene

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5
Q

What four things distinguished kekule’s model from empirical observations of bond enthalpies?

A

Enthalpy of hydrogenation, reactivity with halogens, regularity of structure and IR spectroscopy.

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6
Q

Why is enthalpy of hydrogenation problematic for Kekule’s benzene model?

A

Using standard bond enthalpies, the calculated value for enthalpy of hydrogenation of Kekule’s benzene is much more exothermic than the empirical value (-372 to -208.5) suggesting a difference in reactivity.

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7
Q

Why is the bromination of benzene problematic for Kekule’s benzene model?

A

The C=C bonds in Kekule’s benzene would be expected to open up and accept bromine atoms via electrophilic addition, causing decolourisation of bromine water. This does not occur in practice, suggesting a difference in reactivity.

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8
Q

Why is X-ray crystallography problematic for Kekule’s benzene model?

A

X-ray crystallography showed benzene to have a regular hexagonal structure with equal bond lengths. However, according to Kekule’s model, the stronger C=C double bonds would hold C atoms closer together than the single bonds, giving a shorter bond length and hence an irregular structure.

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9
Q

Why is IR spectroscopy problematic for Kekule’s benzene model?

A

Different bond types absorb different frequencies of IR radiation. The IR spectrum for Kekule’s model should show peaks for C-C and C=C bonds. In reality, benzene does not absorb radiation in the expected range.

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10
Q

Describe the actual structure of benzene.

A

Benzene is formed from a hexagon of C atoms, bound by single bonds, and a pi cloud of electrons.

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11
Q

Describe the pi cloud of electrons.

A

A cloud of delocalised electrons formed by the sideways overlap of p orbitals above and below the plane of electrons.

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12
Q

Describe the relative strength of the bonds in benzene.

A

Bonds in benzene are stronger than C-C single bonds, but weaker than C=C double bonds.

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13
Q

What is the bond angle within benzene?

A

120 degrees

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14
Q

What is a phenyl group?

A

A benzene side group: -C6H5

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15
Q

What is the catalyst for the halogenation of benzene?

A

FeX3 or AlX3 (a halogen carrier catalyst)

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16
Q

Give a general equation for the reaction of benzene with a halogen.

A

Halogen + benzene —-> halobenzene + hydrogen halide

X + C6H6 —–> C6H5X + HX

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17
Q

How many substitutions occur in a standard halogenation of benzene?

A

A single halogen atom is substituted- monosubstitution

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18
Q

What is the nitration of benzene?

A

The substitution of an NO3 group to give nitrobenzene

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19
Q

What is the catalyst for the nitration of benzene?

A

Sulphuric acid

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20
Q

What are the conditions for the nitration of benzene?

A

H2SO4 catalyst and temperature of 50 degrees C

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21
Q

How many substitutions occur in a standard nitration of benzene?

A

1

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22
Q

How many NO3 groups can be substituted onto methylbenzene and in what positions?

A

3 groups in positions 2, 4 and 6

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23
Q

Why can iodine not react with benzene?

A

It is insufficiently reactive.

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24
Q

What reactant is used to substitute iodine onto benzene?

A

Iodine chloride (ICl)

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25
Q

Give an equation for the reaction of benzene with iodine chloride.

A

benzene + iodine chloride —–> iodobenzene + hydrochloric acid

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26
Q

Why is a catalyst not needed for the reaction of iodochlorine and benzene?

A

Because iodine chloride is a polar molecule

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27
Q

Why does the reaction of iodochloride and benzene not produce chlorobenzene?

A

Because iodine has the delta + charge so acts as the electrophile.

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28
Q

What is the alkylation of benzene?

A

The substitution of alkyl groups onto benzene molecules

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29
Q

What reagents are required for the alkylation of benzene?

A

The desired alkyl chloride/ bromide and benzene

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30
Q

What is the catalyst for the alkylation of benzene?

A

A halogen carrier

31
Q

What is acylation?

A

The formation of an aromatic ketone via the substitution of an acylchloride onto the aromatic ring.

32
Q

What reagant is used in the acylation of benzene?

A

An acylchloride (A species with a carbonyl and a chlorine group attached to the last C atom in the chain)

33
Q

Give a general equation for the acylation of benzene.

A

Acyl chloride + benzene —–> aromatic ketone + hydrogen chloride

34
Q

What catalyst is required for acylation?

A

AlCl3 or FeCl3

35
Q

Name the mechanism of nitration and halogenation.

A

Electrophilic substitution

36
Q

How is the electrophile formed in nitration?

A

HNO3 + H2SO4 ——> HSO4- + NO2+ + H2O

37
Q

What is the electrophile in nitration?

A

NO2+

38
Q

Describe the mechanism of nitration.

A

Some electrons from the pi cloud of benzene are attracted to the nucleophile to form a new covalent bond and give benzene a +ve charge. The C-H bond on the carbon that NO3 has added to splits heterolytically and the electrons restore the pi cloud. An H+ ion is formed.

39
Q

Describe the intermediary stage of nitration

A

A benzene ‘smile’ is formed (an incomplete circle with a positive charge to show the incomplete pi cloud), with both the original hydrogen atom and the substituted group of NO3 attached to one carbon atom

40
Q

How is the catalyst in nitration regenerated?

A

HSO4- + H+ ——> H2SO4

41
Q

How does AlCl act as a catalyst in the halogenation of benzene?

A

It accepts a fourth chlorine atom to give AlCl4-, providing the Cl+ ion to act as an electrophile

42
Q

What is the electrophile in halogenation of benzene?

A

X+ (Cl+, I+, Br+)

43
Q

What is phenol?

A

An aromatic ring with an -OH group

44
Q

What is the apearance of phenol at room temperature?

A

A pale pink crystalline solid

45
Q

How does phenol interact with water?

A

It partially dissolves in water, leaving some precipitate

46
Q

How does phenol interact with sodium hydroxide?

A

It completely dissolves

47
Q

How does phenol solution interact with sodium carbonate?

A

No reaction occurs

48
Q

What can be observed when solid phenol reacts with bromine water?

A

Bromine water is decolourised and a white precipitate forms

49
Q

Give the reaction of phenol and water.

A

C6H5OH C6H5O- + H+

50
Q

How does phenol act as a weak acid?

A

It partially releases H+ ions in solution

51
Q

What reaction occurs between NaOH and phenol?

A

Neutralisation- H+ reacts with OH- ions to give water and two salts (Na+ and C6H5O-)

52
Q

Why is phenol soluble in NaOH

A

The products of the neutralisation of NaOH and phenol are ionic, and so soluble.

53
Q

Why does phenol have poor solubility in water?

A

It is a non-polar molecule.

54
Q

Why does phenol not react with carbonates?

A

Phenol is not sufficiently reactive and only very slightly dissociates in water.

55
Q

Describe how the structure of phenol leads to its acidity.

A

The lone pair on oxygen is drawn into the pi cloud of electrons in the aromatic ring, weakening the OH bond and allowing the H+ ion to be released. The pi cloud makes the phenoxide ion more stable.

56
Q

What is the reaction mechanism when phenol reacts with a halogen?

A

Electrophilic substitution

57
Q

Why is phenol more reactive than benzene?

A

The additional electrons in the phenol pi cloud provided by oxygen’s lone pair increase the electron density, making phenol more electronegative and therefore more prone to reaction with electrophiles that benzene.

58
Q

How does the halogenation of phenol differ from that of benzene?

A

It does not require a catalyst

59
Q

What is a deactivating group?

A

A side group that draws electrons away from the pi cloud of an aromatic ring.

60
Q

What is an activating group?

A

A side group that donates electrons to the pi cloud of an aromatic ring

61
Q

What do all activating groups have in common?

A

A lone electron pair

62
Q

What is a directing effect?

A

The tendency of a existing group on an aromatic ring to direct groups to substitute onto certain C atoms around the molecule

63
Q

What is an ortho para directing effect?

A

2, 4, 6 directing

64
Q

What is a meta directing effect?

A

3,5 directing

65
Q

What is the directing effect of a deactivating group?

A

meta directing (3,5 directing)

66
Q

What is the directing effect of an activating group?

A

ortho para directing (2, 4, 6 directing)

67
Q

Give two examples of activating groups.

A

OH, NH2

68
Q

Give an example of a deactivating group.

A

NO2

69
Q

How many substitutions occur with activating groups?

A

Usually two or three

70
Q

How many substitutions occur with deactivating groups?

A

One

71
Q

Which of activating or deactivating groups require a catalyst?

A

Deactivating groups

72
Q

How does an alkyl group behave as a directing group?

A

ortho para directing

73
Q

How does a halogen group behave as a directing group?

A

ortho para directing

74
Q

What is an amine group?

A

NH2

Chemistry (13 decks)

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