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Pharmacy Stage 2 > Aromatics Directing Effects 1+2 > Flashcards

Aromatics Directing Effects 1+2 Flashcards

1
Q

What type of compound is benzene?

A

An planar, aromatic compound

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2
Q

What makes benzene stable?

A

The delocalised electrons within the benzene ring.

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3
Q

The resonance forms of benzene are of equal energy true or false?

A

True

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4
Q

Define Electrophile?

A

A species which attracts electrons towards itself

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5
Q

Define Nucleophile?

A

A lone electron pair donor.

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6
Q

Define Oxidation?

A

The removal of electrons from a molecule (OIL)

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7
Q

Define Reduction?

A

The addition of electrons to a molecule (RIG)

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8
Q

Define Bases

A

Electron Rich species (Can be negatively charged)

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9
Q

Define Acids

A

Electron Deficient species (Can be positively charged)

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10
Q

What is a negative charged species?

A

contains electrons (nucleophiles/bases)

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11
Q

What is a positive charged species?

A

devoid of electrons (electrophiles/acids)

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12
Q

What are arrows used for?

A

A full bladed arrow represents the movement of 2 electrons

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13
Q

What are dots used for?

A

one dot represents 1 valence electron (e.g. radicals)

2 dots represents 2 valence electrons (e.g. nucleophiles)

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14
Q

Why do the resonance forms of benzene have equal energies?

A

because of the uninterrupted cyclic cloud of p electrons above and below the plane of benzene (a planar molecule) and there is an odd number of pairs of p electrons.

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15
Q

What is Huckel’s Rule?

A

For a planar cyclic compound to be aromatic, it must contain and uninterrupted p cloud with (4n+2) p electrons. Where n is any whole number.

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16
Q

What is the aromaticity of benzene?

A

pi system

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17
Q

What is aromaticity?

A

a term used to describe a cyclic, planar molecule with a ring of resonance bonds that exhibits more stability than other geometric or connective arrangements with the same set of atoms.

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18
Q

What structure is preferred for drawing mechanisms using the benzene ring?

A

Kekule’s structure.

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19
Q

Why does benzene not react with electrophiles like alkenes do?

A

Because it is less reactive than alkenes due to it’s aromatic stabilisation.

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20
Q

What mechanism is used to brominate benzene?

A

Electrophilic Substitution

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21
Q

What is required to brominate benzene?

A

An acid catalyst e.g. AlCl3

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22
Q

How does the presence of an acid catalyst allow for benzene to do an electrophilic substitution reaction?

A

The acid catalyst makes Br2 more electrophilic (look at slide 12 for mechanism)

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23
Q

What is a wheland intermediate?

A

The carbocation formed after the addition of an electrophile to the benzene ring.

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24
Q

How is the wheland intermediate stabilised?

A

By the resonance forms

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25
Q

Wha drives re-aromatisation?

A

Aromatic stabilisation

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26
Q

What is used to analyse the wheland intermediate?

A

NMR

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27
Q

What two substances are required for the nutrition of benzene?
And draw the mechanism

A

Nitric acid and sulfuric acid (the catalyst)

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28
Q

What three substances are needed for the sulfonylation of benzene to form crystalline sodium benzene sulfonate?
And draw the mechanism!

A

SO3
Excess H2SO4
NaCl

29
Q

What is required for alkylation and acylation reactions?

A

An acid catalyst (mainly Lewis acids)

30
Q

What is the difference in reactivity of the starting material and product after undergoing the Friedel-Crafts alkylation reaction?

A

The product is more reactive than the starting material

31
Q

What are the three types of precursors that can be employed in the Friedel-Crafts alkylation reaction?

A
  • alkyl halide
  • aliphatic alcohols
  • alkenes
32
Q

What is used as the catalyst for reacting benzene with methyl or ethyl halide?

A

FeCl3

33
Q

What is used as the catalyst to react alkyl halides, from propyl onwards, with benzene

A

AlCl3 (a Lewis acid)

34
Q

Why is there a mixture of products when reacting benzene with alkyl halides with 3 or more carbons?

A

Because the alkyl halide and AlCl3 react to form varying carbocations (primary and secondary carbocations).

35
Q

Why is there more of the products formed by the secondary carbocation than the primary carbocation when reacting alkyl halides with benzene?

A

Because the secondary carbocation is more stable than the primary carbocation.

36
Q

What happens to the stability and energy as you go from methyl to primary to secondary and then to tertiary alkyl carbocations?

A

Stability increases but energy decreases

37
Q

How do we get a monoalkyl product?

A

Use excess benzene

38
Q

What is required in the alkylation process with alcohols?

A

alcohol, strong acid (e.g. H2SO4) and this is used to produce the carbocation to react with benzene.

39
Q

How does alkylation with olefins work?

A

Alkenes react with benzene in the presence of a suitable catalyst (e.g. HF, BF3) to produce alkylbenzenes.

40
Q

How does friedel-crafts acylation work?

A

Acid chlorides (acyl halides) and acid anhydrides can both react with benzene in the presence of molar amounts of Lewis acids to produce ketones.

41
Q

What product is formed from Friedel-Crafts acylation reaction?

A

Aryl ketone which is more basic than the starting product (acyl halide)

42
Q

Why is water and acid added at the end of the Friedel-Crafts acylation reaction?

A

To hydrolyse any Lewis acid remaining hence liberating the ketone.

43
Q

What is the reactive intermediate, formed from the reaction of acyl chloride with AlCl3, of the Friedel-Craft acylation mechanism known as?

A

Acylium ion

44
Q

Which mechanism happens faster acylation or alkylation?

A

Acylation

45
Q

What is electronegativity?

A

A measure of an elements ability to pull a pair of bonding electrons towards itself.

46
Q

What scale is used to measure electronegativity?

A

Pauling’s scale

47
Q

What causes a bond to be polar?

A

Where there’s a difference in the electronegativitiies of the elements bonded together.

48
Q

What is a dipole moment?

A

The permanent electric moment of a molecule

49
Q

What is the negative inductive effect?

A

When a species pulls electrons towards itself

50
Q

What is the positive inductive effect?

A

When a species pushes electrons away from itself.

51
Q

What happens to the inductive effect as the distance increases?

A

The effect falls off (weakens)

52
Q

Why is the meta derivative of benzoic acid stronger than the para isomer (derivative)

A

Because of resonance. It is a result of conjugation of a lone pair of electrons with the pi-system of the ring.

53
Q

What do the resonance structures show us?

A

The regions of increased/decreased electron density.

54
Q

What can stop resonance occurring?

A

Bulky substituents

55
Q

What is hyperconjugation?

A

A mechanism of both electron release and withdrawal. (happens via C-H and C-C bonds).

56
Q

Alkyl substituents donate electron density’s via two mechanisms, what are they?

A

positive inductive effect and hyperconjugation

57
Q

What is the effect of the positive inductive effect and hyperconjugation on the alkyl substituted benzene?

A

They make it more susceptible to electrophilic attack than benzene, making it more reactive than the starting material.

58
Q

Three electronic effects that operate when looking at substituted benzene derivatives?

A

Inductive effect
Resonance Effect
Hyperconjugative Effect

59
Q

Three compounds needed for the bromination of a phenol?

A

Br2, Ethanol, H2o

60
Q

What directing effect does -OH have on the benzene ring?

A

Ortho-Para directing (look at slide 46 of lecture for more info)

61
Q

When looking at HNMR to analyse directing effect, what does the proton shift indicate?

A

If the HNMR value decreases, then it shows an increase in electron density at those positions.

62
Q

When a benzene ring has an O substituent, where does the reacting elements attach to?

A

At the ortho and para sites. (look at slide 48 for more info)

63
Q

Effect of nitrogen substituent on the reactivity of benzene in comparison to the oxygen substituent?

A

Makes benzene even more reactive than the oxygen substituent did. (shown by the lower HNMR values at the para and ortho positions with nitrogen than with oxygen-slide 51)

64
Q

How can we reduce the effect of the nitrogen substituent on the benzene ring?

A

By reducing the electron donating power of the nitrogen substituent e.g. pull electrons away from the ring, making it a poorer nucleophile, hence less reactive.

65
Q

Effect of a methyl substituent on the reactivity of benzene?

A

It increases the reactivity of benzene at the ortho and para positions but not as much as the oxygen and nitrogen substituents do.

66
Q

How does methyl increase the reactivity of benzene?

A

It is a weak conjugative electron donor hence increases electron density in the benzene ring.
(The C-H sigma bond of the methyl overlaps with the pi-system of benzene donating its electron density to benzene’s pi-system. The electrons flow between the orbitals.)

67
Q

What intermediate is formed by the substitution of an electrophile on a benzene ring?

A

A wheland intermediate

68
Q

What is the most favourable resonance form of the benzene with a methyl substituent?

A

The one with the positive charge at the carbon bonded to the methyl group that is pushing electrons towards it.

Pharmacy Stage 2 (85 decks)

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