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S2 Case 3 - asthma & COPD > Heteroaromatic Chemistry > Flashcards

Heteroaromatic Chemistry Flashcards

1
Q

Why is benzene stable?

A

Due to delocalisation of e-

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2
Q

When does aromaticity occur?

A

Compound has uninterrupted cyclic cloud of p e- above + below plane of planar molecule

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3
Q

What should the e- cloud contain in aromaticity?

A

Odd number of pairs of p e-

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4
Q

What is Huckel’s rule?

A

For planar, cyclic compound to be aromatic must contain uninterrupted p cloud with 4n + 2 e-

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5
Q

What are examples of heterocycles in nature?

A

DNA
Haemoglobin

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6
Q

What is a heterocycle?

A

Any ring containing at least 1 heteroatom (NOT C)

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7
Q

What are some heteroatoms?

A

N
O
S

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8
Q

What set of hybrid orbitals are in benzene?

A

sp2

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9
Q

What do the p-orbitals in benzene generate?

A

Pi network

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10
Q

How is the pi system formed?

A

Each p e- is delocalised over 6 Cs creating pi system above + below plane of hexagon

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11
Q

What is pyridine?

A

Derived from benzene
CH is replaced with a sp2 hybridised N atom

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12
Q

Why is there a permanent dipole towards N in pyridine?

A

Inductive effects + resonance work in the same direction

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13
Q

What does the N do in pyridine?

A

Pull e- towards itself
= more electronegative than C

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14
Q

Where are pyridine’s partial positive charges?

A

Alpha position
Gamma position

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15
Q

What are the alpha positions?

A

2 + 6

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16
Q

What are the gamma positions?

A

4

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17
Q

What is pyridine termed as?

A

“Electron poor”
OR
Pi deficient system

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18
Q

What does the electronegative N in pyridine cause?

A

Inductive polarisation = additional structures = resonance

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19
Q

What is the pKa of pyridine?

A

5.2 = WB

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20
Q

What does in the N in pyridine make it?

A

Basic
NOT the reason for it being aromatic

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21
Q

Can pyridine undergo electrophilic aromatic substitution?

A

YES, BUT harder than with benzene

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22
Q

What are the physiochemical properties of pyridine?

A

Soluble in H2O
H bonding possible

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23
Q

What is the N in pyridine?

A

HBA

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24
Q

What are most drugs absorbed by?

A

Passive diffusion

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25
Q

What happens if a drug has a basic centre?

A

Make a salt = more soluble than parent compound

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26
Q

Describe H2O solubility of drug absorption

A

Almost all drugs are H2O soluble to undergo passive diffusion to some extent

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27
Q

Describe lipid solubility of drug absorption

A

Passive diffusion depends mainly on lipid solubility

28
Q

Why must molecule be unionised?

A

As only unionised form diffuses

29
Q

What does ionisation depend on?

A

Depends on pKa of drug + pH of solution

30
Q

What do EDGs do?

A

Increase the HDA ability + pKa value (= increase basicity)

31
Q

Because the N is NOT part of the pi system what can it do?

A

Behave as a nucleophile

32
Q

Describe pyridine N-oxides

A

More polar than pyridine
Made with per-oxy acid or hydrogen peroxide

33
Q

What does N-oxide promote?

A

Both electrophilic + nucleophilic substitution reactions

34
Q

Describe nucleophilic substitution in pyridine

A

Occur more readily than in benzene
Positions 2 + 4

35
Q

What do EDGs facilitate?

A

Reactions via EsAR

36
Q

What do EWGs facilitate?

A

Reactions via SnAR

37
Q

Describe amino pyridines

A

More basic
Resonance between 2 + 4 amino isomers

38
Q

What do 2 + 4 amino isomers form?

39
Q

What do 3-isomer amino form?

A

Double salt

40
Q

What are all 3 hydroxy-pyridines subject to?

A

Tautomerism involving H exchange
= pyridones

41
Q

What positions do pyridones exclusively exist in?

42
Q

In 6 membered rings, where does electrophilic substitution occur?

A

On the benzene ring

43
Q

What happens if you add 2nd N?

A

Decrease pKa = decrease basicity

44
Q

Describe other N containing 6-membered rings

A

Weaker bases than pyridine
All aromatic
Less reactive

45
Q

What does acidic sulphonamide group do?

A

Give up H+
= conjugate base has resonance

46
Q

What are the common 5-membered heterocycles in single system rings?

A

Pyrrole - NH
Furan - O
Imidazole - NNH
Thiophene - S

47
Q

What are some common 5-membered heterocycles in fused ring systems?

A

Indole - NH
Benzimidazole - NNH
Benzothiophene - S
Benzofuran - O

48
Q

What do 5-membered rings not fulfil + why?

A

Huckel criteria
= each molecule only appears to have 4 pi-e-
= 1 L.P used to make system

49
Q

Are 5-membered rings aromatic?

50
Q

Describe pyrrole L.P

A

Delocalised = NOT available for donation to E+

51
Q

What is the pKa of pyrrole?

A

-4
= non-basic

52
Q

What are furan + thiophene?

A

Analogous in structure to pyrrole

53
Q

What are difference between furan & thiophene compared to pyrrole?

A

Heteroatoms (O + S) have 2 L.Ps of e-

54
Q

Describe the L.Ps in furan + thiophene

A

1 L.P delocalised = aromaticity
2nd located in sp2 orbital

55
Q

Describe order of electronegativity in S, O + N

56
Q

Describe the order of aromaticity in pyrrole, thiophene + furan

A

Thiophene > pyrrole > furan

57
Q

What is the reactivity of pyrrole, furan + thiophene dominated by?

A

Their readiness to undergo electrophilic substitution

58
Q

Describe the order of reactivity in pyrrole, thiophene + furan?

A

Pyrrole > furan > thiophene

59
Q

Where does electrophilic attack mostly occur in pyrrole, thiophene + furan?

A

2-position

60
Q

Does pyrrole, thiophene + furan all undergo EAS?

61
Q

Describe oxazole

A

5-membered ring system
N + O
Heterocycle is aromatic
WB = pKa = 0.8

62
Q

Describe imidazole

A

5-membered ring system
NN - 1 like pyridine, 1 like pyrrole
pKa = 7
N = sp2

63
Q

What is important about imidazole?

A

Soluble in H2O
= forms extensive H bonding networks

64
Q

Describe tautomerism in imidazole

A

Rapid transfer of H from one nitrogen to the other
2 tautomers in inseparable eqm mixture

65
Q

What is it called when H-atoms change position in heterocyclic systems?

A

Annular tautomerisations

66
Q

What are bicyclic heterocycles?

A

Indoles
= benzo-fused pyrroles

67
Q

Describe indole reactivity

A

Reactive pyrrole + unreactive benzene

S2 Case 3 - asthma & COPD (25 decks)

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