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CHY3108 Organic Caroll > L1/2/3 pyrrole chemistry > Flashcards

L1/2/3 pyrrole chemistry Flashcards

1
Q

What are the three bond lengths in pyrrole?

A

C-C 1.43 A
C=C 1.37 A
C-N 1.38 A

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2
Q

What is the resonance energy of pyrrole, compared to benzene?

A

90 kJmol-1

whereas benzene is 150kJmol-1

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3
Q

What makes a compound aromatic?

A

Obeys huckles rule of aromaticity (4n+2)

Cyclic series of overlapping p orbitals with delocalised pi bonding

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4
Q

What are the 1H NMR shifts of pyrrole, compare to benzene?

A

6.2 and 6.5 ppm, N-H is 10ppm

Benzene is around 7.3ppm

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5
Q

How does the alkene character of pyrrole compare to benzene?

A

It is more alkene like

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6
Q

What type of heteroaromatic is pyrrole?

A

Electron rich

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7
Q

Where does the lone pair on the nitrogen sit?

A

90 degrees to the plane of the ring which allows delocalisation into the aromatic system

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8
Q

Pyrrole contains both alpha and beta carbons, which is more nucleophillic?

A

Alpha

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9
Q

Which carbons in pyrrole are most reactive to electrophillic substitution?

A

C2 and C5

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10
Q

Which carbons in pyrrole are most reactive to electrophilic substitution?

A

C2 and C5

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11
Q

How is pyrrole as a base?

A

Very weak base

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12
Q

What is the pKa of pyrrole in its basic form?

A

-3.8

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13
Q

How does pyrrole interact with acid?

A

It is protonated with a strong acid at the C2 position

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14
Q

How is pyrrole as an acid?

A

It is a weak acid

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15
Q

What is the pKa of pyrrole in its acidic form?

A

17.5

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16
Q

Why is pyrrole a weak base?

A

The N lone pair is a part of the aromatic ring and protonation disrupts the aromaticity

17
Q

How does pyrrole interact with bases?

A

Deprotonation will occur with strong bases

18
Q

Which carbon atom does protonation of pyrrole occur at?

A

C-2

19
Q

In general, how does pyrrole react?

A

Electrophilic aromatic substitution via wheland intermediate

20
Q

How does resonance stabilisation at C2 compare to C3

A

There is increased resonance stabilisation, it is the preferred site for SarE

21
Q

What conditions are required for the sulfonation of pyrrole?

A

Pyr.SO3

100C

22
Q

What is the role of pyr.SO3 in the sulfonation of pyrrole?

A

Pyridine-sulfur trioxide

Used as a mild sulfonating agent as pyrrole is unstable to acid

23
Q

What is used as the nitrating agent in the nitration of pyrrole?

A

Acetyl nitrate (AcONO2)

24
Q

What is the result of the nitration of pyrrole?

A

Mix of the nitro group at the C2 and C3 positions in a ration of 4:1

25
Q

What is the impact of the nitro group on the pyrrole ring?

A

The electron withdrawing nitro group makes the pyrrole less reactive

26
Q

What conditions are required for the nitration of pyrrole?

A

AcONO2
AcOH
-10C

27
Q

How does the nitration of C2 position of pyrrole compare to benzene?

A

it is 10^5 times faster

Due to the electron rich nature of pyrrole

28
Q

Does the nitro product react any further?

A

No it is a very poor nucleophile

29
Q

Does the bromination product react any further?

A

Yes

Electron rich pyrrole is very reactive so it undergoes multiple rapid bromination

30
Q

What conditions are required for the bromination of pyrrole?

A

Br2
EtOH
O degrees C

31
Q

In which order do the brominations take place?

A

The first two are alpha positioned and the second two are beta positions

32
Q

What is the Mannich reaction?

A

The addition of an amine alkyl chain to an alpha carbon on pyrrole

33
Q

What is the first step of the mannich reaction?

A

The formation of the reactive species, an iminium cation

34
Q

What is the second step of the Mannich reaction?

A

Electrophilic substitution at the C2 position by the iminium salt

35
Q

What is the Vilsmeier reaction?

A

The addition of a carbonyl function to heteroaromatics under mild conditions

36
Q

What is the first step in the Vilsmeier reaction?

A

Formation of the reactive species, iminium cation

37
Q

What is the second step in the vilsmeier reaction?

A

Electrophilic substitution at C-2 by iminium salt and then hydrolysis work up

38
Q

When does the polymerisation of pyrrole occur?

A

In the presence of strong acid

CHY3108 Organic Caroll (5 decks)

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