L1/2/3 pyrrole chemistry

Question 1

What are the three bond lengths in pyrrole?

Answer 1

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

Question 2

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

Answer 2

90 kJmol-1

whereas benzene is 150kJmol-1

Question 3

What makes a compound aromatic?

Answer 3

Obeys huckles rule of aromaticity (4n+2)

Cyclic series of overlapping p orbitals with delocalised pi bonding

Question 4

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

Answer 4

6.2 and 6.5 ppm, N-H is 10ppm

Benzene is around 7.3ppm

Question 5

How does the alkene character of pyrrole compare to benzene?

Answer 5

It is more alkene like

Question 6

What type of heteroaromatic is pyrrole?

Answer 6

Electron rich

Question 7

Where does the lone pair on the nitrogen sit?

Answer 7

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

Question 8

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

Answer 8

Alpha

Question 9

Which carbons in pyrrole are most reactive to electrophillic substitution?

Answer 9

C2 and C5

Question 10

Which carbons in pyrrole are most reactive to electrophilic substitution?

Answer 10

C2 and C5

Question 11

How is pyrrole as a base?

Answer 11

Very weak base

Question 12

What is the pKa of pyrrole in its basic form?

Answer 12

-3.8

Question 13

How does pyrrole interact with acid?

Answer 13

It is protonated with a strong acid at the C2 position

Question 14

How is pyrrole as an acid?

Answer 14

It is a weak acid

Question 15

What is the pKa of pyrrole in its acidic form?

Answer 15

17.5

Question 16

Why is pyrrole a weak base?

Answer 16

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

Question 17

How does pyrrole interact with bases?

Answer 17

Deprotonation will occur with strong bases

Question 18

Which carbon atom does protonation of pyrrole occur at?

Answer 18

C-2

Question 19

In general, how does pyrrole react?

Answer 19

Electrophilic aromatic substitution via wheland intermediate

Question 20

How does resonance stabilisation at C2 compare to C3

Answer 20

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

Question 21

What conditions are required for the sulfonation of pyrrole?

Answer 21

Pyr.SO3

100C

Question 22

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

Answer 22

Pyridine-sulfur trioxide

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

Question 23

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

Answer 23

Acetyl nitrate (AcONO2)

Question 24

What is the result of the nitration of pyrrole?

Answer 24

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

Question 25

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

Answer 25

The electron withdrawing nitro group makes the pyrrole less reactive

Question 26

What conditions are required for the nitration of pyrrole?

Answer 26

AcONO2 AcOH -10C

Question 27

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

Answer 27

it is 10^5 times faster | Due to the electron rich nature of pyrrole

Question 28

Does the nitro product react any further?

Answer 28

No it is a very poor nucleophile

Question 29

Does the bromination product react any further?

Answer 29

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

Question 30

What conditions are required for the bromination of pyrrole?

Answer 30

Br2 EtOH O degrees C

Question 31

In which order do the brominations take place?

Answer 31

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

Question 32

What is the Mannich reaction?

Answer 32

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

Question 33

What is the first step of the mannich reaction?

Answer 33

The formation of the reactive species, an iminium cation

Question 34

What is the second step of the Mannich reaction?

Answer 34

Electrophilic substitution at the C2 position by the iminium salt

Question 35

What is the Vilsmeier reaction?

Answer 35

The addition of a carbonyl function to heteroaromatics under mild conditions

Question 36

What is the first step in the Vilsmeier reaction?

Answer 36

Formation of the reactive species, iminium cation

Question 37

What is the second step in the vilsmeier reaction?

Answer 37

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

Question 38

When does the polymerisation of pyrrole occur?

Answer 38

In the presence of strong acid