21

Pyrrole(likewise Furanand Thiophene)

•
•

is non-basic
its electron lone pair is involved in π cloud

3.1.1.Reactivity and Orientation

•reaction occur mainly in 2-position
extra canonical involved in substitution at the 2-position

3.2.

Six Membered Rings

Pyridine is very different from pyrrole:

•
•
•
•
•
•

aromatic
resonance energy 23 kcal/mol
uses only 1electron in π cloud
acts as a base
undergoes nucleophilic substitution
highly deactivated (ring nitrogen similar to nitro-substituent); 3-position

3.2.1.Reactivity and Orientation

also for ortho attack
destabilising
no such destabilisation for meta attack!

especially

N

H

N

O

S

H

N

SO3

Cl

O

N+Cl-
2

N

O

S

H

N

N

Ph

SO3H

Ph

O

+

+

+

SnCl4

modified reagents = milder conditions

N

H

N

H

N

H

N

H

N

H

N

H

N

H

H

E

H

E

H

E

H

E

H

E

+E+

+E+

N

N

N

N

NO2

SO3H

Br

N

Br

Br

HNO3/ H2SO4
300oC

H2SO4
550oC

Br2
300oC

+

RX / RCOX
AlCl3

no reaction !