Electrophilic and Nucleophilic Substitution in Aromatic Systems *
Substitution is complicated, however, by the fact that if protonation
is forced on pyrrole in strongly acid solution (this probably takes
place on an a-carbon atom rather than on nitrogen, XXVII, cf. p.
56), the aromatic character is lost, the compound behaves like a
conjugated diene and undergoes extremely rapid polymerisation:
HC=CH
I. I,.H
HC^N/C*H
H
(XXVII) •
Electrophilic substitution can, however, be carried out under
highly specialised conditions leading to preferential attack at the
a-position, reflecting the greater delocalisation, and hence stabilisa
tion, possible in the metastable intermediate leading to a-, as com
pared with /J-, substitution:
H H
The difference in stability between the two is not very strongly
marked, however, reflecting the highly activated state of the nucleus,
and ready attack will take place at the j8-position if the a- is already
substituted.
NUCLEOPHILIC ATTACK ON AROMATIC SPECIES
(i) Substitution of hydrogen
- As it :s the IT electrons that are initially responsible for the normal
substitution of benzene being an electrophilic process, the presence
of a strongly electron-withdrawing substituent might be expected to
render attack by a nucleophile possible provided electron-withdrawal