A Guidebook to Mechanism in Organic Chemistry

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