The attack on the electrophiles occurs from position 3, because the pyrrole ring is richer in charge density than the carbocycle. Said attack is favored by the transfer of the lone pair of nitrogen.

electrophile substitution indole 01

a) Nitration:

The indole is nitrated with the nitric mixture in acetic anhydride.

indole 02 electrophile substitution

b) Halogenation

The halogenation of the indole is carried out with dilute halogens and cooling. NBS and NCS can also be used
indole 03 electrophile substitution
c) Sulfonation:
The indole sulfonation is performed with the SO3-pyridine complex under mild conditions to avoid possible polymerization side reactions.
indole 04 electrophile substitution
d) Vilsmeier formulation
It allows to introduce formyl groups in position 3 of the ring. The reaction is carried out with dimethylfomamide and phosphorous oxytrichloride, followed by basic hydrolysis.
indole 05 electrophile substitution
e) Mannich reaction
The Mannich reaction uses methanal and a primary or secondary amine in a hydrochloric medium as reagents.
indole 06 electrophile substitution