Chemical Properties or Reactions of Indole
Important chemical reactions of indole are given below-
Acidic and Basic Character
Like pyrrolee, indole is a weak base and also a weak acid. It is polymerized by strong acids and reacts with KOH and Grignard Reagents.
Electrophilic Substitution Reactions
The pyrrole ring in indole is very electron rich, in comparison to the benzene ring, therefore, electrophile's attack always takes place in the five-membered ring, except in special circumstances. Unlike pyrrole, indole undergoes electrophylic substitution reactions at carbon-3 because two resonance forms can be written for intermediate cation obtained from attack at carbon-3, whereas only one such form is possible for substitution at carbon-2.
Nitration, Sulphonation, Bromination, Alkylation and Friedel Crafts Acylation of Indole
Nitration: At low temperature with ethyl nitrate in the presence of sodium ethoxide to yield 3-nitroindole.
Sulphonation: With sulphur trioxide in pyridine at 110oC give indole 3-sulphonic acid.
Bromination: With bromine in dioxan at 0oC to form 3-bromo indole.
Friedel Craft Acylation: With acetyl chloride in the presence of SnCl4 to yield 3-acetylindole.
Alkylation: With methyl iodide in dimethylsulphoxide (DMF) at about 80oC to give 3-methylindole (skatole).
Reimer-Tiemann Formylation
Indole reacts with chloroform in the presence of alkali to yield indole 3-aldehyde ()3-formylindole) and 3-chloroquinoline.
Diazo Coupling
Indole couples with benzenediazonium chloride is a widely acidic solution to yield 3-phenylazoindole.
Mannich Reaction
Indole undergoes Mannich reaction with formaldehyde and dimethylamine to give 3-dimethylaminomethylindole (Gramine).
Vilsmeier Haack Reaction
Indoles can be readily formylated to 3-formyl-indoles via Vilsmeier Haack reaction.
Oxidation of Indole
Indole may be oxidized by ozone in formamide to give 2-formamidobenzaldehyde.
Reduction of Indole
Mild reduction of indole with zinc or tin and HCl yields 2,3-dihydroindole (indoline). Catalytic reduction hydrogenates both rings and produces octahydroindole. However, lithium/liquid ammonia reduces the benzene ring to 4,7-dihydroindole as major product. The heterocyclic ring can be reduced in acidic reagents such as Zn/HCl or NaCNBH3/CH3COOH to give indoline.
Also read Preparation of Indole and other Heterocyclic Compounds
