Preparation or Synthesis of Indole
Indole may be prepared by number of ways, some of which are given below-
Lipp Synthesis of Indole
When o-amino ω-chlorostyrene is heated with sodium ethoxide at 160-170°C, indole is obtained.
Fischer Indole Synthesis
Pyruvic acid is first treated with phenylhydrazine to form the corresponding phenylhydrazone which upon heating with anhydrous ZnCl2 or polyphosphoric acid gives idole 2-carboxylic acid which decomposes yields indole.
Reissert Synthesis of Indole
o-nitrotoluene is condensed with diethyloxalate in the presence of a base to form a 2-keto ester which is reduced with zinc and glacial acetic acid to indole 2-carboxylic acid which decomposes to indole.
Madelung Synthesis of Indole
o-toluidine treated with formic acid to form N-formyl-o-toluidine which undergoes dehydration on heating with potassium t-butoxide to yield indole.
Preparation of Indole by o-nitrophenylacetaldehyde
Reduction of o-nitrophenylacetaldehyde with Fe-powder and sodium bisulphite to give o-aminophenylacetaldehyde which cyclises spontaneously to yield indole.
Preparation of Indole by trans-indigo
Oxidation of trans-indigo with potassium permanganate isatin. Isatin on reduction with zinc and glacial acetic acid first gives dioxindole and finally oxindole which on distillation with Zn-dust to give indole.
Also read Chemical Reactions of Indole and other Heterocyclic Compounds
