Indole is an aromatic heterocyclic organic compound. It has a bicyclic structure, consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring.

Indole is solid at room temperature and has an intense fecal smell. At very low concentrations however it has a flowery smell and is a constituent of many flower scents (such as orange blossoms) and perfumes.

The indole structure can be found in many organic compounds like the amino acid tryptophan and in tryptophan containing protein, in alkaloids, or in pigments.

Indole undergoes electrophilic substitution, mainly at position 3. Substituted indoles are structural elements of (and for some compounds the synthetic precursors for) the tryptophan-derived tryptamine alkaloids like the neurotransmitter serotonin, melatonin, the hallucinogens psilocybin, DMT, 5-MeO-DMT, or the ergolines like LSD.

Other indolic compounds include the plant hormone heteroauxin (indolyl-3-acetic acid, IAA), the anti-inflammatory drug indomethacin, or the betablocker pindolol.

The name indole is derived from indigo, a blue pigment obtained from some plant species. The indigo molecule comprises two indole units joined together.

Although the indole nitrogen atom has a lone pair of electrons, indole is not basic like the amines because the lone pair is delocalised and contributes to the aromatic system.

See also

• Skatole (3-methyl-indole)
• Tryptamines
• Fischer indole synthesis