Carbazole is an aromatic heterocyclic organic compound. It has a tricyclic structure, consisting of two six-membered benzene ring fused on either side of a five-membered nitrogen-containing ring. The compound’s structure is based on the indole structure but in which a second benzene ring is fused onto the five-membered ring at the 2-3 position of indole (equivalent to the 4a-9a double bond in carbazole).

A classic laboratory organic synthesis for carbazole is the Borsche-Drechsel cyclization ^{[1] [2]}. In the first step phenylhydrazine is condensed with cyclohexanone to the corresponding imine. The second step is a hydrochloric acid catalyzed rearrangement reaction and ring-closing reaction to tetrahydrocarbazole. In one modification both steps are rolled into one by carrying out the reaction in acetic acid ^[3]. In the third step this compound is oxidized by Red lead to carbazole itself. Another classic is the Bucherer carbazole synthesis.