Imidazole

Imidazole is a heterocyclic aromatic organic compound with formula C3H4N2. It comprises a five-membered ring containing two nitrogen atoms at nonadjacent positions (a 1,3-diazole). The ring is planar and aromatic, contributing six pi electrons. In the neutral molecule, one nitrogen is pyrrolic (N–H) and the other is pyridinic (non-protonated). Imidazole is a weak base; the pKa of its conjugate acid is about 7.0, so it is protonated under physiological conditions. The ring can exist in tautomeric forms in solution.

Imidazole occurs as part of the essential amino acid histidine, and its side chain plays a key role in enzyme active sites and metal coordination. Industrially, imidazole can be prepared by the Debus synthesis from glyoxal, formaldehyde, and ammonia, among other routes, making it a useful synthon in heterocycle chemistry.

Applications include use as a versatile building block for imidazole derivatives, as a ligand in coordination chemistry and catalysis, and as a buffering reagent in biochemistry (imidazole buffers). It is also employed in the purification of histidine-tagged proteins, where imidazole competes with histidine for binding to metal resins to elute the target protein. Imidazole rings form the core of many pharmaceuticals, and salts of imidazole are common components of ionic liquids (imidazolium cations).