In organic chemistry, a diazirine is an organic molecule consisting of a carbon bound to two nitrogen atoms, which are double-bonded to each other, forming a cyclopropene-like ring, 3H-diazirine (>CN₂). Diazirines are isomeric with diazocarbon groups (>C=N=N), and like them can serve as precursors for carbenes by loss of a molecule of dinitrogen. For example, irradiation of diazirines with ultraviolet light leads to carbene insertion into various C−H, N−H, and O−H bonds. Hence, diazirines have grown in popularity as small, photo-reactive, crosslinking reagents. They are often used in photoaffinity labeling studies to observe a variety of interactions, including ligand-receptor, ligand-enzyme, protein-protein, and protein-nucleic acid interactions.

A number of methods exist in the literature for the preparation of diazirines, which begin from a variety of reagents.

Generally, synthetic schemes that begin with ketones (>C=O) involve conversion of the ketone with the desired substituents to diaziridines (>CN₂H₂). These diaziridines are then subsequently oxidized to form the desired diazirines.

Diaziridines can be prepared from ketones by oximation, followed by tosylation (or mesylation), and then finally by treatment with ammonia (NH₃). Generally, oximation reactions are performed by reacting the ketone with hydroxylammonium chloride (NH₃OH⁻Cl⁺) under heat in the presence of a base such as pyridine. Subsequent tosylation or mesylation of the alpha-substituted oxygen with tosyl or mesyl chloride in the presence of base yields the tosyl or mesyl oxime. The final treatment of the tosyl or mesyl oxime with ammonia produces the diaziridine.

Diaziridines can be also produced directly by the reaction of ketones with ammonia in the presence of an aminating agent such as a monochloramine or hydroxyl amine O-sulfonic acid.

Diaziridines can be oxidized to diazirines by a number of methods. These include oxidation by chromium-based reagents such as the Jones oxidation, oxidation by iodine and triethylamine, oxidation by silver oxide, oxidation by oxalyl chloride, or even electrochemical oxidation on a platinum-titanium anode.

Diazirines can be alternatively formed in a one-pot process using the Graham reaction, starting from amidines. This reaction yields a halogenated diazirine, whose halogen can be displaced by various nucleophilic reagents.

Upon irradiation with UV light, diazirines form reactive carbene species. The carbene may exist in the singlet form, in which the two free electrons occupy the same orbital, or the triplet form, with two unpaired electrons in different orbitals.