diazo

Diazo refers to chemical species that contain a diazo functional group, characterized by a diazo bond between nitrogen atoms and, in many organic compounds, a diazo substituent attached to carbon. In organic chemistry, the most common forms are diazo compounds with the general motif R2C=N2 or, when the diazo group is on an aryl ring, aryl diazonium salts with the structure Ar-N2+ X−. The N2 unit is a relatively good leaving group and a source of nitrogen gas in many reactions, enabling a variety of transformations.

Diazomethane and related diazo compounds are among the simplest and most widely used members of this class. Diazomethane (CH2N2) is a gaseous, highly reactive species that is typically prepared and used in situ due to its toxicity and tendency to decompose. Diazomethyl esters and other diazo carbonyl derivatives are employed as carbonyl equivalents, methylating agents, or carbenoid precursors in a range of synthetic transformations, including cyclopropanations and insertions into C–H or heteroatom–H bonds. The reactivity of diazo compounds is harnessed under controlled conditions, often with metal catalysts, to generate reactive metal carbenes.

Diazonium salts arise from diazotization, the acid‑catalyzed reaction of primary aromatic amines with nitrous acid. These salts are versatile intermediates in aromatic substitution and dye chemistry, most notably in azo coupling reactions that form azo dyes. They also participate in various substitution and coupling processes, including Sandmeyer-type transformations.

Safety and handling considerations are important for diazo compounds, as many are toxic, potentially explosive, and sensitive to heat, light, or shock. Proper facilities and protocols are essential for their preparation, storage, and use in synthesis.