Diazo

The diazo group is an organic moiety consisting of two linked nitrogen atoms (azo) at the terminal position. Overall charge neutral organic compounds containing the diazo group bound to a carbon atom are called diazo compounds or diazoalkanes and are described by the general structural formula R2C=N+=N–, and the simplest example of a diazo compound is diazomethane. Compounds with the diazo moiety should be distinguished from diazonium compounds, which have the same terminal azo group but bear an overall positive charge, and azo compounds in which the azo group bridges two organic substituents. The electronic structure of diazo compounds is characterized by π electron density delocalized over the α-carbon and two nitrogen atoms, along with an orthogonal π system with electron density delocalized over only the terminal nitrogen atoms. Because all octet rule-satisfying resonance forms of diazo compounds have formal charges, they are members of a class of compounds known as 1,3-dipoles. Some of the most stable diazo compounds are α-diazo-β-diketones and α-diazo-β-diesters with the electron density further delocalized into an electron-withdrawing carbonyl group. In contrast, most diazoalkanes without electron-withdrawing substituents, including diazomethane itself, are explosive. A commercially relevant diazo compound is ethyl diazoacetate (N2CHCOOEt). A group of isomeric compounds with only few similar properties are the diazirines, where the carbon and two nitrogens are linked as a ring. Four resonance structures can be drawn: Diazo compounds (R2C=N2) should not be confused with azo compounds of the type R-N=N-R or with diazonium compounds of the type R-N2+. Diazo compounds were first produced by Peter Griess who had discovered a versatile new chemical reaction, as detailed in his 1858 paper 'Preliminary notice on the influence of nitrous acid on aminonitro- and aminodinitrophenol.' Several methods exist for the preparation of diazo compounds. Alpha-acceptor-substituted primary aliphatic amines R-CH2-NH2 (R = COOR, CN, CHO, COR) react with nitrous acid to generate the diazo compound. An example of an electrophilic substitution using a diazomethyl compound is that of a reaction between an acyl halide and diazomethane, for example the first step in the Arndt-Eistert synthesis.