Azo Compounds and Diazene

Azo compounds have the general formula R -N=N-R, where R and R are alkyl or aryl groups. Aromatic azo compounds are generally strongly colored and many are easily prepared via a so-called diazo coupling reaction, in which a diazonium cation is coupled with a relatively active (i.e., highly nucleophilic) arene such as a phenol or aniline. The dye called aniline yellow, for example, is prepared as follows  

Aliphatic azo compounds are also important, most notably as initiators of radical reactions, including radical polymerizations. The most common reagent used for this purpose is azobisisobutyronitrile (AIBN), which decomposes on gentle heating to N2 and a pair of 2-cyanoprop-2-yl radicals  

The driving force for this decomposition is clearly the formation of Nj. The 2-cyanoprop-2-yl radicals can then abstract a hydrogen from a substrate and set off a radical chain reaction. For example, when heated with a small quantity of AIBN, styrene and maleic anhydride yield a copolymer with almost perfectly alternating monomer units  

The parent azo compound HN=NH, called diimide or diazene, is unstable but is useful as a reducing agent in organic chemistry, where it is typically generated in situ. Some standard methods for the generation of diimide are as follows  

The first of the above three methods, involving aerial or H202-mediated oxidation of hydrazine, is the traditional route to diimide. Acidification of dipotassium azodicarboxylate is another common route, which is analyzed below in some detail. Protonation of the nitrogens leads to spontaneous decarboxylation of the azodicarboxylate anion, as shown below  

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