Azo compound

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In chemistry, azo compounds generally have a molecular formula of the form R-N=N-R', in which R and R' can be either aromatic or aliphatic. The N=N group is called an azo or diimide functional group. This helps to stabilise the N=N group by making it part of an extended delocalised system. This also has the effect of making many azo compounds coloured, as delocalised or conjugated systems often absorb visible frequencies of light.

A typical Azo compound, 4-hydroxyphenylazobenzene, a.k.a. yellow azo dye

The name azo comes from azote, an old name of nitrogen that originates in French and is derived from the Greek a (not) + zoe (to live). Aromatic azo compounds (R = R' = aromatic) are usually stable and have vivid colors such as red, orange, and yellow. Therefore, they are used as dyes, which are called azo dyes. Some azo compounds, eg. Methyl orange, can also be used as acid-base indicators, due to their ability to function as weak acids, and the different colours of the acid and salt forms. Azobenzene is another typical aromatic azo compound. Their colour originates from absorbance in the visible region of the spectrum due to the delocalization of electrons in the benzene and azo groups forming a conjugated system, whereby the N=N group is the chromophore.

Aliphatic azo compounds (R and/or R' = aliphatic) are rather unstable. At an elevated temperature or by irradiation, two carbon-nitrogen (R-N) bonds are cleaved simultaneously with the loss of nitrogen gas to generate carbon-centered radicals. Owing to this process, some aliphatic azo compounds are utilized as radical initiators. Azobisisobutylonitrile (AIBN) is a typical one and is widely used in industrial processes and in laboratory experiments.

Aromatic azo compounds can be synthesized by using an azocoupling reaction, that is, an electrophilic substitution reaction on aromatic rings with diazonium salts. Diazonium salts decompose at temperatures warmer than about 5 degrees Celsius, so the reaction must take place in solution under freezing conditions:

An oxidation reaction of hydrazine (R-NH-NH-R') also gives an azo compound.

Because of their instability, especially for aliphatic ones, care should be taken with the handling of azo compounds or an explosion may occur.