Azo compounds refer to synthetic inorganic chemical compounds bearing the functional group R-N=N-R', in which R and R' can be either aryl or alkyl. The N=N group is called an azo or diimide. Many of the more stable derivatives contain two aryl groups due to the electron delocalisation. Because of this delocalisation, many azo compounds are coloured and in fact many are used as dyes.
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 colours 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 part of the chromophore.
Aliphatic azo compounds (R and/or R' = aliphatic) are less common. One example is diethyldiazene, EtN=NEt.
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. Representative is Azobisisobutylonitrile (AIBN) which is widely used as an initiator in polymerization.
Aromatic azo compounds can be synthesized by using an azo coupling 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:
Because of their instability, especially for aliphatic ones, care should be taken with the handling of azo compounds or an explosion may occur.
- Ohme, R.; Preuschhof, H.; Heyne, H.-U. "Azoethane" Organic Syntheses, Collected Volume 6, p.78 (1988). http://www.orgsyn.org/orgsyn/pdfs/CV6P0078.pdf