page_title: Azobenzene
section_title: Structure and synthesis
hierarchical_section_title: Azobenzene / Structure and synthesis
context_page_description: Azobenzene is a chemical compound composed of two phenyl rings linked by a N=N double bond. It is the simplest example of an aryl azo compound. The term 'azobenzene' or simply 'azo' is often used to refer to a wide class of similar compounds. These azo compounds are considered as derivatives of diazene, and are sometimes referred to as 'diazenes'. The diazenes absorb light strongly and are common dyes.
context_section_description: trans-Azobenzene is planar.  The N-N distance is 1.189 Å.  cis-Azobenzene is nonplanar with a C-N=N-C dihedral angle of 173.5°.  The N-N distance is 1.251 Å.
  Azobenzene was first described by Eilhard Mitscherlich in 1834. Yellowish-red crystalline flakes of azobenzene were obtained in 1856. Its original preparation is similar to the modern one. According to the 1856 method, nitrobenzene is reduced by iron filings in the presence of acetic acid. In the modern synthesis, zinc is the reductant in the presence of a base. Industrial electrosynthesis using nitrobenzene is also employed.
  The trans isomer is more stable by approximately 50 kJ/mol, and the barrier to isomerization in the ground state is approximately 100 kJ/mol.
caption: Azobenzene photoisomerization. The trans form (left) can be converted to the cis form (right) using a UV wavelength of 300–400 nm. Visible illumination at >400 nm converts the molecule back to the trans form. Alternately, the molecule will thermally relax to the stable trans form.
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