Azobenzene

Di ( phenyl) diazene (IUPAC)

Orange-red leaves

• (E)- azobenzene 68 ° C.
• (Z)- azobenzene: 71.4 ° C

(E)- azobenzene: 293 ° C ( without decomposition )

Sparingly soluble in water, but readily in organic solvents

Risk

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Azobenzene is the simplest aromatic azo compound and thus the parent substance of the azo dyes. It is a chemical substance that is composed of two phenyl groups linked by an azo bridge -N = N- with each other.

History

Just six years after the commonly accepted beginning of the " organic synthesis " (F. Wöhler, urea from ammonium cyanate ) and 22 years before WH Perkin ( mauve ) reported E. Mitscherlich on a red compound, which he obtained by distillation of nitrobenzene with potassium hydroxide solution. He called them azobenzene.

Long time did not know anything about the constitution of this compound. Mitscherlich himself suggested as a molecular formula C12 H5 N. Due to vapor density measurements of other authors came to the molecular formula C24 H10 N2. Only in 1860 the correct molecular formula was postulated. FA Kekulé made ​​in 1866 finally the first correct structure proposal.

Unresolved the question remained the configuration of the N = N double bond. For analog connections ( Diazohydroxide ) suggested A. Hantzsch 1921 before a isomerism, then called " syn-/anti-Isomerie ". (Now the IUPAC recommends the use of (E ) / ( Z) nomenclature, see below. )

Isomers

1937 was S. Hartley by exposure of azobenzene a second yellow form ( see below). The yellow isomer he could separate chromatographically. The exact configuration of the two isomers was established in 1939 by HP Robertson et al. proved by X-ray analysis. Azobenzene therefore exists in the form of two isomers (see cis- trans isomerism ), which differ in color, solubility in the chromatographic behavior, etc..

During the irradiation of a solution of (E)- azobenzene with ultraviolet light that is in an equilibrium reaction partially in the (Z)- form over incurred while 15-40 % ( Z)- azobenzene, depending on the solvent.

The more stable is normally present (E)- azobenzene has no dipole moment ( μ = 0, D), in contrast to the metastable (Z)- azobenzene ( μ = 3 D).

Representation and extraction

Azobenzene (5 ) can be prepared in the following ways (see also picture below):

• By the reduction of nitrobenzene (1) with sodium amalgam or lithium aluminum hydride ( route A)
• By oxidation of hydrazobenzene (2) with sodium hypobromite solution (route B )
• By condensation of nitrosobenzene (3) with an aniline ( 4) in acetic acid solution (in particular for the preparation of asymmetric derivatives ) (route C)

Use

(E)- azobenzene is used as a test substance for the Kofler hot stage microscope, or as calibration substance for the Kofler heating use.