Azobenzene

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Structural formula
Structural formula of azobenzene
Structural formula of ( E ) -azobenzene
General
Surname Azobenzene
other names

Diphenyldiazene ( IUPAC )

Molecular formula C 12 H 10 N 2
Brief description

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External identifiers / databases
CAS number
  • 103-33-3 (unspec.)
  • 17082-12-1 ( E ) -azobenzene
EC number 203-102-5
ECHA InfoCard 100.002.820
PubChem 2272
ChemSpider 2185
Wikidata Q8884513
properties
Molar mass 182.22 g mol −1
Physical state

firmly

density

1.203 g cm −3

Melting point
  • 68 ° C [( E ) -azobenzene]
  • 71.4 ° C [( Z ) -azobenzene]
boiling point

293 ° C [( E ) -azobenzene, undecomposed]

solubility
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary
08 - Dangerous to health 07 - Warning 09 - Dangerous for the environment

danger

H and P phrases H: 350-341-302 + 332-373-410
P: 201-273-314
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Azobenzene is the simplest aromatic azo compound . It consists of two phenyl groups linked by an azo bridge (–N = N–). It is the parent substance of numerous azo dyes .

history

Six years after the start of the generally accepted "organic synthesis" ( F. Wohler , urea from ammonium cyanate ) and 22 years before WH Perkin ( Mauvein ) reported E. Mitscherlich a red compound, which he distillation of nitrobenzene with potassium hydroxide solution obtained. He called it azobenzene.

For a long time nothing was known about the constitution of this connection. Mitscherlich himself suggested C 12 H 5 N as the empirical formula . On the basis of steam density measurements , other authors came up with the empirical formula C 24 H 10 N 2 . It was not until 1860 that the correct sum formula was postulated. FA Kekulé finally made the first correct structural proposal in 1866.

The question of the configuration of the N = N double bond remained unresolved . For analogous compounds ( diazo hydroxides ), A. Hantzsch proposed isomerism in 1921 , then called "syn / anti isomerism". The IUPAC now recommends the use of ( E , Z ) nomenclature.

Isomers

In 1937, S. Hartley found a second, yellow modification by exposing azobenzene to light . He was able to separate the yellow isomer by chromatography . The exact configuration of the two isomers was proven by an X-ray structure analysis in 1939 . Azobenzene therefore exists in the form of two isomers (see cis - trans isomerism ), which differ in color, solubility, chromatographic behavior, etc.

cis-trans isomerism of azobenzene

When a solution of ( E ) -azobenzene is irradiated with UV light , this partially changes into the ( Z ) form in an equilibrium reaction ; depending on the solvent , 15-40% ( Z ) -azobenzene is formed. Pure ( Z ) -azobenzene slowly converts thermally in the solid or in the melt into the more stable ( E ) -isomer. The enthalpy of reaction of isomerization in the melt is −48.2 kJ mol −1 or −264.5 J g −1 .

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

Presentation and extraction

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

Synthesis of azobenzene

use

( E ) azobenzene is used as a test substance for the Kofler hot stage microscope or as a calibrator for the Kofler hot bench use.

literature

  • H. Zollinger: chemistry of azo dyes . ( Chemical series , vol. 13). Birkhäuser Verlag, Basel, 1958.

Individual evidence

  1. ^ A b c d e Hans Beyer , Wolfgang Walter : Textbook of Organic Chemistry , 20th Edition, Hirzel, Stuttgart 1984. P. 529.
  2. ↑ a b c d Entry on azobenzene in the GESTIS substance database of the IFA , accessed on February 1, 2016(JavaScript required) .
  3. ↑ Entry on Azobenzene in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on February 1, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
  4. ^ E. Mitscherlich: Annalen der Physik und Chemie XXXII (1834), p. 224.
  5. ↑ E. Mitscherlich: Annalen der Chemie and Pharmacie XII, p. 311.
  6. ↑ Otto Linné Erdmann: Journal for practical chemistry . tape 82 . Publisher by Johann Ambrosius Barth, 1861, p. 444 ( read online in Google Book Search).
  7. ↑ P. Hofmann: Annals of Chemistry and Pharmacy CXV. P. 362.
  8. ↑ Peter Griefs: On the knowledge of the so-called azobenzene and related compounds . In: Justus Liebig, Freiherr von (Ed.): Annals of Chemistry and Pharmacy . Volumes 131-132. CF Winter'sche, 1864 ( read online in the Google book search).
  9. ↑ P. Hofmann: Annalen der Chemie und Pharmacie (1860), p. 324.
  10. ↑ Dr. Carl Glaser: About a new way of forming azobenzene . In: Journal of Chemistry . 1866 ( read online in Google book search).
  11. ↑ A. Hantzsch, G. Reddelien: Die Diazoverbindungen . Springer, Berlin, 1921.
  12. ↑ GS Hartley: Nature 140 (1937). P. 281.
  13. ↑ Robertson, JM: Crystal structure and configuration of the isomeric azobenzenes in J. Chem. Soc. 1939, pp. 232-236, doi : 10.1039 / JR9390000232 .
  14. ↑ Cook, AH: The preparation of some cis-azo compounds in J. Chem. Soc. 1938, pp. 876-881, doi : 10.1039 / JR9380000876 .
  15. ↑ a b Wolf, E .; Cammenga, HK: Thermodynamic and Kinetic Investigation of the Thermal Isomerization of Cis-azobenzene in Z. Phys. Chem. 107 (1977) pp. 21-38, doi : 10.1524 / zpch.1977.107.1.021 .
  16. ↑ a b Eckardt, N .; Flammersheim, HJ; Cammenga, HK: The cis-trans isomerization of azobenzene in the molten state in J. Therm. Anal. Calorim. 52 (1998), pp. 177-185, doi : 10.1023 / A: 1010178610642 .
  17. ↑ RJW Le Fevre, GS Hartley: The dipole moments of cis and trans azobenzenes and of some related compounds in J. Chem. Soc. 1939, pp. 531-535, doi : 10.1039 / JR9390000531 .
  18. ^ M. Kuhnert-Brandstätter: Thermomicroscopy in the Analysis of Pharmaceuticals, Pergamon Press, Oxford (1971).