Heumann synthesis
The Heumann synthesis (also Heumann's indigo syntheses ) describes the synthetic representation of indigo . Based on aniline and anthranilic acid , the German chemist Karl Heumann developed two synthesis variants, which formed the basis of the first large-scale indigo production (1897).
First Heumann synthesis
The first variant for the synthesis of indigo was developed in 1890.
In the first step, aniline ( 1 ) is reacted with chloroacetic acid ( 2 ) to form N- phenylglycine ( 3 ). Cyclization to indoxyl ( 4 ) takes place in a potassium hydroxide melt at 300 ° C , which is oxidized to indigo ( 5 ) with oxygen in an alkaline solution . The critical point in this synthesis variant is the high temperature that is required for the cyclization. Since the phenylglycine partially decomposes under these conditions, the yield is unsatisfactory.
This synthesis route could be significantly improved in 1901 by a modification by Johannes Pfleger . The use of sodium amide enables the ring closure under milder conditions (180–200 ° C). Current large-scale indigo syntheses are based on this Heumann-Pfleger synthesis .
Second Heumann synthesis
The poor yields of the first indigo synthesis according to Heumann could be improved by using anthranilic acid ( 1 ).
Analogously to the first synthesis variant, the reaction with chloroacetic acid ( 2 ) gives the corresponding phenylglycine-o-carboxylic acid ( 3 ), which is converted to 2-indoxylcarboxylic acid ( 4 ) in a potassium hydroxide melt . When heated, carbon dioxide is split off and the indoxyl ( 5 ) can in turn be oxidized to indigo ( 6 ). Based on this synthesis variant, indigo was first produced on an industrial scale at BASF in 1897 .
Individual evidence
- ↑ Heumann's indigo syntheses . ( Spektrum.de [accessed on November 15, 2018]).
- ^ Matthias Dietrich: ( The Heumann-Pfleger process. ) In: Didaktik der Chemie. Uni Bayreuth, accessed on November 19, 2018.
- ↑ 1865–1901: The Age of Colors - 1897 / "Indigo pure BASF" . basf.com. Retrieved November 19, 2018.