Indigo - chemical synthesis


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Plant based products were almost entirely replaced in the 20th century by synthetic indigo. An early synthesis used by Meister, Lucius & Brunning (Hoechst) was from o-nitrobenzaldehyde was first described by Adolf Baeyer and Viggo Drewson (Darstellung von Indigblau aus Orthonitrobenzaldehyd, Berichte der Deutschen Chemische Gesellschaft 1882 14 2856-2864) and this is easily repeated in the laboratory. 

This description is from J R McKee and M Zanger, Journal of Chemical Education 1991 68 A242 

Read the warnings in the article first!
To synthesise indigo in microscale, we use the Baeyer-Drewson Reaction. o-Nitrobenzaldehyde (0.5 g) is dissolved in 5 mL of acetone in a 6-in. test tube. Five milliliters of water is added and the suspension stirred. Sodium hydroxide (2.5 mL of a 1.0 N solution) is added dropwise. As the base is added, the blue color of indigo can be seen almost at once, and the solution may boil. 
After the exothermic reaction has subsided, the reaction is allowed to stand for 5 min. The precipitated indigo is collected by suction filtration. The precipitate is washed successively with 10 mL of water and 10 mL of ethyl alcohol. 
For another way, see B T Golding and C Pierpoint, Indigo blue in Education in Chemistry, May 1986 71-73 Another version with illustrations can be found here

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