The indigo color has been known to humans for thousands of years. Its precursors can be naturally found in a plant of the indigofera genus. Frequently mentioned ones are indigofera tinctoria and indigofera suffructiosa. Through the time more than 300 hundred plant spieces were told to contain the precursor of indigo. The precursor orginally found in the plants is called indican. The compound is easily hydrolyzed to one molecule of β-D-glucose and one molecule of indoxyl that is readily oxidized by air to indigo.
Indigo is an organic compound that is at normal pressure and temperature blue crystalline solid insoluble in water, alcohol and ether but soluble in organic solvents. For dyeing process, indigo must undergo reduction to a form soluble leuco form – indigo white – that is “colorless”. There are few ways indigo color can be obtained. The extraction from the plants rich in indican is generally done in three steps:
1. soaking in hot water (effective due to epidermal wax melting)
2. adjusting pH value (up to 11)
3. reduction of indigo (different concentrations of reducer had insignificant effect)
Synthetically, indigo color is on laboratory scale mostly produced by Bayer-Drewson indigo synthesis (1882). The reaction between 2-nitrobenzaldehyde and acetone in basic enviroment. Basically, reaction involves aldol condensation, cyclization and few elimination steps. The mechanism of this this reaction is quite complex. Many modifications of Bayer-Drewson reaction were described since the nineteenth century. Other routes involves starting compounds such as naphtalene, anline, etc. On industrial scale, indigo is produced from N-phenylglycine (Pfleger´s process).
2-nitrobenzaldehyde (1.0 g, 6.6 mmol) was dissolved in 20 ml of acetone in 100 mL beaker. The solution was diluted with 35 mL of water. Under vigorous stirring 5 mL of 2M sodium hydroxide solution were added. The solution under constant stirring turned to deep yellow and continued to darken until the blue precipitate of indigo formed. The precipitate was collected by suction filtration. The product was washed with several portions of water until the washings became colorless, then with 20 mL of ethanol. The product was dried at room temperature and weighted. Yield: yet to come.
20 mg of indigo were placed in 50 mL beaker and few drops of ethanol were added. To the paste 3 mL of sodium hydroxide solution and 20 mL of sodium hydrosulfite were added and the mixture was gently heated up to 40 °C. After obtaining clear yellow solution cotton was immersed into the vat. The cotton was removed after half an hour, squeezed and hang in the air to develop the color.