Synthesis Of Indigo

To demonstrate synthesis of Indigo through the Baeyer-Drewson reaction, and subsequent vat dying of cotton through conversion to and from Leucoindigo.

Indigo (Vat Blue 1) is a vat dye with a molecular formula of C16H10N2O2. A characteristic of vat dyes is that they are insoluble in most aqueous environments. A reducing agent is required to convert them into a soluble “leuco” form, meaning “white” or colourless. The dye can subsequently be reversed back into its original coloured form through oxidation. The Baeyer-Drewson reaction with use of 2-nitrobenzaldehyde was originally developed in 1880, as one of the pioneer methods in production of synthetic Indigo.

The Baeyer-Drewson Indigo synthesis reaction is an organic reaction, which utilizes

This conversion was required to perform a conjugated addition of the alpha-carbon of acetone to 2-nitrobenzaldehyde, resulting in formation of an aldol, which is subsequently converted to Indoxyl. 5mL of 2M Sodium Hydroxide was diluted by the presence of 35mL of water, effectively reducing its concentration to 0.25M. A low hydroxide ion concentration was required to prevent aldol from condensing, which will result in the hydroxyl group leaving as water. Ethanol was subsequently added to reduce the time required for drying

It was able to support itself as a thin sheet, but easily fragmented when a small force was applied. 3mL of 2M Sodium Hydroxide and 1mL of water, effectively 4mL of 1.5M Sodium Hydroxide, was added to a small amount of Indigo, forming a paste. Sodium Hydrosulphite the acted as a reducing agent, converting Indigo into Leucoindigo, an acidic phenolic compound that reacts with hydroxide ions provided by Sodium Hydroxide to form a water-soluble salt. The solution turns colourless, and the dying process can begin. A 60°C water bath was chosen as Sodium Hydrosulphite will decompose into Sodium Sulfate and Sulfur Dioxide in presence of air at 90°C. After soaking a piece of cotton for an hour, it was taken out to be oxidised through exposure to air. Oxidation converts Leucoindigo back into Indigo, and a colour change from yellow to green to purple was observed. This is because Leucoindigo absorbs light poorly in the yellow(570nm) wavelength, while Indigo absorbs light in the blue(475nm) wavelength. The visual change in colours can be seen due to the changes in amount of each form as oxidation