Explain synthesis of Indigo dye by traditional way and by green pathway.

Traditional (conventional) synthesis – Heumann–Pfleger process:

\( \underset{\text{aniline}}{\mathrm{C_6H_5NH_2}} + \mathrm{HCHO} + \mathrm{HCN} \rightarrow \mathrm{C_6H_5NHCH_2CN} \xrightarrow{\text{hydrolysis}} \underset{\text{N-phenylglycine}}{\mathrm{C_6H_5NHCH_2COOH}} \)

\( \text{N-phenylglycine} \xrightarrow[\text{fusion, about 200 °C}]{\mathrm{NaOH/KOH} + \mathrm{NaNH_2}} \text{indoxyl} \xrightarrow{\mathrm{O_2}\ (\text{air})} \text{indigo} \)

Older route: aniline + chloroacetic acid gives N-phenylglycine, which goes on in the same way.

Drawbacks: it uses highly toxic HCN and formaldehyde, carcinogenic aniline, corrosive alkali fusion and sodamide at high temperature. Toxic waste is produced, and the feedstock comes from petroleum.

Green (biocatalytic) synthesis: genetically engineered E. coli converts glucose (a renewable feedstock) into indigo in water at room temperature.

\( \text{glucose} \xrightarrow{E.\ coli} \text{tryptophan} \xrightarrow{\text{tryptophanase}} \text{indole} \xrightarrow{\text{naphthalene dioxygenase}} \textit{cis}\text{-indole-2,3-dihydrodiol} \xrightarrow{-\mathrm{H_2O}} \text{indoxyl} \xrightarrow{\mathrm{O_2}\ (\text{air})} \text{indigo} \)

Indigo: 2 indoxyl + O₂ → indigo (C₁₆H₁₀N₂O₂) + 2H₂O

Green features: renewable feedstock; no HCN, aniline or strong alkali; aqueous medium; ambient temperature and pressure; enzyme catalysis; biodegradable waste (principles 3, 5, 6, 7 and 9).