In one line
Conventionally, indigo is made from aniline, formaldehyde and hydrogen cyanide using strong alkali at high temperature, while the green route uses genetically engineered bacteria to convert glucose (via tryptophan/indole) into indigo under mild conditions.
Explained simply
Indigo is the blue dye in your jeans. Making it chemically is like cooking with poison: it uses toxic aniline and cyanide, molten caustic alkali and lots of energy. The green route lets microbes do the cooking: feed sugar to engineered E. coli and they produce indigo in water at about body temperature.
How it works
Conventional route (Heumann–Pfleger type process)
- Aniline + formaldehyde + hydrogen cyanide (as NaCN) → phenylglycinonitrile, hydrolysed to N-phenylglycine (as its salt).
- N-phenylglycine (salt) is fused with a strong base mixture (NaOH/KOH with sodamide, NaNH2) at high temperature → indoxyl.
- Indoxyl is oxidised by air → indigo (two indoxyl units join).
Drawbacks: toxic and hazardous reagents (aniline, HCN, formaldehyde, sodamide), corrosive molten alkali, high energy demand, petroleum-based feedstock, and residual aniline in the product and waste water.
Green route (biotechnological)
- Genetically modified E. coli is grown on glucose (renewable feedstock).
- Its tryptophan pathway is engineered to produce indole (from tryptophan, by the enzyme tryptophanase).
- An inserted dioxygenase enzyme (naphthalene dioxygenase, from Pseudomonas) oxidises indole to indole-cis-2,3-dihydrodiol, which loses water to form indoxyl.
- Indoxyl is oxidised by air to indigo, which precipitates from the aqueous broth.
Key formula
Final step common to both routes: \( \mathrm{2\ indoxyl + O_2 \rightarrow indigo + 2H_2O} \) (indigo = C16H10N2O2).
| Point | Conventional | Green |
|---|---|---|
| Feedstock | Aniline (petroleum) | Glucose (renewable) |
| Reagents | HCN, HCHO, NaNH2, strong alkali | Enzymes in microbes, air |
| Conditions | High temperature fusion | Mild, aqueous, near 30–37 °C |
| Waste | Toxic, alkaline effluent | Biodegradable broth |
Quick example
Global denim production uses tens of thousands of tonnes of indigo every year, so replacing cyanide- and aniline-based steps has a large environmental benefit. The bio-route is a classic textbook example of using renewable feedstock and biocatalysis.
Common mistakes
- Saying the green route uses no oxygen; the final oxidation to indigo still uses air.
- Writing aniline as the green feedstock.
- Omitting the role of the engineered enzyme (dioxygenase).
How MU asks it
"Compare the conventional and green synthesis of indigo" (5–6 marks). For full marks: both routes with reactants, intermediates and steps (structures/equations), list of drawbacks of the conventional route, advantages of the green route, and the green chemistry principles satisfied (renewable feedstock, less hazardous synthesis, safer conditions, catalysis).