In one line
Adipic acid is conventionally made from benzene via cyclohexanol/cyclohexanone oxidation with nitric acid, which releases the greenhouse gas N2O, whereas green routes use glucose with engineered microbes or hydrogen peroxide oxidation of cyclohexene.
Explained simply
Adipic acid, HOOC–(CH2)4–COOH, is a key raw material for nylon 6,6. The old method starts from benzene (carcinogenic) and uses nitric acid that gives off nitrous oxide, a gas that traps far more heat than CO2 and damages the ozone layer. Green chemists found ways to start from sugar or to use H2O2, whose only by-product is water.
How it works
Conventional route
- Benzene + 3H2 (Ni catalyst) → cyclohexane.
- Cyclohexane + air (Co catalyst) → mixture of cyclohexanol and cyclohexanone ("KA oil").
- KA oil + HNO3 (Cu/V catalyst) → adipic acid + N2O (and other nitrogen oxides).
Drawbacks: carcinogenic benzene from petroleum, corrosive nitric acid, N2O emission (a potent greenhouse gas and ozone-depleting gas), high energy.
Green route 1 (Draths and Frost, biocatalytic)
- D-glucose is converted by genetically engineered E. coli (via the shikimate pathway intermediates such as 3-dehydroshikimate and catechol) into cis,cis-muconic acid in water.
- cis,cis-Muconic acid is hydrogenated (H2, Pt catalyst) → adipic acid.
Green route 2 (Noyori, H2O2 oxidation)
- Cyclohexene + 30% H2O2 with sodium tungstate catalyst and a phase-transfer catalyst, without organic solvent → adipic acid + water.
Key formula
Hydrogenation step: \( \mathrm{HOOC{-}CH{=}CH{-}CH{=}CH{-}COOH + 2H_2 \rightarrow HOOC{-}(CH_2)_4{-}COOH} \)
Atom economy of this step: muconic acid (142.11) + 2H2 (4.03) → adipic acid (146.14): 100% (addition reaction).
H2O2 route overall: \( \mathrm{C_6H_{10} + 4H_2O_2 \rightarrow HOOC(CH_2)_4COOH + 4H_2O} \)
Quick example
| Point | Conventional | Green (glucose) |
|---|---|---|
| Feedstock | Benzene (petroleum, carcinogen) | Glucose (renewable) |
| Oxidant | HNO3 | Microbial enzymes; then H2 |
| By-product | N2O (greenhouse, ozone-depleting) | No N2O |
| Medium | Organic / acidic | Water |
Common mistakes
- Writing CO2 instead of N2O as the main harmful by-product of the HNO3 step.
- Writing adipic acid as a C4 acid; it is a C6 dicarboxylic acid.
- Forgetting the hydrogenation step after muconic acid.
How MU asks it
"Give the conventional and green synthesis of adipic acid. State the green chemistry principles involved" (5–6 marks). For full marks: both routes with equations, the N2O problem explained, advantages of the green route, and principles satisfied (renewable feedstock, waste prevention, safer solvent (water), catalysis).