Answer: Traditional: benzene → cyclohexane → cyclohexanol/cyclohexanone → HNO₃ oxidation, releasing N₂O. Green: glucose → cis,cis-muconic acid (by genetically modified E. coli) → hydrogenation to adipic acid. Principles: renewable feedstock, safer synthesis, waste prevention, safer solvent (water), catalysis.
Adipic acid, HOOC–(CH₂)₄–COOH, is the raw material for nylon-6,6.
1. Traditional route
\[\text{C}_6\text{H}_6\ (\text{benzene})\xrightarrow[\text{Ni, high T, P}]{3\text{H}_2}\text{C}_6\text{H}_{12}\ (\text{cyclohexane})\]
\[\text{C}_6\text{H}_{12}\xrightarrow[\text{Co catalyst}]{\text{O}_2\ (\text{air})}\text{cyclohexanol}+\text{cyclohexanone}\ (\text{KA oil})\]
\[\text{KA oil}\xrightarrow[\text{Cu/V catalyst}]{\text{HNO}_3\ (\text{conc.})}\text{HOOC(CH}_2)_4\text{COOH}+\text{N}_2\text{O}\uparrow\]
Drawbacks: benzene is carcinogenic and comes from non-renewable petroleum; high temperature and pressure are needed; concentrated HNO₃ is corrosive; N₂O is released, a strong greenhouse gas (about 300 times CO₂) which also depletes the ozone layer.
2. Green route (Draths and Frost)
\[\text{D-glucose}\xrightarrow[\text{in water}]{\text{genetically modified }E.\ coli}\textit{cis,cis}\text{-muconic acid}\xrightarrow[\text{Pt catalyst, about 3.4 atm}]{\text{H}_2}\text{adipic acid}\]
(Another green method oxidises cyclohexene with 30% H₂O₂ using a sodium tungstate catalyst and a phase-transfer catalyst, without organic solvent; the only by-product is water.)
Principles of green chemistry involved:
- Use of renewable feedstock: glucose from starch/cellulose instead of benzene from petroleum.
- Less hazardous chemical synthesis: no carcinogenic benzene and no corrosive nitric acid.
- Prevention of waste/pollution: no N₂O is formed.
- Safer solvents: the biological step runs in water.
- Catalysis and energy efficiency: enzymes and a Pt catalyst work under mild conditions.
