Write the conventional and green methods for the synthesis of adipic acid.

Conventional (traditional) route

\( \mathrm{C_6H_6} \xrightarrow[\text{Ni / Pt}]{3\mathrm{H_2}} \underset{\text{cyclohexane}}{\mathrm{C_6H_{12}}} \xrightarrow[\text{Co catalyst}]{\mathrm{O_2\ (air)}} \underset{\text{cyclohexanol + cyclohexanone (KA oil)}}{\mathrm{C_6H_{11}OH + C_6H_{10}O}} \xrightarrow{\mathrm{HNO_3}} \underset{\text{adipic acid}}{\mathrm{HOOC(CH_2)_4COOH}} + \mathrm{N_2O}\uparrow \)

Drawbacks:

• The starting material, benzene, comes from petroleum. It is non-renewable and carcinogenic.
• The HNO₃ oxidation releases N₂O. N₂O is a greenhouse gas about 300 times stronger than CO₂, and it also depletes ozone.
• The process needs high temperature and pressure and uses corrosive acid.

Green route 1 (Frost and Draths – biocatalytic)

\( \underset{\text{D-glucose}}{\mathrm{C_6H_{12}O_6}} \xrightarrow{\text{genetically modified } E.\ coli} \underset{\textit{cis,cis}\text{-muconic acid}}{\mathrm{HOOC{-}CH{=}CH{-}CH{=}CH{-}COOH}} \xrightarrow[\text{3 atm, room temp.}]{\mathrm{H_2},\ \mathrm{Pt}} \mathrm{HOOC(CH_2)_4COOH} \)

Green route 2 (Noyori): cyclohexene is oxidised with 30% H₂O₂. The catalyst is Na₂WO₄ with a phase-transfer catalyst, and no organic solvent is used.

\( \mathrm{C_6H_{10}} + 4\,\mathrm{H_2O_2} \xrightarrow{\mathrm{Na_2WO_4},\ \text{PTC}} \mathrm{HOOC(CH_2)_4COOH} + 4\,\mathrm{H_2O} \)

Why these routes are green: they use a renewable feedstock (glucose) in place of benzene. No N₂O is formed, and in the H₂O₂ route water is the only by-product. They need no toxic solvents, run under mild conditions and use catalysts.