What do you mean by green solvent, explain it with suitable example.

Green solvents are environmentally benign solvents that replace volatile, toxic and flammable organic solvents (benzene, CCl₄, CHCl₃). They are non-toxic or of low toxicity, non-flammable and non-volatile, and they are recyclable, often renewable and easy to separate from products. Examples are water, supercritical CO₂, ionic liquids and bio-based solvents such as ethyl lactate.

1. Supercritical CO₂: above its critical point (T_c = 31.1 °C, P_c = 72.8 atm), CO₂ has a gas-like viscosity and diffusivity and a liquid-like density and dissolving power. It is non-toxic, non-flammable, cheap and easily removed by releasing the pressure.

Applications: decaffeination of coffee; extraction of flavours and essential oils; dry cleaning in place of perchloroethylene; polymerisation; hydrogenation reactions.

2. Ionic liquids: salts that are liquid below 100 °C, e.g. 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF₄]. They have negligible vapour pressure, are non-flammable and thermally stable, dissolve many substances and are recyclable.

Applications: solvents for organic synthesis and catalysis (Diels–Alder, Friedel–Crafts), dissolving cellulose, electrolytes in batteries.