What is green chemistry and how is atom economy used to compare conventional and green routes?

In one line

Green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances, and atom economy measures what fraction of the reactant atoms end up in the desired product.

Explained simply

Conventional chemistry asked "Does the reaction give a good yield?" Green chemistry also asks "How much waste did we make, how toxic were the chemicals, and how much energy did we burn?" It is like cooking: a good recipe is not just tasty, it also uses ingredients fully, avoids dangerous gas and does not leave a sink full of waste. Green chemistry was formalised by Paul Anastas and John Warner as 12 principles (prevention of waste, atom economy, less hazardous synthesis, safer chemicals, safer solvents, energy efficiency, renewable feedstocks, fewer derivatives, catalysis, design for degradation, real-time analysis, inherently safer chemistry).

How it works

To compare a conventional route with a green route for the same product (as in this module for indigo, adipic acid and carbaryl), check:

1. Feedstock: petroleum-based or renewable (glucose, biomass)?
2. Reagents: toxic (HCN, phosgene, methyl isocyanate, HNO₃) or benign?
3. By-products and waste: N₂O, salts, acids?
4. Solvent: volatile organic solvent or water/supercritical CO₂/ionic liquid?
5. Energy: high temperature and pressure, or mild conditions with catalysts or enzymes?
6. Atom economy: how many atoms are wasted?

Key formula

\( \%\ \text{Atom economy} = \dfrac{\text{Molecular weight of desired product}}{\text{Sum of molecular weights of all reactants}} \times 100 \)

• Use stoichiometric coefficients (multiply molecular weight by moles in the balanced equation).
• Catalysts are not counted as reactants.

Atom economy differs from % yield: yield measures how much product was actually obtained compared with theory; atom economy measures how much of the reactants could possibly become product even at 100% yield.

Quick example

• Addition: CH₂=CH₂ + H₂ → CH₃CH₃: all atoms end in the product, atom economy = 100%.
• Substitution: C₂H₅OH (46.07) + HBr (80.91) → C₂H₅Br (108.97) + H₂O. Atom economy = 108.97/126.98 × 100 = 85.8%.

Addition and rearrangement reactions are the most atom-economical; substitution and elimination reactions always waste some atoms.

Common mistakes

• Confusing atom economy with percentage yield.
• Forgetting stoichiometric coefficients when adding reactant masses.
• Counting a catalyst or solvent as a reactant.

How MU asks it

"Define atom economy. Calculate % atom economy for the reaction..." (2–3 marks), or used inside a comparison of conventional and green synthesis (5–6 marks). For full marks: definition, formula, balanced equation, molecular weights shown, and final percentage with a comment.