3.10 Atom economy

Atom economy

Use the balanced equation, including the numbers in front of each formula.

Example 1: making ethanol by fermentation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

• M_\mathrm{r}: glucose = 180, ethanol = 46
• atom economy = (2 × 46) ÷ 180 × 100 = 51.1%

Example 2: making ethanol by hydration of ethene: C₂H₄ + H₂O → C₂H₅OH

• the only product is ethanol, so atom economy = 46 ÷ (28 + 18) × 100 = 100%

Reactions with only one product, such as addition reactions, always have an atom economy of 100%.

Why high atom economy matters

• Less waste is produced, so it is better for the environment.
• Fewer raw materials are needed, which is more sustainable and cheaper.
• A waste product may be useful and could be sold, which improves the economics of the process.

Choosing a reaction pathway

Atom economy is not the only thing that matters. Chemists also consider:

• the percentage yield
• the rate of reaction
• the position of equilibrium (for reversible reactions)
• whether any by-products are useful
• the cost and availability of raw materials and energy

For example, fermentation has a low atom economy but uses renewable sugar from plants, while hydration of ethene has 100% atom economy but uses ethene from crude oil, which is non-renewable.