Improving Atom Economy

Definition of Atom Economy

Atom economy is a measure of how efficiently a chemical reaction uses the atoms of the reactants to form the desired product. It shows the proportion of the total mass of reactants that ends up in the useful product, rather than in waste or by-products.

Atom economy is expressed as a percentage and calculated using the formula:

$$\text{Atom Economy} = \left( \frac{\text{Mr of desired product}}{\text{Sum of Mr of all reactants}} \right) \times 100$$

A higher atom economy means more of the reactants are converted into useful product, making the process more efficient and environmentally friendly.

For instance, in the reaction of ethene with bromine to form dibromoethane, all atoms from the reactants are incorporated into the product, so the atom economy is 100%.

Importance of Improving Atom Economy

Improving atom economy is important because:

• It reduces the amount of waste produced, which is better for the environment.
• It lowers the environmental impact by minimising harmful by-products and pollution.
• It increases sustainability by making better use of raw materials, conserving resources for the future.

Chemicals with poor atom economy often produce large amounts of waste, which can be costly to dispose of and harmful to ecosystems.

Methods to Improve Atom Economy

There are several ways to improve atom economy in chemical reactions:

• Use alternative reactions: Choose reactions that produce fewer or no by-products, so more atoms end up in the desired product.
• Select reactions with fewer by-products: For example, addition reactions tend to have higher atom economies than substitution reactions because they add atoms to molecules without losing any.
• Use catalysts to increase selectivity: Catalysts can help direct the reaction to produce mainly the desired product, reducing waste from side reactions. Catalysts are not consumed during the reaction and help improve efficiency.

Improving atom economy often involves redesigning processes to be more efficient and environmentally friendly.

Examples of High Atom Economy Reactions

Some reactions have naturally high atom economies, such as:

• Addition reactions: These reactions add atoms to a molecule without producing by-products. For example, the addition of bromine to ethene forms dibromoethane with 100% atom economy.
• Atom-efficient synthesis routes: Designing synthetic pathways that avoid unnecessary steps or wasteful side products improves atom economy.

In contrast, substitution reactions often have lower atom economies because they replace one atom or group with another, producing by-products. For example, when making a haloalkane by substituting a hydroxyl group with a halogen, water is produced as a by-product, lowering atom economy.

For example, consider the reaction of ethene with bromine:

Ethene (C₂H₄) + Bromine (Br₂) 12 Dibromoethane (C₂H₄Br₂)

All atoms from the reactants are in the product, so atom economy is:

$$\frac{\text{Mr of C}_2\text{H}_4\text{Br}_2}{\text{Mr of C}_2\text{H}_4 + \text{Mr of Br}_2} \times 100 = \frac{187}{28 + 160} \times 100 = 100\%$$

For example, if a reaction produces only the desired product, the atom economy is 100%.