Chemical Synthesis and Design

In short: Designing a chemical synthesis means choosing a route and conditions that balance yield, rate, purity, cost, energy, safety and waste. The Haber process for ammonia uses a compromise temperature, high pressure and a catalyst, and green chemistry aims for high atom economy and less waste.

What is chemical synthesis?

Chemical synthesis is making a wanted product from starting materials (reactants) through one or more reactions. Industrial chemists must consider many factors at once.

Factors in designing a synthesis

Using equilibrium and rate together

Many industrial reactions are reversible. Chemists use Le Chatelier's principle and collision theory to choose conditions:

• Temperature: for an exothermic reaction, a low temperature gives a better yield but a slow rate. A compromise temperature and a catalyst are used.
• Pressure: high pressure favours the side with fewer gas moles and also speeds up gas reactions, but costs more and is riskier.
• Concentration: using an excess of a cheap reactant, or removing the product, pushes the equilibrium right.

Choosing between routes

When two processes make the same product, compare them fairly:

For example, phenol can be made from benzene through chlorobenzene (producing waste NaCl and HCl) or by the cumene process (producing propanone, which can be sold as a solvent). The cumene route has higher yield and purity, lower energy costs and a useful by-product.

Green chemistry

Green chemistry aims to design processes that are safer and produce less waste. Its ideas include:

• Prevent waste rather than clean it up.
• High atom economy: as many reactant atoms as possible end up in the product.
• Use renewable raw materials.
• Use catalysts instead of large amounts of reagents.
• Use safer solvents (like water) and lower temperatures.
• Design products that break down safely after use.

Scale and monitoring

A reaction that works in a test tube must be scaled up. In a large reactor:

• Heat from exothermic reactions builds up and must be removed, or the reaction can run out of control.
• Mixing is harder.
• Conditions (temperature, pressure, concentrations, pH) are monitored constantly with sensors.

Chemists also analyse samples of the product (using techniques from the last two chapters) to check its purity.

Quick summary

• Synthesis design weighs reactants, yield, rate, purity, energy, safety, waste, environment and cost.
• For equilibria, use a compromise temperature, a catalyst, suitable pressure, excess reactant and product removal.
• Haber (ammonia) and Contact (sulfuric acid) processes show these choices.
• Compare routes by yield, purity, cost, energy, by-products and environmental impact.
• Green chemistry: prevent waste, high atom economy, renewable raw materials, catalysts, safer solvents.

Quiz