Choosing Reaction Pathways

A reaction pathway describes the series of reactions that lead to a desired product. There is often more than one way to make a particular substance. The pathway chosen for a product depends on several factors:

  • Percentage yield – Compares the actual yield of the product to the theoretical yield. A higher percentage yield means less waste and more efficient use of the reactants.
  • Atom economy – Measures the amount of the reactants that end up as useful products. A higher atom economy means less waste, a lower cost of production, and fewer environmental concerns.
  • Equilibrium position – If a reaction is reversible, we have to consider if it will favour the useful product
  • Rate of reaction – This is the speed at which the product can be made. A faster reaction rate can be more profitable but might require more energy, which can add to the cost.
  • Usefulness of by-products – If we can’t convert 100% of the reactants into useful products, we have to consider if the by-products are useful. This will reduce waste.

Manufacturing Ethanol

The manufacture of ethanol is a common example used to explain the process of choosing reaction pathways. The two main methods of producing alcohol are fermentation and the hydration of ethene.

Fermentation

Fermentation is a process by which glucose is broken down with yeast to form ethanol and carbon dioxide. The chemical equation for this process is:

Glucose Ethanol + Carbon dioxide

C6H12O6 (aq)2C2H5OH (aq) + 2CO2 (g)

The factors we need to consider in this pathway are:

  • Percentage yield – 15%
  • Atom economy – 51.1%
  • As this is not a reversible reaction, the position of equilibrium is not important
  • Slow rate of reaction
  • Usefulness of by-products – The by-product is CO2. which can be used to carbonate drinks, making them fizzy.

Hydration of ethene

The hydration of ethene involves reacting ethene with water to form ethanol. The chemical equation for this process is:

Ethene + Water Ethanol

C2H4(g) + H2O(g)C2H5OH(l)

The factors we need to consider in this pathway are:

  • Percentage yield – 95%
  • Atom economy – 100% (only one product)
  • This is a reversible reaction, and the position of equilibrium lies to the left. So at any given time, there will be more ethene than ethanol.
  • Fast rate of reaction
  • No by-products

Comparing the two methods

ProcessPercentage yieldAtom economyRate of reaction
Fermentation of glucose15%51.1%Slow
Hydration of ethene 95%100%Fast

The hydration of ethene has a higher yield than the fermentation of glucose, but it has a reversible reaction with an equilibrium position that favours the reactants. This means that only 5% of the ethene supplied is converted to ethanol, and reaching the 95% yield requires recirculating unreacted ethene until it reacts.

Also, the hydration of ethene has an atom economy of 100%, meaning that all of the reactants form the desired product. In contrast, the atom economy of fermenting glucose is only 51.1%, meaning that just over half of the reactants form the desired product.

However, fermenting glucose produces carbon dioxide as a by-product, which can be used to make carbonated drinks, which increases the atom economy.

It is also important to consider that glucose comes from plants, which is a renewable resource. In contrast, ethene comes from crude oil, which is a non-renewable resource.

Conclusion

Overall, the hydration of ethene seems to be a more efficient method due to its 100% atom economy and a fast rate of reaction. Even though it’s a reversible reaction, the unreacted ethene can be recycled, which increases the yield. However, the fermentation of glucose is still a viable option as it uses a renewable resource.

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