Atom economy Key words: Atom economy Percentage yield

Learning objectives Define the atom economy of a reaction and describe the benefits of developing chemical processes with a high atom economy. Explain that addition reactions have an atom economy of 100%, whereas substitution reactions are less efficient. Carry out calculations to determine the atom economy of a reaction. Explain that a reaction may have a high percentage yield but a low atom economy.

Percentage yield is used to assess the efficiency of a chemical reaction. Calculating the percentage yield only tells part of the story. A reaction may not only produce the desired product, but by products as well What happens to the by-products? Considered as a waste and disposed of – Costly – Potential environmental problems May be sold on or used elsewhere in the plant – Preserves Earth’s resources – Minimises waste

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Atom economy considers not only the desired product, but also all the by-products of a chemical reaction. It describes the efficiency of a reaction in terms of ALL the atoms involved. A reaction with a high atom economy uses atoms with minimal waste We are now much more aware of our environment. By using processes with a higher atom economy, chemical companies can reduce the amount of waste produced. This is good news, especially as we are running out of landfill sites. It has also been suggested that about 5-10% of the total expenditure of a chemical company goes on waste treatment

1.Calculate the molecular mass of the desired product 2.Divide this by the sum of the molecular masses of all products

The type of reaction used for a chemical process is a major factor in achieving a higher atom economy. Addition reactions have an atom economy of 100% Reactions involving substitution or elimination have an atom economy less than 100% This doesn’t mean that we should never carry out substitution or elimination reactions but we need to find a use for all the products to improve their atom economy. This can be problematic when the undesired products are toxic