A5.8 Atom Economy

How “green” is a chemical manufacturing process?

The answer to this question can be expressed as the chemical’s percentage atom economy. To evaluate the atom economy, this equation is used:
Atom economy=molar mass of desired productstotal molar mass of all the reactants×100\textsf{Atom economy} = {\frac{\textsf{molar mass of desired products}}{\textsf {total molar mass of all the reactants}}} \times \textsf{100}

Worked example
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What is the atom economy of the manufacture of ethanol by: i. fermentation of glucose?
C6H12O62C2H5OH + 2CO2\textsf{C}_\textsf{6}\textsf{H}_\textsf{12}\textsf O_\textsf6 \rightarrow \textsf{2C}_\textsf2\textsf H_\textsf5\textsf{OH + 2CO}_\textsf2
ii. hydration of ethane?
C2H4 + H2OC2H5OH\textsf{C}_\textsf{2}\textsf H_\textsf4 \textsf{ + H}_\textsf2\textsf O \rightarrow \textsf{C}_\textsf2\textsf H_\textsf5\textsf{OH}
Method i. molar mass of desired products
=2×[(12×2)+5+16+1]=92=\textsf{2} \times [(\textsf{12} \times \textsf{2})+\textsf{5}+{\textsf{16}}+\textsf{1}]=\textsf{92}
molar mass of reactants
=[(12×6)+12+(16×6)]=180=[(\textsf{12} \times \textsf{6})+\textsf{12}+\textsf{(16}\times \textsf{6})]=\textsf{180}
Atom economy=molar mass of desired productstotal molar mass of all the reactants×100=50%\textsf{Atom economy} = {\frac{\textsf{molar mass of desired products}}{\textsf{total molar mass of all the reactants}}} \times \textsf{100}=\textsf{50\%}
(of course, in practice this reaction is even less efficient as the reaction usually stops before all the glucose has been changed into ethanol!) Method ii. Atom economy = 100% as there is only one product.
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