Atom Economy

This lesson covers: 

  1. What atom economy measures
  2. How atom economy is calculated
  3. Examples of atom economy in different reaction types
  4. Why high atom economy is important

Atom economy measures reaction efficiency

The atom economy of a reaction indicates how efficient the chemical reaction is at incorporating the reactants into the desired product. Specifically, atom economy measures the percentage of reactant atoms that end up in the final desired product, rather than in by-products.


A reaction with 100% atom economy converts all reactant atoms into the desired product. Reactions with lower atom economies generate more waste.

Calculating atom economy

Atom economy is calculated using the following formula:

% atom economy=ΣMr​ of all reactantsMr​ of desired product​×100


Where:

  • Σ = sum of
  • Mr = relative molecular mass


This formula essentially compares the mass of the desired product to the total mass of all reactants. The higher the percentage, the more efficient the reaction is at converting reactants to the desired product.


Some key points about atom economy:

  • It is based on the balanced chemical equation - make sure you are using the correct mole ratios.
  • Atom economy gives no indication of the percentage yield or losses during processing.

Worked example 1 - Calculating atom economy for the production of methanol

Calculate the atom economy for the production of methanol (CH3OH) from chloromethane (CH3Cl) and potassium hydroxide (KOH). The balanced equation is:

CH3Cl + KOH ➔ CH3OH + KCl


Step 1: Calculate Mr of desired product

Mr of CH3OH = 12.0 + 4(1.0) + 16.0 = 32.0


Step 2: Calculate ΣMr of all reactants

Mr of CH3Cl = 12.0 + 3(1.0) + 35.5 = 50.5

Mr of KOH = 39.1 + 16.0 + 1.0 = 56.1

ΣMr = 50.5 + 56.0 = 196.6


Step 3: Equation

% atom economy=ΣMr​ of all reactantsMr​ of desired product​×100


Step 4: Substitution and correct evaluation

% atom economy=106.632.0​×100=30.0%


This calculation shows that only 30.0% of the reactant atoms are incorporated into the desired product, indicating a low atom economy for this substitution reaction.

Worked example 2 - Calculating atom economy for the fermentation of glucose

Calculate the atom economy for the production of ethanol (C2H5OH) from the fermentation of glucose (C6H12O6). The balanced chemical equation is:

C6H12O6 ➔ 2C2H5OH + 2CO2


Step 1: Calculate Mr of desired product

Mr of C2H5OH = 2(12.0) + 6(1.0) + 16.0 = 46.0


Step 2: Multiply by reaction stoichiometry

Since the equation produces 2 moles of C2H5OH the total Mr for C2H5OH = 2 × 46.0 = 92.0


Step 2: Calculate ΣMr of all reactants

Mr of C6H12O6 = 6(12.0) + 12(1.0) + 6(16.0) = 180.0


Step 4: Equation

% atom economy=ΣMr​ of all reactantsMr​ of desired product​×100


Step 5: Substitution and correct evaluation

% atom economy=180.092.0​×100=51.1%


This calculation shows that 51.1% of the reactant atoms are incorporated into the desired product, indicating a moderate atom economy for this reaction.

Atom economy in different reaction types

Addition reactions

In an addition reaction, two reactants combine to form a single product. For example, ethene and hydrogen undergo an addition reaction:

C2H4 + H2 ➔ C2H6

Since all reactant atoms end up in the desired product ethane, addition reactions always have 100% atom economy.


Substitution reactions

In a substitution reaction, some atoms are substituted between reactants, generating at least two products - the desired product and an unavoidable by-product. For example, chloromethane reacts with potassium hydroxide in a substitution reaction:

CH3Cl + KOH ➔ CH3OH + KCl

Here the desired product is methanol, but potassium chloride is also produced as waste. The atom economy will be less than 100%.

Importance of high atom economy

Environmental: Reactions with higher atom economies are more sustainable and better for the environment because:

  • Less waste is generated, reducing disposal impacts.
  • Raw materials are used more efficiently, preserving limited resources.
  • Less energy intensive conditions are often required.


Economic: High atom economy also has financial advantages:

  • Reduced costs for separating products from waste.
  • Lower raw material demands decrease expenses.
  • Energy costs for heating and pressurising reactions are minimised.


Therefore, companies aim to develop chemical processes with the highest atom economy wherever possible to maximise sustainability and profits.