Efficiency of organic reaction pathways

Efficiency of Organic Reaction Pathways

In organic chemistry, reaction pathways refer to the series of steps that transform reactants into products. The efficiency of these pathways is a key consideration, as it determines how effectively a reaction converts starting materials into the desired products with minimal side reactions or waste.

Efficient reaction pathways are essential for optimising yields, reducing costs, and ensuring the sustainability of chemical processes. This is particularly important in industrial applications such as pharmaceuticals, materials science, and energy production. In recent years, the principles of green chemistry have become increasingly important in improving the efficiency of organic reactions.

Green chemistry focuses on designing chemical processes that minimise the use of hazardous substances, reduce waste and conserve energy. It encourages the development of reaction pathways that are safer, more sustainable , and more environmentally friendly, aligning with both economic and ecological goals. By integrating green chemistry principles into the design of reaction pathways, chemists can enhance both the efficiency and sustainability of organic synthesis.

Use this page to revise the following concepts within efficiency of organic reaction pathways:

Relevant Green Chemistry Principles

Green chemistry principles (VCAA 2024):

  1. Atom economy: Processes/pathways should be designed to maximise incorporation of all reactant materials used in the process into the final product.
  2. Catalysis: Catalysts should be selected to generate the same desired product(s) with less waste and using less energy and reagents in reaction processes/pathways.
  3. Design for degradation: Chemical products should be designed so that at the end of their use they break down into harmless degradation products and do not persist in the environment.
  4. Design for energy efficiency: Processes/pathways should be designed for maximum energy efficiency and with minimal negative environmental and economic impacts.
  5. Designing safer chemicals: Chemical products should be designed to achieve their intended function while minimising toxicity.
  6. Prevention of wastes: It is better to prevent waste than to treat or clean up waste after it has been produced.
  7. Use of renewable feedstocks: Raw materials or feedstocks should be made from renewable (mainly plant‑based) materials, rather than from fossil fuels, whenever practicable.

(2024 VCAA - Data Book VCE Chemistry)

Percentage yield measures the efficiency of a chemical reaction by comparing the actual and theoretical yields, while atom economy assesses how effectively reactants are converted into desired products, minimising waste. Developing chemical processes with a high atom economy benefits society and industry by reducing waste, lowering costs, improving sustainability, and enhancing environmental and economic efficiency.

Atom economy

Atom economy evaluates the efficiency of a reaction in terms of how well atoms in the reactants are utilised in the final product. It is calculated using the formula:

Note

Atom Economy = (Molar Mass of Desired Product/Total Molar Mass of Reactants) × 100

A high atom economy indicates that a large proportion of the reactants have been converted into the desired product, minimising waste.

Example

One common example of atom economy is the synthesis of aspirin (acetylsalicylic acid) from salicylic acid and acetic anhydride.

Synthesising acetylsalicylic acid

The reaction is: Salicylic Acid + Acetic Anhydride → Aspirin + Acetic Acid

  • Molar mass of salicylic acid (C7H6O3) = 138.12 gmol-1
  • Molar mass of acetic anhydride (C4H6O3) = 102.09 gmol-1
  • Molar mass of aspirin (C9H8O4) = 180.16 gmol-1
  • Molar mass of acetic acid (C2H4O2) = 60.05 gmol-1

The total molar mass of reactants: 138.12 (salicylic acid) +102.09 (acetic anhydride) = 240.21 gmol-1

The molar mass of the desired product (aspirin) is 180.16 gmol-1

Atom economy = 180.16/240.21 × 100 = 75%

The atom economy of this reaction is 75%, meaning that 75% of the mass of the reactants is incorporated into the desired aspirin product, while the rest (mostly acetic acid) is waste.

If the reaction were modified to reduce or eliminate waste products (for example, by finding a way to reuse acetic acid), the atom economy could be improved, making the process more sustainable and efficient.

Percentage yield

Percentage yield measures the efficiency of a chemical reaction by comparing the actual yield of a product obtained to the theoretical yield based on stoichiometric calculations. It is calculated using the formula:

Percentage Yield = (Actual Yield/Theoretical Yield)×100

Percentage yield example

4.0 grams of Hydrogen gas reacts with oxygen gas to produce water. The actual yield of water obtained from the reaction was only 32.0 grams.

The balanced chemical equation is: 2H2 (g) + O2 (g) → 2H2O (l)

  1. Calculate moles of hydrogen

    Molar mass of H2 = 2.02 gmol-1

    Moles of H2 = 4.0 g/ 2.02 gmol-1 = 1.98 mol

  2. Use stoichiometry to calculate moles of water produced:

    From the balanced equation, 2 moles of H2 produce 2 moles of H2O.

    Therefore, 1.98 moles of H2 will produce 1.98 moles of H2O.

  3. Calculate the theoretical yield of water.

    Molar mass of H2O = 18.02 gmol-1.

    Theoretical yield of H2O = 1.98 mol× 18.02 gmol-1 = 35.7 g

  4. Calculate the percentage yield.

    Percentage yield = (32.0 g/35.7 g) × 100 = 89.7%

Example: Ibuprofen Synthesis - Brown vs. Green

The brown and green routes for the synthesis of ibuprofen, an anti-inflammatory drug, offer a compelling comparison in terms of atom economy and environmental impact. The brown route involves six steps, generating numerous intermediate products, and results in a low atom economy of approximately 40%. This method also produces substantial waste, including toxic inorganic salts like aluminium trichloride hydrate, which pose a threat to aquatic life.

In contrast, the green route, shown below, streamlines the process to just three steps, achieving a significantly higher atom economy of around 77%. This approach minimises waste by converting more reactants into the final product, and its by-product, acetic acid, has valuable applications in the food and cleaning industries. Overall, the green synthesis route not only reduces waste but also offers a more environmentally friendly alternative to the traditional method of ibuprofen production.

Green synthesis of Ibuprofen comparison of two pathways vertical

Table 1: Atom Economy in the Brown Synthesis of Ibuprofen

Reagent

Utilised in ibuprofen

Unutilised in ibuprofen

Formula

FW

Formula

FW

Formula

FW

C10H14

134

C10H13

133

H

1

C4H6O3

102

C2H3

27

C2H3O3

75

C4H7ClO2

122.5

CH

13

C4H6ClO2

109.5

C2H5ONa

68

0

C2H5ONa

68

H3O

19

0

H3O

19

NH3O

33

0

NH3O

33

H4O2

36

HO2

33

H3

3

Total

Ibuprofen

Waste products

C20H42NOClNa

514.5

C13H18O2

206

C7H24NO8ClNa

308.5

Percentage Atom Economy = (FW Ibuprofen/FW all reactants) x 100 = (206/514.5) x 100 = 40%

Table 2: Atom Economy in the Green Synthesis of Ibuprofen

Reagent

Utilised in ibuprofen

Unutilised in ibuprofen

Formula

FW

Formula

FW

Formula

FW

C10H14

134

C10H13

133

H

1

C4H6O3

102

C2H3O1

43

C2H3O2

59

H2

2

H2

2

-

0

CO

28

CO

28

-

0

Total

Ibuprofen

Waste products

C15H22O4

266

C13H18O2

206

C2H4O2

60

Percentage Atom Economy = (FW Ibuprofen/FW all reactants) x 100 = (206/266) x 100 = 77%

Source:The BHC Company Synthesis of Ibuprofen: A Greener Synthesis of Ibuprofen Which Creates Less Waste and Fewer Byproducts (from Cann, M.C.; and Connelly, M.E. Real World Cases in Green Chemistry, American Chemical Society: Washington, DC, 2000)

Advantages for society and industry

Environmental impact: Processes with a high atom economy generate less waste, reducing the environmental footprint and the need for waste disposal, which is crucial for sustainable development.

Cost efficiency: High atom economy often correlates with lower material costs since fewer raw materials are wasted. This can lead to reduced production costs for industries, making products more affordable for consumers.

Resource conservation: Efficient use of resources is essential as natural resources are finite. High atom economy processes help conserve raw materials, which is increasingly important in a resource-limited world.