Chapter 9: Synthesis reactions Notes

Introduction

  • Chapter 9 focuses on synthesis reactions, defined as producing chemicals in a cost-efficient, safe, and timely manner.
  • Examples of commonly synthesized chemicals include medicines, paints, plastics and beauty products.
  • The content emphasizes planning from reactants to products, considering pathways, conditions, and practical constraints.

Uses of synthesis reactions

  • Chemical synthesis is the process of selecting suitable reactants, conditions, and pathways to form a desired product.
  • Specific medicines developed through synthesis include aspirin (acetylsalicylic acid) and penicillin.

Choosing reactants (retrosynthetic analysis)

  • Retrosynthetic analysis: identify the final product, then work backwards to determine necessary reactants.
  • For organic compounds, identify the product structure and simplify by removing carbons or breaking down the molecule until appropriate starting materials are reached.

Choosing a pathway

  • Most chemical reactions occur in multiple steps; choosing a pathway involves considering:
    • Availability and cost of reactants
    • Required conditions (temperature, pressure)
    • Other chemicals required (e.g., catalysts)
    • Potential side reactions that produce unwanted products
  • For each step, control the reactants, products, and conditions.
  • Intermediates: products formed in one step and used in the next.
  • Example: an energy profile diagram for the formation of D from A, where B and C are intermediates.

Multistep process – the Contact Process

  • Example of a multistep process: production of sulfuric acid (the Contact Process).
  • Design goal: minimize waste, side reactions, and undesired products.
  • Steps are made energy-efficient, and catalysts are used where possible to reduce energy use.

Multistep process – production of ethyl ethanoate

  • Ethene reacts with steam to form ethanol (intermediate).
  • Ethanol then reacts with acetic acid (ethanoic acid) to form ethyl ethanoate.
  • Conditions and reagents are chosen to maximize product yield.

Linear pathways

  • Linear pathway: product A → B → C → … → final product.
  • The Contact Process is an example of a linear pathway.

Convergent pathways

  • Convergent pathway: two independent reactions produce two components that are combined to form the final product.

Convergent pathways – ester formation

  • Example: Ethanol and butanoic acid are produced separately and then combined to form the ester ethyl butanoate.

Yields of reactions

  • What is yield? The actual amount of product formed in a reaction.
  • Often less than the theoretical maximum due to equilibrium limitations or suboptimal conditions.
  • A high yield is desirable but may incur higher costs; a balance between yield and cost is common.

Maximising yield

  • Ways to maximise yield by controlling reactions:
    • Manipulate equilibrium to shift to the right (toward products):
    • Remove product as it forms
    • Choose appropriate temperature and pressure for the equilibrium reaction
  • Other strategies:
    • Use catalysts to speed up reactions (increases production rate, not necessarily yield, but increases product formed over time)
    • Recycle unused reactants to minimize waste

The Haber process

  • The Haber process synthesizes ammonia from nitrogen and hydrogen.
  • Overall reaction (classic form): \mathrm{N2 + 3\,H2 \rightleftharpoons 2\,NH_3}
  • Industrial compromise: ideal conditions for yield and rate are conflicting; temperature and pressure are chosen to balance yield and rate.

The Haber process – temperature

  • Since the reaction is exothermic (forward direction favored at lower temperature), Le Chatelier’s principle predicts higher yields at lower temperatures.
  • However, lower temperatures slow the reaction rate; a compromise is used.
  • Typical operating temperature: $$400!-\