Organic Synthesis Techniques

Overview

This lecture covers essential techniques in organic synthesis, including apparatus setup, purification methods, and a summary of key reaction types and their conditions. Accurate drawing and labeling of apparatus are emphasized, along with safety considerations and practical tips for successful organic reactions.

Reflux Technique

• Reflux is used to heat organic reaction mixtures for extended periods, preventing the loss of volatile substances.
• Apparatus setup:
  • Round-bottomed flask
  • Condenser with water inlet and outlet (outer tube sealed at top and bottom, water flows in at the bottom and out at the top)
  • Electric heater (preferred, as organic chemicals are often flammable)
  • Anti-bumping granules to ensure smooth, even boiling by forming small bubbles
• Safety notes:
  • Never seal the end of the condenser; pressure buildup can cause explosions.
  • This applies to any apparatus where volatile liquids are heated.
• Example reaction:
  • Primary alcohol + excess potassium dichromate(VI) solution and dilute sulfuric acid, heated under reflux, produces a carboxylic acid.
  • Equation: CH₃CH₂CH₂OH + 2[O] → CH₃CH₂CO₂H + H₂O
  • Observation: Orange dichromate solution turns green due to formation of Cr³⁺ ions.
• After the reaction, the product is distilled off using a distillation setup.

Distillation Technique

• Distillation is used to separate organic products from reaction mixtures based on boiling points and to prevent over-oxidation.
• Apparatus setup:
  • Round-bottomed flask
  • Liebig condenser (with water in/out)
  • Thermometer
  • Heat source
• Key points for drawing apparatus:
  • Do not draw lines between flask, adaptor, and condenser.
  • The top of the condenser must not be sealed.
  • The condenser must have two open water connections.
• Example reaction:
  • Primary alcohol + limited potassium dichromate(VI) solution and dilute sulfuric acid, gently warmed and distilled, produces an aldehyde.
  • Equation: CH₃CH₂CH₂OH + [O] → CH₃CH₂CHO + H₂O
  • Observation: Orange dichromate solution turns green (Cr³⁺ ions), indicating reaction progress.
  • Distil out the aldehyde as it forms to prevent further oxidation to carboxylic acid.

Purifying Organic Liquids

• Use a separating funnel to wash the organic product:
  • Add sodium hydrogencarbonate solution to neutralize any remaining acid, shake, and release pressure from CO₂ produced.
  • Alternatively, use saturated sodium chloride solution to help separate the organic and aqueous layers.
• Allow the layers to separate:
  • The layer with lower density (usually the organic layer) will be on top.
  • Run off and discard the aqueous layer.
• Transfer the organic layer to a clean, dry conical flask.
• Add a drying agent (e.g., anhydrous sodium sulfate or calcium chloride) to remove water. Use about three spatula loads.
  • The organic liquid should become clear when dry.
• Decant (carefully pour off) the dry organic liquid, leaving the drying agent behind.
• Distil the dried organic liquid to collect the pure product.

Common Reaction Conditions and Synthetic Routes

• Free radical substitution:
  • Alkane + Br₂ or Cl₂, UV light → Haloalkane
• Electrophilic addition:
  • Alkene + Br₂ or Cl₂, room temperature
  • Alkene + HBr or HCl, room temperature
• Oxidation of alcohols:
  • Primary alcohol + Na₂Cr₂O₇/H⁺, heat gently and distil → Aldehyde (partial oxidation)
  • Primary alcohol + Na₂Cr₂O₇/H⁺, heat under reflux with excess oxidant → Carboxylic acid
  • Secondary alcohol + Na₂Cr₂O₇/H⁺, heat under reflux → Ketone
• Polymerization:
  • Alkene, high pressure, catalyst → Poly(alkene)
• Nucleophilic substitution:
  • Haloalkane + aqueous KOH, heat under reflux
• Addition/reduction:
  • Alkene + H₂, nickel catalyst
• Substitution:
  • Alcohol + concentrated H₂SO₄ and sodium halide, heat under reflux
• Dehydration:
  • Alcohol + concentrated H₂SO₄ or H₃PO₄

Key Terms & Definitions

• Reflux: Heating a reaction mixture with a condenser to prevent loss of volatile substances.
• Distillation: Technique to separate compounds based on differences in boiling points.
• Separating funnel: Apparatus for separating immiscible liquid layers.
• Anti-bumping granules: Added to liquids to prevent violent, uneven boiling by forming small bubbles.
• Drying agent: Substance (e.g., anhydrous sodium sulfate, calcium chloride) used to remove water from organic liquids.
• Decant: Carefully pouring off a liquid, leaving solids (such as drying agents) behind.

Action Items / Next Steps

• Practice drawing and accurately labeling reflux and distillation apparatus, ensuring all key features are included.
• Review and memorize the reaction conditions and reagents for common organic synthesis routes.
• Complete assigned problems on purification techniques, focusing on the correct use of separating funnels and drying agents.
• Familiarize yourself with safety precautions for heating organic chemicals and handling volatile substances.