Organic Synthesis

A functional group is a specific group of atoms in a molecule that determines how that molecule behaves in chemical reactions. Think of it like the "active part" of the molecule that does the chemistry.

When you have a molecule with several functional groups, you need to be able to spot each one and predict what reactions it can undergo.

Here's a summary of the main functional groups you need to know:

• Test: No specific chemical test available
• How to make them:
  • Add hydrogen (H₂) to alkenes using a Pt or Ni catalyst with heat (this is called hydrogenation)
  • Break down crude oil using heat and an Al₂O₃ catalyst (called cracking)

• Test: They turn bromine water from orange to colourless (we say they "decolourise" bromine water)
• How to make them:
  • Heat halogenoalkanes with ethanolic NaOH (this removes a small molecule, called elimination)
  • Remove water from alcohols using a hot Al₂O₃ catalyst (called dehydration)
  • Crack crude oil using heat and Al₂O₃ catalyst

• Test: Add dilute nitric acid, then silver nitrate, then dilute ammonia solution. You'll see a coloured precipitate:
  • AgCl = white precipitate
  • AgBr = cream precipitate
  • AgI = yellow precipitate
• How to make them:
  • React alkanes with halogens under UV light (free-radical substitution)
  • Add hydrogen halides (like HBr) to alkenes (electrophilic addition)
  • React alcohols with reagents like HX gas, or KCl with concentrated H₂SO₄, or PCl₃ with heat, or PCl₅, or SOCl₂ (nucleophilic substitution)

• Tests:
  • Forms a silver mirror with Tollens' reagent
  • Forms a red precipitate with Fehling's solution
  • Forms an orange precipitate with 2,4-DNPH
• How to make them:
  • Oxidise primary alcohols using acidified K₂Cr₂O₇ or KMnO₄, with distillation to stop the aldehyde oxidising further into a carboxylic acid

• Test: Forms an orange precipitate with 2,4-DNPH (but does NOT react with Fehling's or Tollens')
• How to make them:
  • Oxidise secondary alcohols using acidified K₂Cr₂O₇ or KMnO₄ under distillation

• Test: React with carbonates to produce CO₂ gas, which turns limewater cloudy
• How to make them:
  • Oxidise aldehydes and primary alcohols using acidified K₂Cr₂O₇ or KMnO₄ under reflux (heating with a condenser so nothing escapes)
  • Break down nitriles using dilute acid or dilute alkali, then add acid (hydrolysis)
  • Break down esters using dilute acid or dilute alkali, then add acid (hydrolysis)

• Test: Have sweet, fruity smells
• How to make them:
  • React alcohols with carboxylic acids using hot, concentrated H₂SO₄ as catalyst (this is a condensation reaction because a small molecule—water—is eliminated)

• Tests:
  • Turn damp red litmus paper blue
  • Turn universal indicator blue/purple
• How to make them:
  • Heat halogenoalkanes under pressure with ammonia (NH₃) in ethanol (nucleophilic substitution)

• Test: No official test
• How to make them:
  • Heat halogenoalkanes under pressure with KCN in ethanol (nucleophilic substitution)