21: Organic Synthesis - CAIE Chemistry A-level

Synthetic Routes

• Definition: Routes to produce a product from a starting organic compound.
• Importance: Understanding methods and conditions to convert compounds.

Homologous Series and Typical Reactions

Alkanes (C-C)

• Combustion
• Electrophilic substitution/free radical substitution with Br₂ or Cl₂ (forms haloalkanes)
• Cracking (forms short chain alkenes and alkanes)

Alkenes (C=C)

• Electrophilic Addition:
  • Steam (forms alcohols)
  • Hydrogen halides (forms haloalkanes)
  • Halogens (forms dihaloalkanes)
  • Hydrogen (forms alkanes)
• Oxidation with H⁺/MnO₄⁻ (forms diols)
• Addition polymerisation (forms polymers)
• Combustion
• Identification: React with bromine water - decolorizes in presence of C=C.

Halogenoalkanes (C-F, C-Cl, C-Br, C-I)

• Nucleophilic Substitution:
  • Hydrolysis (forms alcohols)
  • Reaction with ethanolic cyanide (forms nitriles)
  • Reaction with ammonia (forms primary amines)
• Elimination of hydrogen halide using ethanolic hydroxide ions (forms alkenes)
• Identification: React with AgNO₃(aq), test precipitate with NH₃(aq).

Alcohols (-OH)

• Combustion
• Substitution with hydrogen halides, sulfur dichloride oxide, or phosphorus(III) halides (forms haloalkanes)
• Reaction with sodium (forms sodium ethoxide and hydrogen gas)
• Oxidation with H⁺/Cr₂O₇²⁻ (forms carbonyls and carboxylic acids)
• Dehydration using an acid catalyst (forms alkenes)
• Esterification with carboxylic acids or acyl chlorides
• Identification: React with H⁺/Cr₂O₇²⁻: Orange to green color change in presence of primary/secondary alcohols.

Aldehydes (-CHO)

• Oxidation with H⁺/Cr₂O₇²⁻ (forms carboxylic acids)
• Reduction using NaBH₄ or LiAlH₄ (forms primary alcohols)
• Nucleophilic addition with HCN (forms hydroxynitriles)
• Identification: React with 2,4-DNPH, Tollens reagent, Fehlings reagent, and acidified potassium dichromate(VI).

Ketones (RCOR)

• Reduction using NaBH₄ or LiAlH₄ (forms secondary alcohols)
• Nucleophilic addition with HCN (forms hydroxynitriles)
• Identification: React with 2,4-DNPH.

Carboxylic Acids (-COOH)

• Reaction with metals, alkalis, or carbonates (forms a salt and inorganic products)
• Esterification with alcohols
• Reduction with LiAlH₄ (forms alcohols)
• Reaction with SOCl₂ (forms acyl chlorides, sulfur dioxide, and hydrochloric acid)
• Reaction with phosphorus(V) chloride or phosphorus(III) chloride (forms acyl chlorides)
• Identification: Test pH (less than 7) and react with a carbonate (effervescence as CO₂ is formed).

Esters (RCOOR)

• Acid hydrolysis (forms a carboxylic acid and an alcohol)
• Alkali hydrolysis (forms a carboxylate salt and an alcohol)

Amines (-NH₂)

• Reaction with acids (forms a salt)

Nitriles (CN)

• Acid hydrolysis (forms a carboxylic acid and a salt)
• Alkali hydrolysis (forms a carboxylate salt and ammonia)

Multi-Stage Synthesis

• Organic molecules can be prepared in multiple stages: reactant → intermediate → product.
• Example: Ethanoic acid from chloroethane; 2-propylamine from propene.

Analyzing Synthetic Routes

• Factors:
  • Type of reaction: addition reactions are more sustainable.
  • Reagents: prefer renewable and safe reagents.
  • By-products: less harmful by-products are preferred, and those usable in other industries are more sustainable.
  • Conditions: most energy-efficient and safe reactions are preferred.
• For multi-functional organic molecules:
  1. Identify organic functional groups using syllabus reactions.
  2. Predict properties and reactions.
  3. Devise multi-step synthetic routes.
  4. Analyze synthetic routes for reaction types, reagents, and by-products.