Synthesis

(a) Synthesis of Organic Compounds by a Sequence of Reactions

Concept: Organic synthesis involves a stepwise conversion of starting materials into a desired product through a sequence of reactions.

Reaction Pathways:

• Functional group interconversions (e.g., alcohols → aldehydes → carboxylic acids).

• Use of reagents to control selectivity (e.g., oxidising/reducing agents, protecting groups).

Common Reactions in Synthesis:

• Nucleophilic substitution (SN1/SN2).

• Electrophilic addition (alkenes → alcohols, halogenoalkanes).

• Electrophilic substitution (aromatic compounds).

• Reduction and oxidation reactions.

• Esterification and amide formation.

Retrosynthetic Analysis: Breaking down a target molecule into simpler precursors to plan synthesis.

(b) Principles Underlying Techniques of Manipulation, Separation, and Purification

Filtration: Separation of solids from liquids (gravity and vacuum filtration).

Distillation: Separation based on boiling points (simple and fractional distillation).

Recrystallisation: Purification of solids using a suitable solvent (dissolution, filtration, cooling).

Solvent Extraction: Use of immiscible solvents to separate compounds based on solubility.

Chromatography: TLC, GC, HPLC for separating components based on polarity and interactions with a stationary phase.

(c) Distinction Between Condensation Polymerisation and Addition Polymerisation

Addition Polymerisation:

• Involves unsaturated monomers (alkenes).

• No loss of small molecules.

• Example: Polyethene, PVC, polystyrene.

Condensation Polymerisation:

• Involves monomers with two different functional groups.

• Produces small molecules (e.g., H₂O, HCl) as by-products.

• Example: Polyesters, polyamides.

(d) How Polyesters and Polyamides are Formed

Polyesters:

• Formed by condensation polymerisation of a dicarboxylic acid and a diol.

• Example: Terylene (PET) from ethane-1,2-diol and benzene-1,4-dicarboxylic acid.

Polyamides:

• Formed by condensation polymerisation of a dicarboxylic acid (or acyl chloride) and a diamine.

• Example: Nylon-6,6 from hexanedioic acid and hexane-1,6-diamine.

(e) Use of Melting Temperature in Determination of Purity

the Pure substances have a sharp, well-defined melting point.

• Impurities cause:

  • Depression of melting point.

  • Broadening of melting range.

• Technique: Compare experimental melting point with literature values.