6.2.4 Organic Synthesis

Question 1

Haloalkane → Nitriles

Answer 1

Mechanism: Nucleophilic substitution
Reagents: NaCN in CH3CH2OH
R-Cl + NaCN → R-CN + NaCl

Question 2

Aldehyde → hydroxynitrile

Answer 2

Mechanism: Nucleophilic addition
Reagents: NaCN / H2SO4
R-CHO + NaCN → R(OH)-CN + NaOH

Question 3

Nitriles → Amines

Answer 3

Mechanism: Reduction
Reagents: Ni catalyst
R-CN + 2H2 → R-NH2

Question 4

Nitriles → Carboxylic acid

Answer 4

Mechanism: Hydrolysis
Reagents: heat, dilute aqueous HCl
R-CN + 2H2O + HCl(aq) → R-COOH + NH4Cl

Question 5

Recrystallisation

Answer 5

• Dissolve in minimum volume hot solvent (ethanol) to dissolve most of the solid to make a saturated solution
• Filter to remove insoluble impurities cool + allow to crystallise
• Filter under reduced pressure to remove soluble impurities
• Wash with cold minimum volume of solvent + dry between filter papers with paper towel

Question 6

Melting point determination

Answer 6

• Sample in suitable melting point apparatus (e.g. capillary in oil bath / Thiele tube / melting point apparatus
• Heat slowly to establish melting point range
• (Broad) range of melting point indicates presence of impurities
• Melts sharply over narrow (agrees with data) indicates purity

Question 7

Filtration under reduced pressure

Answer 7

• Use Buchner flask (with side arm), Buchner funnel (flat-bottomed), filter paper
• Make sure apparatus works, flow through, air-tight connection between flask and funnel to vacuum pump
• Switch on vacuum pump and wet filter paper with solvent, check for suction
• Pour reaction mixture into centre slowly, then rinse out beaker with solvent to allow all crystals to collect in Buchner funnel
• Rinse crystals in funnel with more solvent + leave under suction to dry