1. Planning a synthesis

2. Retrosynthesis involves working backwards from a target molecule to determine suitable starting materials for its preparation.

3. For example, what starting materials are needed to make phenylethanone?

5. Imagine disconnecting the target molecule here. What fragments do we get?

6. Imagine breaking the bond between the benzene ring and the carbonyl group.

7. Two charged fragments would form.

9. Fragments like these are called synthons.

10. These two synthons do not exist but their synthetic equivalents do.

12. They are benzene and ethanoyl chloride. Aluminium chloride will also be needed to act as a Friedel–Crafts catalyst.

14. So we can synthesise phenylethanone by the Friedel–Crafts acylation of benzene, using ethanoyl chloride and aluminium chloride.

18. There may be several different routes to a target molecule. The shortest one is not always the best.

19. The cost of starting materials and reagents must be considered.

20. Different reagents pose different hazards. The safe disposal of waste must be considered.

21. The overall yield and atom economy must be considered, to avoid unnecessary waste and use of resources.

22. The presence of other functional groups may influence the order of the steps taken, if they also react with the reagents.

23. Sometimes more than one isomer of the product forms, which reduces the yield.

25. For example, the nitration of methylbenzene produces three isomers.

26. Separating isomers can be expensive and time consuming.

27. Optical isomers are particularly difficult to separate.

28. In the example seen here, the products could be separated by chromatography or fractional crystallisation.

29. There are three main stages in carrying out an organic preparation.

30. The reaction or reactions needed.

31. The extraction of the product from the reaction mixture.

32. The purification of the product.

33. It may be necessary to heat the reaction mixture for a long time without losing volatile solvents or products.

35. Heating under reflux is used to achieve this.

36. The product may need extracting from the reaction mixture.

38. Vacuum filtration may be used to achieve this if the product is an insoluble solid.

39. The product must be purified. There are several ways to do this.

41. Simple distillation may be used to separate a mixture of miscible liquids.

42. A separating funnel may be used to separate a mixture of immiscible liquids.

43. Thin-layer chromatography or t.l.c. may be used to separate small amounts of solutes.

44. Recrystallisation involves several steps to purify solid organic products.

45. The impure solid is first dissolved in a minimum volume of hot solvent. Impurities remain undissolved.

46. The hot solution is filtered to remove insoluble impurities. The funnel and flask should be preheated.

47. The filtrate is allowed to cool and pure crystals form.

48. The crystals are collected by vacuum filtration. They are left to air dry or dried in an oven.

49. The melting point may be determined to check the purity of the final product. A pure compound will melt within 0.5°C of its true melting point.