2. New Way Chemistry for Hong Kong A-Level Book 3B1 Syntheses and Interconversions of Organic Compounds 37.1Planning Organic Syntheses 37.2Interconversions of Functional Groups of Organic Compounds 37.3Chain Lengthening or Shortening of Carbon Skeleton Chapter 37

3. New Way Chemistry for Hong Kong A-Level Book 3B2 37.1 Planning Organic Syntheses (SB p.122) The organic compounds that are now known: Only small fractions of them can be isolated from natural resources All the remaining are synthesized by organic chemists Reasons for carrying out syntheses: e.g.To make a new medicine, dye, plastics, pesticide; To make a new compound for studying reaction mechanisms or metabolic pathways

4. New Way Chemistry for Hong Kong A-Level Book 3B3 37.1 Planning Organic Syntheses (SB p.122) The way to plan the synthesis is to think backwards  From the desired product to simpler molecules that can act as the precursor for our target molecule A synthesis usually involves more than one step

5. New Way Chemistry for Hong Kong A-Level Book 3B4 37.1 Planning Organic Syntheses (SB p.123) There are usually more than one way to carry out a synthesis

6. New Way Chemistry for Hong Kong A-Level Book 3B5 37.1 Planning Organic Syntheses (SB p.123) The feasibility of an organic synthesis depends on a number of factors: Numbers of steps involved in the synthesis Availability of starting materials and reagents Duration of the synthetic process

7. New Way Chemistry for Hong Kong A-Level Book 3B6 37.1 Planning Organic Syntheses (SB p.123) Most organic reactions are reversible and seldom proceed to completion As the backward reaction takes place, it is impossible to have a 100% yield of product from each step of the synthetic route Numbers of Steps involved in the Synthesis

8. New Way Chemistry for Hong Kong A-Level Book 3B7 The yield of desired product is 12.96% ∴ an efficient route of synthesis consists of a minimal number of steps Usually the number of steps is limited to not more than four 37.1 Planning Organic Syntheses (SB p.123) e.g.

9. New Way Chemistry for Hong Kong A-Level Book 3B8 37.1 Planning Organic Syntheses (SB p.123) There are often a restricted number of simple, relatively cheap organic compounds available e.g.simple haloalkanes, alcohols of not more than four carbon atoms, simple aromatic compounds such as benzene and methylbenzene Availability of Starting Materials and Reagents

10. New Way Chemistry for Hong Kong A-Level Book 3B9 37.1 Planning Organic Syntheses (SB p.124) The time factor must be considered when planning the synthetic pathway ∵ many organic reactions proceed at a relatively slow rate e.g. acid-catalyzed esterification requires the reaction mixture to be refluxed for a whole day Involvement of slow reactions in the synthetic route is impractical as the reaction will be too long Duration of the Synthetic Process

11. New Way Chemistry for Hong Kong A-Level Book 3B10 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.124) Oxidation of an organic compound usually corresponds to increasing its oxygen content or decreasing its hydrogen content Common oxidizing agents used: KMnO 4, K 2 Cr 2 O 7 or H 2 CrO 4 KMnO 4 is the strongest oxidizing agent and can be used in acidic, neutral or alkaline medium Other oxidizing agents: Tollens’ reagent, Fehling’s reagent, ozone Oxidation

12. New Way Chemistry for Hong Kong A-Level Book 3B11 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.125) 1.Mild Oxidation by Potassium Manganate(VII) Under mild oxidation by alkaline KMnO 4, alkenes are oxidized to diols Potassium Manganate(VII)

13. New Way Chemistry for Hong Kong A-Level Book 3B12 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.125) 2.Vigorous Oxidation by Potassium Manganate(VII) Occur in acidic or alkaline medium Heating is to ensure the vigour of the reaction

14. New Way Chemistry for Hong Kong A-Level Book 3B13 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.125) Alkylbenzenes are converted to benzoic acid by vigorous oxidation of KMnO 4

15. New Way Chemistry for Hong Kong A-Level Book 3B14 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.126) 1° alcohols are oxidized to carboxylic acids by vigorous oxidation of KMnO 4 The oxidation is difficult to stop at the aldehyde stage. The way to obtain aldehyde from oxidation is to remove the aldehyde by distillation as soon as they formed

16. New Way Chemistry for Hong Kong A-Level Book 3B15 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.126) The most commonly used chromium(VI) reagent is H 2 CrO 4 which is prepared by adding CrO 3, Na 2 Cr 2 O 7 or K 2 Cr 2 O 7 to aqueous H 2 SO 4 It oxidizes 1° alcohols or aromatic side chains to aldehydes but not in good yields Potassium Dichromate(VI) or Sodium Dichromate(VI)

17. New Way Chemistry for Hong Kong A-Level Book 3B16 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.126) It is most often used to oxidize 2° alcohols to ketones in excellent yields

18. New Way Chemistry for Hong Kong A-Level Book 3B17 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.126) Tollens’ and Fehling’s reagents are weak oxidizing agents which are able to oxidize aldehydes to carboxylate ions Tollens’ and Fehling’s Reagents

19. New Way Chemistry for Hong Kong A-Level Book 3B18 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.126) Ozonolysis is the most important oxidation reaction of alkenes This reaction provides a method for locating the double bond of an alkene Ozone reacts vigorously with alkenes to form ozonides and then reduced by treatment with Zn and H 2 O to give carbonyl compounds Ozone

20. New Way Chemistry for Hong Kong A-Level Book 3B19 Check Point 37-1 Show how each of the following transformations could be accomplished. (a) (b) Answer 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.127) (a) (b)

21. New Way Chemistry for Hong Kong A-Level Book 3B20 Check Point 37-1 Show how each of the following transformations could be accomplished. (c) (d) Answer 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.127) (c) (d)

22. New Way Chemistry for Hong Kong A-Level Book 3B21 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.128) Reduction of an organic compound usually corresponds to increasing its hydrogen content or decreasing its oxygen content Examples: Reduction

23. New Way Chemistry for Hong Kong A-Level Book 3B22 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.128) Hydrogen is added to the C = C and C  C bonds in alkenes and alkynes in the presence of transition metal catalysts Hydrogen with Transition Metal Catalyst

24. New Way Chemistry for Hong Kong A-Level Book 3B23 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.129) Nitriles or nitro compounds are reduced to amines by hydrogen in the presence of transition metal catalysts

25. New Way Chemistry for Hong Kong A-Level Book 3B24 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.129) LiAlH 4 is a powerful reducing agent It reduces carboxylic acids, esters, aldehydes, ketones, amides, nitriles and nitro compounds Lithium Tetrahydridoaluminate

26. New Way Chemistry for Hong Kong A-Level Book 3B25 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.129)

27. New Way Chemistry for Hong Kong A-Level Book 3B26 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.129) LiAlH 4 cannot normally reduce unsaturated centres like C = C and C  C bonds and benzene ring Reduction with LiAlH 4 must be carried out in anhydrous solutions ∵ LiAlH 4 reacts violently with water

28. New Way Chemistry for Hong Kong A-Level Book 3B27 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.130) NaBH 4 is a less powerful reducing agent than LiAlH 4 It reduces aldehydes and ketones only It can be used in water or alcohols Sodium Tetrahydridoborate

29. New Way Chemistry for Hong Kong A-Level Book 3B28 Check Point 37-2 Which reducing agent, LiAlH 4 or NaBH 4, would you use to carry out the following transformations? (a) (b) (c) 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.130) (a)LiAlH 4 (b)LiAlH 4 (c)NaBH 4 Answer

30. New Way Chemistry for Hong Kong A-Level Book 3B29 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.130) Aromatic nitro group can be reduced to amines by treatment with HCl and Fe, Zn or Sn, or a metal salt such as SnCl 2 Zinc, Tin, Tin(II) Chloride or Iron with Hydrochloric acid

31. New Way Chemistry for Hong Kong A-Level Book 3B30 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.131) Free radical substitution of alkanes with halogens forms haloalkanes A mixture of mono-, di- and poly-substituted haloalkanes is formed RH + X 2  RX + HX Substitution

32. New Way Chemistry for Hong Kong A-Level Book 3B31 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.131) Various aromatic compounds can be prepared from benzene by electrophilic substitution of a hydrogen atom by substituents

33. New Way Chemistry for Hong Kong A-Level Book 3B32 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.131) Haloalkanes can be converted into alcohols, nitriles or amines by substitution reactions R – X + OH –  R – OH + X –

34. New Way Chemistry for Hong Kong A-Level Book 3B33 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.131) Addition of alkenes and alkynes with various reagents can produce haloalkanes, haloalcohols, alcohols, alkanes and polymers Addition (X = Cl or Br)

35. New Way Chemistry for Hong Kong A-Level Book 3B34 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.132)

36. New Way Chemistry for Hong Kong A-Level Book 3B35 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.132) Carbonyl compounds undergo addition reaction with hydrogen cyanide for form hydroxyalkanenitriles Addition of hydrogen to nitriles yields amines

37. New Way Chemistry for Hong Kong A-Level Book 3B36 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.132) Haloalkanes can be converted to alkenes by elimination with alcoholic KOH or NaOH Elimination Dihaloalkanes (with one halogen atom on each of two adjacent carbon atoms) can be converted to alkynes by elimination

38. New Way Chemistry for Hong Kong A-Level Book 3B37 37.2 Interconversions of Functional Groups of Organic Compounds (SB p.132) Alcohols undergo dehydration to give alkenes by treatment with conc. H 2 SO 4

39. New Way Chemistry for Hong Kong A-Level Book 3B38 37.2 Chain Lengthening or Shortening of Carbon Skeleton (SB p.133) The carbon chain is lengthened by one when haloalkanes react with NaCN to form nitriles Hydrolysis of nitriles gives carboxylic acids, while hydrogenation of nitriles gives 1° amines Methods of Chain Lengthening

40. New Way Chemistry for Hong Kong A-Level Book 3B39 37.2 Chain Lengthening or Shortening of Carbon Skeleton (SB p.133) The carbon chain is also lengthened by one when carbonyl compounds react with HCN to form 2-hydroxyalkanenitriles Hydrolysis of the 2-hydroxyalkanenitriles yields 2-hydroxycarboxylic acids

41. New Way Chemistry for Hong Kong A-Level Book 3B40 37.2 Chain Lengthening or Shortening of Carbon Skeleton (SB p.133) In Hofmann degradation of amides, the carbon chain is reduced by one carbon atom Methods of Chain Shortening

42. New Way Chemistry for Hong Kong A-Level Book 3B41 37.2 Chain Lengthening or Shortening of Carbon Skeleton (SB p.134) In the triiodomethane formation reaction (iodoform reaction) of alcohols containing the group and aldehydes or ketones containing the group, the carbon chain is also degraded by one carbon atom

43. New Way Chemistry for Hong Kong A-Level Book 3B42 37.2 Chain Lengthening or Shortening of Carbon Skeleton (SB p.134)

44. New Way Chemistry for Hong Kong A-Level Book 3B43 37.2 Chain Lengthening or Shortening of Carbon Skeleton (SB p.134) In ozonolysis, alkenes react with ozone to from ozonide which is reduced by using Zn and H 2 O to produce a mixture of carbonyl compounds resulting from the cleavage of the C = C bond The cleavage of the C = C bond results in the degradation of carbon chain

45. New Way Chemistry for Hong Kong A-Level Book 3B44 Check Point 37-3 By means of simple chemical equations, indicate how you would carry out the following conversion, which may involve more than one step. Give the reagents for each step and indicate the major product formed. (a) 37.3 Chain Lengthening or Shortening of Carbon Skeleton (SB p.133) Answer (a)

46. New Way Chemistry for Hong Kong A-Level Book 3B45 Check Point 37-3 By means of simple chemical equations, indicate how you would carry out the following conversion, which may involve more than one step. Give the reagents for each step and indicate the major product formed. (b) 37.3 Chain Lengthening or Shortening of Carbon Skeleton (SB p.133) Answer (b)

47. New Way Chemistry for Hong Kong A-Level Book 3B46 Check Point 37-3 By means of simple chemical equations, indicate how you would carry out the following conversion, which may involve more than one step. Give the reagents for each step and indicate the major product formed. (c) 37.3 Chain Lengthening or Shortening of Carbon Skeleton (SB p.133) Answer (c)

48. New Way Chemistry for Hong Kong A-Level Book 3B47 Check Point 37-3 By means of simple chemical equations, indicate how you would carry out the following conversion, which may involve more than one step. Give the reagents for each step and indicate the major product formed. (d) 37.3 Chain Lengthening or Shortening of Carbon Skeleton (SB p.133) Answer (d)

49. New Way Chemistry for Hong Kong A-Level Book 3B48 The END