Organic Chemistry II Aldehydes and Ketones

1. Organic Chemistry IIAldehydes and Ketones Dr. Ralph C. Gatrone Department of Chemistry and Physics Virginia State University

2. Chapter Objectives • Nomenclature • Preparation • Reactions • Spectroscopy

3. Nomenclature • Aldehydes • Identify the alkane • Parent alkane must contain the CHO group • CHO group C is numbered 1 • Replace the “e” with “al”

4. Examples

5. Nomenclature • Aldehydes • Aldehyde carbon is bonded to ring • Suffix used is “carbaldehyde”

6. Examples

7. Common Names

8. Nomenclature • Ketones • Identify the alkane • Parent alkane • The longest chain containing the carbonyl group • The carbonyl C gets the lowest number possible • Replace the “e” with “one”

9. Examples

10. Nomenclature • If present with another functional group • Prefix “oxo” is used

11. Common Names

12. As a Substituent • When R-C=O is used as a substituent • Referred to as an acyl group • Ending “yl” is used

13. Preparation • [O] of primary ROH • [H] of RCO2H • [O] of secondary ROH

14. Preparation • Ozonolysis of Alkenes

15. Hydration of Alkynes • Hydration of terminal alkynes in the presence of Hg2+ (catalyst)

16. Preparation From Organometallics

17. Preparation • Friedal-Crafts Acylation • Recall: • Reaction does not occur on deactivated rings

18. Reactions • Oxidation of Aldehydes • [O] = KMnO4/acid; hot HNO3, and CrO3/acid • Ketones are generally inert to oxidation

19. Reactions • Resonance contribution • Carbon is electrophilic • Oxygen is nucleophilic

20. Nucleophilic Addition • Provides a tetrahedral intermediate

22. The Tetrahedral Intermediate • Aldehydes are more reactive than ketones • Consider several nucleophiles

23. Nucleophile = Water • Product is a 1,1-diol, a gem-diol, a hydrate • Reaction is equilibrium process • Position of equilibrium depends upon structure • Reaction is readily reversible

24. Equilibrium Process

25. Nucleophile = Y in HY • Reaction of C=O with H-Y, where Y is electronegative, gives an addition product • Formation is readily reversible

26. Nu = HCNCyanohydrin Formation • HCN – very weak acid • pKa = 9.1 • Equilibrium favors HCN • Availability of CN as nucleophile is reduced • Base catalysis favors cyanohydrin formation

27. Uses of Cyanohydrins • The nitrile group (CN) can be reduced with LiAlH4 to yield a primary amine (RCH2NH2) • Can be hydrolyzed by hot acid to yield a carboxylic acid

28. Nucleophile = Organometallic Reagent • Grignard reagent • Effectively a carbanion

29. Grignard Additions

30. Nucleophile = Hydride • Reduction of Carbonyl compounds • Can use NaBH4 or LiAlH4

31. Hydride Addition • Convert C=O to CH-OH • LiAlH4 and NaBH4 react as donors of hydride ion • Source of H-1 (not real but useful formally) • Protonation after addition yields the alcohol

32. Nucleophile = AmineImine and Enamine Formation • Amines – organic derivatives of ammonia • Classified by number of substituents on N • Primary and Secondary amines react • Tertiary amines do not react with carbonyls

34. Imines and Enamines • Requires an acid catalyst • pH dependent reaction • Reaction is slow at high and low pH • At high pH – not enough acid to protonate • At low pH – the amine is protonated

36. Imine Formation is Reversible • Drive reaction to right • Add excess amine • Remove water • Dean Stark Trap • Removes water • Azeotrope formation

37. Derivatives of Imines • Hydroxylamine (NH2OH) • Hydrazine (NH2NH2)

38. Uses of Oximes • Beckmann rearrangement • Synthesis of Nylon

39. Uses of Hydrazones The Wolff–Kishner Reaction • Reduction under basic conditions • Ketone or Aldehyde into an alkane • Originally carried out at high temperatures but with dimethyl sulfoxide as solvent takes place near room temperature

40. Uses of HydrazonesThe Clemmensen Reduction • Reduction under acidic conditions • Provides alkane from Ketone/aldehyde • Through Hydrazone

41. Uses of HydrazonesReduction of Carbonyls • Reduction under neutral conditions • Tosylhydrazone

42. Nucleophile = Alcohol • Two equivalents of ROH and acid catalyst • Acetal formation

43. Uses of Acetals • Acetals can serve as protecting groups for aldehydes and ketones • It is convenient to use a diol, to form a cyclic acetal (the reaction goes even more readily)

44. Uses of Acetals • Thioacetals • Prepared in same manner as acetals • Reduction under neutral conditions

45. Acetals and Hemiacetals • Common in carbohydrate chemistry

46. Glucopyranoses

47. Glucose • a-D-glucopyranose • mp = 146 oC and [a] = +112.2o • b-D-glucopyranose • mp = 148 - 155 oC and [a] = +18.7o • Dissolve either in water, mutarotation occurs • Alpha become beta, beta becomes alpha • Equilibrium mixture results (37:63 a:b)

48. Some Phosphorus Chemistry • Amines react with alkyl halides • Quaternary ammonium salt • Phosphines also react with alkyl halides

49. Phosphorus Chemistry • Positive charge on P stabilizes negative charge that can form on an alpha carbon (must have a H atom) • Ylides are nucleophilic • React with carbonyl compounds

50. Nucleophile = Phosphorus YlideThe Wittig Reaction • Extends carbon chain by one carbon atom • Adds a double bond into system • Known to be able to control stereochemistry of double bond