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Chapter 12

Chapter 12. Alcohols from Carbonyl Compounds Oxidation-Reduction & Organometallic Compounds. About The Authors. These PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife, Dr. Phillis Chang.

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Chapter 12

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  1. Chapter 12 Alcohols from Carbonyl Compounds Oxidation-Reduction & Organometallic Compounds

  2. About The Authors These PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife, Dr. Phillis Chang. Professor William Tam received his B.Sc. at the University of Hong Kong in 1990 and his Ph.D. at the University of Toronto (Canada) in 1995. He was an NSERC postdoctoral fellow at the Imperial College (UK) and at Harvard University (USA). He joined the Department of Chemistry at the University of Guelph (Ontario, Canada) in 1998 and is currently a Full Professor and Associate Chair in the department. Professor Tam has received several awards in research and teaching, and according to Essential Science Indicators, he is currently ranked as the Top 1% most cited Chemists worldwide. He has published four books and over 80 scientific papers in top international journals such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem. Dr. Phillis Chang received her B.Sc. at New York University (USA) in 1994, her M.Sc. and Ph.D. in 1997 and 2001 at the University of Guelph (Canada). She lives in Guelph with her husband, William, and their son, Matthew.

  3. Aldehyde Ketone Carboxylic acid Ester Amide Structure of the Carbonyl Group • Carbonyl compounds

  4. Structure • Carbonyl carbon: sp2 hybridized • Planar structure

  5. Polarization and resonance structure

  6. 1A. Reactions of Carbonyl Compoundswith Nucleophiles • One of the most important reactions of carbonyl compounds is nucleophilic addition to the carbonyl group

  7. Two important nucleophiles: • Hydride ions (from NaBH4 and LiAlH4) • Carbanions (from RLi and RMgX) • Another important reactions:

  8. Oxidation-Reduction Reactions inOrganic Chemistry • Reduction of an organic molecule usually corresponds to increasing its hydrogen content or decreasing its oxygen content oxygen content decreases hydrogen content decreases carboxylic acid aldehyde

  9. The opposite reaction of reduction is oxidation. Increasing the oxygen content of on organic molecule or decreasing its hydrogen content is oxidation lowest oxidation state highest oxidation state

  10. Oxidation of an organic compound may be more broadly defined as a reaction that increases its content of any element more electronegative than carbon

  11. 2A. Oxidation States in Organic Chemistry • Rules • For each C–H (or C–M) bond  -1 • For each C–C bond  0 • For each C–Z bond  +1 (where M = electropositive element and is equivalent to H, e.g. Li, K, etc.; Z = electronegative heteroatom, e.g. OR, SR, PR2, halogen, etc.) • Calculate the oxidation state of each carbon based on the number of bonds it is forming to atoms more (or less) electronegative than carbon

  12. Examples Bonds to C: 4 to H = (- 1) x 4 = - 4 Total = - 4 Oxidation state of C = - 4

  13. Examples Bonds to C: 3 to H = - 3 1 to O = +1 Total = - 2 Oxidation state of C = - 2

  14. Examples Bonds to C: 2 to H = - 2 2 to O = +2 Total = 0 Oxidation state of C = 0

  15. Examples Bonds to C: 1 to H = - 1 3 to O = +3 Total = +2 Oxidation state of C = +2

  16. Overall order oxidation state lowest oxidation state of carbon highest oxidation state of carbon

  17. Alcohols by Reduction of Carbonyl Compounds (1o alcohol)

  18. 3A. Lithium Aluminum Hydride • LiAlH4 (LAH) • Not only nucleophilic, but also very basic • React violently with H2O or acidic protons (e.g. ROH) • Usually reactions run in ethereal solvents (e.g. Et2O, THF) • Reduces all carbonyl groups

  19. Examples

  20. Mechanism Esters are reduced to 1o alcohols

  21. 3B. Sodium Borohydride • NaBH4 • less reactive and less basic than LiAlH4 • can use protic solvent (e.g. ROH) • reduces only more reactive carbonyl groups (i.e. aldehydes and ketones) but not reactive towards esters or carboxylic acids

  22. Examples

  23. Mechanism Aldehydes are reduced to 1° alcohols & ketones are reduced to 2° alcohols

  24. 3C. Overall Summary of LiAlH4 and NaBH4 Reactivity reduced by LiAlH4 reduced by NaBH4 ease of reduction

  25. Oxidation of Alcohols 4A. Oxidation of Primary Alcohols to Aldehydes • The oxidation of aldehydes to carboxylic acids in aqueous solutions is easier than oxidation of 1o alcohols to aldehydes • It is, therefore, difficult to stop the oxidation of a 1o alcohol to the aldehyde stage unless specialized reagents are used

  26. PCC oxidation • Reagent

  27. PCC oxidation

  28. 4B. Oxidation of Primary Alcohols toCarboxylic Acids • Chromic acid (H2CrO4) usually prepared by Jones reagent

  29. Jones oxidation • Reagent: CrO3 + H2SO4 • A Cr(VI) oxidant

  30. 4D. Mechanism of Chromate Oxidations • Formation of the Chromate Ester

  31. The oxidation step

  32. 4E. A Chemical Test for Primary andSecondary Alcohols

  33. 4F. Spectroscopic Evidence for Alcohols • Alcohols give rise to broad O-H stretching absorptions from 3200 to 3600 cm-1 in IR spectra • The alcohol hydroxyl hydrogen typically produces a broad 1H NMR signal of variable chemical shift which can be eliminated by exchange with deuterium from D2O • Hydrogen atoms on the carbon of a 1o or 2o alcohol produce a signal in the 1H NMR spectrumbetween d 3.3 and d 4.0ppm that integrates for 2 and 1 hydrogens, respectively • The 13C NMR spectrum of an alcohol shows a signal between d 50 and d 90 ppm for the alcohol carbon

  34. Organometallic Compounds • Compounds that contain carbon-metal bonds are called organometallic compounds

  35. Preparation of Organolithium &Organomagnesium Compounds 6A. Organolithium Compounds • Preparation of organolithium compounds • Order of reactivity of RX • RI > RBr > RCl

  36. Example

  37. 6B. Grignard Reagents • Preparation of organomagnesium compounds (Grignard reagents) • Order of reactivity of RX • RI > RBr > RCl

  38. Example

  39. Reactions of Organolithium andOrganomagnesium Compounds 7A. Reactions with Compounds Con-taining Acidic Hydrogen Atoms • Grignard reagents and organolithium compounds are very strong bases

  40. Examples • As base

  41. Examples • As base A good method for the preparation of alkynylmagnesium halides

  42. 7B. Reactions of Grignard Reagentswith Epoxides (Oxiranes) • Grignard reagents react as nucleophiles with epoxides (oxiranes), providing convenient synthesis of alcohols

  43. Via SN2 reaction

  44. Also work for substituted epoxides

  45. 7C. Reactions of Grignard Reagentswith Carbonyl Compounds

  46. Mechanism

  47. Alcohols from Grignard Reagents

  48. R, R’ = H (formaldehyde) • 1o alcohol

  49. R = alkyl, R’ = H (higher aldehydes) • 2o alcohol

  50. R, R’ = alkyl (ketone) • 3o alcohol

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