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Chapter 12

Alcohols from Carbonyl Compounds Oxidation-Reduction & Organometallic Compounds. Chapter 12. Aldehyde. Ketone. Carboxylic acid. Ester. Amide. Structure of the Carbonyl Group. Carbonyl compounds. Structure. Carbonyl carbon: sp 2 hybridized Planar structure.

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Chapter 12

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

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

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

  4. Polarization and resonance structure

  5. 1A. Reactions of Carbonyl Compoundswith Nucleophiles • One of the most important reactions of carbonyl compounds is nucleophilic addition to the carbonyl group

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

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

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

  9. 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

  10. Examples

  11. 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

  12. Examples

  13. 3C. Overall Summary of LiAlH4 and NaBH4 Reactivity reduced by LiAlH4 reduced by NaBH4 ease of reduction

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

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

  16. Example

  17. 6B. Grignard Reagents • Preparation of organomagnesium compounds (Grignard reagents) • Order of reactivity of RX • RI > RBr > RCl

  18. 7B. Reactions of Grignard Reagentswith Epoxides (Oxiranes) • Grignard reagents react as nucleophiles with epoxides (oxiranes), providing convenient synthesis of alcohols

  19. Via SN2 reaction

  20. Also work for substituted epoxides

  21. 7C. Reactions of Grignard Reagentswith Carbonyl Compounds

  22. Mechanism

  23. Alcohols from Grignard Reagents

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

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

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

  27. Reaction with esters • 3o alcohol

  28. Mechanism

  29. Examples

  30. Examples

  31. Examples

  32. Examples

  33. 8A. How to Plan a Grignard Synthesis • Synthesis of

  34. Method 1 • Retrosynthetic analysis • Synthesis

  35. Method 2 • Retrosynthetic analysis • Synthesis

  36. Method 3 • Retrosynthetic analysis • Synthesis

  37. 8B. Restrictions on the Use ofGrignard Reagents • Grignard reagents are useful nucleophiles but they are also very strong bases • It is not possible to prepare a Grignard reagent from a compound that contains any hydrogen more acidic than the hydrogen atoms of an alkane or alkene

  38. A Grignard reagent cannot be prepared from a compound containing an –OH group, an –NH– group, an –SH group, a –CO2H group, or an –SO3H group • Since Grignard reagents are powerful nucleophiles, we cannot prepare a Grignard reagent from any organic halide that contains a carbonyl, epoxy, nitro, or cyano (–CN) group

  39. Grignard reagents cannot be prepared in the presence of the following groups because they will react with them:

  40. 8C. The Use of Lithium Reagents • Organolithium reagents have the advantage of being somewhat more reactive than Grignard reagents although they are more difficult to prepare and handle

  41. 8D. The Use of Sodium Alkynides • Preparation of sodium alkynides • Reaction via ketones (or aldehydes)

  42. Protecting Groups

  43. Retrosynthetic analysis • However

  44. Need to “protect” the –OH group first

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