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Chapter 12

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

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

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

  4. Polarity 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 reaction:

  7. 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 increases carboxylic acid aldehyde

  8. 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 other than CO2

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

  10. 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.). • The oxidation state of each carbon is based on the number of bonds it forms to atoms more (or less) electronegative than carbon.

  11. Examples Bonds to C: 4 to H = (- 1) x 4 = - 4 Total = - 4 Oxidation state of C = - 4 Or as indicated in class, the more electronegative atom controls the shared electrons. Here C controls 8 es.

  12. Examples Bonds to C: 3 to H = - 3 1 to O = +1 Total = - 2 Oxidation state of C = - 2 Here C controls 6 es compared to its 4 valence es.

  13. Examples Bonds to C: 2 to H = - 2 2 to O = +2 Total = 0 Oxidation state of C = 0 Here C controls 4 es which equals its # of valence es.

  14. Examples Bonds to C: 1 to H = - 1 3 to O = +3 Total = +2 Oxidation state of C = +2 Here C controls 2 es compared to its 4 valence es.

  15. Overall order of oxidation states of C oxidation state lowest oxidation state of carbon highest oxidation state of carbon

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

  17. 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. dry Et2O or THF). • Reduces all carbonyl groups and requires two separate steps.

  18. Examples

  19. Mechanism Esters are reduced to 1o alcohols

  20. 3B. Sodium Borohydride • NaBH4 • less reactive and less basic than LiAlH4. • can use protic solvent (e.g. ROH); separate acidification is not needed. • reduces only more reactive carbonyl groups (i.e. aldehydes and ketones) but not reactive towards esters or carboxylic acids.

  21. Examples

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

  23. 3C. Overall Summary of LiAlH4 and NaBH4 Reactivity reduced by LiAlH4 reduced by NaBH4 ease of reduction

  24. 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. • Therefore, it is difficult to stop the oxidation of a 1o alcohol to the aldehyde stage unless specialized reagents are used.

  25. PCC oxidation (mild oxidant): Reagent:

  26. PCC oxidation: 1o 2o 3o

  27. 4B. Oxidation of Primary Alcohols toCarboxylic Acids • Chromic acid (H2CrO4) usually prepared by Jones reagent

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

  29. 4D. Mechanism of Chromate Oxidations Formation of the Chromate Ester intermediate:

  30. The oxidation step:

  31. 4E. A Chemical Test for Primary andSecondary Alcohols

  32. 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.

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

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

  35. Example:

  36. 6B. Grignard Reagents • Preparation of organomagnesium compounds (Grignard reagents). • Order of reactivity of RX • RI > RBr > RCl

  37. Example:

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

  39. Examples: • As base:

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

  41. 7B. Reactions of Grignard Reagentswith Epoxides (Oxiranes) • Grignard reagents react as nucleophiles with epoxides (oxiranes), providing convenient synthesis of alcohols.

  42. Proceeds via an SN2 reaction:

  43. Also work for substituted epoxides: (attacks the least substituted side).

  44. 7C. Reactions of Grignard Reagentswith Carbonyl Compounds

  45. Mechanism:

  46. Alcohols from Grignard Reagents

  47. R, R’ = H (formaldehyde): • Yields a 1o alcohol.

  48. R = alkyl, R’ = H (higher aldehydes) • Yields a 2o alcohol.

  49. R, R’ = alkyl (ketone) • Yields a 3o alcohol.

  50. Reaction with esters: • Yields a 3o alcohol:

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