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Fischer-Rosanoff Convention

Fischer-Rosanoff Convention. Before 1951, only relative configurations could be known. Sugars and amino acids with same relative configuration as (+)-glyceraldehyde were assigned D and same as (-)-glyceraldehyde were assigned L.

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Fischer-Rosanoff Convention

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  1. Fischer-Rosanoff Convention • Before 1951, only relative configurations could be known. • Sugars and amino acids with same relative configuration as (+)-glyceraldehyde were assigned D and same as (-)-glyceraldehyde were assigned L. • With X-ray crystallography, now know absolute configurations: D is (R) and L is (S). • No relationship to dextro- or levorotatory. =>

  2. => * * D and L Assignments *

  3. Properties of Diastereomers • Diastereomers have different physical properties: m.p., b.p. • They can be separated easily. • Enantiomers differ only in reaction with other chiral molecules and the direction in which polarized light is rotated. • Enantiomers are difficult to separate.=>

  4. Resolution of Enantiomers React a racemic mixture with a chiral compound to form diastereomers, which can be separated. =>

  5. ChromatographicResolution of Enantiomers =>

  6. Organic Chemistry, 5th EditionL. G. Wade, Jr. Chapter 6Alkyl Halides: Nucleophilic Substitution and Elimination

  7. => Classes of Halides • Alkyl: Halogen, X, is directly bonded to sp3 carbon. • Vinyl: X is bonded to sp2 carbon of alkene. • Aryl: X is bonded to sp2 carbon on benzene ring. • Examples:

  8. Polarity and Reactivity • Halogens are more electronegative than C. • Carbon-halogen bond is polar, so carbon has partial positive charge. • Carbon can be attacked by a nucleophile. • Halogen can leave with the electron pair. =>

  9. Classes of Alkyl Halides • Methyl halides: only one C, CH3X • Primary: C to which X is bonded has only one C-C bond. • Secondary: C to which X is bonded has two C-C bonds. • Tertiary: C to which X is bonded has three C-C bonds. =>

  10. => Classify These:

  11. => Dihalides • Geminal dihalide: two halogen atoms are bonded to the same carbon • Vicinal dihalide: two halogen atoms are bonded • to adjacent carbons.

  12. => IUPAC Nomenclature • Name as haloalkane. • Choose the longest carbon chain, even if the halogen is not bonded to any of those C’s. • Use lowest possible numbers for position.

  13. => Systematic Common Names • Name as alkyl halide. • Useful only for small alkyl groups. • Name these:

  14. “Trivial” Names • CH2X2 called methylene halide. . • CHX3 is a haloform. • CX4 is carbon tetrahalide. • Examples: • CH2Cl2 is methylene chloride • CHCl3 is chloroform -CHI3 is iodoform • CCl4 is carbon tetrachloride

  15. Preparation of RX • Free radical halogenation (Chapter 4) - REVIEW • Free radical allylic halogenation • produces alkyl halide with double bond on the neighboring carbon. LATER =>

  16. Substitution Reactions • The halogen atom on the alkyl halide is replaced with another group. • Since the halogen is more electronegative than • carbon, the C-X bond breaks heterolytically and • X- leaves. The group replacing X- is a nucleophile. =>

  17. Elimination Reactions • The alkyl halide loses halogen as a halide ion, and also loses H+ on the adjacent carbon to a base. • The alkyl halide loses halogen as a halide ion, and • also loses H+ on the adjacent carbon to a base. • A pi bond is formed. Product is alkene. • Also called dehydrohalogenation (-HX).

  18. Ingold

  19. Ingold Sir Christopher Father of Physical Organic Chemistry Coined such names and symbols as: SN1, SN2, E1, E2, nucleophile, electrophile resonance effect, inductive effect/ In print, he often attacked enemies vigorously and sometimes in vitrolic manner.

  20. SN2 Mechanism • Rate is first order in each reactant • Both reactants are involved in RDS Note: one-step reaction with no intermediate • Bimolecular nuleophilic substitution. • Concerted reaction: new bond forming • and old bond breaking at same time INVERSION OF CONFIGURATION

  21. SN2 Energy Diagram • One-step reaction. • Transition state is highest in energy. =>

  22. Uses for SN2 Reactions • Synthesis of other classes of compounds. • Halogen exchange reaction. =>

  23. => SN2: Nucleophilic Strength • Stronger nucleophiles react faster. • Strong bases are strong nucleophiles, but not all strong nucleophiles are basic.

  24. Trends in Nuc. Strength • Of a conjugate acid-base pair, the base is stronger: OH- > H2O, NH2- > NH3 • Decreases left to right on Periodic Table. More • electronegative atoms less likely to form new bond: • OH- > F-, NH3 > H2O • Increases down Periodic Table, as size and polarizability • increase: I- > Br- > Cl-

  25. Polarizability Effect =>

  26. => Bulky Nucleophiles Sterically hindered for attack on carbon, so weaker nucleophiles.

  27. => Solvent Effects (1) Polar protic solvents (O-H or N-H) reduce the strength of the nucleophile. Hydrogen bonds must be broken before nucleophile can attack the carbon.

  28. => Solvent Effects (2) • Polar aprotic solvents (no O-H or N-H) do not form hydrogen bonds with nucleophile • Examples:

  29. => Crown Ethers • Solvate the cation, so nucleophilic strength of the anion increases. • Fluoride becomes a good nucleophile.

  30. => SN2: Reactivity of Substrate • Carbon must be partially positive. • Must have a good leaving group • Carbon must not be sterically hindered.

  31. => Leaving Group Ability • Electron-withdrawing • Stable once it has left (not a strong base) • Polarizable to stabilize the transition state.

  32. Structure of Substrate • Relative rates for SN2: CH3X > 1° > 2° >> 3° • Tertiary halides do not react via the SN2 mechanism, due to steric hindrance. =>

  33. Stereochemistry of SN2 Walden inversion =>

  34. SN1 Reaction • Unimolecular nucleophilic substitution. • Two step reaction with carbocation intermediate. • Rate is first order in the alkyl halide, zero order in the nucleophile. • Racemization occurs. =>

  35. => SN1 Mechanism (1) Formation of carbocation (slow)

  36. => SN1 Mechanism (2) • Nucleophilic attack • Loss of H+ (if needed)

  37. SN1 Energy Diagram • Forming the carbocation is endothermic • Carbocation intermediate is in an energy well. =>

  38. Rates of SN1 Reactions • 3° > 2° > 1° >> CH3X • Order follows stability of carbocations (opposite to SN2) • More stable ion requires less energy to form • Better leaving group, faster reaction (like SN2) • Polar protic solvent best: It solvates ions strongly with hydrogen bonding. =>

  39. Stereochemistry of SN1 Racemization: inversion and retention =>

  40. Rearrangements • Carbocations can rearrange to form a more stable carbocation. • Hydride shift: H- on adjacent carbon bonds with C+. • Methyl shift: CH3- moves from adjacent carbon if no H’s are available. =>

  41. => Hydride Shift

  42. => Methyl Shift

  43. Primary or methyl Strong nucleophile Polar aprotic solvent Rate = k[halide][Nuc] Inversion at chiral carbon No rearrangements Tertiary Weak nucleophile (may also be solvent) Polar protic solvent, silver salts Rate = k[halide] Racemization of optically active compound Rearranged products => SN2 or SN1?

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