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CHE 240 Unit IV Stereochemistry, Substitution and Elimination Reactions CHAPTER SIX

CHE 240 Unit IV Stereochemistry, Substitution and Elimination Reactions CHAPTER SIX. Terrence P. Sherlock Burlington County College 2004. Polarity and Reactivity. Halogens are more electronegative than C. Carbon-halogen bond is polar, so carbon has partial positive charge.

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CHE 240 Unit IV Stereochemistry, Substitution and Elimination Reactions CHAPTER SIX

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  1. CHE 240Unit IVStereochemistry, Substitution and Elimination ReactionsCHAPTER SIX Terrence P. Sherlock Burlington County College 2004

  2. 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. => Chapter 6

  3. 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. => Chapter 6

  4. => Classify These: Chapter 6

  5. => Dihalides • Geminal dihalide: two halogen atoms are bonded to the same carbon • Vicinal dihalide: two halogen atoms are bonded to adjacent carbons. Chapter 6

  6. => 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. Chapter 6

  7. => Systematic Common Names • Name as alkyl halide. • Useful only for small alkyl groups. • Name these: Chapter 6

  8. “Trivial” Names • CH2X2 called methylene halide. • CHX3 is a haloform. • CX4 is carbon tetrahalide. • Examples: • CH2Cl2 is methylene chloride • CHCl3 is chloroform • CCl4 is carbon tetrachloride. => Chapter 6

  9. Uses of Alkyl Halides • Solvents - degreasers and dry cleaning fluid • Reagents for synthesis of other compounds • Anesthetic: Halothane is CF3CHClBr • CHCl3 used originally (toxic and carcinogenic) • Freons, chlorofluorocarbons or CFC’s • Freon 12, CF2Cl2, now replaced with Freon 22, CF2CHCl, not as harmful to ozone layer. • Pesticides - DDT banned in U.S. => Chapter 6

  10. Dipole Moments • m= 4.8 x d x d, where d is the charge (proportional to DEN) and d is the distance (bond length) in Angstroms. • Electronegativities: F > Cl > Br > I • Bond lengths: C-F < C-Cl < C-Br < C-I • Bond dipoles: C-Cl > C-F > C-Br > C-I1.56 D 1.51 D 1.48 D 1.29 D • Molecular dipoles depend on shape, too! => Chapter 6

  11. Boiling Points • Greater intermolecular forces, higher b.p. • dipole-dipole attractions not significantly different for different halides • London forces greater for larger atoms • Greater mass, higher b.p. • Spherical shape decreases b.p. (CH3)3CBr CH3(CH2)3Br 73C 102C => Chapter 6

  12. Densities • Alkyl fluorides and chlorides less dense than water. • Alkyl dichlorides, bromides, and iodides more dense than water. => Chapter 6

  13. Halogenation of Alkanes • All H’s equivalent. Restrict amount of halogen to prevent di- or trihalide formation • Highly selective: bromination of 3 C => Chapter 6

  14. => Allylic Halogenation • Allylic radical is resonance stabilized. • Bromination occurs with good yield at the allylic position (sp3 C next to C=C). • Avoid a large excess of Br2 by using N-bromosuccinimide (NBS) to generate Br2 as product HBr is formed. Chapter 6

  15. => Reaction Mechanism Free radical chain reaction • initiation, propagation, termination. Chapter 6

  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. => Chapter 6

  17. Elimination Reactions • 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). => Chapter 6

  18. SN2 Mechanism • Bimolecular nucleophilic substitution. • Concerted reaction: new bond forming and old bond breaking at same time. • Rate is first order in each reactant. • Walden inversion. => Chapter 6

  19. SN2 Energy Diagram • One-step reaction. • Transition state is highest in energy. => Chapter 6

  20. Uses for SN2 Reactions • Synthesis of other classes of compounds. • Halogen exchange reaction. => Chapter 6

  21. => SN2: Nucleophilic Strength • Stronger nucleophiles react faster. • Strong bases are strong nucleophiles, but not all strong nucleophiles are basic. Chapter 6

  22. 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- => Chapter 6

  23. Polarizability Effect => Chapter 6

  24. => Bulky Nucleophiles Sterically hindered for attack on carbon, so weaker nucleophiles. Chapter 6

  25. => 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. Chapter 6

  26. => Solvent Effects (2) • Polar aprotic solvents (no O-H or N-H) do not form hydrogen bonds with nucleophile • Examples: Chapter 6

  27. => Crown Ethers • Solvate the cation, so nucleophilic strength of the anion increases. • Fluoride becomes a good nucleophile. Chapter 6

  28. => SN2: Reactivity of Substrate • Carbon must be partially positive. • Must have a good leaving group • Carbon must not be sterically hindered. Chapter 6

  29. => Leaving Group Ability • Electron-withdrawing • Stable once it has left (not a strong base) • Polarizable to stabilize the transition state. Chapter 6

  30. Structure of Substrate • Relative rates for SN2: CH3X > 1° > 2° >> 3° • Tertiary halides do not react via the SN2 mechanism, due to steric hindrance. => Chapter 6

  31. Stereochemistry of SN2 Walden inversion => Chapter 6

  32. 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. => Chapter 6

  33. => SN1 Mechanism (1) Formation of carbocation (slow) Chapter 6

  34. => SN1 Mechanism (2) • Nucleophilic attack • Loss of H+ (if needed) Chapter 6

  35. SN1 Energy Diagram • Forming the carbocation is endothermic • Carbocation intermediate is in an energy well. => Chapter 6

  36. 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. => Chapter 6

  37. Stereochemistry of SN1 Racemization: inversion and retention => Chapter 6

  38. 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. => Chapter 6

  39. => Hydride Shift Chapter 6

  40. => Methyl Shift Chapter 6

  41. 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? Chapter 6

  42. E1 Reaction • Unimolecular elimination • Two groups lost (usually X- and H+) • Nucleophile acts as base • Also have SN1 products (mixture) => Chapter 6

  43. E1 Mechanism • Halide ion leaves, forming carbocation. • Base removes H+ from adjacent carbon. • Pi bond forms. => Chapter 6

  44. => A Closer Look Chapter 6

  45. E1 Energy Diagram • Note: first step is same as SN1 => Chapter 6

  46. E2 Reaction • Bimolecular elimination • Requires a strong base • Halide leaving and proton abstraction happens simultaneously - no intermediate. => Chapter 6

  47. E2 Mechanism => Chapter 6

  48. major minor => Saytzeff’s Rule • If more than one elimination product is possible, the most-substituted alkene is the major product (most stable). • R2C=CR2>R2C=CHR>RHC=CHR>H2C=CHR tetra > tri > di > mono Chapter 6

  49. E2 Stereochemistry => Chapter 6

  50. Tertiary > Secondary Weak base Good ionizing solvent Rate = k[halide] Saytzeff product No required geometry Rearranged products Tertiary > Secondary Strong base required Solvent polarity not important Rate = k[halide][base] Saytzeff product Coplanar leaving groups (usually anti) No rearrangements => E1 or E2? Chapter 6

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