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Chapter 10

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  1. Chapter 10 Nucleophilic Substitution: The SN1 and SN2 Mechanisms WWU -- Chemistry

  2. Assignment for Chapter 10 • We will cover all the sections in this chapter, except Sections 10.12 and 10.13 WWU -- Chemistry

  3. Problem Assignment for Chapter 10 In-Text Problems 1 - 15 17, 18 19 (SN2 react) 20 (SN1 reaction), 21, 22, 24, 25, 26, 27, 28 End-of-Chapter Problems 30 - 37 39 - 42 44 – 49 51 - 55 WWU -- Chemistry

  4. Sect. 10.1: Nomenclature of alkyl halides -- common names methylene chloride CH2Cl2 chloroform CHCl3 carbon tetrachloride CCl4 WWU -- Chemistry

  5. More common and IUPAC names isopropyl chloride (2-chloropropane) sec-butyl chloride (2-chlorobutane) isobutyl chloride (1-chloro-2-methylpropane) tert-butyl chloride (2-chloro-2-methylpropane) allyl chloride (3-chloro-1-propene) vinyl chloride (chloroethene) benzyl chloride (chloromethylbenzene) phenyl chloride (chlorobenzene) WWU -- Chemistry

  6. Sect. 10.2: Overview of nucleophilic substitution • The substitution reaction: SN1 and SN2 • Primary halides = SN2 • Secondary halides = both mechanisms! • Tertiary halides = SN1 • Leaving groups: halogens most common • There are a number of different nucleophiles!! WWU -- Chemistry

  7. Nucleophilic Substitution (SN2) WWU -- Chemistry

  8. Nitrogen as a nucleophile (SN2) WWU -- Chemistry

  9. Carbon as a nucleophile (SN2) WWU -- Chemistry

  10. d- d- energy Reaction coordinate WWU -- Chemistry

  11. The SN1 Mechanism carbocation WWU -- Chemistry

  12. d+ d+ d- d- + energy intermediate Reaction coordinate WWU -- Chemistry

  13. Sect. 10.3: SN2 Mechanism • reaction and mechanism • kinetics • stereochemistry • substrate structure • nucleophiles • leaving groups • solvents WWU -- Chemistry

  14. The SN2 Reaction Sterically accessible compounds react by this mechanism!! Methyl group is small WWU -- Chemistry

  15. SN2 Mechanism: kinetics • The reactions follows second order (bimolecular) kinetics • Rate = k [R-Br]1 [OH-]1 WWU -- Chemistry

  16. d- d- energy Reaction coordinate WWU -- Chemistry

  17. SN2 Reaction: stereochemistry Inversion of configuration WWU -- Chemistry

  18. For an SN2 Reaction: EVERY REACTION EVENT ALWAYS LEADS TO INVERSION OF CONFIGURATION WWU -- Chemistry

  19. SN2 Reaction: substrate structure (Table 10-5) KI in Acetone at 25° WWU -- Chemistry

  20. Chloromethane + Iodide as the Nucleophile Fast I- WWU -- Chemistry

  21. tert-Butyl Chloride + Iodide as the Nucleophile No reaction I- WWU -- Chemistry

  22. SN2 Reaction: substrate structure Reactivity order---- fastest to slowest! WWU -- Chemistry

  23. SN2 Reaction: nucleophilicity WWU -- Chemistry

  24. is morenucleophilic! Predict which is more nucleophilic WWU -- Chemistry

  25. Relative Nucleophilicity 1) In general, stronger bases are better nucleophiles 2) However, iodide doesn’t fit that pattern (weak base, but great nucleophile!) 3) Cyanide is an excellent nucleophile because of its linear structure 4) Sulfur is better than oxygen as a nucleophile WWU -- Chemistry

  26. SN2 Reaction: Leaving Groups • Best leaving groups leave to form weakLewis bases. • Good leaving groups: • Br, I, Cl, OTs, OH2+ • “Lousy” leaving groups: • OH, OR, NH2,, F WWU -- Chemistry

  27. Sulfonate Leaving Groups WWU -- Chemistry

  28. Tosylate leaving group WWU -- Chemistry

  29. Inversion of Configuration WWU -- Chemistry

  30. SN2 Reaction: solvents SN2 reactions are accelerated in polar, aprotic solvents. Consider Na+-OEt as an example of a nucleophile. Why are reactions accelerated? The Na+ cation is complexed by the negative part of the aprotic solvent molecule pulling it away from –OEt. Now that the sodium ion is complexed, the oxygen in the nucleophile –OEt is more available for attack. WWU -- Chemistry

  31. Aprotic solvents • These solvents do not have OH bonds in them. They complex the cation through the lone pairs on oxygen or nitrogen: WWU -- Chemistry

  32. How cations are complexed with aprotic solvents WWU -- Chemistry

  33. Now that the Na+ is complexed, the –OEt can react more easily WWU -- Chemistry

  34. SN2 Reaction: solvents SN2 reactions areretarded (slowed) in polar, protic solvents. Protic solvents have O-H groups. Why are reactions retarded? Nucleophile is hydrogen bonded to solvent! WWU -- Chemistry

  35. Protic solvents abbreviations Typical protic solvents: WWU -- Chemistry

  36. Sect. 10.4: SN1 Mechanism • reaction and mechanism • kinetics • stereochemistry • substrate structure • nucleophiles • leaving groups • solvents WWU -- Chemistry

  37. Solvolysis of tert-Butyl Bromide Acetone is used to dissolve everything! Water is the solvent and nucleophile (solvolysis). WWU -- Chemistry

  38. The SN1 Mechanism carbocation 1935: Hughes & Ingold WWU -- Chemistry

  39. d+ d- intermediate + energy intermediate Reaction coordinate WWU -- Chemistry

  40. SN1 Reaction: kinetics • The reactions follows first order (unimolecular) kinetics • Rate = k [R-Br]1 WWU -- Chemistry

  41. SN1 Reaction: stereochemistry With chiral R-X compounds, the product will be racemic (50% of each enantiomer). WWU -- Chemistry

  42. Stereochemistry in SN1 reactions – racemic product WWU -- Chemistry

  43. d+ d- intermediate + energy intermediate Reaction coordinate WWU -- Chemistry

  44. SN1 Reaction: substrate structure Solvolysis in water at 50°C WWU -- Chemistry

  45. SN1 Reaction: substrate structure tertiary>secondary>primary > methyl Primary and methyl halides are very unreactive! They don’t go by SN1 reactions. WWU -- Chemistry

  46. WWU -- Chemistry

  47. Nucleophiles • Usually SN1 reactions are run in polar protic solvents; compounds with O-H groups. • The polar protic solvent acts as BOTH nucleophile as well as the solvent. • Common solvent/nucleophiles include: water, ethanol, methanol, acetic acid, and formic acid. WWU -- Chemistry

  48. A protic solvent acts as both a solvent and nucleophile in SN1 reactions - solvolysis: abbreviations WWU -- Chemistry

  49. Typical solvolysis reaction Polar solvent stabilizes the carbocation! Solvent is the nucleophile WWU -- Chemistry

  50. Leaving groups • Leaving groups are the same as in SN2 reactions: • Cl, Br, I, OTs are the usual ones. WWU -- Chemistry