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Chapter 8

Chapter 8. Substitution Reaction of Alkyl Halide. Substitution versus elimination reaction. Nucleophilic substitution reactions. concerted. two steps. stitution nucleophile bimolecular, S N 2 reaction. Bimolecular Nucleophlic Substitution reaction.

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Chapter 8

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  1. Chapter 8 Substitution Reaction of Alkyl Halide

  2. Substitution versus elimination reaction

  3. Nucleophilic substitution reactions concerted two steps

  4. stitution nucleophile bimolecular, SN2 reaction Bimolecular Nucleophlic Substitution reaction

  5. HOMO - LUMO interaction : good orbital overlapping

  6. The approach of the hydroxide ion to alkyl halides steric hindrance

  7. Reaction coordinate diagrams for SN2 reactions

  8. Inversion of configuration in an SN2 reaction

  9. Factors That Affect SN2 Reactions Leaving group : The weaker the basicity of a leaving group, the better is its leaving ability. Acidity : HI > HBr > HCl >> HF Basicity : I- < Br- < Cl- << F-

  10. Leaving group

  11. Basicity vs. Nucleophilicity • When lone pair electrons of a compound • abstract proton from the other compound : Base • (2) attacks carbon to kick out leaving group : Nucleophiles Nucleophiles : stronger bases are better nucleophiles.

  12. Comparison of iodide and fluoride ions Page 354 교과서의 표현이 정확하지 않음 higher TS energy lower TS energy faster more stable product stronger bond energy An iodide ion is larger and more polarizable than a fluoride ion →The relatively loosely held electrons of the ion can overlap the orbital of carbon from farther away → Greater degree of bonding in the transition state, making the transition state more stable

  13. Size, basicity, and nucleophilicity comparisons of halide ions faster slower faster slower

  14. In polar protic solvents Cations are strongly solvated : via coordination Anions are strongly solvated : hydrogen bonding smaller anions are more solvated show poorer nucleophilicities larger anions are less solvated show better nucleophilicities

  15. In polar aprotic solvents Cations are strongly solvated : via coordination Anions are poorly solvated : no hydrogen bonding become naked anions or bare anions anions reveal their own nucleophilicities

  16. Nucleophilicity is affected by steric effects

  17. List the following species in order of decreasing nucleophilicity in an aqueous solution

  18. Various nucleophiles used in SN2 reactions

  19. Reaction coordinate diagrams for SN2 reactions

  20. An SN2 reaction is reversible when the basicities of the leaving groups are similar

  21. The reaction of chloroethane with methanol The first step is reversible because the difference between the basicities of the nucleophile and the leaving group is not very large. HCl → H+ + Cl- pKa = -7 CH3OH2 + → H+ + CH3OH pKa = - 2.5 , DpKa ≈ 5 one out of 105 (10만) 평형 가능 CH3OH → H+ + CH3O- pKa = 16, DpKa ≈ 23 평형 불가능 If the reaction is carried out in a neutral solution, the protonated product will lose a proton, disturbing the equilibrium and driving the reaction toward the products

  22. Problem 9 The reaction of methyl chloride with hydroxide ion at 30º has a DGº value of –21.7 kcal/mol

  23. Mechanism of an SN1 reaction Unimolecular Nucleophlic Substitution reaction

  24. Reaction coordinate diagram for an SN1 reaction

  25. relative inversion of configuration inversion and retention product

  26. Solvent-solvated ion pair : Solvent cage

  27. Benzylic and allylic halides, unless they are tertiary, will undergo SN2 reactions Benzylic and allylic halides readily undergo SN1 reactions

  28. the resonance contributors of the allylic carbocation intermediate

  29. Reactivity of vinylic and aryl halides in SN2 reactions Reactivity of vinylic and aryl halides in SN1 reactions

  30. Identify the alkyl halide that would be more stable in an SN1 solvolysis reaction

  31. Competition between SN2 and SN1 Reaction

  32. Pure SN1 and SN2 reactions alkyl halide that can undergo both SN1 and SN2 reactions simultaneously

  33. Predict an SN1 or SN2 pathway

  34. Predict an SN1 or SN2 pathway

  35. Predict an SN1 or SN2 pathway

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