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Chapter 8 - Nucleophilic Substitution at sp 3 C

Chapter 8 - Nucleophilic Substitution at sp 3 C. nucleophile is a Lewis base (electron-pair donor) often negatively charged and used as Na + or K + salt substrate is usually an alkyl halide. YSU. 8.1 Functional Group Transformation by S N 2.

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Chapter 8 - Nucleophilic Substitution at sp 3 C

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  1. Chapter 8 - Nucleophilic Substitution at sp3 C • nucleophile is a Lewis base (electron-pair donor) • often negatively charged and used as Na+ or K+ salt • substrate is usually an alkyl halide YSU

  2. 8.1 Functional Group Transformation by SN2 Table 8.1 Examples of Nucleophilic Substitution Alkoxide ion as the nucleophile gives an ether • Referred to as the Williamson ether synthesis • Limited to primary alkyl halides • Run in solvents such as diethyl ether and THF YSU

  3. Carboxylate ion as the nucleophile gives an ester • Not very useful – carboxylates are poor nucleophiles • Limited to primary alkyl halides • Run in solvents such as diethyl ether and THF • Better ways of forming esters later in 3720 YSU

  4. Cyanide as nucleophile Azide as nucleophile YSU

  5. Halides as Nucleophiles – Finkelstein Reaction NaI is soluble in acetone, NaCl and NaBr are not 8.2 Relative reactivity of halide leaving groups • Halides are very good leaving groups • I- better than Br- which is better than Cl- F- is not used as a leaving group YSU

  6. 8.3 The SN2 mechanism of Nucleophilic Substitution Example: CH3Br + HO – CH3OH + Br – rate = k[CH3Br][HO – ] inference: rate-determining step is bimolecular P.E. R.C. YSU

  7. Inversion of configuration during SN2 reaction – Figure 8.1 YSU

  8. 8.4 Steric effects in substitution (SN2) reactions Figure 8.2 YSU

  9. Table 8.3 - Relative rates of reaction of different primary alkyl bromides YSU

  10. 8.5 – Nucleophiles and Nucleophilicity – Table 8.4 YSU

  11. Solvation of a chloride by ion-dipole Figure 8.3 YSU Choice of solvent is important for SN2 - polar aprotic used most often

  12. 8.6 The SN1 reaction revisited Tertiary system - favours SN1 - carbocation possible Carbocation will be the electrophile Water will be the nucleophile YSU

  13. Solvolysis of t-BuBr with water Figure 8.5 YSU

  14. 8.7 Relative rates of reaction by the SN1 pathway Table 8.5 YSU

  15. 8.8 Stereochemical consequences in SN1 reactions Figure 8.6 YSU

  16. 8.9 Carbocation rearrangements also possible in SN1 • Look for change in the product skeleton. • Rearrangement (in this case hydride shift) to generate a more stable carbocation. YSU

  17. 8.10 Choice of solvent for SN1 is important – Table 8.6 More polar solvents (higher dielectric constant) will help stabilize the ionic intermediates. YSU

  18. 8.10 Proper solvent can stabilize transition states Figure 8.7 YSU

  19. 8.10 Choice of solvent important – Table 8.7 YSU

  20. 8.11 Substitution vs. Elimination – SN2 vs. E2 YSU

  21. 8.11 Substitution vs. Elimination Figure 8.8 YSU

  22. 8.12 Sulfonate ester leaving groups – Table 8.8 YSU

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