2. remember from chapter 6 (alkyne chapter):

3. The SN2 mechanism Two possible mechanistic pictures for SN displacement:

4. The SN1 mechanismThe SN1 mechanism

5. Which mechanism? Depends on: structure of electrophile (alkyl halide) structure, concentration of nucleophile solvent

6. example of an SN2

7. SN2 has second order kinetics – rds is collision between two molecules

8. Structure of the alkyl halide: less hindered = faster

11. (demonstrate with models)

12. SN2 results in inversion of configuration

13. Rate of SN2 reaction influenced by leaving group: weak base = good leaving group

14. ...but not so fast!

15. polarizability also is a factor!

16. which factor predominates? depends on the solvent

17. protic solvent shields nucleophile, stronger solvent interactions with stronger bases

18. in protic sovent, polarizability determines nucleophilicity

19. polar, aprotic solvent solvates nucleophiles, but with less shielding. base strength determines nucleophilicity These solvents used for SN2

20. less hindered nucleophiles are better nucleophiles quiz: why is tert-butoxide stronger base? (pKa of alcohols 18 vs. 15.9)

22. The SN1 reaction first order rate expression: rds is unimolecular rate = k[bromide] [H2O] does not influence rate

26. think about stability of carbocation intermediate!

27. SN1 results in racemization of asymmetric center

29. Again, weaker bases are better LG Notice: reactivity of Nu does NOT influence rate of SN1 rxn Solvent effect: more polar solvents better at stabilizing carbocation, so polar solvents used in SN1 reactions (solvolysis – solvent is nucleophile)

30. SN1 reactions can undergo carbocation rearrangements!

31. not always 50:50 mix – why?

32. intimate ion pair influences stereochemical outcome

33. Benzylic and allylic alkyl halides: primary can undergo SN1, because stable intermediate

34. can get mixed products (compare to 1,2 vs 1,4 addition)

35. vinylic, aryl halides do not undergo SN reactions

37. SN1 or SN2? methyl, primary: SN2 only secondary: both primary and secondary allylic/benzylic: both tertiary: SN1 only vinylic/benzylic: neither When both are possible: high concentration of good Nu, polar aprotic solvent favors SN2 poor Nu, polar solvent favors SN1 (typically solvolysis)

38. SN2!

39. SN1

40. Which is likely to occur? Depends on concentration of reactant, ring size

41. 5,6-membered rings: intra favored tethering: entropy!