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Alkenes II-continued 1. Types of Rxn.: elimination, addition, substitution

Alkenes II-continued 1. Types of Rxn.: elimination, addition, substitution 2. Mechanisms: How rxn’s of molecules occur. 3. Electrophilic (Markovnikov) addition A. Addition of HX, regioselective R 3 C + > R 2 HC + > RH 2 C + > H 3 C + B. Hydration H + /H 2 O C. Rearrangements

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Alkenes II-continued 1. Types of Rxn.: elimination, addition, substitution

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  1. Alkenes II-continued 1. Types of Rxn.: elimination, addition, substitution 2. Mechanisms: How rxn’s of molecules occur. 3. Electrophilic (Markovnikov) addition A. Addition of HX, regioselective R3C+ > R2HC+ > RH2C+ > H3C+ B. Hydration H+/H2O C. Rearrangements D. Bromonium ions, (anti addition) E. Halohydrin formation, (anti) F. H+ - hydration, Hg+2-hydration - (anti) 4. Hydroboration (syn addition) 5. Oxidation (syn) 6. Reduction (syn), olefin and diene stability 7. Reactions relative to stereocenters For PRACTICE (“worksheet”) problems go to the home page Work/Study Sheet 6 due in recitation 3/26 or 3/27 SS! 28 19

  2. Chapter 6 synthesis reaction types olefin additions mechanisms energy progress of rx specific additions and reactions

  3. Chapter 6 synthesis hydration rearrangements hydroboration other additions stereochemistry halogenation halohydration mercury hydration

  4. H H OH C C R H R H C C H H H F. Alternative “Hydration” of Alkenes Oxymercuration/Reduction Hg(OAc)2 NaBH4 + Hgo +HOAc H2O R = H, alkyl, Ph, etc. 2 rx sequence

  5. Last lecture:

  6. Last lecture: 3o>2o

  7. “general” antiaddition

  8. 1 2 Issues: no rearrangement regiochemistry stereochemistry

  9. Issues: rearrangement regiochemistry stereochemistry

  10. NaBH4 H Na H B H ~[ :H(-)] H [1] Issues: rearrangement regiochemistry stereochemistry [2]

  11. 6.4 HYDROBORATION [ BH or B H ] 3 2 6 +  H   H B H 2 B H H H H CH CH 3 3 mechanism: syn addition H moves with 2 electrons [H:](-)  H H B H (+) H CH 3 

  12. 6.4 HYDROBORATION [BH3 or B2H6] rx [ts] synchronous B& H addition

  13. OXIDATION synchronous B & H addition 6.4 HYDROBORATION / rx ts

  14. OXIDATION 6.4 HYDROBORATION / rx

  15. Note regioselectivity H OH H BH2 H H BH3 H2O2 H H C C H H C C C C NaOH ether H R H H R R Hg(OAc)2 NaBH4 H2O OH H H H H H C C C C H H R R H+ H2O

  16. Hydroxylation [O] of olefins Note: cold KMnO4/(-)OH = OsO4/ROOH/H2O

  17. mechanism: syn addition of O’s H2O ROOH isoelectronic cold/basic/H2O

  18. H C 3 CH C 3 O H C 3 CH + C 3 O H C C 3 H C C 3 note! H H H2O2 H C 3 CH C 3 O + O H C C 3 OH Ozonolysis reducing agent S(CH3)2 (or Zn:) + O3 oxidizing reagent mechanism: syn addition,ignore H oxidized to OH

  19. Ozonolysis

  20. metal surface H H H H H H R R C C R R R C C R R R R R C C R R Catalytic addition of H2 to  bonds break one C-C bond, gain two C-H bonds How? H2

  21. H2

  22. other double bonds Catalytic addition of H2 to  bonds

  23. H H R C H R R C C < C H H C < C R H C H H H C H R H R C R R C < C R R H R C < C H R C R R R R H H metal C C C C  + H + cat. H 2 H H H H H H stability of olefins (based on energy released)

  24. 60.6 kcal -56.5 kcal ~60.6 kcal ~56.5 kcal 4.1 kcal Conjugated Dienes - similar to simple olefins but resonance makes them special. Classification: isolated conjugated resonance energy

  25. Reaction: conjugated vs unconjugated 1 equivalent Br2 1 equivalent Br2 + “1,4-addition” vs “1,2-addition”

  26. Stereochemistry of Reactions regio constitutional isomers selective specific excess of product X over Y only product X, no Y (product X’s, no Y’s) stereo stereoisomers

  27. 100% 0% Stereochemistry of Reactions Regioselectiverx - of 2 possible constitutional isomers, rx forms 1 > other (Markovnikov addition). 30 | 70 Regiospecificrx - of 2 possible constitutional isomers, rx forms 1 (Markovnikov addition).

  28. Stereospecific rx stereoisomers  stereoisomers products

  29. Stereoselectiverx: Rx stereoisomer [B] > [C] major

  30. equal opportunityeither side Review Rx forming stereogenic center - ionic addition: 7 regiospecific not stereo-selective or -specific

  31. Br- Review: Br2, Cl2, I2, Hg++ - anti addition 7

  32. OsO4, KMnO4, HBR2, H2 , O3 - syn addition No RR ionic addition - carbocations - possible RR racemic 7

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