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Chapter 6; Addition Reaction of Alkenes part 2

Chapter 6; Addition Reaction of Alkenes part 2. HX Addition Electrophilic Addition (Markovnikov Product) Free Radical Mechanism (Anti-Mark Product) Hydration (+ H 2 O) Halogenation/ Hydrohalogenation Reduction or Hydrogenation (+ H 2 ) Oxidation Multi-step Synthesis. + X 2. C. C. C.

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Chapter 6; Addition Reaction of Alkenes part 2

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  1. Chapter 6; Addition Reaction of Alkenespart 2 • HX Addition • Electrophilic Addition (Markovnikov Product) • Free Radical Mechanism (Anti-Mark Product) • Hydration (+ H2O) • Halogenation/ Hydrohalogenation • Reduction or Hydrogenation (+ H2 ) • Oxidation • Multi-step Synthesis

  2. +X2 C C C X C X Addition of Halogens to Alkenes • electrophilic addition to double bond • forms a vicinal dihalide X2 = Cl2 or Br2 F2; explosive I2 ; endothermic

  3. Example Br2 CH3CHCHCH(CH3)2 CH3CH CHCH(CH3)2 Br Br (100%)

  4. H Br2 H Br Br H H trans-1,2-Dibromocyclopentane80% yield; only product Stereochemistry of Halogen Addition • anti addition Anti Addition ; Two Bromines add to opposite sides of the ring

  5. H Cl H Cl Example H Cl2 H trans-1,2-Dichlorocyclooctane73% yield; only product

  6. Mechanism is electrophilic addition • Br2 is not polar, but it is polarizable • two steps (1) formation of bromonium ion & • electrophilic attack • (2) nucleophilic attack on bromonium ion by bromide NET REACTION CH2=CH2 + Br2 -> Br-CH2-CH2-Br

  7. Br – Br Br Br + + Step 1a: Formation of Bromonium Ion Mutual polarizationof electron distributionsof Br2 and alkene Electrons flow from alkenetoward Br2

  8. Step 1b; Electrophilic Addition to form Bromonium Ion Part i + Br-

  9. + Br Step 1b; Lone Pair on Bromine Stabalizes Carbocation and Forms Cyclic Bromonium Ion Part ii + Br-

  10. Br + Br Step 2; Bromide Ion Must Attack from Oppositte Side of Cyclic Bromonium Ion (anti addition)

  11. H H Br Br H H Example Br2 trans-1,2-Dibromocyclopentane80% yield; only product

  12. C C C C C C C C + X2 X X alkenes react with X2 to form vicinal dihalides alkenes react with X2 in water to give vicinal halohydrins + H2O + X2 X OH + H—X

  13. H H OH Cl H H Examples H2O + Br2 BrCH2CH2OH H2C CH2 (70%) Cl2 H2O anti addition: only product

  14. Mechanism; 1) Cl2 is polarized and adds across double bond. 2) Ion formed is stabalized by lone pair of Cl.

  15. 3) Water attacks chloronium ion from side opposite (anti addition) carbon-chlorine bond. This gives trans isomer

  16. CH3 H3C Br2 CH3 C C CH2Br CH2 H2O H3C OH Regioselectivity • Markovnikov's rule applied to halohydrin formation: the halogen adds to the carbon having the greater number of hydrogens. (77%)

  17. H H H H C C H H C C H H H H Hydrogenation (Reduction, +H2) of Ethylene Metal Catalyst • exothermic H° = –136 kJ/mol • catalyzed by finely divided Pt, Pd, Rh, Ni     + H—H

  18. H CO2CH3 CO2CH3 H2, Pt CO2CH3 CO2CH3 H Two spatial (stereochemical) aspects ofalkene hydrogenation: • (1) syn addition of both H atoms to double bond • (2) hydrogenation is stereoselective, corresponding to addition to less crowded face of double bond

  19. syn-Additon versus anti-Addition syn addition anti addition

  20. H H H H syn-Addition; Metal catalyst breaks H-H bonds. B Y C C A X

  21. B Y X A H H C C H H syn-Addition; Addition of H2 across double bonds takes place in two steps.

  22. H3C CH3 H H3C H3C H3C CH3 CH3 H H H H H3C H H3C H Example of Stereoselective Reaction H2, cat Both productscorrespond tosyn additionof H2.

  23. H3C CH3 H H3C H3C CH3 H H H3C H Example of Stereoselective Reaction H2, cat Top face of doublebond blocked bythis methyl group But only thisone is formed. H2 adds to bottom face of double bond.

  24. Oxidation of Alkenes • Oxidation without bond cleavage (epoxidation) • Oxidation with bond cleavage

  25. O + C RCOOH C peroxy acid CHCH3 H2C CH2 H2C O O O RCOH C C O Epoxides; three-membered rings that contain oxygen Epoxidation of AlkenesNo C-C Sigma Bond Breaking +

  26. O + CH3COOH O + CH3COH O (52%) Example

  27. Mechanism of Epoxidation

  28. CH2CH3 CH3 C C H CH2CH3 CH2CH3 CH3 + O O C C H CH2CH3 (38%) (57%) Ozonolysis; Strong Oxidizing Agent Breaks C=C Bond 1. O3 2. H2O, Zn

  29. Multi-step Synthesis • Making Alkenes • Reactions of Alkenes • Alkenes are intermediates in multi-step synthesis

  30. C C C C C C C C -Elimination Reactions to Make Alkenes dehydration of alcohols: X = H; Y = OH H+ + H2O OH H   dehydrohalogenation of alkyl halides: X = H; Y = Cl, Br, I Base HBr + Br H  

  31. R R + H—X R H2O2 + H—X H X H H OH X C C C C C C C C C C C C C C C Specific Addition Reactions R R R H+ HO + H—OH H C R R 1) BH3 2) H2O2/OH-

  32. X + X—X + X—X C C C C C C C C X 2)H2O C C C OH X Specific Addition Reactions anti addition syn addition H H C + H—H

  33. Oxidation of Alkenes • Oxidation without bond cleavage • Oxidation with bond cleavage

  34. H2SO4 H2 OH OH Pt heat Prepare cyclohexane from cyclohexanol ? Multi-step synthesis uses alkenes as intermediate

  35. (CH3)2CCH2Br OH (CH3)2C CH2 Prepare 1-bromo-2-methyl-2-propanol from tert-butyl alcohol ? (CH3)3COH 1) H2SO4 heat 2) Br2 H2O

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