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6.17 Mechanism of Halogen Addition to Alkenes: Halonium Ions

6.17 Mechanism of Halogen Addition to Alkenes: Halonium Ions. Mechanism is electrophilic addition. Br 2 is not polar, but it is polarizable two steps involved (1) formation of bromonium ion (2) nucleophilic attack on bromonium ion by bromide. Relative Rates. Bromination.

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6.17 Mechanism of Halogen Addition to Alkenes: Halonium Ions

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  1. 6.17Mechanism of Halogen Additionto Alkenes:Halonium Ions

  2. Mechanism is electrophilic addition Br2 is not polar, but it is polarizable two steps involved (1) formation of bromonium ion (2) nucleophilic attack on bromonium ion by bromide

  3. Relative Rates Bromination ethylene H2C=CH2 1 propene CH3CH=CH2 61 2-methylpropene (CH3)2C=CH2 5400 2,3-dimethyl-2-butene (CH3)2C=C(CH3)2 920,000 More highly substituted double bonds react faster.Alkyl groups on the double bond make itmore “electron rich.”

  4. .. + – + : : Br C C .. : : Br .. Mechanism? + Br2 BrCH2CH2Br H2C CH2 No obvious explanation for anti addition provided by this mechanism. ?

  5. : : .. : : Br + Mechanism + Br2 BrCH2CH2Br H2C CH2 Cyclic bromonium ion .. – + Br C C

  6. Formation of bromonium ion Br Mutual polarizationof electron distributionsof Br2 and alkene Br

  7. Formation of bromonium ion – Electrons flow from alkene toward Br2 Br Br + +

  8. Formation of bromonium ion – Br  electrons of alkenedisplace Br– from Br + Br

  9. .. – : : Br .. .. + Br .. Stereochemistry .. : Br .. attack of Br– from side oppositeC—Br bond of bromonium ion givesanti addition .. : Br ..

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

  11. Cyclopentene +Br2

  12. Bromonium ion –

  13. – – Bromide ion attacks the bromonium ion from side opposite carbon-bromine bond

  14. trans-Stereochemistry in vicinal dibromide

  15. 6.18Conversion of Alkenesto Vicinal Halohydrins

  16. C C C C + X2 X X alkenes react with X2 to form vicinal dihalides

  17. 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

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

  19. O .. .. Br .. O .. Mechanism : • bromonium ion is intermediate • water is nucleophile that attacks bromonium ion + + .. : Br ..

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

  21. Cyclopentene + Cl2

  22. Chloronium ion

  23. Water attacks chloronium ion from side opposite carbon-chlorine bond

  24. trans-Stereochemistry in oxonium ion

  25. trans-2-Chlorocyclopentanol

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

  27. H H .. ..  H  O H O Explanation transition state for attack of water on bromonium ion has carbocation character; more stable transition state (left) has positive charge on more highly substituted carbon H3C H3C   H3C H3C CH2 CH2 C C : : : : Br Br  

  28. 6.19Epoxidation of Alkenes

  29. CHCH3 H2C CH2 H2C O O Epoxides • are examples of heterocyclic compounds • three-membered rings that contain oxygen ethylene oxide propylene oxide

  30. H3C CHCH3 CHCH3 H2C C H3C O O Epoxide Nomenclature Substitutive nomenclature: • named as epoxy-substituted alkanes. • “epoxy” precedes name of alkane 1,2-epoxypropane 2-methyl-2,3-epoxybutane 1 4 2 3

  31. O H H Problem 6.17 Give the IUPAC name, including stereochemistry, for disparlure. cis-2-Methyl-7,8-epoxyoctadecane

  32. O C RCOOH C O RCOH C C O Epoxidation of Alkenes + peroxy acid +

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

  34. O C RCOOH C O RCOH C C O Epoxidation of Alkenes + syn addition +

  35. Relative Rates Epoxidation ethylene H2C=CH2 1 propene CH3CH=CH2 22 2-methylpropene (CH3)2C=CH2 484 2-methyl-2-butene (CH3)2C=CHCH3 6526 More highly substituted double bonds react faster.Alkyl groups on the double bond make itmore “electron rich.”

  36. Mechanism of Epoxidation

  37. Mechanism of Epoxidation

  38. Mechanism of Epoxidation

  39. Mechanism of Epoxidation

  40. Mechanism of Epoxidation

  41. 6.20Ozonolysis of Alkenes Ozonolyis has both synthetic and analytical applications. • synthesis of aldehydes and ketones • identification of substituents on the double bond of an alkene

  42. O C C C C O O Ozonolysis of Alkenes First step is the reaction of the alkene with ozone. The product is an ozonide. + O3

  43. O C C C C O O O C Ozonolysis of Alkenes Second step is hydrolysis of the ozonide. Two aldehydes, two ketones, or an aldehyde and a ketone are formed. + O3 H2O, Zn + O C

  44. O C C C C O O O O C C Ozonolysis of Alkenes As an alternative to hydrolysis, the ozonidecan be treated with dimethyl sulfide. + O3 (CH3)2S +

  45. CH2CH3 CH3 C C H CH2CH3 CH2CH3 CH3 O O C C H CH2CH3 Example 1. O3 2. H2O, Zn + (57%) (38%)

  46. 6.21Introduction to OrganicChemical Synthesis

  47. Prepare cyclohexane from cyclohexanol devise a synthetic plan reason backward from the target molecule always use reactions that you are sure will work OH

  48. Prepare cyclohexane from cyclohexanol ask yourself the key question "Starting with anything, how can I make cyclohexane in a single step by a reaction I am sure will work?" OH

  49. Prepare cyclohexane from cyclohexanol The only reaction covered so far for preparing alkanes is catalytic hydrogenation of alkenes. This leads to a new question. "Starting with anything, how can I prepare cyclohexene in a single step by a reaction I am sure will work?" H2 OH Pt

  50. H2SO4 heat Prepare cyclohexane from cyclohexanol Alkenes can be prepared by dehydration of alcohols. The synthesis is complete. H2 OH Pt

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