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6.14 Addition of Halogens to Alkenes

6.14 Addition of Halogens to Alkenes. C. C. C. C. General features. electrophilic addition to double bond forms a vicinal dihalide. + X 2. X. X. Example. Br 2. CH 3 CHCH CH(CH 3 ) 2. CH 3 CH. CH CH(CH 3 ) 2. CHCl 3 0°C. Br. Br. (100%). Scope. limited to Cl 2 and Br 2.

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6.14 Addition of Halogens to Alkenes

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  1. 6.14Addition of Halogensto Alkenes

  2. C C C C General features • electrophilic addition to double bond • forms a vicinal dihalide +X2 X X

  3. Example Br2 CH3CHCHCH(CH3)2 CH3CH CHCH(CH3)2 CHCl3 0°C Br Br (100%)

  4. Scope limited to Cl2 and Br2 • F2 addition proceeds with explosive violence • I2 addition is endothermic: vicinal diiodidesdissociate to an alkene and I2

  5. 6.15Stereochemistry of Halogen Addition • anti addition

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

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

  8. 6.16Mechanism of Halogen Addition to Alkenes: Halonium Ions

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

  10. Relative Rates of 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.”

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

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

  13. Formation of Bromonium Ion Br Mutual polarizationof electron distributionsof Br2 and alkene Br

  14. Formation of Bromonium Ion d– Electrons flow from alkene toward Br2 Br Br d+ d+

  15. + Br Formation of Bromonium Ion – Br p electrons of alkenedisplace Br– from Br

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

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

  18. Cyclopentene +Br2

  19. Bromonium ion –

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

  21. trans-Stereochemistry in vicinal dibromide

  22. 6.17Conversion of Alkenes to Vicinal Halohydrins

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

  24. 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

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

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

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

  28. Cyclopentene + Cl2

  29. Chloronium ion

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

  31. trans-Stereochemistry in oxonium ion

  32. trans-2-Chlorocyclopentanol

  33. 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%)

  34. H H .. .. d+ H d+ O H O Explanation • transition state has 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 d+ d+ H3C H3C CH2 CH2 C C : : : : Br Br d+ d+

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