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6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

6.4 Electrophilic Addition of Hydrogen Halides to Alkenes. C. C. E. C. Y. C. General Equation for Electrophilic Addition. .  –. + E — Y. C. C. H. C. X. C. When the Electrophile is a Hydrogen Halide. .  –. + H — X. C. C. CH 3 CH 2 C H 2 C HCH 2 CH 3. Br. (76%).

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6.4 Electrophilic Addition of Hydrogen Halides to Alkenes

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  1. 6.4Electrophilic Addition of Hydrogen Halides to Alkenes

  2. C C E C Y C General Equation for Electrophilic Addition  – +E—Y

  3. C C H C X C When the Electrophile is a Hydrogen Halide  – +H—X

  4. C C CH3CH2CH2CHCH2CH3 Br (76%) Example CH2CH3 CH3CH2 HBr CHCl3, -30°C H H

  5. Mechanism • Electrophilic addition of hydrogen halides to alkenes proceeds by rate-determining formation of a carbocation intermediate.

  6. Mechanism • Electrons flow fromthe  system of thealkene (electron rich) toward the positivelypolarized proton of the hydrogen halide.

  7. + C C H .. – : : X .. .. : .. .. C : C C H C X .. Mechanism H X

  8. 6.5Regioselectivity of Hydrogen Halide Addition:Markovnikov's Rule

  9. Markovnikov's Rule • When an unsymmetrically substituted alkene reacts with a hydrogen halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen adds to the carbon that has the fewer hydrogen substituents.

  10. CH3CH2CH CH3CH2CHCH3 CH2 Br Markovnikov's Rule HBr acetic acid (80%) Example 1

  11. CH3 CH3 CH3 C C C Br Markovnikov's Rule CH3 H HBr acetic acid CH3 H (90%) Example 2

  12. CH3 CH3 Cl Markovnikov's Rule HCl 0°C (100%) Example 3

  13. 6.6Mechanistic Basis for Markovnikov's Rule • Protonation of double bond occurs in direction that gives more stable of two possible carbocations.

  14. HBr CH3CH2CH CH3CH2CHCH3 CH2 acetic acid Br Mechanistic Basis for Markovnikov's Rule:Example 1

  15. + CH3CH2CH2—CH2 primary carbocation is less stable: not formed + CH3CH2CH—CH3 + Br– CH3CH2CH CH3CH2CHCH3 CH2 Br Mechanistic Basis for Markovnikov's Rule:Example 1 HBr

  16. H CH3 CH3 Cl Mechanistic Basis for Markovnikov's Rule:Example 3 HCl 0°C

  17. H secondary carbocation is less stable: not formed + CH3 H H H + CH3 H CH3 CH3 Cl Cl– HCl

  18. 6.7Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes

  19. CHCH(CH3)2 H2C H + + CH3CHC(CH3)2 CH3CHCH(CH3)2 CH3CH2C(CH3)2 CH3CHCH(CH3)2 Cl Cl (40%) (60%) Rearrangements Sometimes Occur HCl, 0°C

  20. 6.8Free-radical Addition of HBr to Alkenes • The "peroxide effect"

  21. CH3CH2CH CH3CH2CHCH3 CH2 Br Markovnikov's Rule HBr acetic acid (80%)

  22. CH3CH2CH CH2 CH3CH2CHCH3 Br only product when peroxides added to reaction mixture only product inabsence of peroxides Addition of HBr to 1-Butene HBr CH3CH2CH2CH2Br

  23. CH3CH2CH CH2 Addition of HBr to 1-Butene HBr addition opposite to Markovnikov's rule occurs with HBr (not HCl or HI) CH3CH2CH2CH2Br only product when peroxides added to reaction mixture

  24. CH2Br CH2 H Photochemical Addition of HBr • Addition of HBr with a regiochemistry oppositeto Markovnikov's rule can also occur wheninitiated with light with or without added peroxides. h + HBr (60%)

  25. .. .. .. .. . . + R O O R R O O R .. .. .. .. .. .. .. .. . : + . Br H R + O : H O R Br .. .. .. .. Mechanism • Addition of HBr opposite to Markovnikov's rule proceeds by a free-radical chain mechanism. • Initiation steps:

  26. .. . : + Br CH3CH2CH CH2 .. .. . : CH2 Br CH3CH2CH .. .. .. : : CH2 Br CH3CH2CH2CH2 Br CH3CH2CH .. .. . + .. : Br H .. .. . : Br .. Mechanism Propagation steps:

  27. 6.9Addition of Sulfuric Acid to Alkenes

  28. CH3CHCH3 CH3CH CH2 OSO2OH Addition of H2SO4 HOSO2OH • follows Markovnikov's rule: • yields an alkyl hydrogen sulfate Isopropylhydrogen sulfate

  29. CH3CH CH2 slow – + .. + : CH3CH SO2OH O CH3 .. fast CH3CHCH3 OSO2OH : .. Mechanism .. + SO2OH O H ..

  30. CH3CHCH3 SO2OH O heat + CH3CHCH3 HO—SO2OH O H Alkyl Hydrogen Sulfates Undergo Hydrolysis in Hot Water + H—OH

  31. OH Application: Conversion of Alkenes to Alcohols 1. H2SO4 2. H2O, heat (75%)

  32. But... • not all alkenes yield alkyl hydrogen sulfateson reaction with sulfuric acid • these do: • H2C=CH2, RCH=CH2, and RCH=CHR' • these don't: • R2C=CH2, R2C=CH2, and R2C=CR2

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