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Addition of Hydrogen Halides to Alkynes

Addition of Hydrogen Halides to Alkynes. CH 3 (CH 2 ) 3 C. CH 3 (CH 2 ) 3 C. CH 2. CH. Br. Follows Markovnikov's Rule. HBr. Alkynes are slightly less reactive than alkenes. (60%). Br. :. H. Br. :. H. Termolecular Rate-Determining Step. CH. RC.

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Addition of Hydrogen Halides to Alkynes

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  1. Addition of Hydrogen Halidesto Alkynes

  2. CH3(CH2)3C CH3(CH2)3C CH2 CH Br Follows Markovnikov's Rule HBr • Alkynes are slightly less reactive than alkenes (60%)

  3. .. Br : H .. .. Br : H .. Termolecular Rate-Determining Step CH RC Observed rate law: rate = k[alkyne][HX]2

  4. H F C C CH2CH3 CH3CH2 H F Two Molar Equivalents of Hydrogen Halide CCH2CH3 CH3CH2C 2 HF (76%)

  5. Hydration of Alkynes

  6. H+ + RCH CR' H2O RC CR' OH RCH2CR' RC CR' O Hydration of Alkynes • expected reaction: enol observed reaction: H+ + H2O ketone

  7. RCH CR' RCH2CR' OH O Enols • enols are regioisomers of ketones, and exist in equilibrium with them • keto-enol equilibration is rapid in acidic media • ketones are more stable than enols andpredominate at equilibrium enol ketone

  8. H O C C Mechanism of conversion of enol to ketone .. : H + : H O H

  9. .. : H O H C C + : H O H Mechanism of conversion of enol to ketone

  10. Mechanism of conversion of enol to ketone .. : H O H H C C + : : O H

  11. : : O Mechanism of conversion of enol to ketone H .. : H O H H C C +

  12. H .. : : O : H O H H C C + Mechanism of conversion of enol to ketone

  13. Mechanism of conversion of enol to ketone H .. : O H + : O H H C C

  14. .. .. + : H H O O H H C C C C + Key Carbocation Intermediate Carbocation is stabilized by electron delocalization (resonance)

  15. CH3(CH2)2C C(CH2)2CH3 OH Hg2+ CH3(CH2)2CH C(CH2)2CH3 O Example of Alkyne Hydration via H2O, H+ CH3(CH2)2CH2C(CH2)2CH3 (89%)

  16. O CH3(CH2)5C CH OH CH2 CH3(CH2)5C Regioselectivity Markovnikov's rule followed in formation of enol H2O, H2SO4 CH3(CH2)5CCH3 HgSO4 (91%) via

  17. Addition of Halogens to Alkynes

  18. Cl Cl2CH CH3 HC C CCH3 Cl (63%) Example + 2 Cl2

  19. C CCH2CH3 CH3CH2C C Addition is anti Br CH3CH2 Br2 CH2CH3 Br (90%)

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