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9.8 Reactions of Alkynes

9.8 Reactions of Alkynes. Reactions of Alkynes. Acidity (Section 9.5) Hydrogenation (Section 9.9) Metal-Ammonia Reduction (Section 9.10) Addition of Hydrogen Halides (Section 9.11) Hydration (Section 9.12) Addition of Halogens (Section 9.13) Ozonolysis (Section 9.14).

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9.8 Reactions of Alkynes

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  1. 9.8Reactions of Alkynes

  2. Reactions of Alkynes • Acidity (Section 9.5) • Hydrogenation (Section 9.9) • Metal-Ammonia Reduction (Section 9.10) • Addition of Hydrogen Halides (Section 9.11) • Hydration (Section 9.12) • Addition of Halogens (Section 9.13) • Ozonolysis (Section 9.14)

  3. 9.9Hydrogenation of Alkynes

  4. + 2H2 RC CR' Hydrogenation of Alkynes cat • alkene is an intermediate RCH2CH2R' catalyst = Pt, Pd, Ni, or Rh

  5. CH3CH2C CH CCH3 CH3C Heats of Hydrogenation 292 kJ/mol 275 kJ/mol Alkyl groups stabilize triple bonds in the same way that they stabilize doublebonds. Internal triple bonds are more stable than terminal ones.

  6. H2 H2 RCH RC CHR' CR' cat cat Partial Hydrogenation • Alkenes could be used to prepare alkenes if acatalyst were available that is active enough to catalyze the hydrogenation of alkynes, but notactive enough for the hydrogenation of alkenes. RCH2CH2R'

  7. H2 H2 RCH RC CHR' CR' cat cat Lindlar Palladium • There is a catalyst that will catalyze the hydrogenationof alkynes to alkenes, but not that of alkenes to alkanes. • It is called the Lindlar catalyst and consists ofpalladium supported on CaCO3, which has been poisoned with lead acetate and quinoline. • syn-Hydrogenation occurs; cis alkenes are formed. RCH2CH2R'

  8. C(CH2)3CH3 CH3(CH2)3C C C Example + H2 Lindlar Pd CH3(CH2)3 (CH2)3CH3 H H (87%)

  9. 9.10Metal-Ammonia Reductionof Alkynes • Alkynes Ætrans-Alkenes

  10. Partial Reduction • Another way to convert alkynes to alkenes isby reduction with sodium (or lithium or potassium)in ammonia. • trans-Alkenes are formed. RCH2CH2R' RCH RC CHR' CR'

  11. CCH2CH3 CH3CH2C C C Example Na, NH3 CH3CH2 H CH2CH3 H (82%)

  12. Mechanism Metal (Li, Na, K) is reducing agent; H2 is not involved • four steps • (1) electron transfer • (2) proton transfer • (3) electron transfer • (4) proton transfer

  13. M+ – . .. . R + R' R' C C R M C C Mechanism • Step (1): Transfer of an electron from the metalto the alkyne to give an anion radical.

  14. R . R' C C H .. : NH2 – Mechanism • Step (2) Transfer of a proton from the solvent (liquid ammonia) to the anion radical. . – R' R C C .. .. NH2 H

  15. M+ R – .. C C R' H Mechanism • Step (3): Transfer of an electron from the metalto the alkenyl radical to give a carbanion. R . . + R' M C C H

  16. .. H NH2 .. : H R NH2 R – .. – C C C C R' R' H H Mechanism • Step (4) Transfer of a proton from the solvent(liquid ammonia) to the carbanion .

  17. Problem 9.12 • Suggest efficient syntheses of (E)- and (Z)-2-heptene from propyne and any necessary organic or inorganic reagents.

  18. Problem 9.12 Strategy

  19. Problem 9.12Synthesis 1. NaNH2 2. CH3CH2CH2CH2Br Na, NH3 H2, Lindlar Pd

  20. 9.11Addition of Hydrogen Halidesto Alkynes

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

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

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

  24. CHBr CH3(CH2)3C CH CH3(CH2)3CH Free-radical Addition of HBr • regioselectivity opposite to Markovnikov's rule HBr peroxides (79%)

  25. 9.12Hydration of Alkynes

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

  27. 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

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

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

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

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

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

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

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

  35. 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%)

  36. 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

  37. 9.13Addition of Halogens to Alkynes

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

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

  40. 9.14Ozonolysis of Alkynes • gives two carboxylic acids by cleavage of triple bond

  41. CH3(CH2)3C CH 1. O3 2. H2O O O CH3(CH2)3COH HOCOH (51%) Example +

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