1. 9.8 Reactions of Alkynes

2. 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) Reactions of Alkynes

3. 9.9 Hydrogenation of Alkynes

4. RCH 2 CH 2 R' cat catalyst = Pt, Pd, Ni, or Rh alkene is an intermediate RC CR'+ 2H22H22H22H2 Hydrogenation of Alkynes

5. Heats of Hydrogenation 292 kJ/mol 275 kJ/mol Alkyl groups stabilize triple bonds in the same way that they stabilize double bonds. Internal triple bonds are more stable than terminal ones. CH 3 CH 2 C CH CH 3 C CCH 3

6. RCH 2 CH 2 R' Alkenes could be used to prepare alkenes if a catalyst were available that is active enough to catalyze the hydrogenation of alkynes, but not active enough for the hydrogenation of alkenes. cat H2H2H2H2 RC CR' cat H2H2H2H2RCH CHR' Partial Hydrogenation

7. There is a catalyst that will catalyze the hydrogenation of alkynes to alkenes, but not that of alkenes to alkanes. It is called the Lindlar catalyst and consists of palladium supported on CaCO 3, which has been poisoned with lead acetate and quinoline. syn-Hydrogenation occurs; cis alkenes are formed. RCH 2 CH 2 R' cat H2H2H2H2 RC CR' cat H2H2H2H2RCH CHR' Lindlar Palladium

8. + H 2 Lindlar Pd CH 3 (CH 2 ) 3 (CH 2 ) 3 CH 3 HH (87%) CH 3 (CH 2 ) 3 C C(CH 2 ) 3 CH 3 C C Example

9. Alkynes  trans-Alkenes 9.10 Metal-Ammonia Reduction of Alkynes

10. RCH 2 CH 2 R' Another way to convert alkynes to alkenes is by reduction with sodium (or lithium or potassium) in ammonia. trans-Alkenes are formed. RC CR' RCH CHR' Partial Reduction

11. CH 3 CH 2 CH 2 CH 3 H H (82%) CH 3 CH 2 C CCH 2 CH 3 C C Na, NH 3 Example

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

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

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

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

16. Step (4) Transfer of a proton from the solvent (liquid ammonia) to the carbanion...H NH 2 R' R C C H.. – R' H H C C R..– : Mechanism

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

18. Problem 9.12 Strategy

19. 1. NaNH 2 2. CH 3 CH 2 CH 2 CH 2 Br Na, NH 3 H 2, Lindlar Pd Problem 9.12 Synthesis

20. 9.11 Addition of Hydrogen Halides to Alkynes

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

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

23. (76%) CH 3 CH 2 C CCH 2 CH 3 2 HF F F C CHH CH 3 CH 2 CH 2 CH 3 Two Molar Equivalents of Hydrogen Halide

24. HBr (79%) regioselectivity opposite to Markovnikov's rule CH 3 (CH 2 ) 3 C CH CH 3 (CH 2 ) 3 CH CHBr peroxides Free-radical Addition of HBr

25. 9.12 Hydration of Alkynes

26. expected reaction: enol observed reaction: RCH 2 CR' O H+H+H+H+RC CR' H2OH2OH2OH2O + H+H+H+H+RCCR' H2OH2OH2OH2O + OHRCHCR' ketone Hydration of Alkynes

27. 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 and predominate at equilibrium enol OHRCHCR' RCH 2 CR' O ketone Enols

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

29. O H C C H + O H H :..:

30. O H C C H + O H H :.. : :

31. O H C C H H O ::H +.. :

32. O H C CHH O : : H +.. :

33. O H C C H H O : H +.. :

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

35. H 2 O, H + (89%) via CH 3 (CH 2 ) 2 C C(CH 2 ) 2 CH 3 Hg 2+ O CH 3 (CH 2 ) 2 CH 2 C(CH 2 ) 2 CH 3 OH CH 3 (CH 2 ) 2 CH C(CH 2 ) 2 CH 3 Example of Alkyne Hydration

36. H 2 O, H 2 SO 4 HgSO 4 CH 3 (CH 2 ) 5 CCH 3 (91%) via Markovnikov's rule followed in formation of enol CH 3 (CH 2 ) 5 C CH 2 OH CH 3 (CH 2 ) 5 C CH ORegioselectivity

37. 9.13 Addition of Halogens to Alkynes

38. + 2 Cl 2 ClCl (63%) C Cl 2 CH CH 3 HC CCH 3 Example

39. Br 2 CH 3 CH 2 CH 2 CH 3 Br Br (90%) CH 3 CH 2 C CCH 2 CH 3 C C Addition is anti

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

41. 1. O 3 2. H 2 O + CH 3 (CH 2 ) 3 C CHCHCHCH CH 3 (CH 2 ) 3 COH (51%)O HOCOH OExample