Alkynes

1. C C Alkynes

2. coke lime * *This reaction was used to produce light for miners’ lamps and for the stage. Synthesis of Acetylene • Heating coke with lime in an electric furnace to forms calcium carbide. • Then drip water on the calcium carbide.

3. The Structure of Alkynes A triple bond is composed of a s bond and two p bonds

4. C C Acidity of Acetyleneand Terminal Alkynes H

5. H2C CH2 Acidity of Hydrocarbons In general, hydrocarbons are excedingly weak acids • Compound pKa • HF 3.2 • H2O 16 • NH3 36 • 45 • CH4 60

6. HC CH H2C CH2 Acetylene Acetylene is a weak acid, but not nearlyas weak as alkanes or alkenes. • Compound pKa • HF 3.2 • H2O 16 • NH3 36 • 45 • CH4 60 26

8. C H H C C H C C C C Carbon: Hybridization and Electronegativity 10-60 sp3 : H++ C sp2 : 10-45 H++ C C 10-26 sp : H++ Electrons in an orbital with more s character are closer to thenucleus and more strongly held.

9. The stronger the acid, the weaker its conjugate base top 252

10. + + NaNH2 NaC NH3 CH HC CH CH C CH C H Sodium Acetylide Solution: Use a stronger base. Sodium amideis a stronger base than sodium hydroxide. – .. .. – + : : + H2N H H2N weaker acidpKa = 36 stronger acidpKa = 26 Ammonia is a weaker acid than acetylene.The position of equilibrium lies to the right.

11. Preparation of Various Alkynes by alkylation reactions withAcetylide or Terminal Alkynes

12. Synthesis Using Acetylide Ions: Formation of C–C Bond

13. Alkylation of Acetylene and Terminal Alkynes C—H H—C R—C C—H C—R R—C

14. – : : R X X– H—C H—C C C—R Alkylation of Acetylene and Terminal Alkynes SN2 • The alkylating agent is an alkyl halide, andthe reaction is nucleophilic substitution. • The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) alkyne. + +

15. HC HC CNa CH CH2CH2CH2CH3 HC C Example: Alkylation of Acetylene NaNH2 NH3 CH3CH2CH2CH2Br (70-77%)

16. (CH3)2CHCH2C CH (CH3)2CHCH2C CNa (CH3)2CHCH2C C—CH3 (81%) Example: Alkylation of a Terminal Alkyne NaNH2, NH3 CH3Br

17. H—C C—H 1. NaNH2, NH3 2. CH3CH2Br CH3CH2—C C—H 1. NaNH2, NH3 2. CH3Br C—CH3 CH3CH2—C Example: Dialkylation of Acetylene (81%)

18. Limitation • Effective only with primary alkyl halides • Secondary and tertiary alkyl halides undergo elimination

19. Reactions of Alkynes

20. Reactions of Alkynes • Acidity • Hydrogenation • Metal-Ammonia Reduction • Addition of Hydrogen Halides • Hydration

21. Hydrogenation of Alkynes

22. + 2H2 RC CR' Hydrogenation of Alkynes cat • alkene is an intermediate RCH2CH2R' catalyst = Pt, Pd, Ni, or Rh

23. 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'

24. 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'

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

28. Metal-Ammonia Reductionof Alkynes • Alkynes trans-Alkenes

29. 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'

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

33. Mechanism Metal (Li, Na, K) is reducing agent; H2 is not involved; proton comes from NH3 • four steps • (1) electron transfer • (2) proton transfer • (3) electron transfer • (4) proton transfer

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

35. Problem Strategy

36. Problem Strategy

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

38. Addition of Hydrogen Halidesto Alkynes

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

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

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

42. Hydration of Alkynes

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

45. 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

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

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

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

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