1. CCAlkynes

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

3. The Structure of Alkynes A triple bond is composed of a  bond and two  bonds

4. H CC Acidity of Acetylene and Terminal Alkynes

5. In general, hydrocarbons are excedingly weak acids CompoundpK a HF3.2 H 2 O16 NH 3 36 45 CH 4 60 H2CH2CH2CH2C CH 2 Acidity of Hydrocarbons

6. Acetylene is a weak acid, but not nearly as weak as alkanes or alkenes. CompoundpK a HF3.2 H 2 O16 NH 3 36 45 CH 4 60 H2CH2CH2CH2C CH 2 HCCH 26 Acetylene

8. C H H + + 10 -60 10 -45 10 -26 sp 3 C : sp 2 spHC C C CHC C CC : : Electrons in an orbital with more s character are closer to the nucleus and more strongly held. Carbon: Hybridization and Electronegativity

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

10. Solution: Use a stronger base. Sodium amide is a stronger base than sodium hydroxide. NH 3 NaNH 2 + HC CH NaC CH+– H2NH2NH2NH2N.. : H CCHH.. + + CCH: – stronger acid pK a = 26 weaker acid pK a = 36 Ammonia is a weaker acid than acetylene. The position of equilibrium lies to the right. H2NH2NH2NH2N Sodium Acetylide

11. Preparation of Various Alkynes by alkylation reactions with Acetylide or Terminal Alkynes

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

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

14. RX SN2SN2SN2SN2 X–X–X–X–:+ C–: H—C C—RH—C+ The alkylating agent is an alkyl halide, and the reaction is nucleophilic substitution. The nucleophile is sodium acetylide or the sodium salt of a terminal (monosubstituted) alkyne. Alkylation of Acetylene and Terminal Alkynes

15. NaNH 2 NH 3 CH 3 CH 2 CH 2 CH 2 Br (70-77%)HCCH HC CNa HC C CH 2 CH 2 CH 2 CH 3 Example: Alkylation of Acetylene

16. NaNH 2, NH 3 CH 3 Br CHCHCHCH (CH 3 ) 2 CHCH 2 C CNa (CH 3 ) 2 CHCH 2 C (81%) C—CH 3 (CH 3 ) 2 CHCH 2 C Example: Alkylation of a Terminal Alkyne

17. 1. NaNH 2, NH 3 2. CH 3 CH 2 Br (81%)H—CC—H 1. NaNH 2, NH 3 2. CH 3 Br C—H CH 3 CH 2 —C C—CH 3 CH 3 CH 2 —C Example: Dialkylation of Acetylene

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

19. Reactions of Alkynes

20. AcidityHydrogenation Metal-Ammonia Reduction Addition of Hydrogen Halides Hydration Reactions of Alkynes

21. Hydrogenation of Alkynes

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

23. 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

24. 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

25. + 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

28. Alkynes  trans-Alkenes Metal-Ammonia Reduction of Alkynes

29. 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

31. 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

33. 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; proton comes from NH 3 Mechanism

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

35. Problem Strategy

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

38. Addition of Hydrogen Halides to Alkynes

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

40. (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

41. 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

42. Hydration of Alkynes

44. 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

45. 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

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

48. O H C C H + O H H :..:

49. O H C C H + O H H :.. : :

50. O H C C H H O ::H +.. :

51. O H C CHH O : : H +.. :

52. Useful for symmetrical starting alkynes to produce a single product. Unsymmetrical starting alkynes produce a mixture of ketones… not so useful.

53. Aldehyde vs. Ketone

54. Can you identify and name the function?

59. Example