Chapter 9 Alkynes

1. Chapter 9Alkynes

2. Sources of Alkynes

3. + H2 + H2 HC CH CH2 H2C Acetylene Industrial preparation of acetylene isby dehydrogenation of ethylene 800°C CH2 H2C CH3CH3 1150°C cost of energy makes acetylene a more expensive industrial chemical than ethylene

4. Nomenclature

5. Acetylene and ethyne are both acceptableIUPAC names for HC CH HC CCH3 HC CCH2CH3 Propyne (CH3)3CC CCH3 Nomenclature Higher alkynes are named in much the sameway as alkenes except using an -yne suffixinstead of -ene. 1-Butyne 4,4-Dimethyl-2-pentyne

6. Physical Properties of Alkynes The physical properties of alkynes are similar to those of alkanes and alkenes.

7. Structure and Bonding in Alkynes:sp Hybridization

8. 120 pm H C C H 106 pm 106 pm Structure linear geometry for acetylene 121 pm C CH3 C H 146 pm 106 pm

9. Cyclononyne is the smallest cycloalkyne stable enough to be stored at room temperaturefor a reasonable length of time. Cyclooctyne polymerizeson standing.

10. Bonding in acetylene is based on sp-hybridizationfor each carbon Mix together (hybridize) the 2s orbital and one of the three 2p orbitals 2p 2p 2sp 2s

11. Bonding in acetylene is based on sp-hybridizationfor each carbon Mix together (hybridize) the 2s orbital and one of the three 2p orbitals 2p Each carbon has two half-filled sp orbitalsavailable to form s bonds. 2sp

12. s Bonds in Acetylene Each carbon isconnected to ahydrogen by as bond. The twocarbons are connectedto each other by as bond and two p bonds.

13. p Bonds in Acetylene One of the twop bonds in acetylene isshown here.The second pbond is at rightangles to the first.

14. p Bonds in Acetylene This is the secondof the twop bonds in acetylene.

15. The region of highest negative charge encirclesthe molecule around itscenter in acetylene. The region of highest negative charge lies aboveand below the molecular plane in ethylene.

16. Table 9.1 Comparison of ethane, ethylene, and acetylene Ethane Ethylene Acetylene C—C distance 153 pm 134 pm 120 pm C—H distance 111 pm 110 pm 106 pm H—C—C angles 111.0° 121.4° 180° C—C BDE 368 kJ/mol 611 kJ/mol 820 kJ/mol C—H BDE 410 kJ/mol 452 kJ/mol 536 kJ/mol hybridization of C sp3 sp2 sp % s character 25% 33% 50% pKa 62 45 26

17. C C H Acidity of Acetylene and Terminal Alkynes

18. H2C CH2 In general, hydrocarbons are exceedingly weak acids Compound pKa HF 3.2 H2O 16 NH3 36 45 CH4 60

19. HC CH H2C CH2 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

20. C H H C C H C C C C Electronegativity of carbon increases with its s character 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.

21. NaC CH + + NaOH NaC H2O CH HC CH Objective: Prepare a solution containing sodium acetylideWill treatment of acetylene with NaOH be effective?

22. + + NaOH NaC H2O CH HC CH .. .. CH C : CH C H H HO HO .. .. No. Hydroxide is not a strong enough base to deprotonate acetylene. – – + : + stronger acidpKa = 16 weaker acidpKa = 26 In acid-base reactions, the equilibrium lies tothe side of the weaker acid.

23. + + NaNH2 NaC NH3 CH HC CH CH C CH C H 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.

24. Preparation of Alkynes byAlkylation of Acetylene and Terminal Alkynes

25. Preparation of Alkynes There are two main methods for the preparationof alkynes: Carbon-carbon bond formationalkylation of acetylene and terminal alkynes Functional-group transformationselimination

26. Alkylation of acetylene and terminal alkynes C—H H—C R—C C—H C—R R—C

27. – : R : X– H—C H—C C X 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. + +

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

29. (CH3)2CHCH2C CH (CH3)2CHCH2C CNa (CH3)2CHCH2C C—CH3 (81%) Example: Alkylation of a terminal alkyne NaNH2, NH3 CH3Br

30. 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%)

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

32. – : H—C C + —H + : C C C H—C X– E2 predominates over SN2 when alkyl halide is secondary or tertiary C H C— X E2

33. Preparation of Alkynes by Elimination Reactions

34. H H H X C C C C X X H X Preparation of Alkynes by "Double" Dehydrohalogenation Geminal dihalide Vicinal dihalide The most frequent applications are in preparation of terminal alkynes.

35. 1. 3NaNH2, NH3 2. H2O (CH3)3CC CH (56-60%) Geminal dihalide ® Alkyne (CH3)3CCH2—CHCl2

36. CHCl (CH3)3CCH CH (CH3)3CC CNa (CH3)3CC Geminal dihalide ® Alkyne (CH3)3CCH2—CHCl2 (slow) NaNH2, NH3 (slow) NaNH2, NH3 H2O (fast) NaNH2, NH3

37. CH3(CH2)7CH—CH2Br Br 1. 3NaNH2, NH3 2. H2O CH3(CH2)7C CH (54%) Vicinal dihalide ® Alkyne

38. Reactions of Alkynes

39. 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)

40. Hydrogenation of Alkynes

41. + 2H2 RC CR' Hydrogenation of Alkynes cat alkene is an intermediate RCH2CH2R' catalyst = Pt, Pd, Ni, or Rh

42. 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.

43. H2 H2 RCH RC CHR' CR' cat cat Partial Hydrogenation Alkynes 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'

44. 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'

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

46. Metal-Ammonia Reduction of Alkynes Alkynes ®trans-Alkenes

47. 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'

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

49. 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

50. 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.