Chapter 9 Alkynes

1. Chapter 9Alkynes

2. 9.1Sources 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 moreexpensive industrial chemical than ethylene.

4. Tariric acid: occurs in seed of a Guatemalan plant. C(CH2)4COH O CH3(CH2)10C Naturally Occurring Alkynes Some alkynes occur naturally. For example,

5. 9.2Nomenclature

6. 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 or But-1-yne 4,4-Dimethyl-2-pentyne or 4,4-Dimethyl-pent-2-yne

7. 9.3Physical Properties of Alkynes The physical properties of alkynes are similar to those of alkanes and alkenes.

8. 9.4Structure and Bonding in Alkynes:sp Hybridization

9. H C C H C CH3 C H Structure • Linear geometry for acetylene sp hybridized

10. 120 pm H C C H 106 pm 106 pm 121 pm C CH3 C H 146 pm 106 pm Structure • Linear geometry for acetylene sp hybridized

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

12. sp Hybridization in Acetylene Mix together (hybridize) the 2s orbital and one of the three 2p orbitals. 2p 2p 2sp 2s

13. sp Hybridization in Acetylene Mix together (hybridize) the 2s orbital and one of the three 2p orbitals. 2p Each carbon has two half-filled sp orbitalsavailable to form  bonds. 2sp

14.  Bonds in Acetylene Each carbon isconnected to ahydrogen by a bond. The twocarbons are connectedto each other by a bond and two  bonds. Figure 9.2 (a)

15.  Bonds in Acetylene One of the two bonds in acetylene isshown here.The second bond is at rightangles to the first. Figure 9.2 (b)

16.  Bonds in Acetylene This is the secondof the two bonds in acetylene. Figure 9.2 (c)

17. The region of highest negative charge encircles the molecule around itscenter in acetylene. Figure 9.3 Electrostatic Potential in Acetylene The region of highest negative charge lies above and below the molecular plane in ethylene.

18. 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 Table 9.1Comparison of Ethane, Ethylene, and Acetylene

19. H C C 9.5Acidity of Acetyleneand Terminal Alkynes

20. HC CH H2C CH2 Acidity of Hydrocarbons In general, hydrocarbons are exceedingly weak acids, but acetylene is not nearlyas weak as alkanes or alkenes. • Compound pKa • 26 • 45 • CH4 60

21. pKa = 62 – sp3 : C H++ H C H sp2 : pKa = 45 H++ C C C C – pKa = 26 – : sp H C C C C H++ Carbon: Hybridization and Electronegativity Electrons in an orbital with more s character are closer to thenucleus and more strongly held.

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

23. – .. .. – stronger acidpKa = 15.7 weaker acidpKa = 26 + CH C : + CH C H : H HO HO .. .. Sodium Acetylide No. Hydroxide is not a strong enough base to deprotonate acetylene. In acid-base reactions, the equilibrium lies tothe side of the weaker acid.

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

25. 9.6Preparation of AlkynesbyAlkylation of Acetylene and Terminal Alkynes

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

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

28. – : : 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. + +

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

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

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

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

33. – C : H H—C C C X E2 + —H + : C C C H—C X– Acetylide Ion as a Base E2 predominates over SN2 when alkyl halide is secondary or tertiary.

34. 9.7Preparation of Alkynesby Elimination Reactions

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

36. 1. 3NaNH2, NH3 2. H2O (CH3)3CC CH (56-60%) Geminal dihalide  Alkyne (CH3)3CCH2—CHCl2

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

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

39. 9.8Reactions of Alkynes

40. 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)

41. 9.9Hydrogenation of Alkynes

42. + 2H2 RC CR' Hydrogenation of Alkynes cat • alkene is an intermediate RCH2CH2R' catalyst = Pt, Pd, Ni, or Rh

43. 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.

44. 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'

45. H2 H2 RCH RC CHR' CR' cat cat Lindlar Catalyst • 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'

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

47. 9.10Metal-Ammonia Reductionof Alkynes • Alkynes trans-Alkenes

48. 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'

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

50. 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