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Alkynes

C. C. Alkynes. 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. The Structure of Alkynes.

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Alkynes

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  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. Question • Arrange ethane, ethene, and ethyne in order of increasing C-C bond length. • A) ethane < ethene < ethyne • B) ethene < ethane < ethyne • C) ethyne < ethene < ethane • D) ethane < ethyne < ethene

  5. C C Acidity of Acetyleneand Terminal Alkynes H

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

  7. 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. Question • Which one of the following is the strongest acid? • A) water • B) ammonia • C) 1-butene • D) 1-butyne

  9. 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.

  10. Question • Which one of the following statements best explains the greater acidity of terminal alkynes (RCºCH) compared with monosubstituted alkenes (RCH=CH2)? • A) The sp-hybridized carbons of the alkyne are less electronegative than the sp2 carbons of the alkene. • B) The two p bonds of the alkyne are better able to stabilize the negative charge of the anion by resonance. • C) The sp-hybridized carbons of the alkyne are more electronegative than the sp2 carbons of the alkene. • D) The question is incorrect - alkenes are more acidic than alkynes.

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

  12. + + 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.

  13. Question • Which of the following bases is strong enough to completely deprotonate propyne? • A) NH3 • B) CH3OH • C) NaNH2 • D) NaOH

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

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

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

  17. – : : 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. + +

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

  19. Question • Which alkyl halide will react faster with the acetylide ion (HCºCNa) in an SN2 reaction? • A) bromopropane • B) 2-bromopropane • C) tert-butyl iodide • D) 1-bromo-2-methylbutane

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

  21. 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%)

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

  23. – 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.

  24. Question • Consider the reaction of each of the following with cyclohexyl bromide. For which one is • the ratio of substitution to elimination highest? • A) NaOCH2CH3, ethanol, 60°C • B) NaSCH2CH3, ethanol-water, 25°C • C) NaNH2, NH3, -33°C • D) NaCºCH, NH3, -33°C

  25. Preparation of Alkynesby Elimination Reactions

  26. 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.

  27. 1. 3NaNH2, NH3 2. H2O (CH3)3CC CH (56-60%) Geminal dihalide  Alkyne (CH3)3CCH2—CHCl2

  28. 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

  29. Question • In addition to NaNH2, what other base can be used to convert 1,1-dichlorobutane into • 1-butyne? • A) NaOCH3 • B) NaOH • C) NaOCH2CH3 • D) KOC(CH3)3

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

  31. Question • Which of the following compounds yield 1-heptyne on being treated with three moles of sodium amide (in liquid ammonia as the solvent) followed by adding water to the reaction mixture? • A) 1,1,2,2-tetrachloroheptane • B) 1-bromo-2-chloroheptane • C) 1,1,2-trichloropentane • D) all of the above

  32. Reactions of Alkynes

  33. Reactions of Alkynes • Acidity • Hydrogenation • Metal-Ammonia Reduction • Addition of Hydrogen Halides • Hydration • Addition of Halogens • Ozonolysis

  34. Hydrogenation of Alkynes

  35. H C C H Atomic Force Microscopy of Acetylene Lawrence Berkeley Laboratory (LBL)

  36. H C C H TIP pz H + O porbital 1 cm (± 1 μm) Imaging: acetylene on Pd(111) at 28 K Molecular Image Tip cruising altitude ~700 pm Δz = 20 pm Why don’t we see the Pd atoms? Because the tip needs to be very close to image the Pd atoms and would knock the molecule away Surface atomic profile Tip cruising altitude ~500 pm Δz = 2 pm Calculated image (Philippe Sautet) If the tip was made as big as an airplane, it would be flying at 1 cm from the surface and waving up an down by 1 micrometer The STM image is a map of the pi-orbital of distorted acetylene M. Salmeron (LBL)

  37. Tip e- ((( ) ( ))) Excitation of frustrated rotational modes in acetylene molecules on Pd(111) at T = 30 K M. Salmeron (LBL)

  38. + 2H2 RC CR' Hydrogenation of Alkynes cat • alkene is an intermediate RCH2CH2R' catalyst = Pt, Pd, Ni, or Rh

  39. 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'

  40. 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'

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

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

  43. 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'

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

  45. Question • How would you accomplish the following conversion? • A) NaNH2 • B) H2, Lindlar Pd • C) Na, NH3 • D) either B or C

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