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(9) Alkuna.ppt

  1. Chapter 9 Alkynes Organic Chemistry, 6th Edition L. G. Wade, Jr.
  2. Chapter 9 2 Introduction • Alkynes contain a triple bond. • General formula is CnH2n-2. • Two elements of unsaturation for each triple bond. • Some reactions are like alkenes: addition and oxidation. • Some reactions are specific to alkynes. =>
  3. Chapter 9 3 Nomenclature: IUPAC • Find the longest chain containing the triple bond. • Change -ane ending to -yne. • Number the chain, starting at the end closest to the triple bond. • Give branches or other substituents a number to locate their position. =>
  4. Chapter 9 4 Name these: CH3 CH CH3 CH2 C C CH CH3 CH3 CH3 C C CH2 CH2 Br CH3 C CH propyne 5-bromo-2-pentyne 5-bromopent-2-yne => 2,6-dimethyl-3-heptyne 2,6-dimethylpept-3-yne
  5. Chapter 9 5 Additional Functional Groups • All other functional groups, except ethers and halides have a higher priority than alkynes. • For a complete list of naming priorities, look inside the back cover of your text. =>
  6. Chapter 9 6 Examples CH2 CH CH2 CH CH3 C CH 4-methyl-1-hexen-5-yne 4-methylhex-1-en-5-yne CH3 C C CH2 CH OH CH3 4-hexyn-2-ol hex-4-yn-2-ol =>
  7. Chapter 9 7 Common Names Named as substituted acetylene. CH3 C CH methylacetylene (terminal alkyne) CH3 CH CH3 CH2 C C CH CH3 CH3 isobutylisopropylacetylene (internal alkyne) =>
  8. Chapter 9 8 Physical Properties • Nonpolar, insoluble in water. • Soluble in most organic solvents. • Boiling points similar to alkane of same size. • Less dense than water. • Up to 4 carbons, gas at room temperature. =>
  9. Chapter 9 9 Acetylene • Acetylene is used in welding torches. • In pure oxygen, temperature of flame reaches 2800C. • It would violently decompose to its elements, but the cylinder on the torch contains crushed firebrick wet with acetone to moderate it. =>
  10. Chapter 9 10 Synthesis of Acetylene • Heat coke with lime in an electric furnace to form calcium carbide. • Then drip water on the calcium carbide. H C C H Ca(OH)2 CaC2 + 2 H2O + C CaO 3 + + CaC2 CO coke lime *This reaction was used to produce light for miners’ lamps and for the stage. => *
  11. Chapter 9 11 Electronic Structure • The sigma bond is sp-sp overlap. • The two pi bonds are unhybridized p overlaps at 90, which blend into a cylindrical shape. =>
  12. Chapter 9 12 Bond Lengths • More s character, so shorter length. • Three bonding overlaps, so shorter. Bond angle is 180, so linear geometry. =>
  13. Chapter 9 13 Acidity of Alkynes • Terminal alkynes, R-CC-H, are more acidic than other hydrocarbons. • Acetylene  acetylide by NH2 -, but not by OH- or RO-. • More s character, so pair of electrons in anion is held more closely to the nucleus. Less charge separation, so more stable. =>
  14. Chapter 9 14 Acidity Table =>
  15. Chapter 9 15 Forming Acetylide Ions • H+ can be removed from a terminal alkyne by sodium amide, NaNH2. • NaNH2 is produced by the reaction of ammonia with sodium metal. =>
  16. Chapter 9 16 Alkynes from Acetylides • Acetylide ions are good nucleophiles. • SN2 reaction with 1 alkyl halides lengthens the alkyne chain. =>
  17. Chapter 9 17 Must be 1 • Acetylide ions can also remove H+ • If back-side approach is hindered, elimination reaction happens via E2. =>
  18. Chapter 9 18 Addition to Carbonyl Acetylide ion + carbonyl group yields an alkynol (alcohol on carbon adjacent to triple bond). + H2O O H H H R C C C O H => C O + R C C R C C C O
  19. Chapter 9 19 Add to Formaldehyde Product is a primary alcohol with one more carbon than the acetylide. + C O H H CH3 C C CH3 C C C H H O => + H2O O H H H CH3 C C C O H H H
  20. Chapter 9 20 Add to Aldehyde Product is a secondary alcohol, one R group from the acetylide ion, the other R group from the aldehyde. + C O CH3 H CH3 C C CH3 C C C CH3 H O => + H2O O H H H CH3 C C C O H CH3 H
  21. Chapter 9 21 Add to Ketone Product is a tertiary alcohol. + C O CH3 CH3 CH3 C C CH3 C C C CH3 CH3 O => + H2O O H H H CH3 C C C O H CH3 CH3
  22. Chapter 9 22 Synthesis by Elimination • Removal of two molecules of HX from a vicinal or geminal dihalide produces an alkyne. • First step (-HX) is easy, forms vinyl halide. • Second step, removal of HX from the vinyl halide requires very strong base and high temperatures. =>
  23. Chapter 9 23 Reagents for Elimination • Molten KOH or alcoholic KOH at 200C favors an internal alkyne. • Sodium amide, NaNH2, at 150C, followed by water, favors a terminal alkyne. CH3 C C CH2 CH3 200°C KOH (fused) CH3 CH CH CH2 CH3 Br Br => , 150°C CH3 CH2 C CH H2O 2) NaNH2 1) CH3 CH2 CH2 CHCl2
  24. Chapter 9 24 Migration of Triple Bond =>
  25. Chapter 9 25 Addition Reactions • Similar to addition to alkenes. • Pi bond becomes two sigma bonds. • Usually exothermic. • One or two molecules may add. =>
  26. Chapter 9 26 Addition of Hydrogen • Three reactions: • Add lots of H2 with metal catalyst (Pd, Pt, or Ni) to reduce alkyne to alkane, completely saturated. • Use a special catalyst, Lindlar’s catalyst, to convert an alkyne to a cis-alkene. • React the alkyne with sodium in liquid ammonia to form a trans-alkene. =>
  27. Chapter 9 27 Lindlar’s Catalyst • Powdered BaSO4 coated with Pd, poisoned with quinoline. • H2 adds syn, so cis-alkene is formed. =>
  28. Chapter 9 28 Na in Liquid Ammonia • Use dry ice to keep ammonia liquid. • As sodium metal dissolves in the ammonia, it loses an electron. • The electron is solvated by the ammonia, creating a deep blue solution. NH3 + Na + Na + NH3 e - =>
  29. Chapter 9 29 Mechanism Step 1: An electron adds to the alkyne, forming a radical anion Step 2: The radical anion is protonated to give a radical Step 3: An electron adds to the alkyne, forming an anion => Step 4: Protonation of the anion gives an alkene
  30. Chapter 9 30 Addition of Halogens • Cl2 and Br2 add to alkynes to form vinyl dihalides. • May add syn or anti, so product is mixture of cis and trans isomers. • Difficult to stop the reaction at dihalide. CH3 C C CH3 Br2 CH3 C Br C Br CH3 + CH3 C Br C CH3 Br Br2 CH3 C Br Br C Br Br CH3 =>
  31. Chapter 9 31 Addition of HX • HCl, HBr, and HI add to alkynes to form vinyl halides. • For terminal alkynes, Markovnikov product is formed. • If two moles of HX is added, product is a geminal dihalide. CH3 C C H CH3 C CH2 Br HBr HBr CH3 C CH3 Br Br =>
  32. Chapter 9 32 HBr with Peroxides Anti-Markovnikov product is formed with a terminal alkyne. HBr CH3 C C H H H Br Br ROOR => CH3 C C H CH3 C C H H Br HBr ROOR mixture of E and Z isomers
  33. Chapter 9 33 Hydration of Alkynes • Mercuric sulfate in aqueous sulfuric acid adds H-OH to one pi bond with a Markovnikov orientation, forming a vinyl alcohol (enol) that rearranges to a ketone. • Hydroboration-oxidation adds H-OH with an anti-Markovnikov orientation, and rearranges to an aldehyde. =>
  34. Chapter 9 34 Mechanism for Mercuration • Mercuric ion (Hg2+) is electrophile. • Vinyl carbocation forms on most-sub. C. • Water is the nucleophile. CH3 C C H CH3 C + C Hg + H Hg +2 H2O CH3 C H Hg + C O + H H H2O CH3 C H Hg + C OH H3O + CH3 C H H C OH an enol =>
  35. Chapter 9 35 Enol to Keto (in Acid) • Add H+ to the C=C double bond. • Remove H+ from OH of the enol. CH3 C C OH H H H H2O CH3 C C O H H H CH3 C H H C OH H3O + CH3 C C OH H H H A methyl ketone =>
  36. Chapter 9 36 Hydroboration Reagent • Di(secondary isoamyl)borane, called disiamylborane. • Bulky, branched reagent adds to the least hindered carbon. • Only one mole can add. => B CH CH H CH3 CH CH3 H3C H3C HC CH3 H3C
  37. Chapter 9 37 Hydroboration - Oxidation • B and H add across the triple bond. • Oxidation with basic H2O2 gives the enol. CH3 C C H CH3 C H C H BSia2 Sia2 BH CH3 C OH H C H H2O2 NaOH =>
  38. Chapter 9 38 Enol to Keto (in Base) • H+ is removed from OH of the enol. • Then water gives H+ to the adjacent carbon. CH3 C O H C H HOH CH3 C O H C H H OH CH3 C OH H C H CH3 C O H C H An aldehyde =>
  39. Chapter 9 39 Oxidation of Alkynes • Similar to oxidation of alkenes. • Dilute, neutral solution of KMnO4 oxidizes alkynes to a diketone. • Warm, basic KMnO4 cleaves the triple bond. • Ozonolysis, followed by hydrolysis, cleaves the triple bond. =>
  40. Chapter 9 40 Reaction with KMnO4 • Mild conditions, dilute, neutral • Harsher conditions, warm, basic CH3 C O C O CH2 CH3 H2O, neutral KMnO4 CH3 C C CH2 CH3 O C O CH2 CH3 CH3 C O O + H2O, warm , KOH KMnO4 CH3 C C CH2 CH3 =>
  41. Chapter 9 41 Ozonolysis • Ozonolysis of alkynes produces carboxylic acids (alkenes gave aldehydes and ketones). • Used to find location of triple bond in an unknown compound. => HO C O CH2 CH3 CH3 C O OH H2O (2) O3 (1) CH3 C C CH2 CH3 +
  42. Chapter 9 42 End of Chapter 9