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Chapter 3 Alkenes and Alkynes


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Chapter 3 Alkenes and Alkynes

  1. 1. Chapter 3 Alkenes & Alkynes Elfi Susanti VH SBI CLASS Chemistry Department FKIP UNS
  2. 2. Slide 9 of 50
  3. 3. 2-etil-1-pentena etil- 6,6-dimetil- 6,6-dimetil-3-heptena 2-metil-1,3-sikloheksadiena metil-1,3- Bukan 1-metil-1,5-sikloheksadiena 1-metil-1,5-
  4. 4. Z-1,3-dikloro-2-metil-2-butena 1,3-dikloro- metil-
  5. 5. E-3-metil-1,3-pentadiena metil-1,3-
  6. 6. Reaction of alkene
  7. 7. Alkene Addition Reactions:
  8. 8. 4.1 4 1 addition of HX to alkenes Hydrohalogenation
  9. 9. 4.2 Orientation of Alkene Addition Reaction (1) Regiospecific Rxn. (2) Markovnikov observed in the 19th century that in the addition of HX to alkene, the H attaches to the carbon with the most H’s and X attaches to the other end (to the one with the most alkyl substituents) • This is Markovnikov’s rule
  10. 10. Example of Markovnikov’s Rule Addition of HCl to 2-methylpropene Regiospecific If both ends have similar substitution, then not regiospecific
  11. 11. Example of Markovnikov’s Rule
  12. 12. Example of Markovnikov’s Rule
  13. 13. Example of Markovnikov’s Rule Mixture products
  14. 14. Energy of Carbocations and Markovnikov’s Rule More stable carbocation forms faster Tertiary cations and associated transition states are more stable than primary cations
  15. 15. Mechanistic Source of Regiospecificity in Addition Reactions If addition involves a carbocation intermediate and th d there are two t possible ways to add the route producing the more alkyl substituted cationic center is lower in energy alkyl groups stabilize y g p carbocation
  16. 16. The basis of Markovnikov’s rule
  17. 17. Problem
  18. 18. 4.3 Carbocation Structure & Sability Electronic structure of carbocation Stability:
  19. 19. Problem
  20. 20. 4.4 Addition of H2O Hydration of an alkene is the addition of H-OH to H OH to give an alcohol Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol
  21. 21. Mechanism of the acid-catalyzed hydration
  22. 22. 4.5 Addition of X2 Halogenation
  23. 23. The stereochemistry of the addition reaction of Br2→anti stereochemistry Fig 4-1 2 3 4 1,2,3
  24. 24. Mechanism
  25. 25. 4.6 Addition of H2 Hydrogenation Syn. stereochemistry S
  26. 26. mechanism of alkene hydrogenation
  27. 27. 4.7 Oxidation of Alkenes: Hydroxylation and Cleavage Cl Hydroxylation: in basic solution addition solution, of one or more –OH group to a molecule.
  28. 28. Cleavage: in acidic solution No H on C: C=O; O N C C O; One H on C: COOH; t C=O C COOH; two Hs H on C: CO2
  29. 29. Alkene Cleavage: Ozone Ozone, O3, adds to alkenes to form molozonide Reduce molozonide t obtain k t R d l id to bt i ketones and/or d/ aldehydes
  30. 30. Examples of Ozonolysis of Alkenes
  31. 31. Examples of Ozonolysis of Alkenes Cleavage products reveal an alkene’s alkene s structure
  32. 32. 4.8 Alkene polymers A polymer is a very large molecule consisting of repeating units of simpler molecules, f ti it f i l l l formed by polymerization Alkenes react with radical catalysts to undergo radical polymerization Ethylene is polymerized to poyethylene, for poyethylene example
  33. 33. Free Radical Polymerization: Initiation Initiation - a few radicals are generated by the reaction of a molecule that readily forms radicals from a nonradical molecule A bond is broken homolytically
  34. 34. Polymerization: Propagation Radical from intiation adds to alkene to generate alkene derived radical This radical adds to another alkene, and so s ad ca a ot e a e e, a d on many times
  35. 35. Polymerization: Termination Chain propagation ends when two radical chains combine Not controlled specifically but affected by ot co t o ed spec ca y a ected reactivity and concentration
  36. 36. PRACTICE PROBLEM 4.5
  37. 37. 4.9 Conjugate Dienes Conjugated dienes: they are more than one double & separated by only one single bond and their t db l i l b d d th i orbitals interact H2C=CH—CH=CH2 H2C=CH—CH2—CH=CH2 1,3-Butadiene 1,4-Pentadien (conjugated;alternating (nonconjugated; nonalterinating double and single bonds) double and single bonds) Conjugated dienes are somewhat more stable than nonconjugated dienes
  38. 38. 1. electrophilic addition
  39. 39. 2. 1,2-addition & 1,4-addition
  40. 40. Carbocations from Conjugated Dienes j g Addition of H+ leads to delocalized secondary allylic carbocation
  41. 41. 4.10 Stability y why are conjugated dienes so stable? orbital hybridization p orbital overlap
  42. 42. Stability of Allylic Carbocation Resonance eso a ce
  43. 43. The Diels-Alder Cycloaddition Reaction Conjugate dienes can combine with alkenes to form six-membered six membered cyclic compounds The formation of the ring involves no intermediate ( (concerted formation of two bonds) ) Discovered by Otto Paul Hermann Diels and Kurt Alder in Germany in the 1930’s
  44. 44. Diels-Alder cycloaddition
  45. 45. 4.11 Drawing and Interpreting Resonance Forms
  46. 46. Resonance forms are imaginary g y Resonance forms differ only in the placement of their π or non-bonding electrons.
  47. 47. ■ Different resonance forms of a substance don’t have to be equivalent equivalent.
  48. 48. ■ Resonance forms must be valid Lewis structures and obey normal rules of valency valency. ■ Resonance leads to stability stability.
  49. 49. 4.12 Alkynes’ Reactions Naming Alkynes: General hydrocarbon rules apply wuith “- yne yne” as a suffix indicating an alkyne
  50. 50. 1. Reduction of Alkynes Addition of H2 using chemically deactivated palladium on calcium carbonate as a catalyst (the Lindlar catalyst) produces a cis alkene The two hydrogens add syn (from the same side of y g y ( the triple bond)
  51. 51. 2. Reactions of Alkynes: Addition of HX and X2 Addition reactions of alkynes are similar to those of alkenes th f lk Intermediate alkene reacts further with excess eagent e cess reagent Regiospecificity according to Markovnikov
  52. 52. Addition of Bromine and Chlorine Initial addition gives trans intermediate Product with excess reagent is tetrahalide
  53. 53. 3. Hydration of Alkynes Alkynes do not react with aqueous protic acids Mercuric ion (as the sulfate) is a Lewis acid catalyst that promotes addition of water in Markovnikov orientation The immediate product is a vinylic alcohol, or enol, which spontaneously transforms to a ketone
  54. 54. Keto-enol Enols rearrange to the isomeric ketone by the rapid transfer of a proton from the hydroxyl to the alkene carbon The keto form is usually so stable compared to the enol that only the keto form can be observed
  55. 55. Hydration of Unsymmetrical Alkynes If the alkyl groups at either end of the C-C triple bond are not the same, both products can form and this is same not normally useful If the triple bond is at the first carbon of the chain (then H is what is attached to one side) this is called a terminal alkyne Hydration of a terminal always gives the methyl ketone, which is useful
  56. 56. 4. Oxidative Cleavage of Alkynes Strong oxidizing reagents (O3 or KMnO4) cleave internal alkynes, producing t i t l lk d i two carboxylic acids b li id Terminal alkynes are oxidized to a carboxylic acid and carbon dioxide Neither process is useful in modern synthesis – were used to elucidate structures because the products indicate the structure of the alkyne precursor i di h f h lk
  57. 57. 5. Alkyne Acidity: Formation of Acetylide Anions Terminal alkynes are weak Brønsted acids y (alkenes and alkanes are much less acidic (pKa ~ 25) Reaction of strong anhydrous bases with a terminal acetylene produces an acetylide ion
  58. 58. Alkylation of Acetylide Anions Acetylide ions can react as nucleophiles as well as bases Reaction with a primary alkyl halide p y y produces a hydrocarbon that contains carbons from both partners, providing a general route to larger alkynes