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5.6 Relative Stabilities of Alkenes PowerPoint Presentation
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5.6 Relative Stabilities of Alkenes

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  1. 5.6 Relative Stabilities of Alkenes

  2. H R C C H H H R C C C C C C R' H disubstituted Double bonds are classified according tothe number of carbons attached to them. monosubstituted R' H R R R' H H H disubstituted disubstituted

  3. R" R" R R C C C C R' H R' R"' trisubstituted tetrasubstituted Double bonds are classified according tothe number of carbons attached to them.

  4. Substituent effects on alkene stability Electronic disubstituted alkenes are more stable than monosubstituted alkenes Steric trans alkenes are more stable than cis alkenes

  5. Fig. 5.4 Heats of combustion of C4H8isomers. 2717 kJ/mol + 6O2 2710 kJ/mol 2707 kJ/mol 2700 kJ/mol 4CO2 + 8H2O

  6. Substituent effects on alkene stability Electronic alkyl groups stabilize double bonds more than H more highly substituted double bonds are morestable than less highly substituted ones.

  7. C C Problem 5.8 Give the structure or make a molecular model of the most stable C6H12 alkene.

  8. C C Problem 5.8 Give the structure or make a molecular model of the most stable C6H12 alkene. H3C CH3 H3C CH3

  9. Substituent effects on alkene stability Steric effects trans alkenes are more stable than cis alkenes cis alkenes are destabilized by van der Waalsstrain

  10. van der Waals straindue to crowding ofcis-methyl groups Figure 5.5 cis and trans-2-Butene cis-2-butene trans-2-butene

  11. Fig. 5.5cis and trans-2-butene van der Waals straindue to crowding ofcis-methyl groups cis-2-butene trans-2-butene

  12. H3C CH3 CH3 H3C CH3 H3C C C C C H H Van der Waals Strain Steric effect causes a large difference in stabilitybetween cis and trans-(CH3)3CCH=CHC(CH3)3 cis is 44 kJ/mol less stable than trans

  13. 5.7Cycloalkenes

  14. Cycloalkenes Cyclopropene and cyclobutene have angle strain. Larger cycloalkenes, such as cyclopenteneand cyclohexene, can incorporate a double bond into the ring with little or no angle strain.

  15. H H Stereoisomeric cycloalkenes cis-cyclooctene and trans-cycloocteneare stereoisomers cis-cyclooctene is 39 kJ/ mol more stablethan trans-cyclooctene H H cis-Cyclooctene trans-Cyclooctene

  16. H H Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability trans-Cyclooctene

  17. Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability cis-Cyclododecene trans-Cyclododecene

  18. Stereoisomeric cycloalkenes trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability When there are more than 12 carbons in thering, trans-cycloalkenes are more stable than cis.The ring is large enough so the cycloalkene behavesmuch like a noncyclic one.

  19. 5.8 Preparation of Alkenes:Elimination Reactions

  20. H Y C C C C -Elimination Reactions Overview • dehydrogenation of alkanes: H; Y = H • dehydration of alcohols:H; Y = OH • dehydrohalogenation of alkyl halides:H; Y = Br, etc. + Y H  

  21. + H2 H2C CH2 + H2 H2C CHCH3 Dehydrogenation • limited to industrial syntheses of ethylene, propene, 1,3-butadiene, and styrene • important economically, but rarely used in laboratory-scale syntheses 750°C CH3CH3 750°C CH3CH2CH3

  22. 5.9Dehydration of Alcohols

  23. H2C CH2 OH CH3 H3C H2SO4 CH2 C OH H3C C heat H3C CH3 Dehydration of Alcohols H2SO4 + H2O CH3CH2OH 160°C H2SO4 + H2O 140°C (79-87%) + H2O (82%)

  24. R' tertiary:most reactive R OH C R" R' OH C R H H OH C R primary:least reactive H Relative Reactivity

  25. 5.10Regioselectivity in Alcohol Dehydration:The Zaitsev Rule

  26. H2SO4 HO 80°C Regioselectivity • A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective. + 90 % 10 %

  27. CH3 CH3 CH3 H3PO4 + OH heat 16 % 84 % Regioselectivity • A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective.

  28. OH R C R CH2R C CH3 H The Zaitsev Rule • When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the  carbon having thefewest hydrogens. three protons on this  carbon

  29. OH R C R CH2R C CH3 H The Zaitsev Rule • When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the  carbon having thefewest hydrogens. two protons on this  carbon

  30. OH R C R CH2R C CH3 H The Zaitsev Rule • When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the  carbon having thefewest hydrogens. only one proton on this  carbon

  31. CH2R R OH R C C C R CH2R C CH3 R CH3 H The Zaitsev Rule • When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the  carbon having thefewest hydrogens. only one proton on this  carbon

  32. The Zaitsev Rule • Zaitsev Rule states that the elimination • reaction yields the more highly substituted • alkene as the major product. • The more stable alkene product predominates.

  33. 5.11StereoselectivityinAlcohol Dehydration

  34. H2SO4 + heat OH (25%) (75%) Stereoselectivity • A stereoselective reaction is one in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amounts than any other.

  35. 5.12The Mechanism of the Acid-Catalyzed Dehydration of Alcohols

  36. A connecting point... • The dehydration of alcohols and the reaction of alcohols with hydrogen halides share thefollowing common features: • 1) Both reactions are promoted by acids • 2) The relative reactivity decreases in the order tertiary > secondary > primary These similarities suggest that carbocationsare intermediates in the acid-catalyzeddehydration of alcohols, just as they are inthe reaction of alcohols with hydrogen halides.

  37. CH3 H3C H2SO4 CH2 C OH H3C C heat H3C CH3 Dehydration of tert-Butyl Alcohol • first two steps of mechanism are identical tothose for the reaction of tert-butyl alcohol withhydrogen halides + H2O

  38. H H O .. H H H : : O (CH3)3C H Mechanism Step 1: Proton transfer to tert-butyl alcohol .. + + : O (CH3)3C H fast, bimolecular + + : O H tert-Butyloxonium ion

  39. H + : O (CH3)3C H H : : O H Mechanism Step 2: Dissociation of tert-butyloxonium ion to carbocation slow, unimolecular + + (CH3)3C tert-Butyl cation

  40. H : : H O H3C + CH2 H C H3C H H3C CH2 : C O H3C H Mechanism Step 3: Deprotonation of tert-butyl cation. + fast, bimolecular + + H

  41. Carbocations are intermediates in the acid-catalyzeddehydration of tertiary and secondary alcohols Carbocations can: • react with nucleophiles • lose a -proton to form an alkene(Called an E1 mechanism)

  42. H2C CH2 Dehydration of primary alcohols H2SO4 + H2O CH3CH2OH • A different mechanism from 3 o or 2 o alcohols • avoids carbocation because primarycarbocations are too unstable • oxonium ion loses water and a proton in abimolecular step 160°C

  43. H H O .. H fast, bimolecular H H + + : : O : CH3CH2 O H H Ethyloxonium ion Mechanism Step 1: Proton transfer from acid to ethanol .. + : CH3CH2 O H

  44. H H + : O CH2 CH2 : : H O H H slow, bimolecular H H + : + O + : : H O H2C CH2 H H Mechanism Step 2: Oxonium ion loses both a proton and a water molecule in the same step. +

  45. Step 2: Oxonium ion loses both a proton and a water molecule in the same step. H H + : + O CH2 CH2 : : H O H H slow, bimolecular H H + : + O + : : H O H2C CH2 H H Mechanism Because rate-determiningstep is bimolecular, thisis called the E2 mechanism.

  46. Sometimes the alkene product does not have the same carbon skeleton as the starting alcohol. 5.13Rearrangements in Alcohol Dehydration

  47. Example OH H3PO4, heat + + 3% 33% 64%

  48. CH3 CHCH3 CH3 C + CH3 Rearrangement involves alkyl group migration • carbocation can lose a proton as shown • or it can undergo a methyl migration • CH3 group migrates with its pair of electrons to adjacent positively charged carbon 3%

  49. CH3 CH3 97% CHCH3 CH3 CHCH3 C C + CH3 Rearrangement involves alkyl group migration + • tertiary carbocation; more stable CH3 CH3 3%

  50. CH3 CH3 97% CHCH3 CH3 CHCH3 C C + CH3 Rearrangement involves alkyl group migration + CH3 CH3 3%