Chapter 8 RX and Elimination Rxns
The E2 Mechanism
The E2 Mechanism
Energy Diagram for the E2 Mechanism
Effect of the Substrate on E2 Reactivity
Effect of the LG on E2 Reactivity
The E2 and SN2 mechanisms sharemany of the same characteristics
Here are four characteristics that theE2 / SN2 mechanisms have in common.
The E1Mechanism
Let ‘s look at an E1 reaction mechanismusing the rxn shown below.
The E1 Mechanism
The first step of the E1 and SN1mechanisms are identical.
The E1 Mechanism
Energy Diagram for E1
The E1 and SN1 mechanisms sharemany of the same characteristics
Here are four characteristics that the E1 / SN1mechanisms have in common.
What about dehydrohalogenationsinvolving RX with hydrogen atoms ondifferent β carbon atomsThe Zaitsev Rule
The Zaitsev Rule- A Regioselective Rxn
The Zaitsev Rule (Z-rule)According to the Z-rule, the major productin a dehydrohalgenation is the …..most stable product.
Let’s ask a different question about theproducts.
More R groups increase the stability ofthe T.S.
Let look at another example from theSmith text.
The Zaitsev (Saytzeff) Rule – A Stereoselective RXN
E2 Stereochemistry
The LG and the H atom can be syn oranti to each other
Bases Used in Elimination Rxns
Sterically Hindered Strong BasesKOC(CH3)3,LiN(CH(CH3)2)2 or LDA (lithium diisopropyl amide)
Exam II Review Problems
Predict the organic product(s) and namethe mechanism
Methylenecyclohexane is not the majorproduct in this rxn. Give the chemicalstructure of the major product and explainwhy i...
Alkyl Halides and Elimination Reactions• Elimination reactions involve the loss of elements fromthe starting material to f...
Alkyl Halides and Elimination Reactions• Equations [1] and [2] illustrate examples of eliminationreactions. In both reacti...
Alkyl Halides and Elimination Reactions• Removal of the elements HX is calleddehydrohalogenation.• Dehydrohalogenation is ...
Alkyl Halides and Elimination Reactions• The most common bases used in elimination reactionsare negatively charged oxygen ...
Alkyl Halides and Elimination Reactions• To draw any product of dehydrohalogenation—Find theα carbon. Identify all β carbo...
Alkyl Halides and Elimination Reactions• Recall that the double bond of an alkene consists of a σbond and a π bond.Alkenes...
Alkyl Halides and Elimination Reactions• Alkenes are classified according to the number ofcarbon atoms bonded to the carbo...
Alkyl Halides and Elimination Reactions• Recall that rotation about double bonds is restricted.Alkenes—The Products of Eli...
Alkyl Halides and Elimination Reactions• Because of restricted rotation, two stereoisomers of 2-butene are possible. cis-2...
Alkyl Halides and Elimination Reactions• Whenever the two groups on each end of a carbon-carbon double bond are different ...
Alkyl Halides and Elimination Reactions• In general, trans alkenes are more stable than cisalkenes because the groups bond...
Alkyl Halides and Elimination Reactions• The stability of an alkene increases as the number of Rgroups bonded to the doubl...
Alkyl Halides and Elimination Reactions• trans-2-Butene (a disubstituted alkene) is more stablethan cis-2-butene (another ...
Alkyl Halides and Elimination Reactions• There are two mechanisms of elimination—E2 and E1,just as there are two mechanism...
Alkyl Halides and Elimination Reactions• The most common mechanism for dehydrohalogenationis the E2 mechanism.• It exhibit...
The Zaitsev (Saytzeff) Rule
Zaitsev Rule in an E1 Type Reaction
Alkyl Halides and Elimination Reactions• There are close parallels between E2 and SN2 mechanisms inhow the identity of the...
Alkyl Halides and Elimination ReactionsMechanisms of Elimination—E2
Alkyl Halides and Elimination Reactions• The increase in E2 reaction rate with increasing alkylsubstitution can be rationa...
Alkyl Halides and Elimination Reactions• Increasing the number of R groups on the carbon with theleaving group forms more ...
Alkyl Halides and Elimination ReactionsMechanisms of Elimination—E2
Alkyl Halides and Elimination Reactions• Recall that when alkyl halides have two or more different βcarbons, more than one...
Alkyl Halides and Elimination Reactions• When a mixture of stereoisomers is possible from adehydrohalogenation, the major ...
Alkyl Halides and Elimination Reactions• The dehydrohalogenation of (CH3)3CI with H2O to form(CH3)C=CH2 can be used to ill...
Alkyl Halides and Elimination Reactions• The rate of an E1 reaction increases as the number of R groupson the carbon with ...
Alkyl Halides and Elimination ReactionsTable 8.3 summarizes the characteristics of the E1 mechanism.Mechanisms of Eliminat...
Alkyl Halides and Elimination Reactions• SN1 and E1 reactions have exactly the same first step—formationof a carbocation. ...
Alkyl Halides and Elimination Reactions• The transition state of an E2 reaction consists of four atoms froman alkyl halide...
Stereochemistry of the E2 Reaction
Alkyl Halides and Elimination Reactions• The stereochemical requirement of an anti periplanar geometryin an E2 reaction ha...
Alkyl Halides and Elimination ReactionsStereochemistry of the E2 Reaction
Alkyl Halides and Elimination Reactions• Now consider the E2 dehydrohalogenation of cis- and trans-1-chloro-2-methylcycloh...
Alkyl Halides and Elimination Reactions• Because conformer B has two different axial βhydrogens, labeled Ha and Hb, E2 rea...
Alkyl Halides and Elimination Reactions• The trans isomer of 1-chloro-2-methylcyclohexane existsas two conformers: C, havi...
Alkyl Halides and Elimination Reactions• Because conformer D has only one axial β H, E2 reactionoccurs only in one directi...
Alkyl Halides and Elimination Reactions• The strength of the base is the most important factor indetermining the mechanism...
Alkyl Halides and Elimination Reactions• A single elimination reaction produces a π bond of analkene. Two consecutive elim...
Alkyl Halides and Elimination Reactions• Two elimination reactions are needed to remove twomoles of HX from a dihalide sub...
Alkyl Halides and Elimination Reactions• Stronger bases are needed to synthesize alkynes bydehydrohalogenation than are ne...
Alkyl Halides and Elimination Reactions• The reason that stronger bases are needed for thisdehydrohalogenation is that the...
Alkyl Halides and Elimination ReactionsE2 Reactions and Alkyne Synthesis
Alkyl Halides and Elimination Reactions• Good nucleophiles that are weak bases favorsubstitution over elimination—Certain ...
Alkyl Halides and Elimination Reactions• Bulky nonnucleophilic bases favor elimination oversubstitution—KOC(CH3)3, DBU, an...
Predicting the Mechanism from the Reactants—SN1,SN2, E1 or E2.Alkyl Halides and Elimination Reactions
Predicting the Mechanism from the Reactants—SN1,SN2, E1 or E2.Alkyl Halides and Elimination Reactions
Stereochemistry of the E2 Reaction
Is it SN1, SN2, E1 or E2?
• acstaff.gpc.edu/~jpoteat/chapter8smith.ppt• 31 Maret 2010
Chapter8smith reaksi eliminasi
Chapter8smith reaksi eliminasi
Chapter8smith reaksi eliminasi
Chapter8smith reaksi eliminasi
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Chapter8smith reaksi eliminasi

  1. 1. Chapter 8 RX and Elimination Rxns
  2. 2. The E2 Mechanism
  3. 3. The E2 Mechanism
  4. 4. Energy Diagram for the E2 Mechanism
  5. 5. Effect of the Substrate on E2 Reactivity
  6. 6. Effect of the LG on E2 Reactivity
  7. 7. The E2 and SN2 mechanisms sharemany of the same characteristics
  8. 8. Here are four characteristics that theE2 / SN2 mechanisms have in common.
  9. 9. The E1Mechanism
  10. 10. Let ‘s look at an E1 reaction mechanismusing the rxn shown below.
  11. 11. The E1 Mechanism
  12. 12. The first step of the E1 and SN1mechanisms are identical.
  13. 13. The E1 Mechanism
  14. 14. Energy Diagram for E1
  15. 15. The E1 and SN1 mechanisms sharemany of the same characteristics
  16. 16. Here are four characteristics that the E1 / SN1mechanisms have in common.
  17. 17. What about dehydrohalogenationsinvolving RX with hydrogen atoms ondifferent β carbon atomsThe Zaitsev Rule
  18. 18. The Zaitsev Rule- A Regioselective Rxn
  19. 19. The Zaitsev Rule (Z-rule)According to the Z-rule, the major productin a dehydrohalgenation is the …..most stable product.
  20. 20. Let’s ask a different question about theproducts.
  21. 21. More R groups increase the stability ofthe T.S.
  22. 22. Let look at another example from theSmith text.
  23. 23. The Zaitsev (Saytzeff) Rule – A Stereoselective RXN
  24. 24. E2 Stereochemistry
  25. 25. The LG and the H atom can be syn oranti to each other
  26. 26. Bases Used in Elimination Rxns
  27. 27. Sterically Hindered Strong BasesKOC(CH3)3,LiN(CH(CH3)2)2 or LDA (lithium diisopropyl amide)
  28. 28. Exam II Review Problems
  29. 29. Predict the organic product(s) and namethe mechanism
  30. 30. Methylenecyclohexane is not the majorproduct in this rxn. Give the chemicalstructure of the major product and explainwhy it would be the major product.
  31. 31. Alkyl Halides and Elimination Reactions• Elimination reactions involve the loss of elements fromthe starting material to form a new π bond in theproduct.General Features of Elimination
  32. 32. Alkyl Halides and Elimination Reactions• Equations [1] and [2] illustrate examples of eliminationreactions. In both reactions a base removes theelements of an acid, HX, from the organic startingmaterial.General Features of Elimination
  33. 33. Alkyl Halides and Elimination Reactions• Removal of the elements HX is calleddehydrohalogenation.• Dehydrohalogenation is an example of β elimination.• The curved arrow formalism shown below illustrateshow four bonds are broken or formed in the process.General Features of Elimination
  34. 34. Alkyl Halides and Elimination Reactions• The most common bases used in elimination reactionsare negatively charged oxygen compounds, such as HO¯and its alkyl derivatives, RO¯, called alkoxides.General Features of Elimination
  35. 35. Alkyl Halides and Elimination Reactions• To draw any product of dehydrohalogenation—Find theα carbon. Identify all β carbons with H atoms. Removethe elements of H and X form the α and β carbons andform a π bond.General Features of Elimination
  36. 36. Alkyl Halides and Elimination Reactions• Recall that the double bond of an alkene consists of a σbond and a π bond.Alkenes—The Products of Elimination
  37. 37. Alkyl Halides and Elimination Reactions• Alkenes are classified according to the number ofcarbon atoms bonded to the carbons of the double bond.Alkenes—The Products of Elimination
  38. 38. Alkyl Halides and Elimination Reactions• Recall that rotation about double bonds is restricted.Alkenes—The Products of Elimination
  39. 39. Alkyl Halides and Elimination Reactions• Because of restricted rotation, two stereoisomers of 2-butene are possible. cis-2-Butene and trans-2-butene arediastereomers, because they are stereoisomers that arenot mirror images of each other.Alkenes—The Products of Elimination
  40. 40. Alkyl Halides and Elimination Reactions• Whenever the two groups on each end of a carbon-carbon double bond are different from each other, twodiastereomers are possible.Alkenes—The Products of Elimination
  41. 41. Alkyl Halides and Elimination Reactions• In general, trans alkenes are more stable than cisalkenes because the groups bonded to the double bondcarbons are further apart, reducing steric interactions.Alkenes—The Products of Elimination
  42. 42. Alkyl Halides and Elimination Reactions• The stability of an alkene increases as the number of Rgroups bonded to the double bond carbons increases.Alkenes—The Products of Elimination• The higher the percent s-character, the more readily an atomaccepts electron density. Thus, sp2carbons are more able toaccept electron density and sp3carbons are more able todonate electron density.• Consequently, increasing the number of electron donatinggroups on a carbon atom able to accept electron densitymakes the alkene more stable.
  43. 43. Alkyl Halides and Elimination Reactions• trans-2-Butene (a disubstituted alkene) is more stablethan cis-2-butene (another disubstituted alkene), butboth are more stable than 1-butene (a monosubstitutedalkene).Alkenes—The Products of Elimination
  44. 44. Alkyl Halides and Elimination Reactions• There are two mechanisms of elimination—E2 and E1,just as there are two mechanisms of substitution, SN2and SN1.• E2 mechanism—bimolecular elimination• E1 mechanism—unimolecular elimination• The E2 and E1 mechanisms differ in the timing of bondcleavage and bond formation, analogous to the SN2 andSN1 mechanisms.• E2 and SN2 reactions have some features in common, asdo E1 and SN1 reactions.Mechanisms of Elimination
  45. 45. Alkyl Halides and Elimination Reactions• The most common mechanism for dehydrohalogenationis the E2 mechanism.• It exhibits second-order kinetics, and both the alkylhalide and the base appear in the rate equation i.e.Mechanisms of Elimination—E2rate = k[(CH3)3CBr][¯OH]• The reaction is concerted—all bonds are broken andformed in a single step.
  46. 46. The Zaitsev (Saytzeff) Rule
  47. 47. Zaitsev Rule in an E1 Type Reaction
  48. 48. Alkyl Halides and Elimination Reactions• There are close parallels between E2 and SN2 mechanisms inhow the identity of the base, the leaving group and thesolvent affect the rate.• The base appears in the rate equation, so the rate of the E2reaction increases as the strength of the base increases.• E2 reactions are generally run with strong, negatively chargedbases like¯OH and ¯OR. Two strong sterically hinderednitrogen bases called DBN and DBU are also sometimes used.Mechanisms of Elimination—E2
  49. 49. Alkyl Halides and Elimination ReactionsMechanisms of Elimination—E2
  50. 50. Alkyl Halides and Elimination Reactions• The increase in E2 reaction rate with increasing alkylsubstitution can be rationalized in terms of transition statestability.• In the transition state, the double bond is partially formed.Thus, increasing the stability of the double bond with alkylsubstituents stabilizes the transition state (i.e. lowers Ea,which increases the rate of the reaction according to theHammond postulate).Mechanisms of Elimination—E2
  51. 51. Alkyl Halides and Elimination Reactions• Increasing the number of R groups on the carbon with theleaving group forms more highly substituted, more stablealkenes in E2 reactions.• In the reactions below, since the disubstituted alkene is morestable, the 30alkyl halide reacts faster than the 10alkyl halide.Mechanisms of Elimination—E2
  52. 52. Alkyl Halides and Elimination ReactionsMechanisms of Elimination—E2
  53. 53. Alkyl Halides and Elimination Reactions• Recall that when alkyl halides have two or more different βcarbons, more than one alkene product is formed.• When this happens, one of the products usuallypredominates.• The major product is the more stable product—the one withthe more substituted double bond.• This phenomenon is called the Zaitsev rule.The Zaitsev (Saytzeff) Rule
  54. 54. Alkyl Halides and Elimination Reactions• When a mixture of stereoisomers is possible from adehydrohalogenation, the major product is the more stablestereoisomer.• A reaction is stereoselective when it forms predominantly orexclusively one stereoisomer when two or more are possible.• The E2 reaction is stereoselective because one stereoisomeris formed preferentially.The Zaitsev (Saytzeff) Rule
  55. 55. Alkyl Halides and Elimination Reactions• The dehydrohalogenation of (CH3)3CI with H2O to form(CH3)C=CH2 can be used to illustrate the second generalmechanism of elimination, the E1 mechanism.• An E1 reaction exhibits first-order kinetics:Mechanisms of Elimination—E1rate = k[(CH3)3CI]• The E1 reaction proceed via a two-step mechanism: the bondto the leaving group breaks first before the π bond is formed.The slow step is unimolecular, involving only the alkyl halide.• The E1 and E2 mechanisms both involve the same number ofbonds broken and formed. The only difference is timing. In anE1, the leaving group comes off before the β proton isremoved, and the reaction occurs in two steps. In an E2reaction, the leaving group comes off as the β proton isremoved, and the reaction occurs in one step.
  56. 56. Alkyl Halides and Elimination Reactions• The rate of an E1 reaction increases as the number of R groupson the carbon with the leaving group increases.Mechanisms of Elimination—E1• The strength of the base usually determines whether a reactionfollows the E1 or E2 mechanism. Strong bases like ¯OH and ¯ORfavor E2 reactions, whereas weaker bases like H2O and ROHfavor E1 reactions.
  57. 57. Alkyl Halides and Elimination ReactionsTable 8.3 summarizes the characteristics of the E1 mechanism.Mechanisms of Elimination—E1
  58. 58. Alkyl Halides and Elimination Reactions• SN1 and E1 reactions have exactly the same first step—formationof a carbocation. They differ in what happens to the carbocation.SN1 and E1 Reactions• Because E1 reactions often occur with a competing SN1 reaction,E1 reactions of alkyl halides are much less useful than E2reactions.
  59. 59. Alkyl Halides and Elimination Reactions• The transition state of an E2 reaction consists of four atoms froman alkyl halide—one hydrogen atom, two carbon atoms, and theleaving group (X)—all aligned in a plane. There are two ways forthe C—H and C—X bonds to be coplanar.Stereochemistry of the E2 Reaction• E2 elimination occurs most often in the anti periplanar geometry.This arrangement allows the molecule to react in the lowerenergy staggered conformation, and allows the incoming baseand leaving group to be further away from each other.
  60. 60. Stereochemistry of the E2 Reaction
  61. 61. Alkyl Halides and Elimination Reactions• The stereochemical requirement of an anti periplanar geometryin an E2 reaction has important consequences for compoundscontaining six-membered rings.• Consider chlorocyclohexane which exists as two chairconformers. Conformer A is preferred since the bulkier Cl groupis in the equatorial position.Stereochemistry of the E2 Reaction• For E2 elimination, the C-Cl bond must be anti periplanar to theC—H bond on a β carbon, and this occurs only when the H andCl atoms are both in the axial position. The requirement for transdiaxial geometry means that elimination must occur from theless stable conformer, B.
  62. 62. Alkyl Halides and Elimination ReactionsStereochemistry of the E2 Reaction
  63. 63. Alkyl Halides and Elimination Reactions• Now consider the E2 dehydrohalogenation of cis- and trans-1-chloro-2-methylcyclohexane.Stereochemistry of the E2 Reaction• This cis isomer exists as two conformers, A and B, each ofwhich as one group axial and one group equatorial. E2 reactionmust occur from conformer B, which contains an axial Cl atom.
  64. 64. Alkyl Halides and Elimination Reactions• Because conformer B has two different axial βhydrogens, labeled Ha and Hb, E2 reaction occurs in twodifferent directions to afford two alkenes.• The major product contains the more stabletrisubstituted double bond, as predicted by the Zaitsevrule.Stereochemistry of the E2 Reaction
  65. 65. Alkyl Halides and Elimination Reactions• The trans isomer of 1-chloro-2-methylcyclohexane existsas two conformers: C, having two equatorial substituents,and D, having two axial substituents.Stereochemistry of the E2 Reaction• E2 reaction must occur from D, since it contains an axial Clatom.
  66. 66. Alkyl Halides and Elimination Reactions• Because conformer D has only one axial β H, E2 reactionoccurs only in one direction to afford a single product. Thisis not predicted by the Zaitzev rule.Stereochemistry of the E2 Reaction
  67. 67. Alkyl Halides and Elimination Reactions• The strength of the base is the most important factor indetermining the mechanism for elimination. Strongbases favor the E2 mechanism. Weak bases favor the E1mechanism.When is the Mechanism E1 or E2
  68. 68. Alkyl Halides and Elimination Reactions• A single elimination reaction produces a π bond of analkene. Two consecutive elimination reactions producetwo π bonds of an alkyne.E2 Reactions and Alkyne Synthesis
  69. 69. Alkyl Halides and Elimination Reactions• Two elimination reactions are needed to remove twomoles of HX from a dihalide substrate.• Two different starting materials can be used—a vicinaldihalide or a geminal dihalide.E2 Reactions and Alkyne Synthesis
  70. 70. Alkyl Halides and Elimination Reactions• Stronger bases are needed to synthesize alkynes bydehydrohalogenation than are needed to synthesizealkenes.• The typical base used is ¯NH2 (amide), used as thesodium salt of NaNH2. KOC(CH3)3 can also be used withDMSO as solvent.E2 Reactions and Alkyne Synthesis
  71. 71. Alkyl Halides and Elimination Reactions• The reason that stronger bases are needed for thisdehydrohalogenation is that the transition state for thesecond elimination reaction includes partial cleavage ofthe C—H bond. In this case however, the carbon atom issp2hybridized and sp2hybridized C—H bonds arestronger than sp3hybridized C—H bonds. As a result, astronger base is needed to cleave this bond.E2 Reactions and Alkyne Synthesis
  72. 72. Alkyl Halides and Elimination ReactionsE2 Reactions and Alkyne Synthesis
  73. 73. Alkyl Halides and Elimination Reactions• Good nucleophiles that are weak bases favorsubstitution over elimination—Certain anions alwaysgive products of substitution because they are goodnucleophiles but weak bases. These include I¯, Br¯, HS¯,and CH3COO¯.Predicting the Mechanism from the Reactants—SN1,SN2, E1 or E2.
  74. 74. Alkyl Halides and Elimination Reactions• Bulky nonnucleophilic bases favor elimination oversubstitution—KOC(CH3)3, DBU, and DBN are toosterically hindered to attack tetravalent carbon, but areable to remove a small proton, favoring elimination oversubstitution.Predicting the Mechanism from the Reactants—SN1,SN2, E1 or E2.
  75. 75. Predicting the Mechanism from the Reactants—SN1,SN2, E1 or E2.Alkyl Halides and Elimination Reactions
  76. 76. Predicting the Mechanism from the Reactants—SN1,SN2, E1 or E2.Alkyl Halides and Elimination Reactions
  77. 77. Stereochemistry of the E2 Reaction
  78. 78. Is it SN1, SN2, E1 or E2?
  79. 79. • acstaff.gpc.edu/~jpoteat/chapter8smith.ppt• 31 Maret 2010

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