Kompetisi sn 1, sn-2, e-1, dan e-2

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Kompetisi sn 1, sn-2, e-1, dan e-2

  1. 1. Substitution and EliminationReaction of Alkyl Halides
  2. 2. -Organic compounds with anelectronegative atom or anelectron-withdrawing groupbonded to a sp3carbonundergo substitution orelimination reactionsOrganic compounds with anelectronegative atom or anelectron-withdrawing groupbonded to a sp3carbonundergo substitution orelimination reactionsSubstitutionEliminationHalide ions are good leavinggroups. Substitution reactionon these compounds are easyand are used to get a widevariety of compoundsHalide ions are good leavinggroups. Substitution reactionon these compounds are easyand are used to get a widevariety of compoundsalkyl fluoride alkyl chloride alkyl bromide alkyl iodide
  3. 3. Alkyl Halides in NatureSynthesized by red algaeSynthesized by sea harea sea harered algae
  4. 4. sea hare
  5. 5. Alkyl Halides in Nature• Several marine organisms, including sponges,corals, and algae, synthesize organohalides(halogen-containing organic compounds) thatthey use to deterpredators. For example, redalgae synthesize a toxic,foultastingorganohalide that keeps predatorsfrom eating them. One predator, however,that is not deterred is a mollusk called a seahare.
  6. 6. Alkyl Halides in Nature• After consuming red algae, a sea hareconverts the original organohalide into astructurally similar compound it uses for itsown defense. Unlike other mollusks, a seahare doesnot have a shell. Its method ofdefense is to surround itself with a slimymaterial that contains the organohalide,thereby protecting itself from carnivorousfish.
  7. 7. Substitution Reaction with HalidesIf concentration of (1) isdoubled, the rate of thereaction is doubled.If concentration of (1) isdoubled, the rate of thereaction is doubled.bromomethane(1)(2)If concentration of (2) isdoubled, the rate of thereaction is doubled.If concentration of (2) isdoubled, the rate of thereaction is doubled.If concentration of (1) and(2) is doubled, the rate ofthe reaction quadruples.If concentration of (1) and(2) is doubled, the rate ofthe reaction quadruples.methanol
  8. 8. Substitution Reaction with Halidesbromomethane(1)(2)methanolRate law:rate = k [bromoethane][OH-]this reaction is an example of a SN2 reaction.S stands for substitutionN stands for nucleophilic2 stands for bimolecularRate law:rate = k [bromoethane][OH-]this reaction is an example of a SN2 reaction.S stands for substitutionN stands for nucleophilic2 stands for bimolecular
  9. 9. Mechanism of SN2 ReactionsThe rate of reaction depends on theconcentrations of both reactants.The rate of reaction depends on theconcentrations of both reactants.When the hydrogens of bromomethaneare replaced with methyl groups thereaction rate slow down.When the hydrogens of bromomethaneare replaced with methyl groups thereaction rate slow down.The reaction of an alkyl halide in whichthe halogen is bonded to an asymetriccenter leads to the formation of onlyone stereoisomerThe reaction of an alkyl halide in whichthe halogen is bonded to an asymetriccenter leads to the formation of onlyone stereoisomerAlkyl halide Relative rate1200401≈ 0
  10. 10. Mechanism of SN2 ReactionsHughes and Ingold proposed the following mechanism:Hughes and Ingold proposed the following mechanism:Transition stateIncreasing the concentration of either of thereactant makes their collision more probable.Increasing the concentration of either of thereactant makes their collision more probable.
  11. 11. Mechanism of SN2 Reactionsactivationenergy: ∆G1activationenergy: ∆G2Steric effectSteric effectInversion of configurationInversion of configuration(R)-2-bromobutane (S)-2-butanolEnergyreaction coordinate reaction coordinate
  12. 12. Factor Affecting SN2 Reactionsrelative rates of reaction pKa HXHO-+ RCH2I RCH2OH + I-30 000 -10HO-+ RCH2Br RCH2OH + Br-10 000 -9HO-+ RCH2Cl RCH2OH + Cl-200 -7HO-+ RCH2F RCH2OH + F-1 3.2relative rates of reaction pKa HXHO-+ RCH2I RCH2OH + I-30 000 -10HO-+ RCH2Br RCH2OH + Br-10 000 -9HO-+ RCH2Cl RCH2OH + Cl-200 -7HO-+ RCH2F RCH2OH + F-1 3.2The leaving groupThe nucleophileIn general, for halogen substitution thestrongest the base the better thenucleophile.In general, for halogen substitution thestrongest the base the better thenucleophile.pKa Nuclephilicity
  13. 13. SN2 Reactions With Alkyl Halidesan alcohola thiolan ethera thioetheran aminean alkynea nitrile
  14. 14. Substitution Reactions With HalidesIf concentration of (1) isdoubled, the rate of thereaction is doubled.If concentration of (1) isdoubled, the rate of thereaction is doubled.If concentration of (2) isdoubled, the rate of thereaction is not doubled.If concentration of (2) isdoubled, the rate of thereaction is not doubled.Rate law:rate = k [1-bromo-1,1-dimethylethane]this reaction is an example of a SN1reaction.S stands for substitutionN stands for nucleophilic1 stands for unimolecularRate law:rate = k [1-bromo-1,1-dimethylethane]this reaction is an example of a SN1reaction.S stands for substitutionN stands for nucleophilic1 stands for unimolecular1-bromo-1,1-dimethylethane 1,1-dimethylethanol
  15. 15. Mechanism of SN1 ReactionsThe rate of reaction depends on theconcentrations of the alkyl halide only.The rate of reaction depends on theconcentrations of the alkyl halide only.When the methyl groups of 1-bromo-1,1-dimethylethane are replaced withhydrogens the reaction rate slow down.When the methyl groups of 1-bromo-1,1-dimethylethane are replaced withhydrogens the reaction rate slow down.The reaction of an alkyl halide in whichthe halogen is bonded to an asymetriccenter leads to the formation of twostereoisomersThe reaction of an alkyl halide in whichthe halogen is bonded to an asymetriccenter leads to the formation of twostereoisomersAlkyl halide Relative rate≈ 0 *≈ 0 *121 200 000* a small rate is actually observed as a result of a SN2
  16. 16. Mechanism of SN1 ReactionsC-Br bond breaksC-Br bond breaksnucleophile attacks thecarbocationnucleophile attacks thecarbocationProton dissociationProton dissociationslowfast
  17. 17. Mechanism of SN1 Reactions∆GRate determining stepRate determining stepCarbocationintermediateCarbocationintermediateR++ X-R-OH2+R-OH
  18. 18. Mechanism of SN1 ReactionsSame configurationas the alkyl halideSame configurationas the alkyl halideInvertedconfigurationrelative the alkylhalideInvertedconfigurationrelative the alkylhalide
  19. 19. Factor Affecting SN1 reactionTwo factors affect the rate of a SN1 reaction:• The ease with which the leaving group dissociate from the carbon• The stability of the carbocationTwo factors affect the rate of a SN1 reaction:• The ease with which the leaving group dissociate from the carbon• The stability of the carbocationThe more the substituted thecarbocation is, the morestable it is and therefore theeasier it is to form.The more the substituted thecarbocation is, the morestable it is and therefore theeasier it is to form.As in the case of SN2, theweaker base is the leavinggroup, the less tightly it isbonded to the carbon and theeasier it is to break the bondAs in the case of SN2, theweaker base is the leavinggroup, the less tightly it isbonded to the carbon and theeasier it is to break the bondThe reactivity of thenucleophile has no effect onthe rate of a SN1 reactionThe reactivity of thenucleophile has no effect onthe rate of a SN1 reaction
  20. 20. Comparison SN1 – SN2SN1 SN2A two-step mechanism A one-step mechanismA unimolecular rate-determining step A bimolecular rate-determining stepProducts have both retained and invertedconfiguration relative to the reactantProduct has inverted configurationrelative to the reactantReactivity order:3o> 2o> 1o> methylReactivity order:methyl > 1o>2o> 3o
  21. 21. Elimination Reactions1-bromo-1,1-dimethylethane 2-methylpropeneRate law:rate = k [1-bromo-1,1-dimethylethane][OH-]this reaction is an example of a E2 reaction.E stands for elimination2 stands for bimolecularRate law:rate = k [1-bromo-1,1-dimethylethane][OH-]this reaction is an example of a E2 reaction.E stands for elimination2 stands for bimolecular
  22. 22. The E2 ReactionA proton isremovedA proton isremovedBr-is eliminatedBr-is eliminatedThe mechanism shows that an E2reaction is a one-step reactionThe mechanism shows that an E2reaction is a one-step reaction
  23. 23. Elimination ReactionsIf concentration of (1) isdoubled, the rate of thereaction is doubled.If concentration of (1) isdoubled, the rate of thereaction is doubled.If concentration of (2) isdoubled, the rate of thereaction is not doubled.If concentration of (2) isdoubled, the rate of thereaction is not doubled.Rate law:rate = k [1-bromo-1,1-dimethylethane]this reaction is an example of a E1reaction.E stands for elimination1 stands for unimolecularRate law:rate = k [1-bromo-1,1-dimethylethane]this reaction is an example of a E1reaction.E stands for elimination1 stands for unimolecular1-bromo-1,1-dimethylethane 2-methylpropene
  24. 24. The E1 ReactionThe alkyl halidedissociate, forming acarbocationThe alkyl halidedissociate, forming acarbocationThe baseremoves aprotonThe baseremoves aprotonThe mechanism shows that an E1reaction is a two-step reactionThe mechanism shows that an E1reaction is a two-step reaction
  25. 25. Products of Elimination Reaction2-bromobutane2-butene1-butene80%20%The most stable alkene is themajor product of the reactionfor both E1 and E2 reactionThe most stable alkene is themajor product of the reactionfor both E1 and E2 reactionThe greater the number ofalkyl substituent the morestable is the alkeneThe greater the number ofalkyl substituent the morestable is the alkeneFor both E1 and E2 reactions, tertiary alkyl halidesare the most reactive and primary alkyl halidesare the least reactiveFor both E1 and E2 reactions, tertiary alkyl halidesare the most reactive and primary alkyl halidesare the least reactive30% 50%
  26. 26. Competition BetweenSN2/E2 and SN1/E1rate = k1[alkyl halide] + k2[alkyl halide][nucleo.] + k3[alkyl halide] + k2[alkyl halide][base]rate = k1[alkyl halide] + k2[alkyl halide][nucleo.] + k3[alkyl halide] + k2[alkyl halide][base]SN1SN1 SN2SN2 E1E1 E2E2• SN2 and E2 are favoured by a high concentration of a goodnucleophile/strong base• SN1 and E1 are favoured by a poor nucleophile/weak base, because apoor nucleophile/weak base disfavours SN2 and E2 reactions• SN2 and E2 are favoured by a high concentration of a goodnucleophile/strong base• SN1 and E1 are favoured by a poor nucleophile/weak base, because apoor nucleophile/weak base disfavours SN2 and E2 reactions
  27. 27. Competition BetweenSubstitution and Elimination• SN2/E2 conditions:In a SN2 reaction: 1o>2o> 3oIn a E2 reaction: 3o> 2o> 1oIn a SN2 reaction: 1o>2o> 3oIn a E2 reaction: 3o> 2o> 1o90% 10%25% 75%100%
  28. 28. Competition BetweenSubstitution and Elimination• SN1/E1 conditions:All alkyl halides that react under SN1/E1 conditions will giveboth substitution and elimination products (≈50%/50%)All alkyl halides that react under SN1/E1 conditions will giveboth substitution and elimination products (≈50%/50%)
  29. 29. Summary• Alkyl halides undergo two kinds of nucleophilic subtitutions:SN1 and SN2, and two kinds of elimination: E1 and E2.• SN2 and E2 are bimolecular one-step reactions• SN1 and E1 are unimolecular two step reactions• SN1 lead to a mixture of stereoisomers• SN2 inverts the configuration od an asymmetric carbon• The major product of a elimination is the most stable alkene• SN2 are E2 are favoured by strong nucleophile/strong base• SN2 reactions are favoured by primary alkyl halides• E2 reactions are favoured by tertiary alkyl halides

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