Ppt0000015 feed the flame[1]this one (3)
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Ppt0000015 feed the flame[1]this one (3) Presentation Transcript

  • 1. This isThis is Doug Henning!Doug Henning! He performed his first show at the ageHe performed his first show at the age of 14 at the birthday party of a friendof 14 at the birthday party of a friend and was inspired by his audience'sand was inspired by his audience's spellbound reactionspellbound reaction
  • 2. He won The Tony AwardHe won The Tony Award  Debuting in December 1975,Debuting in December 1975, DougDoug Henning's World of MagicHenning's World of Magic capturedcaptured the attention of more than 50 millionthe attention of more than 50 million viewers!viewers!
  • 3. HughesHughes andand Sir Christopher IngoldSir Christopher Ingold InIn 19351935,, Edward D. HughesEdward D. Hughes andand Sir Christopher IngoldSir Christopher Ingold studiedstudied nucleophilic substitution reactions ofnucleophilic substitution reactions of alkyl halidesalkyl halides and related compounds.and related compounds. They proposed that there were twoThey proposed that there were two main mechanisms at work, both ofmain mechanisms at work, both of them competing with each other.them competing with each other.
  • 4. Are just as spectacular!!!!!Are just as spectacular!!!!! The two main mechanisms are theThe two main mechanisms are the SSNN 1 reaction1 reaction and theand the SSNN2 reaction2 reaction.. The “S” stands for chemicalThe “S” stands for chemical substitution,substitution, And the “N” stands for nucleophilic,And the “N” stands for nucleophilic, and the number represents theand the number represents the kinetic orderkinetic order of the reaction.of the reaction.
  • 5. Sir Christopher IngoldSir Christopher Ingold  Known for Organic reactionKnown for Organic reaction mechanismsmechanisms  ““Cahn-Ingold-Prelog” rules !Cahn-Ingold-Prelog” rules ! He received theHe received the Longstaff MedalLongstaff Medal of theof the Royal Society of ChemistryRoyal Society of Chemistry in 1951,in 1951, thethe Royal MedalRoyal Medal of theof the Royal SocietyRoyal Society in 1952, and wasin 1952, and was knightedknighted in 1958in 1958
  • 6. Just like Paul Macartney!!!Just like Paul Macartney!!!
  • 7. Sir Christopher IngoldSir Christopher Ingold Is one bad dude!!!!!Is one bad dude!!!!!
  • 8. This is how he got down!!!This is how he got down!!! A graph showing the relative reactivities of the differentA graph showing the relative reactivities of the different alkyl halides towards Salkyl halides towards SNN1 and S1 and SNN2 reactions2 reactions
  • 9. thethe SSNN1 reaction1 reaction
  • 10. thethe SSNN2 reaction2 reaction
  • 11. Effect of the NucleophileEffect of the Nucleophile  The nucleophile takes part in theThe nucleophile takes part in the slow step (the only step) of the Sslow step (the only step) of the SNN22 reaction but not in the slow step ofreaction but not in the slow step of the Sthe SNN1. Therefore, a strong1. Therefore, a strong nucleophile promotes the Snucleophile promotes the SNN2 but not2 but not the Sthe SNN1.1.
  • 12. Effect of the NucleophileEffect of the Nucleophile  Weak nucleophiles fail to promoteWeak nucleophiles fail to promote the Sthe SNN2 reaction; therefore, reactions2 reaction; therefore, reactions with weak nucleophiles often go bywith weak nucleophiles often go by the Sthe SNN1 mechanism if the substrate is1 mechanism if the substrate is secondary or tertiarysecondary or tertiary
  • 13. Effect of the NucleophileEffect of the Nucleophile  SSNN1: Nucleophile strength is1: Nucleophile strength is unimportant (usually weak).unimportant (usually weak).
  • 14. Effect of the NucleophileEffect of the Nucleophile  SSNN2: Strong nucleophiles are2: Strong nucleophiles are required.required.
  • 15. Effect of the SubstrateEffect of the Substrate  The structure of the substrate (theThe structure of the substrate (the alkyl halide) is an important factor inalkyl halide) is an important factor in determining which of thesedetermining which of these substitution mechanisms mightsubstitution mechanisms might operate.operate.
  • 16. Effect of the SubstrateEffect of the Substrate  Methyl halides and primary halidesMethyl halides and primary halides cannot easily ionize and undergo Scannot easily ionize and undergo SNN11 substitution because methyl andsubstitution because methyl and primary carbocations are high inprimary carbocations are high in energy. They are relativelyenergy. They are relatively unhindered, however, so they makeunhindered, however, so they make good Sgood SNN2 substrates.2 substrates.
  • 17. Tert-butyl-chloride-3DTert-butyl-chloride-3D  Tertiary halides are too hindered toTertiary halides are too hindered to undergo Sundergo SNN2 displacement, but they2 displacement, but they can ionize to form tertiarycan ionize to form tertiary carbocations. Tertiary halidescarbocations. Tertiary halides undergo substitution exclusivelyundergo substitution exclusively through the Sthrough the SNN1 mechanism.1 mechanism. Secondary halides can undergoSecondary halides can undergo substitution by either mechanism,substitution by either mechanism, depending on the conditionsdepending on the conditions
  • 18. Tert-butyl-chloride-3DTert-butyl-chloride-3D will exhibit Steric hinderancewill exhibit Steric hinderance
  • 19. SSNN2 substrates2 substrates  SSNN2 substrates:2 substrates:CHCH33X > 1° > 2°X > 1° > 2°  (3° is not suitable(3° is not suitable
  • 20. SSNN1 substrates1 substrates  SSNN1 substrates: 3° > 2°1 substrates: 3° > 2° (1° and CH(1° and CH33X are unlikely)X are unlikely)
  • 21. silver nitrate (AgNOsilver nitrate (AgNO33))  If silver nitrate (AgNOIf silver nitrate (AgNO33) is added to) is added to an alkyl halide in a good ionizingan alkyl halide in a good ionizing solvent, it removes the halide ion tosolvent, it removes the halide ion to give a carbocation. This techniquegive a carbocation. This technique can force some unlikely ionizations,can force some unlikely ionizations, often giving interestingoften giving interesting rearrangements (see Problem 6-29.)rearrangements (see Problem 6-29.)
  • 22. Effect of the SolventEffect of the Solvent  The slow step of the SThe slow step of the SNN1 reaction1 reaction involves formation of two ions.involves formation of two ions. Solvation of these ions is crucial toSolvation of these ions is crucial to stabilizing them and lowering thestabilizing them and lowering the activation energy for their formation.activation energy for their formation. Very polar ionizing solvents such asVery polar ionizing solvents such as water and alcohols are needed forwater and alcohols are needed for the Sthe SNN1. The solvent may be heated1. The solvent may be heated to reflux (boiling) to provide theto reflux (boiling) to provide the energy needed for ionizationenergy needed for ionization
  • 23. Effect of the SolventEffect of the Solvent  Less charge separation is generatedLess charge separation is generated in the transition state of the Sin the transition state of the SNN22 reaction.reaction.  Strong solvation may weaken theStrong solvation may weaken the strength of the nucleophile becausestrength of the nucleophile because of the energy needed to strip off theof the energy needed to strip off the solvent molecules.solvent molecules.
  • 24. Effect of the SolventEffect of the Solvent  Thus, the SThus, the SNN2 reaction often goes2 reaction often goes faster in less polar solvents if thefaster in less polar solvents if the nucleophile will dissolve. Polarnucleophile will dissolve. Polar aprotic solvents may enhance theaprotic solvents may enhance the strength of weak nucleophilesstrength of weak nucleophiles
  • 25. Effect of the SolventEffect of the Solvent The slow step of the SThe slow step of the SNN1 reaction1 reaction involves formation of two ions.involves formation of two ions. Solvation of these ions is crucial toSolvation of these ions is crucial to stabilizing them and lowering thestabilizing them and lowering the activation energy for their formation.activation energy for their formation. Very polar ionizing solvents such asVery polar ionizing solvents such as water and alcohols are needed forwater and alcohols are needed for the Sthe SNN1.1.
  • 26. Effect of the SolventEffect of the Solvent  The solvent may be heated to refluxThe solvent may be heated to reflux (boiling) to provide the energy(boiling) to provide the energy needed for ionizationneeded for ionization
  • 27. This is . . .This is . . .  ..
  • 28. The same as this . . .The same as this . . .
  • 29. the transition statethe transition state  Less charge separation is generated in theLess charge separation is generated in the transition state of the Stransition state of the SNN2 reaction. Strong2 reaction. Strong solvation may weaken the strength of thesolvation may weaken the strength of the nucleophile because of the energy needednucleophile because of the energy needed to strip off the solvent molecules. Thus,to strip off the solvent molecules. Thus, the Sthe SNN2 reaction often goes faster in less2 reaction often goes faster in less polar solvents if the nucleophile willpolar solvents if the nucleophile will dissolve.dissolve.  Polar aprotic solvents may enhance thePolar aprotic solvents may enhance the strength of weak nucleophilesstrength of weak nucleophiles
  • 30. the transition statethe transition state
  • 31. the transition statethe transition state  ..
  • 32. aprotic solvents:aprotic solvents:  Common characteristics of aproticCommon characteristics of aprotic solvents:solvents:  Examples areExamples are dimethyl sulfoxidedimethyl sulfoxide,, dimethylformamidedimethylformamide,, dioxanedioxane andand hexamethylphosphorotriamidehexamethylphosphorotriamide,, tetrahydrofurantetrahydrofuran
  • 33. dimethyl sulfoxidedimethyl sulfoxide You might see this laterYou might see this later
  • 34. dimethyl sulfoxidedimethyl sulfoxide  ..
  • 35. tetrahydrofurantetrahydrofuran  ..
  • 36. tetrahydrofurantetrahydrofuran You will definitely need this later.You will definitely need this later.  ..
  • 37. Effect of the SolventEffect of the Solvent  Polar Protic SolventsPolar Protic Solvents  Let's start with the meaning of the adjectiveLet's start with the meaning of the adjective protic. In the context used here, protic refers to aprotic. In the context used here, protic refers to a hydrogen atom attached to an electronegativehydrogen atom attached to an electronegative atom. For our purposes that electronegative atomatom. For our purposes that electronegative atom is almost exclusively oxygen.is almost exclusively oxygen.  In other words, polar protic solvents areIn other words, polar protic solvents are compounds that can be represented by thecompounds that can be represented by the general formula ROH. The polarity of the polargeneral formula ROH. The polarity of the polar protic solvents stems from the bond dipole of theprotic solvents stems from the bond dipole of the O-H bond.O-H bond.
  • 38. Effect of the SolventEffect of the Solvent  The large difference in electronegativities of theThe large difference in electronegativities of the oxygen and the hydrogen atom, combined withoxygen and the hydrogen atom, combined with the small size of the hydrogen atom, warrantthe small size of the hydrogen atom, warrant separating molecules that contain an OH groupseparating molecules that contain an OH group from those polar compounds that do not.from those polar compounds that do not.  Examples of polar protic solvents are water (HExamples of polar protic solvents are water (H22O),O), methanol (CHmethanol (CH33OH), and acetic acid (CHOH), and acetic acid (CH33COCO22H).H).
  • 39.  http://myphlip.pearsoncmg.com/altprodhttp://myphlip.pearsoncmg.com/altprod
  • 40. The slow stepThe slow step  The slow step of the SN1 reactionThe slow step of the SN1 reaction involves formation of two ions.involves formation of two ions. Solvation of these ions is crucial toSolvation of these ions is crucial to stabilizing them and lowering thestabilizing them and lowering the activation energy for their formation.activation energy for their formation. Very polar ionizing solvents such asVery polar ionizing solvents such as water and alcohols are needed forwater and alcohols are needed for the SN1.the SN1.
  • 41. The slow stepThe slow step  The solvent may be heated to refluxThe solvent may be heated to reflux (boiling) to provide the energy(boiling) to provide the energy needed for ionization.needed for ionization.
  • 42. The slow stepThe slow step  Less charge separation is generated in theLess charge separation is generated in the transition state of the SN2 reaction.transition state of the SN2 reaction. Strong solvation may weaken the strengthStrong solvation may weaken the strength of the nucleophile because of the energyof the nucleophile because of the energy needed to strip off the solvent molecules.needed to strip off the solvent molecules.
  • 43. less polar solventsless polar solvents  Thus, the SN2 reaction often goesThus, the SN2 reaction often goes faster in less polar solvents if thefaster in less polar solvents if the nucleophile will dissolve. Polarnucleophile will dissolve. Polar aprotic solvents may enhance theaprotic solvents may enhance the strength of weak nucleophiles.strength of weak nucleophiles.
  • 44. SN1SN1  SN1: Good ionizing solvent required.SN1: Good ionizing solvent required.
  • 45. SN2SN2  SN2: May go faster in a less polarSN2: May go faster in a less polar solventsolvent
  • 46. KineticsKinetics  The rate of the SN1 reaction isThe rate of the SN1 reaction is proportional to the concentration ofproportional to the concentration of the alkyl halide but not thethe alkyl halide but not the concentration of the nucleophile. Itconcentration of the nucleophile. It follows a first-order rate equation.follows a first-order rate equation.
  • 47. KineticsKinetics  The rate of the SN2 reaction isThe rate of the SN2 reaction is proportional to the concentrations ofproportional to the concentrations of both the alkyl halide [R—X] and theboth the alkyl halide [R—X] and the nucleophile [Nuc: −]. It follows anucleophile [Nuc: −]. It follows a second-order rate equation.second-order rate equation.
  • 48. KineticsKinetics  SN1 rate = kr[R—X]SN1 rate = kr[R—X]  SN2 rate = kr[R—X][Nuc: −]SN2 rate = kr[R—X][Nuc: −]
  • 49. StereochemistryStereochemistry  The SN1 reaction involves a flatThe SN1 reaction involves a flat carbocation intermediate that can becarbocation intermediate that can be attacked from either face. Therefore,attacked from either face. Therefore, the SN1 usually gives a mixture ofthe SN1 usually gives a mixture of inversion and retention ofinversion and retention of configurationconfiguration
  • 50. StereochemistryStereochemistry  The SN2 reaction takes placeThe SN2 reaction takes place through a back-side attack, whichthrough a back-side attack, which inverts the stereochemistry of theinverts the stereochemistry of the carbon atom. Complete inversion ofcarbon atom. Complete inversion of configuration is the result.configuration is the result.
  • 51. StereochemistryStereochemistry  SN1 stereochemistry:SN1 stereochemistry:  Mixture of retention and inversion;Mixture of retention and inversion; racemization.racemization.
  • 52. StereochemistryStereochemistry  SN2 stereochemistrySN2 stereochemistry  Complete inversionComplete inversion
  • 53. RearrangementsRearrangements  The SN1 reaction involves aThe SN1 reaction involves a carbocation intermediate. Thiscarbocation intermediate. This intermediate can rearrange, usuallyintermediate can rearrange, usually by a hydride shift or an alkyl shift, toby a hydride shift or an alkyl shift, to give a more stable carbocation.give a more stable carbocation.
  • 54. RearrangementsRearrangements  The SN2 reaction takes place in oneThe SN2 reaction takes place in one step with no intermediates.step with no intermediates.  No rearrangement isNo rearrangement is possible in the SN2possible in the SN2 reaction.reaction.
  • 55. rearrangement reactionrearrangement reaction
  • 56. An example of a reaction taking place withAn example of a reaction taking place with an SN1 reaction mechanism is thean SN1 reaction mechanism is the hydrolysis of tert-butyl bromide with waterhydrolysis of tert-butyl bromide with water forming tert-butyl alcoholforming tert-butyl alcohol
  • 57. a tert-butyl carbocationa tert-butyl carbocation  Formation of a tert-butyl carbocationFormation of a tert-butyl carbocation by separation of a leaving group (aby separation of a leaving group (a bromide anion) from the carbonbromide anion) from the carbon atom: this step is slow andatom: this step is slow and reversible!reversible!
  • 58. http://en.wikipedia.org/wiki/SN1_rehttp://en.wikipedia.org/wiki/SN1_re actionaction
  • 59. The hydride shiftThe hydride shift  The mechanism for hydride shiftThe mechanism for hydride shift occurs inoccurs in multiple stepsmultiple steps that includesthat includes various intermediates and transitionvarious intermediates and transition states. Below is the mechanism forstates. Below is the mechanism for the given reaction above:the given reaction above: 
  • 60. The hydride shiftThe hydride shift
  • 61. The hydride shiftThe hydride shift
  • 62. rearrangement reactionrearrangement reaction  AA rearrangement reactionrearrangement reaction is a broadis a broad class of organic reactions where theclass of organic reactions where the carbon skeleton of a molecule iscarbon skeleton of a molecule is rearranged to give a structural isomer ofrearranged to give a structural isomer of the original molecule [1] . Often athe original molecule [1] . Often a substituent moves from one atom tosubstituent moves from one atom to another atom in the same molecule. In theanother atom in the same molecule. In the example below the substituent R movesexample below the substituent R moves from carbon atom 1 to carbon atom 2from carbon atom 1 to carbon atom 2
  • 63. RearrangementsRearrangements  SN1: Rearrangements are common.SN1: Rearrangements are common.
  • 64. RearrangementsRearrangements  SN2: Rearrangements are impossibleSN2: Rearrangements are impossible
  • 65. Nucleophillic substitutionsNucleophillic substitutions SN1:SN1: SN2:SN2: Promoting factorsPromoting factors weak nucleophiles areweak nucleophiles are OKOK strong nucleophilestrong nucleophile neededneeded NucleophileNucleophile 3° > 2°3° > 2° CHCH33X > 1° >3° >2°X > 1° >3° >2° substrate (RX)substrate (RX) good ionizing solventgood ionizing solvent neededneeded wide variety ofwide variety of solventssolvents SolventSolvent good ionizing solventgood ionizing solvent neededneeded wide variety ofwide variety of solventssolvents leaving groupleaving group good one requiredgood one required good one requiredgood one required OtherOther You will use this!You will use this! ***AgNO***AgNO33 force ionization!force ionization!
  • 66. the nucleophile competes . . .the nucleophile competes . . .  In both reactions, the nucleophile competes withIn both reactions, the nucleophile competes with the leaving group. Because of this, one mustthe leaving group. Because of this, one must realize what properties a leaving group shouldrealize what properties a leaving group should have, and what constitutes a good nucleophile.have, and what constitutes a good nucleophile. For this reason, it is worthwhile to know whichFor this reason, it is worthwhile to know which factors will determine whether a reaction followsfactors will determine whether a reaction follows an SN1 or SN2 pathway.an SN1 or SN2 pathway.
  • 67. good leaving groupsgood leaving groups  Very good leaving groups, such asVery good leaving groups, such as triflate, tosylate and mesylate,triflate, tosylate and mesylate, stabilize an incipient negativestabilize an incipient negative charge. The delocalization of thischarge. The delocalization of this charge is reflected in the fact thatcharge is reflected in the fact that these ions are not considered to bethese ions are not considered to be nucleophilicnucleophilic
  • 68. good leaving groupsgood leaving groups  Very good leaving groups, such asVery good leaving groups, such as triflate, tosylate and mesylate,triflate, tosylate and mesylate, stabilize an incipient negativestabilize an incipient negative charge. The delocalization of thischarge. The delocalization of this charge is reflected in the fact thatcharge is reflected in the fact that these ions are not considered to bethese ions are not considered to be nucleophilicnucleophilic
  • 69. good leaving groupsgood leaving groups
  • 70. good leaving groupsgood leaving groups  Hydroxide and alkoxide ions are notHydroxide and alkoxide ions are not good leaving groups; however, theygood leaving groups; however, they can be activated by means of Lewiscan be activated by means of Lewis or Brønsted acidsor Brønsted acids