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Reaction mechanisms

Reaction mechanisms

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Reaction mechanisms

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  1. Reaction mechanisms

  2. Ionic Reactions

  3. Ionic Reactions

  4. Ionic Reactions

  5. Ionic Reactions

  6. Bond Polarity Partial charges

  7. Nucleophiles and Electrophiles

  8. Leaving Groups

  9. Radical Reactions

  10. Type of Reactions

  11. Nucleophilic reactions: nucleophilic substitution (SN) • in the following general reaction, substitution takes place on an sp3 hybridized (tetrahedral) carbon Nucleophilic substitution: -> reagent is nucleophil -> nucleophil replaces leaving group -> competing reaction (elimination + rearrangements)

  12. Nucleophilic Substitution • Some nucleophilic substitution reactions

  13. Mechanism • Chemists propose two limiting mechanisms for nucleophilic displacement • a fundamental difference between them is the timing of bond breaking and bond forming steps • At one extreme, the two processes take place simultaneously; designated SN2 • S = substitution • N = nucleophilic • 2 = bimolecular (two species are involved in the rate-determining step) • rate = k[haloalkane][nucleophile] • In the other limiting mechanism, bond breaking between carbon and the leaving group is entirely completed before bond forming with the nucleophile begins. This mechanism is designated SN1 where • S = substitution • N = nucleophilic • 1 = unimolecular (only one species is involved in the rate-determining step) • rate = k[haloalkane]

  14. SN2 reaction: bimolecular nucleophilic substitution • both reactants are involved in the transition state of the rate-determining step • the nucleophile attacks the reactive center from the side opposite the leaving group

  15. SN2 • An energy diagram for an SN 2 reaction • there is one transition state and no reactive intermediate

  16. SN1 reaction: unimolecular nucleophilic substitution • SN1 is illustrated by the solvolysis of tert-butyl bromide • Step 1: ionization of the C-X bond gives a carbocation intermediate

  17. SN1 • Step 2: reaction of the carbocation (an electrophile) with methanol (a nucleophile) gives an oxonium ion • Step 3: proton transfer completes the reaction

  18. SN1 • An energy diagram for an SN1 reaction

  19. C H C H 6 5 6 5 + CH O C C OCH 3 3 H H Cl Cl (S)-Enantiomer (R)-Enantiomer A racemic mixture SN1 • For an SN1 reaction at a stereocenter, the product is a racemic mixture • the nucleophile attacks with equal probability from either face of the planar carbocation intermediate

  20. Effect of variables on SN Reactions • the nature of substituents bonded to the atom attacked by nucleophile • the nature of the nucleophile • the nature of the leaving group • the solvent effect

  21. Effect of substituents on SN2

  22. Effect of substituents on SN1

  23. Effect of substituents on SN reactions • SN1 reactions • governed by electronic factors, namely the relative stabilities of carbocation intermediates • relative rates: 3° > 2° > 1° > methyl • SN2 reactions • governed by steric factors, namely the relative ease of approach of the nucleophile to the site of reaction • relative rates: methyl > 1° > 2° > 3°

  24. Effect of substituents on SN reactions • Effect of electronic and steric factors in competition between SN1 and SN2 reactions

  25. Nucleophilicity • Nucleophilicity: a kinetic property measured by the rate at which a Nu attacks a reference compound under a standard set of experimental conditions • for example, the rate at which a set of nucleophiles displaces bromide ion from bromoethane • Two important features: • An anion is a better nucleophile than a uncharged conjugated acid • strong bases are good nucleophiles

  26. Nucleophilicity

  27. Nucleophilicity

  28. Leaving Group

  29. Leaving Group

  30. The Leaving Group • the best leaving groups in this series are the halogens I-, Br-, and Cl- • OH-, RO-, and NH2- are such poor leaving groups that they are rarely if ever displaced in nucleophilic substitution reactions

  31. Solvent Effect • Protic solvent: a solvent that contains an -OH group • these solvents favor SN1 reactions; the greater the polarity of the solvent, the easier it is to form carbocationsin it

  32. Solvent Effect • Aprotic solvent: does not contain an -OH group • it is more difficult to form carbocations in aprotic solvents • aprotic solvents favor SN2 reactions

  33. Summary of SN1 and SN2

  34. Competing Reaction: Elimination -Elimination: removal of atoms or groups of atoms from adjacent carbons to form a carbon-carbon double bond • we study a type ofb-eliminationcalled dehydrohalogenation (the elimination of HX)

  35. b-Elimination • There are two limiting mechanisms for β-elimination reactions • E1 mechanism:at one extreme, breaking of the C-X bond is complete before reaction with base breaks the C-H bond • only R-X is involved in the rate-determining step • E2 mechanism:at the other extreme, breaking of the C-X and C-H bonds is concerted • both R-X and base are involved in the rate-determining step

  36. E2 Mechanism • A one-step mechanism; all bond-breaking and bond-forming steps are concerted

  37. E1 Mechanism • Step 1: ionization of C-X gives a carbocation intermediate • Step 2: proton transfer from the carbocation intermediate to a base (in this case, the solvent) gives the alkene Nucleophile -> acting as a strong base

  38. Elimination • Saytzeff rule: the major product of a elimination is the more stable (the more highly substituted) alkene

  39. Elimination Reactions • Summary of E1 versus E2 Reactions for Haloalkanes

  40. Substitution vs Elimination • Many nucleophiles are also strong bases (OH- and RO-) and SN and E reactions often compete • the ratio of SN/E products depends on the relative rates of the two reactions • What favors Elimination reactions: • attacking nucleophil is a strong and large base • steric crowding in the substrate • High temperatures and low polarity of solvent

  41. SN1 versus E1 • Reactions of 2° and 3° haloalkanes in polar protic solvents give mixtures of substitution and elimination products

  42. SN2 versus E2 • It is considerably easier to predict the ratio of SN2 to E2 products

  43. Summary of S vs E for Haloalkanes • for methyl and 1°haloalkanes

  44. Summary of S vs E for Haloalkanes • for 2° and 3° haloalkanes

  45. Summary of S vs E for Haloalkanes • Examples: predict the major product and the mechanism for each reaction Elimination, strong base, high temp. SN2, weak base, good nucleophil SN1 (+Elimination), strong base, good nucleophil, protic solvent No reaction, I is a weak base (SN2) I better leaving group than Cl

  46. Carbocation rearrangements Also 1,3- and other shifts are possible The driving force of rearrangements is -> to form a more stable carbocation !!! Happens often with secondary carbocations -> more stable tertiary carbocation

  47. Carbocation rearrangements in SN + E reactions Rearrangement

  48. Carbocation rearrangements in SN + E reactions -> Wagner – Meerwein rearrangements Rearrangement of a secondary carbocations -> more stable tertiary carbocation Plays an important role in biosynthesis of molecules, i.e. Cholesterol -> (Biochemistry)

  49. Carbocation rearrangements in Electrophilic addition reactions