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The S N 2 Mechanism of Nucleophilic Substitution

The S N 2 Mechanism of Nucleophilic Substitution. Kinetics. Many nucleophilic substitutions follow a second-order rate law. CH 3 Br + HO – CH 3 OH + Br – rate = k [CH 3 Br] [HO – ] What is the reaction order of each starting material?

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The S N 2 Mechanism of Nucleophilic Substitution

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  1. The SN2 Mechanism of Nucleophilic Substitution

  2. Kinetics • Many nucleophilic substitutions follow asecond-order rate law. CH3Br + HO – CH3OH + Br – • rate = k [CH3Br] [HO – ] • What is the reaction order of each starting material? • What can you infer on a molecular level? • What is the overall order of reaction?

  3. HO – CH3Br + HOCH3 + Br – Bimolecular mechanism • one stepconcerted

  4. HO – CH3Br + HOCH3 + Br – Bimolecular mechanism • one stepconcerted

  5. d - d - HO CH3 Br transition state HO – CH3Br + HOCH3 + Br – Bimolecular mechanism • one stepconcerted

  6. Stereochemistry of SN2 Reactions

  7. Generalization • Nucleophilic substitutions that exhibitsecond-order kinetic behavior are stereospecific and proceed withinversion of configuration.

  8. Inversion of Configuration nucleophile attacks carbonfrom side opposite bondto the leaving group

  9. Inversion of Configuration nucleophile attacks carbonfrom side opposite bondto the leaving group three-dimensionalarrangement of bonds inproduct is opposite to that of reactant

  10. Inversion of configuration (Walden inversion) in an SN2 reaction is due to back side attack

  11. Stereospecific Reaction • A stereospecific reaction is one in whichstereoisomeric starting materials givestereoisomeric products. • The reaction of 2-bromooctane with NaOH (in ethanol-water) is stereospecific. • (+)-2-Bromooctane (–)-2-Octanol • (–)-2-Bromooctane (+)-2-Octanol

  12. H H CH3(CH2)5 C HO Br C CH3 CH3 Stereospecific Reaction (CH2)5CH3 NaOH (S)-(+)-2-Bromooctane (R)-(–)-2-Octanol

  13. CH3 CH3 Br H HO H CH2(CH2)4CH3 CH2(CH2)4CH3 • 1) Draw the Fischer projection formula for (+)-S-2-bromooctane. • 2)Write the Fischer projection of the (–)-2-octanol formed from it by nucleophilic substitution with inversion of configuration. R- or S- ?

  14. A conceptual view of SN2 reactions

  15. Why does the nucleophile attack from the back side?

  16. CH3(CH2)5 H – .. .. Br : C HO .. .. H3C

  17. CH3(CH2)5 H d – d – .. .. : Br HO C .. .. CH3 CH3(CH2)5 H – .. .. Br : C HO .. .. H3C

  18. .. – Br : .. CH3(CH2)5 H d – d – .. .. : Br HO C .. .. CH3 CH3(CH2)5 H – .. .. H Br : (CH2)5 CH3 C HO .. .. .. C HO H3C .. CH3

  19. Steric Effects in SN2 Reactions

  20. Crowding at the Reaction Site The rate of nucleophilic substitutionby the SN2 mechanism is governedby steric effects. Crowding at the carbon that bears the leaving group slows the rate ofbimolecular nucleophilic substitution.

  21. Reactivity toward substitution by the SN2 mechanism RBr + LiI RI + LiBr • Alkyl Class Relativebromide rate • CH3Br Methyl 221,000 • CH3CH2Br Primary 1,350 • (CH3)2CHBr Secondary 1 • (CH3)3CBr Tertiary too small to measure

  22. A bulky substituent in the alkyl halide reduces the reactivity of the alkyl halide: steric hindrance

  23. Decreasing SN2 Reactivity CH3Br CH3CH2Br (CH3)2CHBr (CH3)3CBr

  24. Decreasing SN2 Reactivity CH3Br CH3CH2Br (CH3)2CHBr (CH3)3CBr

  25. Reaction coordinate diagrams for (a) the SN2 reaction of methyl bromide and (b) an SN2 reaction of a sterically hindered alkyl bromide

  26. Crowding Adjacent to the Reaction Site The rate of nucleophilic substitutionby the SN2 mechanism is governedby steric effects. Crowding at the carbon adjacentto the one that bears the leaving groupalso slows the rate of bimolecularnucleophilic substitution, but the effect is smaller.

  27. Effect of chain branching on rate of SN2 substitution RBr + LiI RI + LiBr • Alkyl Structure Relativebromide rate • Ethyl CH3CH2Br 1.0 • Propyl CH3CH2CH2Br 0.8 • Isobutyl (CH3)2CHCH2Br 0.036 • Neopentyl (CH3)3CCH2Br 0.00002

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