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8.3 The S N 2 Mechanism of Nucleophilic Substitution

8.3 The S N 2 Mechanism of Nucleophilic Substitution. Kinetics. Many nucleophilic substitutions follow a second-order rate law. CH 3 Br + HO – Æ CH 3 OH + Br – rate = k [CH 3 Br][HO – ] inference: rate-determining step is bimolecular. HO –. CH 3 Br. +. HOCH 3. +. Br –.

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8.3 The S N 2 Mechanism of Nucleophilic Substitution

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  1. 8.3The SN2 Mechanism of Nucleophilic Substitution

  2. Kinetics • Many nucleophilic substitutions follow asecond-order rate law. CH3Br + HO –Æ CH3OH + Br – • rate = k[CH3Br][HO – ] • inference: rate-determining step is bimolecular

  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. 8.4Stereochemistry 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. 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 Æ (–)-2-Octanol • (–)-2-Bromooctane Æ (+)-2-Octanol

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

  12. Problem 8.4 • The Fischer projection formula for (+)-2-bromooctaneis shown. Write the Fischer projection of the(–)-2-octanol formed from it by nucleophilic substitution with inversion of configuration.

  13. CH3 CH3 Br H HO H CH2(CH2)4CH3 CH2(CH2)4CH3 Problem 8.4 • The Fischer projection formula for (+)-2-bromooctaneis shown. Write the Fischer projection of the(–)-2-octanol formed from it by nucleophilic substitution with inversion of configuration.

  14. 8.5How SN2 Reactions Occur

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

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

  17. .. – 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

  18. 8.6Steric Effects in SN2 Reactions

  19. 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.

  20. Table 8.2 Reactivity toward substitution by the SN2 mechanism RBr + LiI Æ 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

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

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

  23. 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.

  24. Table 8.3 Effect of chain branching on rate of SN2 substitution RBr + LiI Æ 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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