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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 –  CH 3 OH + Br – rate = k [CH 3 Br][HO – ] inference: rate-determining step is bimolecular. . . HO. CH 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 – CH3OH + Br – • rate = k[CH3Br][HO – ] • inference: rate-determining step is bimolecular

  3.   HO CH3 Br transition state HO – CH3Br + HOCH3 + Br – Bimolecular Mechanism • one step

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

  5. Nucleophile attacks carbonfrom side opposite bondto the leaving group. Three-dimensionalarrangement of bonds inproduct is opposite to that of reactant. Inversion of Configuration

  6. 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

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

  8. 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.

  9. 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.

  10. 8.4Steric Effects and SN2 Reaction Rates

  11. 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.

  12. Table 8.2 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

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

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

  15. 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.

  16. Table 8.3 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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