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Chapter 8 I. Nucleophilic Substitution ( in depth ) II. Competion with Elimination. X. C. C. Nucleophilic Substitution. Substrate is a sp3 hybridized carbon atom (cannot be an a vinylic halide or an aryl halide except under special conditions to be discussed in Chem 227). X. Kinetics.

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Chapter 8 I. Nucleophilic Substitution ( in depth ) II. Competion with Elimination


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    1. Chapter 8I. Nucleophilic Substitution (in depth)II. Competion with Elimination

    2. X C C Nucleophilic Substitution Substrate is a sp3 hybridized carbon atom (cannot be an a vinylic halide or an aryl halide except under special conditions to be discussed in Chem 227) X

    3. 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?

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

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

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

    7. Stereochemistry of SN2 Reactions

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

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

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

    11. Inversion of configuration (Walden inversion) in an SN2 reaction is due to “back side attack” P.Walden, Berichte, 29(1): 133-138 (1896) Riga Polytechnical College Could there be another mechanism that provides the same results?

    12. Roundabout SN2 Mechanism Traditional SN2 Mechanism Videos courtesy of William L. Hase, Texas Tech University

    13. SN2 Reaction Mechanisms: Gas Phase (2008) http://pubs.acs.org/cen/news/86/i02/8602notw1.html Roundabout Traditional Physicist Roland Wester and his team in Matthias Weidemüller's group at the University of Freiburg, in Germany, in collaboration with William L. Hase's group at Texas Tech University, provide direct evidence for this mechanism in the gas phase. However, they also detected an additional, unexpected mechanism. In this new pathway, called the roundabout mechanism, chloride bumps into the methyl group and spins the entire methyl iodide molecule 360° before chloride substitution occurs. The team imaged SN2 reactions at different collision energies, which depend on the speed at which chloride smashes into methyl iodide. Data at lower collision energies support the traditional SN2 mechanism. However, at higher collision energies, about 10% of the iodide ions fell outside of the expected distribution. "We saw a group of iodide ions with a much slower velocity than the rest," says Wester. "Since energy is conserved, if iodide ions are slow, the energy has to be somewhere else." On the basis of calculations performed by their colleagues at Texas Tech, the team concluded that the energy missing from the iodide transfers to the methyl chloride product in the form of rotational excitation, supporting the proposed roundabout mechanism.

    14. Fig. 1. Calculated MP2(fc)/ECP/aug-cc-pVDZ Born-Oppenheimer potential energy along the reaction coordinate g = RC-I - RC-Cl for the SN2 reaction Cl- + CH3I and obtained stationary points J. Mikosch et al., Science 319, 183 -186 (2008) Published by AAAS

    15. Fig. 2. (A to D) Center-of-mass images of the I- reaction product velocity from the reaction of Cl- with CH3I at four different relative collision energies J. Mikosch et al., Science 319, 183 -186 (2008) Published by AAAS

    16. Fig. 3. View of a typical trajectory for the indirect roundabout reaction mechanism at 1.9 eV that proceeds via CH3 rotation J. Mikosch et al., Science 319, 183 -186 (2008) Published by AAAS

    17. 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

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

    19. H H CH3(CH2)5 C HO Br C CH3 CH3 Question (+)-2-Bromooctane (–)-2-Octanol (CH2)5CH3 NaOH The absolute configurations of (+)-2-bromooctane and (–)-2-octanol are respectively: A) R- & R- B) S- and S- C) R- & S- D) S- & R-

    20. H H CH3(CH2)5 C HO Br C CH3 CH3 Answer (+)-2-Bromooctane (–)-2-Octanol (CH2)5CH3 NaOH The absolute configurations of (+)-2-bromooctane and (–)-2-octanol are respectively: A) R- & R- B) S- and S- C) R- & S- D) S- & R-

    21. 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-

    22. Question • True (A) / False (B) • A racemic mixture of (R- ) and (S- )-2-bromobutane produces an optically active product.

    23. Answer • True (A) / False (B) • A racemic mixture of (R- ) and (S- )-2-bromobutane produces an optically active product. • Optically inactive starting materials produce optically inactive products. The products in this case are also racemic. Inversion occurs with both enantiomers.

    24. A conceptual view of SN2 reactions

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

    26. Steric Effects in SN2 Reactions

    27. 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.

    28. 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

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

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

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

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

    33. Question • Which chloride will react faster with NaI in acetone? • A) B) • C) D)

    34. Answer • Which chloride will react faster with NaI in acetone? • A) B) • C) D)

    35. 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.

    36. 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

    37. Question • Which alkyl chloride will react faster with NaI in acetone? • A) B) • C) D)

    38. Answer • Which alkyl chloride will react faster with NaI in acetone? • A) B) • C) D)

    39. 8.1Functional Group Transformation By Nucleophilic Substitution

    40. – : X Y : Nucleophilic Substitution + + R Y R X • nucleophile is a Lewis base (electron-pair donor) • often negatively charged and used as Na+ or K+ salt • substrate is usually an alkylhalide, (most often 1o)

    41. The nucleophiles described in Sections 8.1-8.6are anions. – – – .. – .. .. : N3 : : : : N C HS HO CH3O .. .. .. .. .. : NH3 CH3OH HOH .. .. Nucleophiles – But, all nucleophiles (neutral electron rich molecules) are Lewis bases.

    42. .. R X R' O: .. gives an ether – + R : X O .. .. R' Table 8.1 Examples of Nucleophilic Substitution Alkoxide ion as the nucleophile +

    43. O R X gives an ester .. – O + R'C R : X O .. Table 8.1 Examples of Nucleophilic Substitution Carboxylate ion as the nucleophile – .. + R'C O: ..

    44. .. R X H S: .. gives a thiol .. – + H R : X S .. Table 8.1 Examples of Nucleophilic Substitution Hydrogen sulfide ion as the nucleophile +

    45. Question • Select the major organic product when (S)-2-propanol is reacted with SOCl2 in pyridine • followed by the addition of NaSH in ethanol. • A) B) • C) D)

    46. Answer • Select the major organic product when (S)-2-propanol is reacted with SOCl2 in pyridine • followed by the addition of NaSH in ethanol. • A) B) • C) D)

    47. Question • The best combination of reactants for preparing (CH3)3CSCH3 is: • A) (CH3)3CCl + CH3SK • B) (CH3)3CBr + CH3SNa • C) (CH3)3CSK + CH3OH • D) (CH3)3CSNa + CH3Br

    48. Answer • The best combination of reactants for preparing (CH3)3CSCH3 is: • A) (CH3)3CCl + CH3SK • B) (CH3)3CBr + CH3SNa • C) (CH3)3CSK + CH3OH • D) (CH3)3CSNa + CH3Br