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A little organic chemistry

A little organic chemistry. Nucleophilic Substitution. substitution reaction. Nucleophilic Substitution. Question. Identify the substrate, nucleophile, leaving group and product for each. Nucleophilic Substitution. Two mechanisms. general: Rate = k 1 [RX] + k 2 [RX][Y – ].

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A little organic chemistry

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  1. A little organic chemistry

  2. Nucleophilic Substitution substitution reaction

  3. Nucleophilic Substitution Question. Identify the substrate, nucleophile, leaving group and product for each.

  4. Nucleophilic Substitution Two mechanisms general: Rate = k1[RX] + k2[RX][Y–] k1 increases RX = CH3X 1º 2º 3º k2 increases k1 ~ 0 Rate = k2[RX][Y–] (bimolecular) SN2 k2 ~ 0 Rate = k1[RX] (unimolecular) SN1

  5. SN2 Mechanism Kinetics e.g., CH3I + OH– CH3OH + I– find: Rate = k[CH3I][OH–], i.e., bimolecular  both CH3I and OH– involved in RLS and recall, reactivity: R-I > R-Br > R-Cl >> R-F  C-X bond breaking involved in RLS  concerted, single-step mechanism: [HO---CH3---I]– CH3I + OH– CH3OH + I–

  6. SN2 Mechanism

  7. SN2 Mechanism Steric effects e.g., R–Br + I– R–I + Br– 1. branching at the a carbon ( X–C–C–C.... ) a b g Compound Rel. Rate methyl CH3Br 150 1º RX CH3CH2Br 1 2º RX (CH3)2CHBr 0.008 3º RX (CH3)3CBr ~0 increasing steric hindrance

  8. SN2 Mechanism Steric effects 1. branching at the a carbon minimal steric hindrance maximum steric hindrance

  9. SN2 Mechanism Steric effects branching at the a carbon • Reactivity toward SN2: CH3X > 1º RX > 2º RX >> 3º RX react readily by SN2 (k2 large) more difficult does not react by SN2 (k2 ~ 0)

  10. SN2 Mechanism Steric effects branching at the b carbon Rel. Rate 1 0.003 0.00001 increasing steric hindrance ~ no SN2 with very hindered substrates

  11. SN2 Mechanism Nucleophiles and nucleophilicity Summary: very good Nu: I–, HS–, RS–, H2N– good Nu: Br–, HO–, RO–, CN–, N3– fair Nu: NH3, Cl–, F–, RCO2– poor Nu: H2O, ROH very poor Nu: RCO2H

  12. Nucleophilic Substitution Leaving groups reactivity: R-I > R-Br > R-Cl >> R-F best L.G. most reactive worst L.G. least reactive precipitate drives rxn (Le Châtelier)

  13. SN2 Mechanism Question. Which reaction will proceed faster in each of the following pairs? What will be the product?

  14. SN1 Mechanism Kinetics e.g., 3º, no SN2 Find: Rate = k[(CH3)3CBr] unimolecular  RLS depends only on (CH3)3CBr

  15. SN1 Mechanism Kinetics

  16. SN1 Mechanism Kinetics Two-step mechanism: R+ RBr + CH3OH ROCH3 + HBr

  17. SN1 Mechanism Carbocation stability R+ stability: 3º > 2º >> 1º > CH3+ R-X reactivity toward SN1: 3º > 2º >> 1º > CH3X CH3+ 1º R+ 2º R+ 3º R+

  18. SN1 Mechanism Question. Which of the following compounds will react fastest by SN1? Which by SN2? A. B.

  19. SN1 vs SN2 Solvent effects nonpolar: hexane, benzene moderately polar: ether, acetone, ethyl acetate polar protic: H2O, ROH, RCO2H polar aprotic: DMSO DMF acetonitrile SN1 mechanism promoted by polar protic solvents stabilize R+, X– relative to RX in less polar solvents in more polar solvents R+X– RX

  20. SN1 vs SN2 Solvent effects SN2 mechanism promoted by moderately polar & polar aprotic solvents destabilize Nu–, make them more nucleophilic e.g., OH– in H2O: strong H-bonding to water makes OH– less reactive OH– in DMSO: weaker solvation makes OH– more reactive (nucleophilic) in DMSO in H2O RX + OH– ROH + X–

  21. SN1 vs SN2 Rate = k1[RX] + k2[RX][Nu] Summary rate of SN1 increases (carbocation stability) RX = CH3X 1º 2º 3º rate of SN2 increases (steric hindrance) react primarily by SN2 (k1 ~ 0, k2 large) may go by either mechanism reacts primarily by SN1 (k2 ~ 0, k1 large) SN2 promoted good nucleophile (Rate = k2[RX][Nu]) -usually in polar aprotic solvent SN1 occurs in absence of good nucleophile (Rate = k1[RX]) -usually in polar protic solvent (solvolysis)

  22. SN1 vs SN2 Question. What would be the predominant mechanism in each of the following reactions? What would be the product?

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