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8.6 Unimolecular Nucleophilic Substitution S N 1

8.6 Unimolecular Nucleophilic Substitution S N 1. Tertiary alkyl halides are very unreactive in substitutions that proceed by the S N 2 mechanism. But they do undergo nucleophilic substitution. But by a mechanism different from S N 2.

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8.6 Unimolecular Nucleophilic Substitution S N 1

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  1. 8.6Unimolecular Nucleophilic Substitution SN1

  2. Tertiary alkyl halides are very unreactive in substitutions that proceed by the SN2 mechanism. But they do undergo nucleophilic substitution. But by a mechanism different from SN2. The most common examples are seen in solvolysis reactions.

  3. CH3 H CH3 .. : : O : C Br .. H CH3 CH3 CH3 .. .. : OH C .. .. CH3 Example of a solvolysis: Hydrolysis of tert-butyl bromide + + Br H

  4. CH3 CH3 H H CH3 CH3 .. : : O : C Br .. H H CH3 CH3 .. – CH3 : : Br CH3 .. .. .. : Br H OH C .. .. CH3 Example of a solvolysis: Hydrolysis of tert-butyl bromide + + : O C + +

  5. CH3 CH3 H H CH3 CH3 : : O H H CH3 CH3 .. – : : Br .. Example of a solvolysis: Hydrolysis of tert-butyl bromide .. + + : : C Br O C .. + This is the nucleophilic substitutionstage of the reaction; the one withwhich we are concerned.

  6. CH3 CH3 H H CH3 CH3 : : O H H CH3 CH3 .. – : : Br .. Example of a solvolysis: Hydrolysis of tert-butyl bromide .. + + : : C Br O C .. + The reaction rate is independentof the concentration of the nucleophileand follows a first-order rate law. rate = k[(CH3)3CBr]

  7. CH3 CH3 H H CH3 CH3 .. + : : : O : C Br O C .. H H CH3 CH3 .. – : : Br .. Example of a solvolysis: Hydrolysis of tert-butyl bromide + + The mechanism of this step isnot SN2. It is called SN1 and begins with ionization of (CH3)3CBr.

  8. rate = k[alkyl halide] First-order kinetics implies a unimolecularrate-determining step. Proposed mechanism is called SN1, which stands forsubstitution nucleophilic unimolecular

  9. CH3 CH3 .. : Br C .. CH3 H3C CH3 .. – + : : C Br .. CH3 unimolecular slow +

  10. H3C CH3 H + C : : O CH3 H CH3 H CH3 + : O C H CH3 bimolecular fast

  11. + ROH2 carbocation formation carbocation capture R+ proton transfer RX ROH

  12. Characteristics of the SN1 mechanism first order kinetics: rate = k[RX] unimolecular rate-determining step carbocation intermediate rate follows carbocation stability rearrangements sometimes observed reaction is not stereospecific much racemization in reactions of optically active alkyl halides

  13. The rate of nucleophilic substitutionby the SN1 mechanism is governedby electronic effects. Carbocation formation is rate-determining.The more stable the carbocation, the fasterits rate of formation, and the greater the rate of unimolecular nucleophilic substitution.

  14. Table 8.5 Reactivity toward substitutionby the SN1 mechanism RBr solvolysis in aqueous formic acid Alkyl bromide Class Relative rate CH3Br Methyl 1 CH3CH2Br Primary 2 (CH3)2CHBr Secondary 43 (CH3)3CBr Tertiary 100,000,000

  15. Decreasing SN1 reactivity (CH3)3CBr (CH3)2CHBr CH3CH2Br CH3Br

  16. 8.8Stereochemistry of SN1 Reactions

  17. Nucleophilic substitutions that exhibitfirst-order kinetic behavior are not stereospecific.

  18. H CH3 Br C CH3(CH2)5 H CH3 OH C CH3(CH2)5 Stereochemistry of an SN1 Reaction R-(–)-2-Bromooctane H CH3 H2O C HO (CH2)5CH3 (R)-(–)-2-Octanol (17%) (S)-(+)-2-Octanol (83%)

  19. Figure 8.8 Ionization stepgives carbocation; threebonds to stereogeniccenter become coplanar + –

  20. Figure 8.8 + Leaving group shieldsone face of carbocation;nucleophile attacks faster at opposite face. –

  21. – – More than 50% Less than 50% Figure 8.8 + –

  22. 8.11Carbocation Rearrangements in SN1 Reactions

  23. Because carbocations are intermediatesin SN1 reactions, rearrangementsare possible.

  24. CH3 CH3 H2O C CHCH3 C CH2CH3 CH3 CH3 (93%) H Br OH Example

  25. Example CH3 CH3 C CHCH3 C CH2CH3 CH3 CH3 (93%) H Br OH H2O CH3 CH3 C CHCH3 C CHCH3 CH3 CH3 + + H H

  26. 8.10Solvent Effects

  27. SN1 Reaction Rates Increase in Polar Solvents

  28. Most polar Fastest rate Table 8.6SN1 Reactivity versus Solvent Polarity Solvent Dielectric Relative constant rate acetic acid 6 1 methanol 33 4 formic acid 58 5,000 water 78 150,000

  29. d+R X d- transition state stabilized by polar solvent R+ energy of RX not much affected by polarity of solvent RX

  30. d+R X d- transition state stabilized by polar solvent activation energy decreases; rate increases R+ energy of RX not much affected by polarity of solvent RX

  31. SN2 Reaction Rates Increase inPolar Aprotic Solvents An aprotic solvent is one that doesnot have an —OH group. So it does not solvate anion well allowing it to be effective nucleophile

  32. Table 8.7SN2 Reactivity versus Type of Solvent Table 8.7 SN2 Reactivity vs Solvent CH3CH2CH2CH2Br + N3– Solvent Type Relative rate CH3OH polar protic 1 H2O polar protic 7 DMSO polar aprotic 1300 DMF polar aprotic 2800 Acetonitrile polar aprotic 5000

  33. Mechanism SummarySN1 and SN2

  34. When... primary alkyl halides undergo nucleophilic substitution, they always react by the SN2 mechanism tertiary alkyl halides undergo nucleophilic substitution, they always react by the SN1 mechanism secondary alkyl halides undergo nucleophilic substitution, they react by the SN1 mechanism in the presence of a weak nucleophile (solvolysis) SN2 mechanism in the presence of a good nucleophile

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