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8.11 Substitution and Elimination as Competing Reactions

8.11 Substitution and Elimination as Competing Reactions.  -elimination. –. +. +. :. H. Y. X. C. C. H. –. +. :. Y. C. C. H. X. –. +. :. X. C. C. Y. nucleophilic substitution. Two Reaction Types.

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8.11 Substitution and Elimination as Competing Reactions

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  1. 8.11Substitution and Eliminationas Competing Reactions

  2. -elimination – + + : H Y X C C H – + : Y C C H X – + : X C C Y nucleophilic substitution Two Reaction Types Alkyl halides can react with Lewis bases by nucleophilic substitution and/or elimination.

  3. – + : H Y X C C – : Y C C – + : X C C Two Reaction Types How can we tell which reaction pathway is followed for a particular alkyl halide? -elimination + H + H X Y nucleophilic substitution

  4. Elimination versus Substitution A systematic approach is to choose as a referencepoint the reaction followed by a typical alkyl halide(secondary) with a typical Lewis base (an alkoxideion). • The major reaction of a secondary alkyl halidewith an alkoxide ion is elimination by the E2mechanism.

  5. CH3CHCH3 Br CH3CHCH3 + CH3CH=CH2 OCH2CH3 (87%) (13%) Example NaOCH2CH3 ethanol, 55°C

  6. – •• CH3CH2 O •• •• Figure 8.11 E2 Br

  7. – •• CH3CH2 O •• •• Figure 8.7 SN2 Br

  8. When is substitution favored? Given that the major reaction of a secondaryalkyl halide with an alkoxide ion is elimination by the E2 mechanism, we can expect the proportion of substitution to increase with: • 1) decreased crowding at the carbon that bears the leaving group

  9. CH3CH2CH2Br NaOCH2CH3 ethanol, 55°C + CH3CH2CH2OCH2CH3 CH3CH=CH2 (9%) (91%) Uncrowded Alkyl Halides Decreased crowding at carbon that bears the leaving group increases substitution relative to elimination. • primary alkyl halide

  10. CH3(CH2)15CH2CH2Br KOC(CH3)3 tert-butyl alcohol, 40°C CH3(CH2)15CH2CH2OC(CH3)3 + CH3(CH2)15CH=CH2 (13%) (87%) But a crowded alkoxide base can favor elimination even with a primary alkyl halide. • primary alkyl halide + bulky base

  11. When is substitution favored? Given that the major reaction of a secondaryalkyl halide with an alkoxide ion is elimination by the E2 mechanism, we can expect the proportion of substitution to increase with: • 1) decreased crowding at the carbon that bears the leaving group • 2) decreased basicity of the nucleophile

  12. CH3CH(CH2)5CH3 Cl KCN pKa (HCN) = 9.1 DMSO CH3CH(CH2)5CH3 (70%) CN Weakly Basic Nucleophile Weakly basic nucleophile increases substitution relative to elimination secondary alkyl halide + weakly basic nucleophile

  13. I NaN3 pKa (HN3) = 4.6 N3 (75%) Weakly Basic Nucleophile Weakly basic nucleophile increases substitution relative to elimination secondary alkyl halide + weakly basic nucleophile

  14. Tertiary Alkyl Halides Tertiary alkyl halides are so sterically hinderedthat elimination is the major reaction with allanionic nucleophiles. Only in solvolysis reactionsdoes substitution predominate over eliminationwith tertiary alkyl halides.

  15. (CH3)2CCH2CH3 Br CH3 CH3 CH3 CH3CCH2CH3 CH3C=CHCH3 CH2=CCH2CH3 OCH2CH3 ethanol, 25°C 36% 64% 2M sodium ethoxide in ethanol, 25°C 99% 1% Example + +

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