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Dehydrohalogenation of Alkyl Halides

Dehydrohalogenation of Alkyl Halides. X. Y. C. C. C. C.  -Elimination Reactions. dehydrohalogenation of alkyl halides: X = H; Y = Br, etc. +. Y. X. . . X. Y. C. C. C. C.  -Elimination Reactions. dehydrohalogenation of alkyl halides: X = H; Y = Br, etc. +. Y.

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Dehydrohalogenation of Alkyl Halides

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  1. Dehydrohalogenation of Alkyl Halides

  2. X Y C C C C -Elimination Reactions • dehydrohalogenation of alkyl halides: X = H; Y = Br, etc. + Y X  

  3. X Y C C C C -Elimination Reactions • dehydrohalogenation of alkyl halides: X = H; Y = Br, etc. + Y X   requires base

  4. Cl Dehydrohalogenation NaOCH2CH3 ethanol, 55°C (100 %) likewise, NaOCH3 in methanol, or KOH in ethanol

  5. KOC(CH3)3 dimethyl sulfoxide CH3(CH2)15CH CH2 (86%) Dehydrohalogenation When the alkyl halide is primary, potassiumtert-butoxide in dimethyl sulfoxide (DMSO), a strong non-protic polar solvent is the base/solvent system that is normally used. CH3(CH2)15CH2CH2Cl

  6. KOCH2CH3 Br ethanol, 70°C Regioselectivity • follows Zaitsev's rule • more highly substituted double bond predominates + 71 % 29 %

  7. KOCH2CH3 ethanol Stereoselectivity • more stable configurationof double bond predominates Br + (23%) (77%)

  8. E2 Energy Diagram

  9. Question • How many alkenes would you expect to be formed from the E2 elimination of • 3-bromo-2-methylpentane? • A)2 • B)3 • C)4 • D)5

  10. Br KOCH2CH3 ethanol Stereoselectivity • more stable configurationof double bond predominates + (85%) (15%)

  11. The E2 Mechanism of Dehydrohalogenation of Alkyl Halides

  12. Empirical Data • (1) Dehydrohalogenation of alkyl halides exhibits second-order kinetics • first order in alkyl halide first order in base rate = k[alkyl halide][base] • implies that rate-determining step involves both base and alkyl halide; i.e., it is bimolecular

  13. Question • The reaction of 2-bromobutane with KOCH2CH3 in ethanol produces trans-2-butene. If the concentration of both reactants is doubled, what would be the effect on the rate of the reaction? • A) halve the rate • B) double the rate • C) quadruple the rate • D) no effect on the rate

  14. Empirircal Data • (2) Rate of elimination depends on halogen • weaker C—X bond; faster rate rate: RI > RBr > RCl > RF • implies that carbon-halogen bond breaks in the rate-determining step

  15. The E2 Mechanism • concerted (one-step) bimolecular process • single transition state • C—H bond breaks •  component of double bond forms • C—X bond breaks

  16. .. : R .. H C C : : X .. The E2 Mechanism – O Reactants

  17. : : X .. The E2 Mechanism – .. H R O .. Transition state C C –

  18. – •• CH3CH2 O •• •• E2 Mechanism / Transition State Br

  19. – .. : : X .. The E2 Mechanism .. H R O .. C C Products

  20. Question • Which one of the following best describes a mechanistic feature of the reaction of 3-bromopentane with sodium ethoxide? • A) The reaction occurs in a single step which is bimolecular. • B) The reaction occurs in two steps, both of which are unimolecular. • C) The rate-determining step involves the formation of the carbocation (CH3CH2)2CH+. • D) The carbon-halogen bond breaks in a rapid step that follows the rate-determining step.

  21. Stereochemistry:Anti Elimination in E2 Reactions • Stereoelectronic Effects

  22. E2 –Stereoelectronic Effect • Considerdehydrohalogenation of chlorocyclohexane. • Ananti-periplanar T.S. is required and only the chair conformation on the left alllowsfor the elimination to occur.

  23. Stereoelectronic Effect • An effect on reactivity that has its origin in the spatial arrangement of orbitals or bonds is called a stereoelectronic effect. • The preference for an anti coplanar arrangement of H and Br in the transition state for E2 dehydrohalogenation is an example of a stereoelectronic effect.

  24. Br (CH3)3C (CH3)3C Stereoelectronic Effect KOC(CH3)3(CH3)3COH cis-1-Bromo-4-tert- butylcyclohexane

  25. (CH3)3C Br (CH3)3C Stereoelectronic Effect trans-1-Bromo-4-tert- butylcyclohexane KOC(CH3)3(CH3)3COH

  26. Br (CH3)3C (CH3)3C Br (CH3)3C Stereoelectronic Effect cis KOC(CH3)3(CH3)3COH • Rate constant for dehydrohalogenation of 1,4- cis is >500 times than that of 1,4- trans KOC(CH3)3(CH3)3COH trans

  27. Br (CH3)3C (CH3)3C Stereoelectronic Effect cis KOC(CH3)3(CH3)3COH • H that is removed by base must be anti coplanar to Br • Two anti coplanar H atoms in cis stereoisomer H H

  28. H Br H (CH3)3C H H (CH3)3C Stereoelectronic Effect trans KOC(CH3)3(CH3)3COH • H that is removed by base must be anti coplanar to Br • No anti coplanar H atoms in trans stereoisomer; all vicinal H atoms are gauche to Br; therefore infinitesimal or no product is formed

  29. Question • Which of the two molecules below will NOT be able to undergo an E2 elimination reaction? B)

  30. Stereoelectronic Effect 1,4- cis more reactive 1,4- trans much less reactive

  31. E2 –Regioselectivity • Sterically unhindered bases favor the Zaitsev product. • Stericallyhindered bases favor the Hofmann product. • See: SKILLBUILDER 8.5.

  32. Question • Which would react with KOC(CH3)3/(CH3)3COH faster? • A)cis-3-tert-butylcyclohexyl bromide • B)trans-3-tert-butylcyclohexyl bromide

  33. Question • Which would react with KOCH2CH3 in ethanol faster? • A)cis-2-tert-butylcyclohexyl bromide • B)trans-2-tert-butylcyclohexyl bromide

  34. Question What is the major product of the following reaction?

  35. Question What is the major product of the following reaction?

  36. The E1 Mechanism ofDehydrohalogenation of Alkyl Halides

  37. C Ethanol, heat H3C CH3 H + C H2C C C CH3 CH2CH3 H3C (75%) (25%) Example CH3 CH2CH3 CH3 Br

  38. The E1 Mechanism • 1. Alkyl halides can undergo elimination in protic solvents in the absence of base. • 2. Carbocation is intermediate. • Rate-determining step is unimolecular ionization of alkyl halide.

  39. CH3 CH2CH3 C CH3 : : Br .. slow, unimolecular CH3 C + CH2CH3 CH3 .. – : : Br .. Step 1

  40. CH3 C + CH2CH3 CH3 – H+ CH3 CH2 + C C CHCH3 CH3 CH2CH3 CH3 Step 2

  41. Question • Which reaction would be most likely to proceed by an E1 mechanism? • A) 2-chloro-2-methylbutane + NaOCH2CH3 in ethanol (heat) • B) 1-bromo-2-methylbutane + KOC(CH3)3 in DMSO • C) 2-bromo-2-methylbutane in ethanol (heat) • D) 2-methyl-2-butanol + KOH

  42. Predicting Substitution vs. Elimination • Analyze the function of the reagent (nucleophile and/ or base). • Analyze the substrate (1°, 2°, or 3°).

  43. Predicting Substitution vs. Elimination • Analyze the function of the reagent (nucleophile and/ or base). • Analyze the substrate (1°, 2°, or 3°).

  44. Predicting Substitution vs. Elimination • Analyze the function of the reagent (nucleophile and/ or base). • Analyze the substrate (1°, 2°, or 3°).

  45. Predicting Substitution vs. Elimination • Analyze the function of the reagent (nucleophile and/ or base). • Analyze the substrate (1°, 2°, or 3°). • See SKILLBUILDER 8.11.

  46. Predicting Products • Analyze the function of the reagent (nucleophile and/ or base). • Analyze the substrate (1°, 2°, or 3°). • Consider regiochemistry and stereochemistry.

  47. Predicting Products • See:SKILLBUILDER 8.12.

  48. QuestionFor each reagent, predict which product will predominate. a. A = 3; B = 1; C = 2; D = 1; E = 1; F = 5 b. A = 4; B = 4; C = 2; D = 4; E = 5; F = 2 c. A = 2; B = 4; C = 2; D = 4; E = 5; F = 2 d. A = 4; B = 4; C = 1; D = 4; E = 3; F = 1 e. A = 3; B = 5; C = 2; D = 1; E = 3; F = 5

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