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5.14 Dehydrohalogenation of Alkyl Halides

5.14 Dehydrohalogenation of Alkyl Halides. H. Y. C. C. C. C.  -Elimination Reactions Overview. dehydrogenation of alkanes:  H; Y = H dehydration of alcohols:  H; Y = OH dehydrohalogenation of alkyl halides:  H; Y = Br, etc. +. Y. H. . . H. Y. C. C. C. C.

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5.14 Dehydrohalogenation of Alkyl Halides

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  1. 5.14 Dehydrohalogenation of Alkyl Halides

  2. H Y C C C C -Elimination Reactions Overview • dehydrogenation of alkanes: H; Y = H • dehydration of alcohols: H; Y = OH • dehydrohalogenation of alkyl halides: H; Y = Br, etc. + Y H  

  3. H Y C C C C -Elimination Reactions Overview • dehydrogenation of alkanes:industrial process; not regioselective • dehydration of alcohols:acid-catalyzed • dehydrohalogenation of alkyl halides:consumes base + Y H  

  4. Dehydrohalogenation A useful method for the preparation of alkenes NaOCH2CH3 Cl ethanol, 55°C (100 %) likewise, NaOCH3 in methanol, or KOH in ethanol

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

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

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

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

  9. 5.15Mechanism of the Dehydrohalogenation of Alkyl Halides: The E2 Mechanism

  10. Facts • (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 (second-order)

  11. Facts • (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

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

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

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

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

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

  17. Stereoelectronic Effects Isotope Effects 5.16 & 5.17Anti Elimination in E2 Reactions

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

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

  20. Br (CH3)3C (CH3)3C Br (CH3)3C Stereoelectronic effect cis KOC(CH3)3(CH3)3COH Rate constant for dehydrohalogenation of cis is 500 times greater than that of trans KOC(CH3)3(CH3)3COH trans

  21. Br (CH3)3C (CH3)3C Stereoelectronic effect cis KOC(CH3)3(CH3)3COH H that is removed by base must be anti periplanar to Br Two anti periplanar H atoms in cis stereoisomer H H

  22. 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 periplanar to Br No anti periplanar H atoms in trans stereoisomer; all vicinal H atoms are gauche to Br

  23. Stereoelectronic effect cis more reactive trans less reactive

  24. 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 periplanar arrangement of H and Br in the transition state for E2 dehydrohalogenation is an example of a stereoelectronic effect.

  25. Isotope effect Deuterium,D, is a heavy isotope of hydrogen but will undergo the same reactions. But the C-D bond is stronger so a reaction where the rate involves breaking a C-H (C-D) bond, the deuterated sample will have a reaction rate 3-8 times slower. In other words comparing the two rates, i.e. kH/kD = 3-8 WHEN the rate determining step involves breaking the C-H bond.

  26. 5.18A Different Mechanism for Alkyl Halide Elimination:The E1 Mechanism

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

  28. The E1 Mechanism 1. Alkyl halides can undergo elimination in absence of base. 2. Carbocation is intermediate 3. Rate-determining step is unimolecular ionization of alkyl halide. 4. Generally with tertiary halide, base is weak and at low concentration.

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

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

  31. End of Chapter 5

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