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5.14 Dehydrohalogenation of Alkyl Halides

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

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

  4. Cl Dehydrohalogenation • is a useful method for the preparation of alkenes NaOCH2CH3 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 Stereoselectivity • more stable configurationof double bond predominates Br + (23%) (77%)

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

  9. 5.15Mechanism of theDehydrohalogenation 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

  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 • p 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 d– .. H R O .. Transition state C C d–

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

  17. 5.16Anti Elimination in E2 Reactions • Stereoelectronic Effects

  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. 5.17A Different Mechanism for Alkyl Halide Elimination:The E1 Mechanism

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

  27. 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.

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

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

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