1. 5.14 Dehydrohalogenation of Alkyl Halides

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

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

4. is a useful method for the preparation of alkenes (100 %) likewise, NaOCH 3 in methanol, or KOH in ethanol NaOCH 2 CH 3 ethanol, 55°C Dehydrohalogenation Cl

5. CH 3 (CH 2 ) 15 CH 2 CH 2 Cl When the alkyl halide is primary, potassium tert-butoxide in dimethyl sulfoxide is the base/solvent system that is normally used. KOC(CH 3 ) 3 dimethyl sulfoxide (86%) CH 2 CH 3 (CH 2 ) 15 CH Dehydrohalogenation

6. Br 29 % 71 % + Regioselectivity follows Zaitsev's rule more highly substituted double bond predominates KOCH 2 CH 3 ethanol, 70°C

7. more stable configuration of double bond predominates Stereoselectivity KOCH 2 CH 3 ethanol Br + (23%)(77%)

8. more stable configuration of double bond predominates Stereoselectivity KOCH 2 CH 3 ethanol + (85%)(15%)Br

9. 5.15 Mechanism 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

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

13. – O R.... : CCCCCCCCHX.. :: Reactants The E2 Mechanism

14. – O R.... : CCCCCCCCHX.. :: Reactants The E2 Mechanism

15. CCCCCCCC –––– OR.... H X..:: –––– Transition state The E2 Mechanism

16. OR.... H CCCCCCCC–X.. ::.. Products

17. Stereoelectronic Effects 5.16 Anti Elimination in E2 Reactions

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

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

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

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

22. (CH 3 ) 3 C KOC(CH 3 ) 3 (CH 3 ) 3 COH trans 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 H H (CH 3 ) 3 C BrHH Stereoelectronic effect

23. cis more reactive trans less reactive Stereoelectronic effect

24. 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. 5.17 A Different Mechanism for Alkyl Halide Elimination: The E1 Mechanism

26. CH 3 CH 2 CH 3 Br CH 3 Ethanol, heat + (25%) (75%) C H3CH3CH3CH3C CH 3 C C H3CH3CH3CH3C H CH 2 CH 3 CH 3 C H2CH2CH2CH2C Example

27. 1. Alkyl halides can undergo elimination in absence of base. 2. Carbocation is intermediate 3. Rate-determining step is unimolecular ionization of alkyl halide. The E1 Mechanism

28. slow, unimolecular C CH 2 CH 3 CH 3 + CH 2 CH 3 Br CH 3 C :.. : :.. : Br..– Step 1

29. C CH 2 CH 3 CH 3 + C CH 2 CH 3 CH 3 CH 2 + C CHCH 3 CH 3 – H + Step 2