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Synthesis of Alkenes

Synthesis of Alkenes. Major approaches to the synthesis of alkenes: Dehydrohalogenation of Alkyl Halides E2 mechanism – most useful E1 mechanism Dehalogenation of Vicinal Dibromides Dehydration of Alcohols. Synthesis of Alkenes.

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Synthesis of Alkenes

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  1. Synthesis of Alkenes • Major approaches to the synthesis of alkenes: • Dehydrohalogenationof Alkyl Halides • E2 mechanism – most useful • E1 mechanism • Dehalogenation of Vicinal Dibromides • Dehydration of Alcohols

  2. Synthesis of Alkenes • Dehydrohalogenation can occur via either an E2 or E1 mechanism. • Loss of H+ and X- ions from adjacent carbons, forming a new pi bond NaOH D

  3. Synthesis of Alkenes • The most synthetically useful dehydrohalogenation reactions occur under E2 reaction conditions. • 3o or bulky 2o alkyl halide • strong bases • strong bulky bases are best when using 2o alkyl halides • less likely to undergo substitution reactions

  4. Synthesis of Alkenes • Common strong bulky bases triethylamine diisopropylamine t-butoxide ion 2,6-dimethylpyridine

  5. Synthesis of Alkenes • Mechanism of E2 Dehydrohalogenation • concerted reaction • anti-coplanar transition state

  6. Synthesis of Alkenes • E2 elimination reactions can take place in cyclohexanes only when proton and leaving group can get into a trans-diaxial arrangement • corresponds to anti-coplanar

  7. Synthesis of Alkenes • Strong, less hindered bases (MeO-, EtO-, etc) generally give the most substituted alkene (Saytzeff’s rule) as the major product.

  8. Synthesis of Alkenes • Strong, bulky bases usually give theHoffmann product (least highly substituted alkene) as the major product • bulky bases often abstract a proton from a less hindered carbon

  9. Synthesis of Alkenes Example: Predict all elimination product(s) of the following reactions. Which one is the major product?

  10. Synthesis of Alkenes Example: Predict all possible elimination products for the following reaction. Which one will be the major product?

  11. Synthesis of Alkenes • Dehalogenation of Vicinal Dibromides • two possible reagents • NaI (E2 mechanism) • Zn/HOAc (redox reaction)

  12. Synthesis of Alkenes • Dehalogenation using I- takes place via a concerted, stereospecific E2 mechanism Anti-coplanar conformation required Trans-diaxial conformation required for cycloalkanes

  13. Synthesis of Alkenes Example: Predict the major elimination product formed in the following reactions.

  14. H2SO4 D Synthesis of Alkenes • Dehydration of Alcohols • removal of water • equilibrium process • drive reaction to completion by removing alkene as formed (LeChatelier’s Principle)

  15. Synthesis of Alkenes • Typical reaction conditions • alcohol substrate • Order of reactivity: • 3o > 2o > 1o alcohol • acid catalyst • conc. H2SO4 • conc. H3PO4 • heat

  16. Synthesis of Alkenes Mechanism of Dehydration (E1) • Step 1: Protonation of the hydroxyl group (fast) • Step 2: Ionization (RDS) +

  17. Synthesis of Alkenes • Step 3: Proton abstraction (fast) • Rearrangements to form more stable carbonium ions are common in dehydration reactions. • Saytzeff’s product preferred.

  18. Synthesis of Alkenes Example: Propose a mechanism for the following reaction. C

  19. Synthesis of Alkenes • Step 1: Protonation of OH group • Step 2: Ionization with Methyl Shift

  20. Synthesis of Alkenes • Step 3: Abstraction of proton

  21. Synthesis of Alkenes Example: Predict the major product formed in the following reaction.

  22. Reactions of Alkenes • The most common reactions of alkenes are addition reactions: • the addition of a reagent to the pi bond with subsequent formation of new sigma bonds • number of elements of unsaturation decreases

  23. Reactions of Alkenes • The electrons in the p bond of C=C are delocalized above and below the sigma bond • more loosely held • In the presence of a strong electrophile, the double bond acts as a nucleophile, donating the p electrons to the electrophile and forming a new s bond.

  24. Reactions of Alkenes • Most reactions of alkenes are electrophilic addition reactions. Step 1: Attack of electrophile on pi bond forming a carbonium ion: Step 2: Nucleophile attacks carbonium ion giving product.

  25. Reactions of Alkenes • Addition of H-X to Alkenes

  26. Reactions of Alkenes • In the previous example, the proton added to the secondary carbon, forming the most stable carbonium ion. • Markovnikov’s Rule: • Asymmetric reagents such as H-X add to a C=C so that the proton adds to the carbon (in the double bond) that already has the greater number of hydrogen atoms. • “The rich get richer”

  27. Reactions of Alkenes • Markovnikov’s Rule (extended): • In an electrophilic addition to an alkene, the electrophile adds in such a way as to give the most stable intermediate.

  28. Reactions of Alkenes Example: Predict the product formed in each of the following reactions.

  29. Reactions of Alkenes • Anti-Markovnikov Addition of HBr • In the presence of peroxides, HBr adds to C=C via a free radical mechanism giving the “Anti-Markovnikov” product. Works only with HBr (not HCl or HI) due to relative bond strengths.

  30. Reactions of Alkenes • Some common peroxides: Benzoyl peroxide Acetyl peroxide Di-t-butyl peroxide Diethyl peroxide

  31. Reactions of Alkenes Example: Predict the product of the following reaction. HBr

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