Chapter 7

1. Chapter 7 Alkenes and Alkynes 1 Properties and Synthesis. Elimination Reactions of Alkyl Halides

2. The (E) and (Z) system for Designating Alkene Diastereoisomers • Z = (zusammen) German for together • E = (Entgegen) German for opposite

3. Rules • 1. Assign an atom with greater atomic number as high priority on each side of the sp2 carbon

4. Rules • 2. If the two subsitutents attached to sp2 carbon start with the same atom (tie), move outward until tie breaks

5. Rules • If an atom is doubly bonded or triply bonded, the priority systems treats it as if it were single bonded

6. Example • Using E and Z designation for the following

7. Relative Stability of Alkenes • Cis isomer is less stable due to the greater strain from crowding the adjacent alkyl group

8. Overall Relative Stabilities of Alkenes • The greater the number of attached alkyl groups • The more highly substituted the carbon atoms of the double bond) the greater the alkene’s stability

9. Synthesis of Alkenes via Elmination • Two methods for alkene synthesis • Dehydrogenation of Alkyl halides • Dehydrogenation of Alcohol

10. Dehydrohalogenation • 1 step reaction • Rate = [Nu:-] [Substrate]

11. E2 mechanism: conditions • 2o or 3o alkyl halide should be used if possible • When a synthesis must begin with a 1o alkyl halide, then a bulky base should be used • Strong and nonpolarizable base such as alkoxides should be used in high concentration • Sodium ethoxide in ethanol and potassium ter-butoxide in tert-butyl alcohol are based typically used in promote E2 reactions • Elevated temperature is usually employed because it generally favors elimination over substitution

12. Zaitsev’s Rule: Formation of the More Substituted Alkene is Favored with a Small Base • Whenever an elimination occurs to give the more stable, more high substituted alkene  Zaitsev’s Rule • Dehydrohalogenation of many alkyl halides, yields more than one product.

13. Zaitsev’s Rule: Formation of the More Substituted Alkene is Favored with a Small Base • Transition state resemble s how alkene will be substituted

14. Formation of the Less subsitituted Alkene using a Bulky Base • Hoffmann Rule applies when elimination yields the less substituted alkene

15. The stereochemistry of E 2 Reactions: The Orientation of groups in the Transition State • Anti coplanar transition state is more preferrable • Staggered conformation • Syn coplanar transition state is prefferred only with rigid molecules • Eclipsed conformation

16. A mechanism Where There Two Axial βHydrogens

17. E2 Elimination Where the Only Axial β Hydrogen is from a less stable conformer

18. Example • When cis-1-bromo-4-tert-butylcyclohexane is treated with sodium ethoxide in ethanol, it reacts rapidly; the product is 4-tert-butylcyclohexene. Under the same condition , trans-1-bromo-4-ter-butylcyclohexane reacts very slowly. Write conformational structures and explain the difference in reactivity of these cis-trans isomers.

19. Acid-Catalyzed Dehydration of Alcohols • Removing water molecule to form an alkene • Dehydration • Favored high temperature • The temperature and concentration of acid required to dehydrate an alcohol depend on the structure of the alcohol substrate • Some primary alcohol and secondary alcohols also undergo rearrangements of their carbon skeletons during dehydration

20. Mechanism for Dehydration of Secondary and Tertiary Alcohol • E1 mechanism • Step 1: protonation – a proton is rapidly transferred from acid to one of the unshared electron pairs of alcohol • Step 2: The carbon-oxygen bond breaks heterolytically. The bonding electrons depart with the water molecule and leave behind a carbocation • Step 3: acid-base protonation

21. Example • Dehydration of 2-propanol occurs in 14M H2SO4 at 100oC • Using curved arrows, write all steps in a mechanism for the dehydration • Explain the essential role of performed in alcohol dehydration by the acid catalyst (hints: consider what would have happen if no acid were present?)