Chapter 5:

1. Chapter 5: Structure and Preparation of Alkenes: Elimination Reactions

2. Alkanes and Alkenes: • Alkanes: CnH2n+2 (saturated) • Alkenes: CnH2n (unsaturated)

3. Some naturally occurring alkenes: unconjugated and conjugated

4. Nomenclature of Alkenes: (-enes) • Step 1: Find longest continuous chain that contains the C=C double bond. This is the parent. Replace “ane” with “ene” • longest chain containing the C=C double bond is 6carbons: parent is hexene

5. Nomenclature of Alkenes: • Step 2: Number carbons to give the C=C the lowest number, regardless of what other alkyl substituents are present.

6. Nomenclature of Alkenes: • But if an OH group is present, it has higher priority than double bond:

7. Nomenclature of Alkenes: • Step 3: Assign numbers to substituents and to the first carbon in the C=C double bond: 6-bromo-5,5-dimethyl-2-heptene (almost finished, but not quite)

8. Nomenclature of Cycloalkenes: • Parent is cycloalkene • Carbon in C=C bond must be number 1 1-isopropylcyclohexene 3,5-difluorocyclopentene 1,8,8-trimethylcyclooctene

9. Nomenclature of Alkenols • If have both C=C and OH, alcohol takes precedence: • Number longest chain containing OH • Number to give OH lowest number • Parent is x-alken-y-ol where x and y are numbers 2-cyclohexenol 6-hepten-2-ol

10. Nomenclature of Alkenyl Substituents: • Common names:

11. Nomenclature of Substituents: • If have C=C attached to ring, name cycloalkane as the parent: • methylenecyclohexane

12. I. Structure • Bonding: • geometry is trigonal planar • all atoms are co-planar (lie in a plane) • hybridization is sp2 • bond angles are 120o • double bond is one sigma (s) bond and one pi (p) bond

13. Geometry: no rotation around double bonds! • How to draw alkenes: • all atoms in double bond in plane- plane of paper hydrogens coming out and going back

14. Geometric isomers in alkenes: • No free rotation around C=C bond: • trans more stable than cis:

15. Geometric isomers (cis-trans isomers : • Lack of rotation around C=C bond can result in different isomers: isomers have different physical properties • for internal, disubstituted alkenes which have two different atoms or groups of atoms attached to each double-bonded carbon

16. Draw line through C=C: • If atoms are both above the line or are both below the line, they are on the sameside of the C=C bond and are cis (A and B are cis to each other and so are D and C) • If atoms are on different sides (one above, one below) of the C=C bond, they are trans. (A and C are trans to each other, and so are B and D) • If they are on the same carbon, they are neither cis nor trans. (A and D are on the same carbon, and so are B and C)

17. Examples:

18. Examples: • Cycloalkanes also have cis and trans isomers (remember Chapter 3?) • Cycloalkenes are usually cis (ring strain):

19. Practice: • Classify the following as cis, trans or neither:

20. Practice:

21. II. Structure- Property Relationships • Physical properties • Similar to alkanes • Dipole moments • alkyl groups are electron-releasing • inductive effect • hyperconjugation • stability depends upon structure

22. Classification of Alkenes and Alkynes • Monosubstituted - one alkyl group, three hydrogens • Disubstituted - two alkyl groups, two hydrogens

23. Classification of Alkenes and Alkynes • Trisubstituted - three alkyl groups, one hydrogen • Tetrasubstituted - four alkyl groups, no hydrogens on the carbons in the double bond.

24. Stability of Alkenes: • The higher the substitution, the more stable the alkene: • tetrasubstituted > trisubstituted > disubstituted > monosubstituted > non-substituted (ethylene) • Internal alkene is more stable than terminal alkene • Trans- alkene is more stable than Cis- alkene • If more than one alkene can form in a reaction, the more highly substituted one will form preferentially.

25. Stability of Alkenes: Combustion Analysis

26. Stability of Alkenes: • Electronic effects: alkyl groups are electron-releasing, help stabilize C=C • Steric effects: (van der Waals strain): large groups are more stable farther apart • for cis- and trans-butene, DH is 3 kcal • for di-tert-butyl groups, DH is 44 kcal

27. Stability of Cycloalkenes: • Cyclopropene and cyclobutene have considerable ring strain • Ring size C=3 to C=7, cis C=C • cyclooctene is big enough to be trans • 12 carbons in rings: trans is more stable

28. III. Preparation of Alkenes: Elimination • If Y = OH, dehydration of an alcohol • If Y = Cl, Br or I, dehydrohalogenation of an alkyl halide

29. Dehydration of an alcohol • C=C double bond forms between carbon with the OH and a H on an adjacent carbon: • acid-catalyzed: uses H2SO4 and H3PO4 to drive equilibrium

30. A. Stereoselectivity in Alcohol Dehydration • Predicting the products: • could form two products:

31. Zaitsev’s Rule: most stable alkene is formed preferentially • remove hydrogen from b-carbon having the fewest hydrogen substituents • i.e. more substituted (more stable!) alkene predominates

32. Stability of Alkenes • Substitution pattern: • tetrasubstituted > trisubstituted > disubstituted > monosubstituted • Geometry: • trans > cis > terminal alkene

33. Predict the product:

34. Mechanism of Alcohol Dehydration (E1) • Step 1: protonation of the alcohol: • Step 2: loss of water to form carbocation: • Step 3: deprotonation of the carbocation

35. Mechanism of Dehydration • Step 1: protonation of the alcohol: • similar to first step of SN1 reaction • acid-base reaction • very fast • low energy of activation • exothermic

36. Mechanism of Acid-Catalyzed E1 Reaction • Step 2: loss of water to form carbocation: • just like SN1 reaction: • unimolecular • rate-determining step • high energy of activation--height depends upon stability of intermediate carbocation • endothermic

37. Dehydration Reactions (E1) • Step 3: dehydration • new reaction: acid-base reaction • carbocation is strong acid, donating a proton to base (water here) to form double bond • reaction is very fast • low energy of activation

38. Acid-catalyzed Dehydration of Primary Alcohols • Energy of primary carbocation too high! • Step 1: protonation of the alcohol: • Step 2: concerted loss of water to form alkene:

39. Mechanism of Acid-Catalyzed Dehydration • Step 1: Protonation of alcohol (fast) • Step 2: Dissociation and loss of water to form carbocation (rate-determining step) • Step 3: Rearrangement to form more stable carbocation (if possible)(fast) • Step 4: Deprotonation of carbocation (fast)

40. Predict the Products: • Give the products:

41. Mechanism for Primary Alcohol • Hydride shift:

42. B. Dehydrohalogenation (E2 Mechanism) • Requires strongbase (OH-, CH3O-, CH3CH2O-, tert-butoxide) in alcohol or water. • No rearrangement: C=C bond forms between carbon with halogen and adjacent carbon

43. C. E1 Mechanism • First order reaction: k = [alkyl halide] • tertiary RX > secondary >>>>>>>>>>primary • primary and methyl don’t react this way • 2-step mechanism: • Step 1: dissociation to yield carbocation • Step 2: deprotonation to yield alkene • polar solvents, weak bases (If strong base, mechanism goes E2)

44. C. E1 Mechanism • Step 1: Dissociation to form carbocation • Step 2: Loss of proton to form alkene

45. Summary of Mechanisms for Alcohols: • Tertiary and secondary alcohols undergo SN1 and E1 reactions: • SN1 predominates if have HCl, HBr, or HI • E1 predominates if H2SO4 or H3PO4 • Both SN1 and E1 involve carbocation intermediate • Rearrangements will occur if possible • Primary alcohols react by hydride or methide shifts to form a more stable carbocation