1. Dr. Wolf's CHM 201 & 202 5-1 Chapter 5 ALKENES

2. Dr. Wolf's CHM 201 & 202 5-2 Alkene Nomenclature

3. Dr. Wolf's CHM 201 & 202 5-3 AlkenesAlkenes Alkenes are hydrocarbons that contain a carbon-carbon double bond also called "olefins" characterized by molecular formula C n H 2n said to be "unsaturated"

4. Dr. Wolf's CHM 201 & 202 5-4 Alkene Nomenclature H2CH2CH2CH2C CH 2 H2CH2CH2CH2C CHCH 3 Etheneor Ethylene (both are acceptable IUPAC names) Propene (Propylene is sometimes used but is not an acceptable IUPAC name)

5. Dr. Wolf's CHM 201 & 202 5-5 Alkene Nomenclature 1) Find the longest continuous chain that includes the double bond. 2) Replace the -ane ending of the unbranched alkane having the same number of carbons by -ene. 3) Number the chain in the direction that gives the lowest number to the doubly bonded carbon. H2CH2CH2CH2C CHCH 2 CH 3 1-Butene

6. Dr. Wolf's CHM 201 & 202 5-6 Alkene Nomenclature 4) If a substituent is present, identify its position by number. The double bond takes precedence over alkyl groups and halogens when the chain is numbered. The compound shown above is 4-bromo-3-methyl-1-butene. H2CH2CH2CH2C CHCHCH 2 Br CH 3

7. Dr. Wolf's CHM 201 & 202 5-7 Alkene Nomenclature 4) If a substituent is present, identify its position by number. Hydroxyl groups take precedence over the double bond when the chain is numbered. The compound shown above is 2-methyl-3-buten-1-ol. H2CH2CH2CH2C CHCHCH 2 OH CH 3

8. Dr. Wolf's CHM 201 & 202 5-8 Alkenyl groups methylenevinylallylisopropenyl CH H2CH2CH2CH2C H2CH2CH2CH2C CHCH 2 H2CH2CH2CH2C CCH 3 H2CH2CH2CH2C

9. Dr. Wolf's CHM 201 & 202 5-9 Cycloalkene Nomenclature 1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. Cyclohexene

10. Dr. Wolf's CHM 201 & 202 5-10 Cycloalkene Nomenclature 1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. 2) Number through the double bond in the direction that gives the lower number to the first-appearing substituent. CH 3 CH 2 CH 3

11. Dr. Wolf's CHM 201 & 202 5-11 Cycloalkene Nomenclature 1) Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. 2) Number through the double bond in the direction that gives the lower number to the first-appearing substituent. 6-Ethyl-1-methylcyclohexene CH 3 CH 2 CH 3

12. Dr. Wolf's CHM 201 & 202 5-12 Structure and Bonding in Alkenes

13. Dr. Wolf's CHM 201 & 202 5-13 Structure of Ethylene bond angles: H-C-H = 117° H-C-C = 121° bond distances: C—H = 110 pm C=C = 134 pm planar

14. Dr. Wolf's CHM 201 & 202 5-14      Framework of  bonds Each carbon is sp 2 hybridized Bonding in Ethylene

15. Dr. Wolf's CHM 201 & 202 5-15 Each carbon has a half-filled p orbital Bonding in Ethylene

16. Dr. Wolf's CHM 201 & 202 5-16 Side-by-side overlap of half- filled p orbitals gives a  bond Bonding in Ethylene

17. Dr. Wolf's CHM 201 & 202 5-17 Isomerism in Alkenes

18. Dr. Wolf's CHM 201 & 202 5-18 IsomersIsomers Isomers are different compounds that have the same molecular formula.

19. Dr. Wolf's CHM 201 & 202 5-19 IsomersIsomers StereoisomersStereoisomers Constitutional isomers

20. Dr. Wolf's CHM 201 & 202 5-20 IsomersIsomers StereoisomersStereoisomers Constitutional isomers different connectivity same connectivity; different arrangement of atoms in space

21. Dr. Wolf's CHM 201 & 202 5-21 IsomersIsomers StereoisomersStereoisomers Constitutional isomers consider the isomeric alkenes of molecular formula C 4 H 8

22. Dr. Wolf's CHM 201 & 202 5-22 2-Methylpropene 1-Butene cis-2-Butene trans-2-Butene C CHH H CH 2 CH 3 H3CH3CH3CH3C C C CH 3 HH H C C H3CH3CH3CH3CH C C H H H3CH3CH3CH3C H3CH3CH3CH3C

23. Dr. Wolf's CHM 201 & 202 5-23 2-Methylpropene 1-Butene cis-2-Butene C CHH H CH 2 CH 3 H CH 3 C C H3CH3CH3CH3CH C C H H H3CH3CH3CH3C H3CH3CH3CH3C Constitutional isomers

24. Dr. Wolf's CHM 201 & 202 5-24 2-Methylpropene 1-Butene trans-2-Butene C CHH H CH 2 CH 3 H3CH3CH3CH3C C C CH 3 HH C C H H H3CH3CH3CH3C H3CH3CH3CH3C Constitutional isomers

25. Dr. Wolf's CHM 201 & 202 5-25 cis-2-Butene trans-2-Butene H3CH3CH3CH3C C C CH 3 HH H C C H3CH3CH3CH3CH Stereoisomers

26. Dr. Wolf's CHM 201 & 202 5-26 Stereochemical Notation cis (identical or analogous substituents on same side) trans (identical or analogous substitutents on opposite sides)

27. Dr. Wolf's CHM 201 & 202 5-27 Figure 5.2 trans cis Interconversion of stereoisomeric alkenes does not normally occur. Requires that  component of double bond be broken.

28. Dr. Wolf's CHM 201 & 202 5-28 Figure 5.2 trans cis

29. Dr. Wolf's CHM 201 & 202 5-29 Naming Stereoisomeric Alkenes by the E-Z Notational System

30. Dr. Wolf's CHM 201 & 202 5-30 Stereochemical Notation cis and trans are useful when substituents are identical or analogous (oleic acid has a cis double bond) cis and trans are ambiguous when analogies are not obvious C C CH 3 (CH 2 ) 6 CH 2 CH 2 (CH 2 ) 6 CO 2 H H H Oleic acid

31. Dr. Wolf's CHM 201 & 202 5-31 ExampleExample What is needed: 1) systematic body of rules for ranking substituents 2) new set of stereochemical symbols other than cis and trans C C H FClBr

32. Dr. Wolf's CHM 201 & 202 5-32 C C The E-Z Notational System E :higher ranked substituents on opposite sides Z :higher ranked substituents on same side higher lower

33. Dr. Wolf's CHM 201 & 202 5-33 C C The E-Z Notational System E :higher ranked substituents on opposite sides Z :higher ranked substituents on same side higher lower

34. Dr. Wolf's CHM 201 & 202 5-34 C C The E-Z Notational System E :higher ranked substituents on opposite sides Z :higher ranked substituents on same side Entgegen higher higherlower lower

35. Dr. Wolf's CHM 201 & 202 5-35 C C C C The E-Z Notational System E :higher ranked substituents on opposite sides Z :higher ranked substituents on same side EntgegenZusammen higher higherlower lower lower higher lower higher

36. Dr. Wolf's CHM 201 & 202 5-36 C C C C The E-Z Notational System Answer: They are ranked in order of decreasing atomic number. EntgegenZusammen higher higherlower lower lower higher lower higher Question: How are substituents ranked?

37. Dr. Wolf's CHM 201 & 202 5-37 The Cahn-Ingold-Prelog (CIP) System The system that we use was devised by R. S. Cahn Sir Christopher Ingold Vladimir Prelog Their rules for ranking groups were devised in connection with a different kind of stereochemistry—one that we will discuss in Chapter 7—but have been adapted to alkene stereochemistry.

38. Dr. Wolf's CHM 201 & 202 5-38 (1)Higher atomic number outranks lower atomic number Br > FCl > H higher lower Br F Cl Hhigherlower C C Table 5.1 CIP Rules

39. Dr. Wolf's CHM 201 & 202 5-39 (1)Higher atomic number outranks lower atomic number Br > FCl > H (Z )-1-Bromo-2-chloro-1-fluoroethene higher lower Br F Cl Hhigherlower C C Table 5.1 CIP Rules

40. Dr. Wolf's CHM 201 & 202 5-40 (2) When two atoms are identical, compare the atoms attached to them on the basis of their atomic numbers. Precedence is established at the first point of difference. —CH 2 CH 3 outranks —CH 3 —C(C,H,H) Table 5.1 CIP Rules —C(H,H,H)

41. Dr. Wolf's CHM 201 & 202 5-41 (3) Work outward from the point of attachment, comparing all the atoms attached to a particular atom before proceeding further along the chain. —CH(CH 3 ) 2 outranks —CH 2 CH 2 OH —C(C,C,H) —C(C,H,H) Table 5.1 CIP Rules

42. Dr. Wolf's CHM 201 & 202 5-42 (4) Evaluate substituents one by one. Don't add atomic numbers within groups. —CH 2 OH outranks —C(CH 3 ) 3 —C(O,H,H) —C(C,C,C) Table 5.1 CIP Rules

43. Dr. Wolf's CHM 201 & 202 5-43 (5)An atom that is multiply bonded to another atom is considered to be replicated as a substituent on that atom. —CH=O outranks —CH 2 OH —C(O,O,H) —C(O,H,H) Table 5.1 CIP Rules (A table of commonly encountered substituents ranked according to precedence is given on the inside back cover of the text.)

44. Dr. Wolf's CHM 201 & 202 5-44 Physical Properties of Alkenes

45. Dr. Wolf's CHM 201 & 202 5-45  = 0 D C CHHHH  = 0.3 D H H H C C H3CH3CH3CH3C Dipole moments What is direction of dipole moment? Does a methyl group donate electrons to the double bond, or does it withdraw them?

46. Dr. Wolf's CHM 201 & 202 5-46  = 0 D C CHHHH  = 1.4 D C C HH ClH  = 0.3 D H H H C C H3CH3CH3CH3C Dipole moments Chlorine is electronegative and attracts electrons.

47. Dr. Wolf's CHM 201 & 202 5-47  = 1.4 D C C HH ClH  = 0.3 D H H H C C H3CH3CH3CH3C = 1.7 D H H Cl C C H3CH3CH3CH3C Dipole moments Dipole moment of 1- chloropropene is equal to the sum of the dipole moments of vinyl chloride and propene. 

48. Dr. Wolf's CHM 201 & 202 5-48  = 1.7 D  = 1.4 D C C HH ClH  = 0.3 D H H H C C H3CH3CH3CH3C H H Cl C C H3CH3CH3CH3C Dipole moments Therefore, a methyl group donates electrons to the double bond.

49. Dr. Wolf's CHM 201 & 202 5-49 Alkyl groups stabilize sp 2 hybridized carbon by releasing electrons.. R—C+ H—C+ is more stable than R—C H—C is more stable than R—C is more stable than H—C

50. Dr. Wolf's CHM 201 & 202 5-50 Relative Stabilities of Alkenes

51. Dr. Wolf's CHM 201 & 202 5-51 Double bonds are classified according to the number of carbons attached to them. H C C R HHmonosubstituted R' C C R HHdisubstituted H C C R H R' disubstituted H C C R H R' disubstituted

52. Dr. Wolf's CHM 201 & 202 5-52 Double bonds are classified according to the number of carbons attached to them. R' C C R HR"trisubstituted R' C C R R"'R"tetrasubstituted

53. Dr. Wolf's CHM 201 & 202 5-53 Substituent effects on alkene stability Electronic disubstituted alkenes are more stable than monosubstituted alkenes Steric trans alkenes are more stable than cis alkenes

54. Dr. Wolf's CHM 201 & 202 5-54 + 6O 2 4CO 2 + 8H 2 O 2700 kJ/mol 2707 kJ/mol 2717 kJ/mol 2710 kJ/mol Fig. 5.4 Heats of combustion of C 4 H 8 isomers.

55. Dr. Wolf's CHM 201 & 202 5-55 Substituent effects on alkene stability Electronic alkyl groups stabilize double bonds more than H more highly substituted double bonds are more stable than less highly substituted ones.

56. Dr. Wolf's CHM 201 & 202 5-56 Problem 5.8 Give the structure or make a molecular model of the most stable C 6 H 12 alkene. C C

57. Dr. Wolf's CHM 201 & 202 5-57 Problem 5.8 Give the structure or make a molecular model of the most stable C 6 H 12 alkene. C C H3CH3CH3CH3C H3CH3CH3CH3C CH 3

58. Dr. Wolf's CHM 201 & 202 5-58 Substituent effects on alkene stability Steric effects trans alkenes are more stable than cis alkenes cis alkenes are destabilized by van der Waals strain

59. Dr. Wolf's CHM 201 & 202 5-59 cis-2-butene trans-2-butene van der Waals strain due to crowding of cis-methyl groups Figure 5.5 cis and trans-2-Butene

60. Dr. Wolf's CHM 201 & 202 5-60 Fig. 5.5 cis and trans-2-butene cis-2-butene trans-2-butene van der Waals strain due to crowding of cis-methyl groups

61. Dr. Wolf's CHM 201 & 202 5-61 Van der Waals Strain Steric effect causes a large difference in stability between cis and trans-(CH 3 ) 3 CCH=CHC(CH 3 ) 3 cis is 44 kJ/mol less stable than trans C C HH CC CH 3 H3CH3CH3CH3C H3CH3CH3CH3C H3CH3CH3CH3C

62. Dr. Wolf's CHM 201 & 202 5-62 Cycloalkenes

63. Dr. Wolf's CHM 201 & 202 5-63 Cyclopropene and cyclobutene have angle strain. Larger cycloalkenes, such as cyclopentene and cyclohexene, can incorporate a double bond into the ring with little or no angle strain. CycloalkenesCycloalkenes

64. Dr. Wolf's CHM 201 & 202 5-64 cis-cyclooctene and trans-cyclooctene are stereoisomers cis-cyclooctene is 39 kJ/ mol more stable than trans-cyclooctene Stereoisomeric cycloalkenes cis-Cyclooctene trans-Cyclooctene H HHH

65. Dr. Wolf's CHM 201 & 202 5-65 trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C 11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability Stereoisomeric cycloalkenes trans-Cyclooctene HH

66. Dr. Wolf's CHM 201 & 202 5-66 Stereoisomeric cycloalkenes trans-Cyclododecene cis-Cyclododecene trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C 11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability

67. Dr. Wolf's CHM 201 & 202 5-67 trans-cyclooctene is smallest trans-cycloalkene that is stable at room temperature cis stereoisomer is more stable than trans through C 11 cycloalkenes cis and trans-cyclododecene are approximately equal in stability When there are more than 12 carbons in the ring, trans-cycloalkenes are more stable than cis. The ring is large enough so the cycloalkene behaves much like a noncyclic one. Stereoisomeric cycloalkenes

68. Dr. Wolf's CHM 201 & 202 5-68 Preparation of Alkenes: Elimination Reactions

69. Dr. Wolf's CHM 201 & 202 5-69 H Y dehydrogenation of alkanes:  H; Y = H dehydration of alcohols:  H; Y = OH dehydrohalogenation of alkyl halides:  H; Y = Br, etc.    -Elimination Reactions Overview C C C C +HY

70. Dr. Wolf's CHM 201 & 202 5-70 DehydrogenationDehydrogenation limited to industrial syntheses of ethylene, propene, 1,3-butadiene, and styrene important economically, but rarely used in laboratory-scale syntheses 750°C CH 3 CH 3 750°C CH 3 CH 2 CH 3 H2CH2CH2CH2C CH 2 + H2H2H2H2 H2CH2CH2CH2C CHCH 3 + H2H2H2H2

71. Dr. Wolf's CHM 201 & 202 5-71 Dehydration of Alcohols

72. Dr. Wolf's CHM 201 & 202 5-72 Dehydration of Alcohols (79-87%) (82%) H 2 SO 4 160°C CH 3 CH 2 OH H2CH2CH2CH2C CH 2 + H2OH2OH2OH2O OH H 2 SO 4 140°C + H2OH2OH2OH2O C OH CH 3 H3CH3CH3CH3C H 2 SO 4 heat CH 2 H3CH3CH3CH3CC H3CH3CH3CH3C + H2OH2OH2OH2O

73. Dr. Wolf's CHM 201 & 202 5-73 RR'R" OH C RR'H OH C RHH OH C Relative Reactivity tertiary: most reactive primary: least reactive

74. Dr. Wolf's CHM 201 & 202 5-74 Regioselectivity in Alcohol Dehydration: The Zaitsev Rule

75. Dr. Wolf's CHM 201 & 202 5-75 10 % 90 % HO H 2 SO 4 80°C + Regioselectivity A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective.

76. Dr. Wolf's CHM 201 & 202 5-76 Regioselectivity A reaction that can proceed in more than one direction, but in which one direction predominates, is said to be regioselective. 84 % 16 % H 3 PO 4 heat CH 3 OH +

77. Dr. Wolf's CHM 201 & 202 5-77 The Zaitsev Rule When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the  carbon having the fewest hydrogens. ROH CH 3 C C H R CH 2 R three protons on this  carbon

78. Dr. Wolf's CHM 201 & 202 5-78 The Zaitsev Rule When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the  carbon having the fewest hydrogens. ROH CH 3 C C H R CH 2 R two protons on this  carbon

79. Dr. Wolf's CHM 201 & 202 5-79 The Zaitsev Rule When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the  carbon having the fewest hydrogens. ROH CH 3 C C H R CH 2 R only one proton on this  carbon

80. Dr. Wolf's CHM 201 & 202 5-80 The Zaitsev Rule When elimination can occur in more than one direction, the principal alkene is the one formed by loss of H from the  carbon having the fewest hydrogens. R R CH 2 R CH 3 C C ROH C C H R CH 2 R only one proton on this  carbon

81. Dr. Wolf's CHM 201 & 202 5-81 The Zaitsev Rule Zaitsev Rule states that the elimination reaction yields the more highly substituted alkene as the major product. The more stable alkene product predominates.

82. Dr. Wolf's CHM 201 & 202 5-82 Stereoselectivity in Alcohol Dehydration

83. Dr. Wolf's CHM 201 & 202 5-83 StereoselectivityStereoselectivity A stereoselective reaction is one in which a single starting material can yield two or more stereoisomeric products, but gives one of them in greater amounts than any other. (25%) (75%) + OH H 2 SO 4 heat

84. Dr. Wolf's CHM 201 & 202 5-84 The Mechanism of the Acid-Catalyzed Dehydration of Alcohols

85. Dr. Wolf's CHM 201 & 202 5-85 The dehydration of alcohols and the reaction of alcohols with hydrogen halides share the following common features: 1)Both reactions are promoted by acids 2) The relative reactivity decreases in the order tertiary > secondary > primary These similarities suggest that carbocations are intermediates in the acid-catalyzed dehydration of alcohols, just as they are in the reaction of alcohols with hydrogen halides. A connecting point...

86. Dr. Wolf's CHM 201 & 202 5-86 first two steps of mechanism are identical to those for the reaction of tert-butyl alcohol with hydrogen halides Dehydration of tert-Butyl Alcohol C OH CH 3 H3CH3CH3CH3C H 2 SO 4 heat CH 2 H3CH3CH3CH3CC H3CH3CH3CH3C + H2OH2OH2OH2O

87. Dr. Wolf's CHM 201 & 202 5-87 Mechanism Step 1: Proton transfer to tert-butyl alcohol (CH 3 ) 3 C O H.. :HO + H O : + H+ fast, bimolecular tert-Butyloxonium ion..HH + O H :H:

88. Dr. Wolf's CHM 201 & 202 5-88 Mechanism Step 2: Dissociation of tert-butyloxonium ion to carbocation + (CH 3 ) 3 C O H :H+ slow, unimolecular (CH 3 ) 3 C O H :H: tert-Butyl cation +

89. Dr. Wolf's CHM 201 & 202 5-89 Mechanism Step 3: Deprotonation of tert-butyl cation. fast, bimolecular + O H :H: CH 2 + H3CH3CH3CH3C C H3CH3CH3CH3CH H3CH3CH3CH3CC H3CH3CH3CH3C + O H :HH +

90. Dr. Wolf's CHM 201 & 202 5-90 CarbocationsCarbocations are intermediates in the acid-catalyzed dehydration of tertiary and secondary alcohols Carbocations can: react with nucleophiles lose a  -proton to form an alkene (Called an E1 mechanism)

91. Dr. Wolf's CHM 201 & 202 5-91 Dehydration of primary alcohols A different mechanism from 3 o or 2 o alcohols avoids carbocation because primary carbocations are too unstable oxonium ion loses water and a proton in a bimolecular step H 2 SO 4 160°C CH 3 CH 2 OH H2CH2CH2CH2C CH 2 + H2OH2OH2OH2O

92. Dr. Wolf's CHM 201 & 202 5-92 Step 1: Proton transfer from acid to ethanol H.. :HO + O CH 3 CH 2..HH H O : + H + fast, bimolecular Ethyloxonium ion CH 3 CH 2 O H : H : Mechanism

93. Dr. Wolf's CHM 201 & 202 5-93 Step 2: Oxonium ion loses both a proton and a water molecule in the same step. + H O :H+ CH 2 H O H :H: slow, bimolecular + O H : H : O H H : H + H2CH2CH2CH2C CH 2 + Mechanism

94. Dr. Wolf's CHM 201 & 202 5-94 Step 2: Oxonium ion loses both a proton and a water molecule in the same step. + H O : H + CH 2 H O H : H : slow, bimolecular + O H : H : O H H : H + H2CH2CH2CH2C CH 2 + Mechanism Because rate-determining step is bimolecular, this is called the E2 mechanism.

95. Dr. Wolf's CHM 201 & 202 5-95 Rearrangements in Alcohol Dehydration Sometimes the alkene product does not have the same carbon skeleton as the starting alcohol.

96. Dr. Wolf's CHM 201 & 202 5-96 ExampleExample OH H 3 PO 4, heat 3% 64% 33% ++

97. Dr. Wolf's CHM 201 & 202 5-97 Rearrangement involves alkyl group migration 3% CH 3 CHCH 3 CH 3 + C carbocation can lose a proton as shown or it can undergo a methyl migration CH 3 group migrates with its pair of electrons to adjacent positively charged carbon

98. Dr. Wolf's CHM 201 & 202 5-98 Rearrangement involves alkyl group migration 3% CH 3 CHCH 3 CH 3 + 97% CHCH 3 CH 3 + C C tertiary carbocation; more stable

99. Dr. Wolf's CHM 201 & 202 5-99 Rearrangement involves alkyl group migration 3% CH 3 CHCH 3 CH 3 + 97% CHCH 3 CH 3 + C C

100. Dr. Wolf's CHM 201 & 202 5-100 Another rearrangement CH 3 CH 2 CH 2 CH 2 OH H 3 PO 4, heat 12% + mixture of cis (32%) and trans-2-butene (56%) CH 2 CH 3 CH 2 CH CHCH 3 CH 3 CH

101. Dr. Wolf's CHM 201 & 202 5-101 Rearrangement involves hydride shift oxonium ion can lose water and a proton (from C-2) to give1-butene doesn't give a carbocation directly because primary carbocations are too unstable CH 3 CH 2 CH 2 CH 2 OHH + : CH 2 CH 3 CH 2 CH

102. Dr. Wolf's CHM 201 & 202 5-102 Rearrangement involves hydride shift hydrogen migrates with its pair of electrons from C-2 to C-1 as water is lost carbocation formed by hydride shift is secondary CH 3 CH 2 CH 2 CH 2 OHH + : CH 2 CH 3 CH 2 CH CH 3 CH 2 CHCH 3 +

103. Dr. Wolf's CHM 201 & 202 5-103 Hydride shift H H CH 3 CH 2 CHCH 2 O H+: + + H OHH : :

104. Dr. Wolf's CHM 201 & 202 5-104 Rearrangement involves hydride shift CH 3 CH 2 CH 2 CH 2 OHH + : CH 2 CH 3 CH 2 CH CH 3 CH 2 CHCH 3 + mixture of cis and trans-2-butene CHCH 3 CH 3 CH

105. Dr. Wolf's CHM 201 & 202 5-105 Carbocations can... react with nucleophiles lose a proton from the  -carbon to form an alkene rearrange (less stable to more stable) (alkyl shift or hydride shift)

106. Dr. Wolf's CHM 201 & 202 5-106 Dehydrohalogenation of Alkyl Halides

107. Dr. Wolf's CHM 201 & 202 5-107 H Y dehydrogenation of alkanes:  H; Y = H dehydration of alcohols:  H; Y = OH dehydrohalogenation of alkyl halides:  H; Y = Br, etc.    -Elimination Reactions Overview C C C C +HY

108. Dr. Wolf's CHM 201 & 202 5-108 H Y dehydrogenation of alkanes: industrial process; not regioselective dehydration of alcohols: acid-catalyzed dehydrohalogenation of alkyl halides: consumes base    -Elimination Reactions Overview C C C C +HY

109. Dr. Wolf's CHM 201 & 202 5-109 DehydrohalogenationDehydrohalogenation A useful method for the preparation of alkenes Cl (100 %) likewise, NaOCH 3 in methanol, or KOH in ethanol NaOCH 2 CH 3 ethanol, 55°C

110. Dr. Wolf's CHM 201 & 202 5-110 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%) DehydrohalogenationDehydrohalogenation CH 2 CH 3 (CH 2 ) 15 CH

111. Dr. Wolf's CHM 201 & 202 5-111 Br 29 % 71 % + Regioselectivity follows Zaitsev's rule: more highly substituted double bond predominates KOCH 2 CH 3 ethanol, 70°C

112. Dr. Wolf's CHM 201 & 202 5-112 more stable configuration of double bond predominates Stereoselectivity KOCH 2 CH 3 ethanol Br + (23%) (77%)

113. Dr. Wolf's CHM 201 & 202 5-113 more stable configuration of double bond predominates Stereoselectivity KOCH 2 CH 3 ethanol + (85%) (15%)Br

114. Dr. Wolf's CHM 201 & 202 5-114 Mechanism of the Dehydrohalogenation of Alkyl Halides: The E2 Mechanism

115. Dr. Wolf's CHM 201 & 202 5-115 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 (second-order)

116. Dr. Wolf's CHM 201 & 202 5-116 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

117. Dr. Wolf's CHM 201 & 202 5-117 The E2 Mechanism concerted (one-step) bimolecular process single transition state C—H bond breaks  component of double bond forms C—X bond breaks

118. Dr. Wolf's CHM 201 & 202 5-118 The E2 Mechanism – O R.... : CCCCCCCCHX.. :: Reactants

119. Dr. Wolf's CHM 201 & 202 5-119 The E2 Mechanism – O R.... : CCCCCCCCHX.. :: Reactants

120. Dr. Wolf's CHM 201 & 202 5-120 CCCCCCCC The E2 Mechanism –––– OR.... H X..:: –––– Transition state

121. Dr. Wolf's CHM 201 & 202 5-121 The E2 Mechanism OR.... H CCCCCCCC–X.. ::.. Products

122. Dr. Wolf's CHM 201 & 202 5-122 Anti Elimination in E2 Reactions Stereoelectronic Effects Isotope Effects

123. Dr. Wolf's CHM 201 & 202 5-123 (CH 3 ) 3 C Br KOC(CH 3 ) 3 (CH 3 ) 3 COH cis-1-Bromo-4-tert- butylcyclohexane Stereoelectronic effect

124. Dr. Wolf's CHM 201 & 202 5-124 (CH 3 ) 3 C Br KOC(CH 3 ) 3 (CH 3 ) 3 COH trans-1-Bromo-4-tert- butylcyclohexane Stereoelectronic effect

125. Dr. Wolf's CHM 201 & 202 5-125 (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

126. Dr. Wolf's CHM 201 & 202 5-126 (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

127. Dr. Wolf's CHM 201 & 202 5-127 (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

128. Dr. Wolf's CHM 201 & 202 5-128 cis more reactive trans less reactive Stereoelectronic effect

129. Dr. Wolf's CHM 201 & 202 5-129 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.

130. Dr. Wolf's CHM 201 & 202 5-130 Isotope effect Deuterium,D, is a heavy isotope of hydrogen but will undergo the same reactions. But the C-D bond is stronger so a reaction where the rate involves breaking a C-H (C-D) bond, the deuterated sample will have a reaction rate 3-8 times slower. In other words comparing the two rates, i.e. k H /k D = 3-8 WHEN the rate determining step involves breaking the C-H bond.

131. Dr. Wolf's CHM 201 & 202 5-131 A Different Mechanism for Alkyl Halide Elimination: The E1 Mechanism

132. Dr. Wolf's CHM 201 & 202 5-132 ExampleExample 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

133. Dr. Wolf's CHM 201 & 202 5-133 1. Alkyl halides can undergo elimination in absence of base. 2. Carbocation is intermediate 3. Rate-determining step is unimolecular ionization of alkyl halide. 4.Generally with tertiary halide, base is weak and at low concentration. The E1 Mechanism

134. Dr. Wolf's CHM 201 & 202 5-134 Step 1 slow, unimolecular C CH 2 CH 3 CH 3 + CH 2 CH 3 Br CH 3 C :.. : :.. : Br..–

135. Dr. Wolf's CHM 201 & 202 5-135 Step 2 C CH 2 CH 3 CH 3 + C CH 2 CH 3 CH 3 CH 2 + C CHCH 3 CH 3 – H +

136. Dr. Wolf's CHM 201 & 202 5-136 End of Chapter 5