2. 西北农林科技大学理学院 By Junru Wang Email: wangjr07@163.com Chapter 08 & 09 Alkenes Organic Chemistry, 6 th Edition L. G. Wade, Jr. Key Notes Electrophile; Electrophilic addition; Carbocation stability; Conjugated dienes

3. Content  Introduction & Properties  Preparation  Reactions of alkenes Electrophilic addition Carbocation stabilization Free-Radical Addition of HBr Reduction and oxidation Allylic halogenation  Conjugated dienes

4. Key Notes  Electrophile  Electrophilic addition  Carbocation stability  Conjugated dienes

5. Sec 1 Introduction & Properties  Hydrocarbon with carbon-carbon double bonds  Sometimes called olefins, “ oil-forming gas ”  Pi bond is the functional group.  More reactive than sigma bond.  Bond dissociation energies: C=C BDE 611 kJ/mol C-C BDE -347 kJ/mol Pi bond264 kJ/mol

6. Alkenes structure  Molecule is planar around the double bond.  The carbon atoms of the C=C bond are sp hybridized and  Double bond is made up of one  bond and one  bond.

7. Alkenes structure  C=C Bond rotation round a C=C bond is not possible Isomers are possible depending on the substituents present.

8. Bond Lengths and Angles  Hybrid orbitals have more s character.  Pi overlap brings carbon atoms closer.  Bond angle with pi orbital increases. Angle C=C-H is 121.7  Angle H-C-H is 116. 6 

9. Pi Bond  Sideways overlap of parallel p orbitals.  Cis and trans isomers cannot be interconverted.  No rotation around the carbon — carbon bond is possible without breaking the pi bond (264 kJ/mol).

10. Cyclic Alkene Compounds  Trans cycloalkenes are not stable unless the ring has at least eight carbons.  Cycloalkenes are assumed to be cis unless otherwise specifically named trans.

11. Commercial Uses: Ethylene

12. Commercial Uses of Propylene

13. Addition Polymers

14. Physical Properties  Low boiling points, increasing with mass.  Branched alkenes have lower boiling points.  Less dense than water.  Slightly polar Pi bond is polarizable, so instantaneous dipole- dipole interactions occur. Alkyl groups are electron-donating toward the pi bond, so may have a small dipole moment.

15. Physical Properties Similar to alkanes, haloalkanes.  = 1.7 D b.p. = 60ºc  = 0 D b.p. = 48ºc Polarity  = 0.33 D  = 0

16. Stability of Alkenes  Measured by heat of hydrogenation: Alkene + H 2  Alkane + energy  More heat released, higher energy alkene.

17. Substituent Effects Among constitutional isomers, more substituted double bonds are usually more stable. Wider separation between the groups means less steric interaction and increased stability.

18. Disubstituted Isomers  trans more stable than cis cis < geminal < trans isomer  Less stable isomer is higher in energy, has a more exothermic heat of hydrogenation. -116 kJTrans-2-butene -117 kJ (CH 3 ) 2 C=CH 2 Isobutylene -120 kJCis-2-butene steric repulsion

19. Substituent Effects  More substituted alkenes are more stable. H 2 C=CH 2 < R-CH=CH 2 < R-CH=CH-R < R-CH=CR 2 < R 2 C=CR 2  unsub. < monosub. < disub. < trisub. < tetra sub.  Alkyl group stabilizes the double bond.  Alkene less sterically hindered.

20. Relative Stabilities

21. Stability of Cycloalkene  Cis isomer more stable  Small rings have additional ring strain.  Must have at least eight carbons to form a stable trans double bond.  For cyclodecene (and larger), the trans double bond is almost as stable as the cis.

22. Bredt ’ s Rule  A bridged bicyclic compound cannot have a double bond at a bridgehead position unless one of the rings contains at least eight carbon atoms.  Examples: Unstable. Violates Bredt’s rule Stable. Double bond in 8-membered ring.

23. Compound (a) is stable. Although the double bond is at a bridgehead, it is not a bridged bicyclic system. The trans double bond is in a ten membered ring. Compound (b) is a Bredt’s rule violation and is not stable. The largest ring contains six carbon atoms, and the trans double bond cannot be stable in this bridgehead position. Compound (c) (norbornene) is stable. The (cis) double bond is not at a bridgehead carbon. Compound (d) is stable. Although the double bond is at the bridgehead of a bridged bicyclic system, there is an eight-membered ring to accommodate the trans double bond. Which of the following alkenes are stable? Solved Problem 1 Solution

24. Sec 2 Preparation of alkenes Alkenes can be synthesized  by the reduction of alkynes, or  by the elimination of alkyl halides and alcohols. E2 dehydrohalogenation (-HX) ； E1 dehydrohalogenation (-HX) ； Dehydration of alcohols (-H 2 O)  Dehalogenation of vicinal dibromides(-X 2 )

25. 顺式烯烃 Lindlar ’ s 催化剂法 : 硼氢化方法 Z-2- 戊烯 顺式烯烃 Reduction of alkynes 反式烯烃 液氨法

26. Preparation of Alkenes: Elimination Reactions A. Dehydration of alcohols Elimination reaction  -elimination or 1,2-elimination K eq < 1 higher b.p. lower b.p(distill alkene to drive rxn  ) - reversible - ease of dehydration: 3º > 2º > 1º -Carbocation intermediate, like E1. -Protic solvent removes adjacent H +.

27. A. Dehydration of alcohols 1. Zaitsev rule: regioselectivity and stereoselectivity -the major product from  -elimination is the more stable alkene regioselective stereoselective Preparation of Alkenes: Elimination Reactions

28. 2. The acid-catalyzed E1 mechanism (E1 CA ) Both products come from same intermediate. Lower energy product has lower E a, is formed faster.

29. Give the expected major product for each reaction. Check your answers. Both major products are the more highly substituted alkenes, which are the more thermodynamically stable.

30. B. Dehydrohalogenation of alkyl halides strong base:KOH/ethanol CH 3 CH 2 ONa/CH 3 CH 2 OH tBuOK/tBuOH preferred for 1º RX Elimination

31. Some Bulky Bases (CH 3 CH 2 ) 3 N : triethylamine

32. Hofmann Product  Bulky bases abstract the least hindered H +  Least substituted alkene is major product.

33. E2: Cyclohexanes Leaving groups must be trans diaxial.

34. Alkenes also can be synthesized  by the Dehalogenation of vicinal dibromides Vicinal dibromides can be debrominated by treatment with zinc dust in acetic acid or with sodium iodide in acetone.

35. E2: Vicinal Dibromides  Remove Br 2 from adjacent carbons.  Bromines must be anti-coplanar (E2).  Use NaI in acetone, or Zn in acetic acid. Br CH 3 H Br CH 3 H

36. Industrial Methods  Catalytic cracking of petroleum Long-chain alkane is heated with a catalyst to produce an alkene and shorter alkane. Complex mixtures are produced.  Dehydrogenation of alkanes Hydrogen (H 2 ) is removed with heat, catalyst. Reaction is endothermic, but entropy-favored.  Neither method is suitable for lab synthesis.

37. Sec 3 Reaction of alkenes  Reactivity of C=C Electrons in pi bond are loosely held. Electrophiles are attracted to the pi electrons. Carbocation intermediate forms.  Electrophilic Addition  Free-Radical Addition of HBr  Reduction and oxidation

38. Electrophilic Additions  Hydrohalogenation using HCl, HBr, HI  Hydration using H 2 O, H 2 SO 4  Halogenation using Cl 2, Br 2  Halohydrination using HOCl, HOBr  Oxymercuration using Hg(OAc) 2, H 2 O  Hydroboration

39. Electrophilic Addition  Step 1: Pi electrons attack the electrophile  Step 2: Nucleophile attacks the carbocation

40. Types of Additions

41. Characteristic Reactions

43. A. Addition of hydrogen halides  Markovnikov's rule states that 'in the addition of HX to an alkene, the hydrogen atom adds to the carbon atom that already has the greater number of hydrogen atoms'.  This produces the more substituted alkyl halide.

44. Addition of HX (1) Protonation of double bond yields the most stable carbocation. Positive charge goes to the carbon that was not protonated. X

45. Addition of HX (2)

46. Regiospecificity  Markovnikov ’ s Rule: The proton of an acid adds to the carbon in the double bond that already has the most H ’ s. “ Rich get richer. ”  More general Markovnikov ’ s Rule: In an electrophilic addition to an alkene, the electrophile adds in such a way as to form the most stable intermediate.  HCl, HBr, and HI add to alkenes to form Markovnikov products.

47. B Carbocation stabilization  Carbocations are stabilized by induction, hyperconjugation, or delocalization The inductive effect  The electron-deficient carbon bearing the positive charge polarizes electrons of the adjacent sigma bonds toward it.  The positive charge on the cation is not localized on the trivalent carbon, but delocalized over nearby atoms.  The larger the volume over which the positive charge is delocalized, the greater the stability of the cation.

48.  Inductive effects ： Alkyl groups have an electrondonating effect on any neighboring positive charge. The more alkyl groups attached, the greater the stabilizing effect.

49.  Hyperconjugation ： Hyperconjugation involves the overlap of the vacant 2p orbital with a neighboring CH σ-bond orbital

50.  The more substituted carbocation (Fig. 4a) can be stabilized by hyperconjugation to nine C-H bonds

51. σ － p 1 超共轭 σ － p 空 超共轭 正碳离子稳定性 3 0 >2 0 >1 0 >CH 3 + CH 2 =CH-CH 2 + ? CH 3 CH 2 + (CH 3 ) 3 C + 稳定效应

52.  The reaction of CF 3 CH=CH 2 with HBr gives CF 3 CH 2 CH 2 Br rather than CF 3 CHBrCH 3  Here, the presence of electron-withdrawing fluorine substituents has a destabilizing influence on the two possible intermediate carbocations

53. Carbocation rearrangements ？？？？

54. Question. Give the expected major product for each reaction.

55. Answer forms tertiary carbocation- no need for rearrangement. forms secondary carbocation, rearranges to a tertiary carbocation via a hydride shift forms a secondary carbocation, rearranges to a tertiary carbocation via a methide shift

56. C. Addition of Halogens  Cl 2, Br 2, and sometimes I 2 add to a double bond to form a vicinal dibromide.  Anti addition, so reaction is stereospecific.

57. Mechanism for Halogenation  Pi electrons attack the bromine molecule.  A bromide ion splits off.  Intermediate is a cyclic bromonium ion.

58. Mechanism (2) Halide ion approaches from side opposite the three-membered ring.

59. Examples of Stereospecificity

60. Test for Unsaturation  Add Br 2 in CCl 4 (dark, red-brown color) to an alkene in the presence of light.  The color quickly disappears as the bromine adds to the double bond.  “ Decolorizing bromine ” is the chemical test for the presence of a double bond.

61. D. Acid-catalyzed hydration  Reverse of dehydration of alcohol(Le Châtelier)  Use very dilute solutions of H 2 SO 4 or H 3 PO 4 to drive equilibrium toward hydration.  Principle of microscopic reversibility

62. D. Acid-catalyzed hydration Markovnikov

63. Mechanism for Hydration

64. Orientation for Hydration  Markovnikov product is formed.

65. Question. Give the products, showing stereochemistry where applicable. Br - is nucleophile H 2 O is nucleophile Interesting! Br + is electrophile,since it is less electronegative; Cl - is nucleophile.

66. Indirect Hydration  Oxymercuration-Demercuration Markovnikov product formed Anti addition of H-OH No rearrangements  Hydroboration Anti-Markovnikov product formed Syn addition of H-OH

67. E Oxymercuration Demercuration  Reagent is mercury(II) acetate which dissociates slightly to form + Hg(OAc).  + Hg(OAc) is the electrophile that attacks the pi bond. The same holds true for the organomercuric synthesis of alcohols.

68. Mechanism  + Hg(OAc) is the electrophile that attacks the pi bond.  The intermediate is a cyclic mercurinium ion

69. Predict the Product Predict the product when the given alkene reacts with aqueous mercuric acetate, followed by reduction with sodium borohydride. anti addition

70. F Alkoxymercuration -Demercuration If the nucleophile is an alcohol, ROH, instead of water, HOH, the product is an ether.

71. G Hydroboration  Borane, BH 3, adds a hydrogen to the most substituted carbon in the double bond.  The alkylborane is then oxidized to the alcohol which is the anti-Mark product.

72. Borane Reagent  Borane exists as a dimer, B 2 H 6, in equilibrium with its monomer.  Borane is a toxic, flammable, explosive gas.  Safe when complexed with tetrahydrofuran. THF THF. BH 3

73. Mechanism  The electron-deficient borane adds to the least-substituted carbon.  The other carbon acquires a positive charge.  H adds to adjacent C on same side (syn).

74. Actually, Trialkyl Borane prefers least-substituted carbon due to steric hindrance as well as charge distribution.

75. Oxidation to Alcohol  Oxidation of the alkyl borane with basic hydrogen peroxide produces the alcohol.  Orientation is anti-Markovnikov.

76. Predict the Product  Predict the product when the given alkene reacts with borane in THF, followed by oxidation with basic hydrogen peroxide. syn addition

77. Addition of Carbenes  Insertion of -CH 2 group into a double bond produces a cyclopropane ring.  Three methods: Diazomethane Simmons-Smith: methylene iodide and Zn(Cu) Alpha elimination, haloform

78. Diazomethane Extremely toxic and explosive.

79. Simmons-Smith Best method for preparing cyclopropanes.

80. Alpha Elimination  Haloform reacts with base.  H and X taken from same carbon

81. Stereospecificity Cis-trans isomerism maintained around carbons that were in the double bond.

82. H Formation of Halohydrin  If a halogen is added in the presence of water, a halohydrin is formed.  Water is the nucleophile, instead of halide.  Product is Markovnikov and anti.

83. Regiospecificity The most highly substituted carbon has the most positive charge, so nucleophile attacks there.

84. Predict the Product Predict the product when the given alkene reacts with chlorine in water.

85. I Epoxidation  Alkene reacts with a peroxyacid to form an epoxide (also called oxirane).  Usual reagent is peroxybenzoic acid.

86. Mechanism One-step concerted reaction. Several bonds break and form simultaneously.

87. Stereospecific syn addition: cis-2,3-epoxybutane trans-2,3-epoxybutane epoxyethane (ethylene oxide) 1,2-epoxypropane (propylene oxide) 1,2-epoxycyclohexane (cyclohexene oxide) cis or trans stereochemistry is maintained

88. Opening the Epoxide Ring  Acid catalyzed.  Water attacks the protonated epoxide.  Trans diol is formed.

89. One-Step Reaction  To synthesize the glycol without isolating the epoxide, use aqueous peroxyacetic acid or peroxyformic acid.  The reaction is stereospecific.

90. J Free-radical addition of HBr Markovnikov orientation antiMarkovnikov orientation (peroxide effect)

91. Free radical chain mechanism: Initiation Propagation

92. Anti-Markovnikov ??  Tertiary radical is more stable, so that intermediate forms faster. X

93. Question Give the major products of the following reactions. goes Markovnikov via electrophilic reaction goes anti- Markovnikov via radical reaction rearranges to more stable carbocation goes anti-Mark via free radical, no rearrangement

94. Regiochemical control in synthesis: Markovnikov antiMarkovnikov

95. Sec 4 Reduction and oxidation of alkenes A. Hydrogenation

96. a.It is used analytically to find the number of mole of double bond or triple bond by the number of mole of hydrogen absorbed per mole of molecule. b.It is used in converting vegetable oil. Hydrogenation

97. 金催化剂的表面 催化加氢过程图解 + H 2 (a) (b) (c) (d)

98. 1 ， 2- 二甲基环戊烯 顺 -1,2- 二甲基环戊烷 催化氢化反应是放热反应。

99. 烯烃氧化的主要类型 烯烃氧化的主要类型 酮、酸 酮、醛 邻二醇 环氧化物 注意双键和 H 的变化 注意双键和 H 的变化

100. B. 臭氧化反应 含 6-8% 臭氧的氧气和烯烃作用，生成臭氧 化合物的反应称为臭氧化反应。 + O 3 6-8% 低温，惰性溶剂 二级臭氧化合物 Zn, H 2 O RCHO ++ Zn(OH) 2

101. B. Ozonolysis Oxidative cleavage forms an ozonide. not isolated, but treated with a mild reducing agent like Zn or dimethyl sulfide. Milder oxidation. Products formed are ketones or aldehydes.

102. Question. What is the structure of the alkene if ozonolysis produced the following: B. Ozonolysis

103. 烯烃臭氧化反应的应用 （ 1 ）测定烯烃的结构 （ 2 ）由烯烃制备醛、酮、醇。 (CH 3 ) 2 C=CH 2 O3O3 H2OH2O Zn (CH 3 ) 2 C=O + CH 2 O H2OH2O Zn O3O3

104. C. 被 KMnO 4 氧化 冷，稀，中性 或碱性 KMnO 4 热，浓，中性 或碱性 KMnO 4 酸性 KMnO 4 + CH 3 COOH

105. Example Glycol initially formed is further oxidized.

106. 合成上的应用 二酮、二酸 或酮酸

107. D. 被 OsO 4 氧化 应用： 1 制邻二醇 2 鉴别双键 3 测双键的位置

108. Mechanism 五元环中间体 五元环中间体 Concerted syn addition of two oxygens to form a cyclic ester.

109. Stereospecificity If a chiral carbon is formed, only one stereoisomer will be produced (or a pair of enantiomers).

110. Polymerization  An alkene (monomer) can add to another molecule like itself to form a chain (polymer).  Three methods: Cationic, a carbocation intermediate Free radical Anionic, a carbanion intermediate (rare)

111. Cationic Polymerization Electrophile, like H + or BF 3, adds to the least substituted carbon of an alkene, forming the most stable carbocation.

112. Radical Polymerization In the presence of a free radical initiator, like peroxide, free radical polymerization occurs.

113.  烯烃与 X 2 反应的两种形式（例：丙烯＋ Cl 2 ）： 烯丙位氯代的条件： 高温（气相）、 Cl 2 低浓度 双键上的亲电加成 饱和碳上的自由基取代 烯丙位 Sec 5 Allylic halogenation 烯丙位卤代反应

114.  烯丙位氯代机理 —— 自由基取代机理 链引发 链转移 烯丙基自由基（稳定，易生成） 链终止 : 略 第 (2), (3) 步重复进行

115. 烯丙位溴代的实验室常用方法 烯丙位溴代的实验室常用方法  NBS 溴代机理（自由基取代机理） N-bromosuccinimide N- 溴代丁二酰亚胺 NBS 持续提 供低浓度 Br 2 链引发 …… 链转移 （请补充完整）

116. Sec 6 Conjugated Dienes twice 1-pentene more substituted Relative Stabilities

117. Structure of 1,3-Butadiene  Most stable conformation is planar.  Single bond is shorter than 1.54 Å.  Electrons are delocalized over molecule.

118. Conformations of 1,3-Butadiene  s-trans conformer is more stable than the s- cis by 12 kJ/mol (2.8 kcal/mol).  Easily interconvert at room temperature.

119. 1,2- and 1,4-Addition to Conjugated Dienes  Electrophilic addition to the double bond produces the most stable intermediate.  For conjugated dienes, the intermediate is a resonance stabilized allylic cation.  Nucleophile adds to either carbon 2 or 4, both of which have the delocalized positive charge.

120. Addition of HBr

121. Carbocations from Conjugated Dienes  Addition of H + leads to delocalized secondary allylic carbocation

122. Products of Addition to Delocalized Carbocation  Nucleophile can add to either cationic site  The transition states for the two possible products are not equal in energy

123. Kinetic vs. Thermodynamic Control Major product at 40  C Major product at -80  C

124. 低温有利于 1,2 加成，升高温度有利于 1,4 加成 a 动力学因素 : 由反应的活化能大小来控制 ( 速度控制 ) b 热力学因素 : 主要由产物平衡稳定性来控制 ( 平衡控制 )

126. Diels-Alder Reaction  Otto Diels, Kurt Alder; Nobel prize, 1950  Produces cyclohexene ring  Diene + alkene or alkyne with electron- withdrawing group (dienophile) 双烯体 Diene 亲双烯体 Dienophile

130. Examples of Diels-Alder Reactions

131. 顺丁烯二酸酐 1,2,5,6- 四氢化苯二甲酸酐 (100%) 2,3- 二甲基 -1,3- 丁二烯 (100%) 30 ° C 丙烯醛

132. ≥90% 双烯合成反应的特点 : a 立体专一性强的顺式加成，亲双烯体的构型保持不变。 顺 - 丁烯二羧酸 顺 -4- 环己烯 -1,2- 二羧酸

133. 反 - 丁烯二酸二甲酯 4- 环己烯基 - 反 -1,2- 二甲酸二甲酯 b 固定为 S- 反式构象的双烯不能发生双烯合成反应 不反应

134. Diels-Alder Examples

135. c 环状共轭双烯与顺 - 丁烯二酸碱类反应得内型 (endo) 加成 外型 (exo) 内型 (endo) 加成

136.  产物为桥环时，一般优先生成内型 (endo) 产物 内型 (endo) 外型 (exo) 主要产物 次要产物 过渡态较稳定 内型（ endo ）： 取代基与大环为同侧 外型（ exo ）： 取代基与小环为同侧

137. Diels-Alder 反应的立体化学 产物与亲二烯体的顺反关系保持一致  Diels-Alder 反应是立体专一性反应（相对于亲二烯体）

138. Diels-Alder 反应在合成上的应用  合成取代环己烯衍生物 反式

139. 例：完成下列合成 反合成分析： 合成： 顺式邻二醇 环己烯衍生物

141. The END Homework 8-23; 8-29; 8-34; 9-30; 9-31(b), (c)