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Chapter 3 Alkenes

Chapter 3 Alkenes. 3.1 Alkene Nomenclature 3.2 Structure of Alkenes 3.3 Isomerism in Alkenes 3.3.1 Stereo-isomerism in Alkenes 3.3.2 Naming Stereoisomeric Alkenes 1. Naming by term Cis-trans 2. Naming by the E, Z Notational System. 3.4 Reactions of Alkenes

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Chapter 3 Alkenes

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  1. Chapter 3 Alkenes 3.1 Alkene Nomenclature 3.2 Structure of Alkenes 3.3 Isomerism in Alkenes 3.3.1 Stereo-isomerism in Alkenes 3.3.2 Naming Stereoisomeric Alkenes 1. Naming by term Cis-trans 2. Naming by the E, Z Notational System 3.4 Reactions of Alkenes 3.4.1 Electrophilic Addition of Alkenes (1) Addition of Hydrogen Halides to Alkenes

  2. Orientation of electrophilic addition Mechanism of the Reaction The stability of carbocations Carbocation Rearrangements Peroxide effect (2) Addition of Sulfuric acid to Alkenes (3) Acid-catalyzed Hydration of Alkenes (4) Hydroboration-Oxidation of alkenes (5) Addition of Halogen to Alkenes

  3. (6) Conversion of Alkenes to Vicinal Halohydrin 3.4.2 Hydrogenation ofAlkenes Heat of hydrogenation Stabilities of alkenes Mechanism of alkenes hydrogenation Stereochemistry of Alkenes hydrogenation Heterogeneous reaction 3.4.3 Oxidation of Alkenes (1) Epoxidation of Alkenes (2) Hydroxylation of alkenes (3) Oxidative cleavage of alkenes: (A) Ozonolysis of Alkenes (B) With KMnO4 solution 3.4.4 Reaction of Alkenes with Alkenes: Polymerization

  4. Alkenes: (Olefin) P76 Hydrocarbon containing carbon-carbon double bond The site of reactions The functional group （反应部位） （官能团） Alkanes cycloalkanes Saturated (饱和烃) Aliphatic hydrocarbons Alkenes Alkynes Unsaturated (不饱和烃)

  5. Farnesene （法呢烯） Isobutylene （异丁烯） α- Pinene （ α- 蒎烯） Terpene （萜烯） 3.1 Nomenclature of Alkenes IUPAC Names: -ene（某烯） 1. Give the base name by selecting the longest continuous carbon chain including the double bond .

  6. 2. Number: Give the boubly bonded carbons the lower number. 3. The location of substituents like alkanes. 4. When C number is over 10: 称某碳烯 5-Undecene 5-十一碳烯 2-Ethyl-1-pentene 4,4-Dimethylcycloheptene

  7. Alkenyl groups（烯基）: Vinyl （乙烯基） Allyl (Allylic group) （烯丙基） Propenyl（丙烯基） Isopropenyl（异丙烯基） Methenecyclohexane （亚甲基环己烷）

  8. 2p 2p sp2 2s 1s 2p 1s An sp2 orbital 2s 1/3 s orbital 2/3 p orbital 1s 3.2 Structure of Alkenes P16, 1.9 Structure of Ethylene: sp2 Hybrid orbitals C: sp2-hybridized state Exited state Ground state Hybri- dization Promotion of electron

  9. Three equivalent sp2 hybrid orbitals lie in a plane at angle of 120° to one another. Geometric structure of C atom with sp2-hybrid: Planar triangle(平面三角) A single unhybridized p orbital perpendicular to the sp2 plane. In the molecule of Ethylene : The formation of C _ Cσbond: sp2 _ sp2 overlap

  10. The formation of C-C πbond: 2p-2p side by side overlap. The formation of C-H σbond : sp2-1s overlap. One C-C σbond and 4 C-H σbond are coplanar. The formation of C-C πbond: 2p-2p side by side overlap.

  11. H H H H p orbital overlap σ- bond p orbital overlap

  12. C C Carbon-carbon double bond σbond πbond

  13. Models of Ethylene

  14. 2-Methyl-2- butene Isobutene 1-Butene cis-2-Butene trans-2-Butene (I) (II) (III) (IV) (I) (II) (III) (IV) (III) (IV) 3.3 Isomerism in Alkenes 3.3.1 Stereo-isomerismin Alkenes P80, 3.3 Constitutional isomers Cis-trans isomers Stereoisomers

  15. Configuration Physical properties: m.p; b.p cis-2-Butene Rotation about C-C double bond is restricted trans-2-Butene The different spacial arrangement of atoms or atomic groups.

  16. Prefix cis- trans- 3.3.2 Naming Stereoisomeric Alkenes 1. Naming by term Cis-trans To disubstituted Alkenes: on the same sides The same atoms or atomic groups of the double bond. onthe opposite sides P83, 3.4 2. Naming by the E, Z Notational system

  17. E, Z Notational system P175, Table 5.1 base on an Sequence Rule（次序规则） －Cahn-Ingold-Prelog priority Rule • Considering each of the double-bonded carbon, • identify the two atoms directly attached and • rank them according to atomic number. Low priority High priority Low priority Br > Cl, C > H High priority (E)-1-Bromo-1-chloro-1-butene (E)-1-氯-1-溴-1-丁烯 E configuration: the high-priority groups are on the opposite sides of the double-bond

  18. Z configuration: the high-priority groups are on the same sides of the double-bond 2. When two atoms directly attached to the double bond are identical, look at the second, third,or fourth atoms away from the double-bonded carbons until the first difference is found. (Z)-3-methyl-2-hexene (Z)-3-甲基-2-己烯

  19. < > < 3. Multiple-bonded atoms are equivalent to the same number of single-bonded atoms. The carbon is bonde to H, O, O The carbon is bonde to H, C, C

  20. solvent Starting material （原料） Substrate （底物） Organic Reactions: P88, 3.6 CCl4 Reagent （试剂） Product The broken of original bonds, the formation of new bonds. Reactants A covalent bond may break in two way: Homolytic bond break(Radical) (均裂) Heterolytic bond break(Polar) (异裂)

  21. Reactional sites of alkene: The πbond is active and is readily attacked by the some reagents. Addition reaction α- H is readily lost 3.4.1. Electrophilic Addition of Alkenes （亲电加成反应） π- bond π electrons lie above and below the plane of double bond, soπ- bonded electrons are exposed (裸露).

  22. 2-Butene 2-Chlorobutane Electronegative species Electropositive species + An reaction rule: The πbond is attacked by electron-seeking reagents－Electrophile （亲电试剂） (1) Addition of Hydrogen Halides(卤化氢) to Alkenes P 109,4.1 Alkane halide

  23. (80%) (20%) • Orientation of electrophilic addition: • Markovnikov’s Rule Addition to an unsymmetrically substituted alkenes: Vladimir Vassilyevich Markovnikov Markovnikov’s Rule: In the addition of HX to an alkene, the H attaches to the carbon with fewer alkyl groups and X attaches to the carbon with more alkyl groups. 1838-1904

  24. slow fast • Mechanism of the Reaction: Reactive intermediate Step 1. P92, 3.8 The formation of the carbocation (正碳离子) Step 2. The formation of the carbocation is the rate-determining step.

  25. + 120° Tertiary(3 ) > Secondary(2 ) > primary(1 ) > Methyl • The stability of carbocations: P112, 4.3 The structure of carbocations: The positively charge carbon atom is sp2-hybridized, The carbocation is trigonal plane. The p orbital is vacant. The one raison that stabilize a carbocation: the electron-donating effect of alkyl groups.

  26. + δ δ + + Inductive effect(诱导效应) of substituents: The electron-donating or electron- withdraw effect of a group that is transmitted through σbond. P22, Ch.9 A more highly substituted carbocation is more stable than a less highly substituted one. • Regioselectivity (区域选择 性) • of the reaction P110, 4.2 Ch.59 The reaction that can proceed in more than one direction, but actually in which one direction is preferred. Regiospecific (区域专一的)

  27. (I) (II) Explanation for “Markovnikov’s rule” The stabilities of carbocation: (I) > (II) Electrophilic addition to an unsymmetri- cally substituted alkene give the more highly substituted carbocation.

  28. 0℃ Problem: Propose a mechanism to account for the following result: • Carbocation Rearrangements (重排) Ch.60, 倒7 (40%) (60%) HCl Cl- Cl- Stabilities of C+: Tertiary > Secondary Hydride-shift (I) (II)

  29. Based on the ability to proton-donating of HX • Reactivity: HI > HBr > HCl >> HF Alkenes? • Peroxideeffect (过氧化物效应) Ch.61,(d) An unsymmetric alkene reacts with HBr in the present of a peroxide (R-O-O-R), the Anti-Markovnikov addition occurs. Free-redical addition （过氧化乙酰） （过氧化苯甲 酰）

  30. (2) Addition of Sulfuric acid to Alkenes Ch. 63, (丙) Cold Concentrated H2SO4 Hydration (水合反应) Alkyl hydrogen Sulfate(硫酸氢酯) Hydrolysis(水解作用): A bond is cleaved by reaction with water. Mono-substituted and Disubstituted alkenes: Ok!

  31. Herbert Charles Brown Got the 1979 Nobel prize (3) Acid-catalyzed Hydration of Alkenes P114, 4.4 (酸催化的烯烃水合反应) Catalyst: Dilute H2SO4, H3PO4 Major method to prepare alcohols in industry (4) Hydroboration-Oxidation(硼氢化-氧化) of alkenes Ch.66, The method for preparation of the alcohols from anti-Markov. Adddition. The hydroxyl group was added on less substituted carbon. Organoboranes (有机硼烷)

  32. He discovered the hydroboration reaction (addition of diborane to alkenes) and developed the multi- faceted and synthetically useful chemistry of the resulting organo- boranes. In this photo, Professor Brown holds a model of 9 -borabicyclo[3.3.1]nonane (9-BBN), prepared by adding borane to 1,5-cyclooctadiene and itself a stable, useful hydroborating reagent. This work is summarized in Brown's book "Organic Synthesis via Boranes" (1975). Brown contri- buted to many other areas of organic chemistry, among which were selective reducing agents, steric effects (in displacement, elimination and acid-base reactions), and directive effects in electrophilic aromatic substitution (the σ+ constant). Brown is perhaps the most prolific organic chemist of the 20th century. He is best known for his work in organoboron chemistry, for which he shared (with G. Wittig) the 1979 Nobel Prize in Chemistry.

  33. Hydroboration : Electronegativity: H 2.1, B 2.0 borane organoborane Reagent: boron hydride B2H6 Solvents ether: Et2O, THF (四氢呋喃) Diglyme: CH3OCH2CH2OCH2CH2OCH3 （二甘醇二甲 醚）

  34. Hydrogen peroxide Oxidation: Equal to the anti-Markov. Addition of H2O to alkenes Feature of the reaction: 1. Regioselectivity: following Markov. Rule. 2. Stereochemistry: Syn-addition （顺式加成）: Two atoms or groups add to the same face of a double bond.

  35. Anti-addition （反式加成）: Two atoms or groups add to the opposite faces of a double bond. The stereoselectivity of Hydroboration- Oxidation: Syn-addition 3. Non rearrangement trans-2-Methylcyclo- pentanol Problem: What products would you obtain from reaction of 1-ethylcyclopentenewith BH3,followed by H2O2,OH-?

  36. 0℃ Bromonium ion ( 型离子) (5) Addition of Halogen to Alkenes P116, 4.5 Identification for C=C. Vicinal dihalide (邻二卤代物) Reagents: Cl2, Br2. Solvents: CH2Cl2,CHCl3, Acetic acid • Mechanism of the reaction: Step 1 is the rate- determining step. Step1. Step2. Bromo-anion attacks from side opposite.

  37. Step 1.

  38. Step 2.

  39. Stereochemistry of halogen addition Anti-addition (6) Conversion of Alkenes to Vicinal Halohydrin(邻卤代醇) Ch.63,(丁) Addition of halogen in aqueous solution. β- Halohydrin

  40. Mechanism of the reaction: Features of the reaction: • Following Markov. Rule, equal to the • addition of one mole of HO-Cl+(次氯酸) 2. Anti-addition

  41. 3.4.2 Hydrogenation of Alkenes P118,4.6 + heat Catalyst: Pt,Pd, Ni Features of the reaction: 1. An exothermic reaction Broken: πbond, H-H σbond Formation: 2 C-H σbond Heat of hydrogenation: The heat evolved on hydrogenation of one mole. of an alkene.

  42. The heat of hydrogenation is relative to the stability of alkenes. The higher is the heat of hydrogenation, the less stable is the alkene. Ch.54,(乙) Stability of alkenes: Cis- < Trans-

  43. 2. The role of the metal catalyst Very slowly without catalyst. Changing the reaction path to lower activation energy （活化能）. Mechanism of alkene hydrogenation The addition of hydrogen to alkene is catalytic hydrogenation （催化氢化）. 3. Stereochemistry of Alkene hydrogenation Alkene hydrogenation: syn-addition

  44. H H2 Catalyst Hydrogen absorbed on catalyst surface Complex of alkene to catalyst H H H H H + Alkane product Regenerated catalyst Insertion of hydrogen into C=C Mechanism of alkene hydrogenation

  45. 4. Heterogeneous reaction（异相反 应） Solvent （溶剂）： ethanol, hexane or acetic acid. To dissolve a alkene Metal: solid The reaction occurs at the interface of two phase. Homogeneous(均相）

  46. 3.4.3 Oxidation of Alkenes P238, 6.18 (1) Epoxidation of Alkenes (环氧化反应) Peroxy acid (过氧酸) Epoxide (环氧化物) Shapless, K. B. got the 2001 Nobel prize. Peroxyacetic acid (过氧乙酸) Reagent: Solvents: acetic acid, CH2Cl2, CHCl3 Preparation of epoxides from alkenes

  47. K. Barry Sharpless, Ph.D. Organic/Inorganic ChemistThe Scripps Research Institutehttp://www.scripps.edu/chem/sharpless/cv.html

  48. cis-cyclohexanediol (37%) Cyclohexene (2) Hydroxylation(羟基化反应) of alkenes: Alkenes react with potassium perman- ganate or Osmium tetraoxide in basic solution to form 1,2-diols (glycol)(二醇). P120,4.7 Cold solution of NaOH syn stereochemistry. NaHSO3

  49. Mechanism of the reaction: (3) Oxidative cleavage of alkenes: (A) Ozonolysis of Alkenes (臭氧化反应) O3(ozone) Reducing agent: Zn Ozonide Ch.70 Hydrolysis Aldehydes or ketones

  50. Isopropylidene- cyclohexene (异亚丙基环己烯) Cyclohexa- none (环己酮) Acetone Ex. 2-Methyl-2-butene Acetone Acetaldehyde (B) With KMnO4 solution In hot OH- solution, neutral or acidic solution: Identification to the structure of a allkene

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