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Chapter 5 Alkenes

Chapter 5 Alkenes. Text 1: Chapter 7: Structure and synthesis of alkenes Chapter 8: Reactions of alkenes Text 2: 第三章 : 单烯烃. Contents. 5.1 Structure of alkenes 5.2 Elements of unsaturation 5.3 Nomenclature of alkenes 5.4 Stability of alkenes

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Chapter 5 Alkenes

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  1. Chapter 5 Alkenes Text 1: Chapter 7: Structure and synthesis of alkenes Chapter 8: Reactions of alkenes Text 2: 第三章: 单烯烃

  2. Contents • 5.1 Structure of alkenes • 5.2 Elements of unsaturation • 5.3 Nomenclature of alkenes • 5.4 Stability of alkenes • 5.5 Physical properties of alkenes • 5.6 Synthesis of alkenes • 5.7 Reactions of alkenes

  3. Alkenes are hydrocarbons whose molecules contain the carbon-carbon double bond. 5.1 Structure of alkenes (7-2) Alkenes, also called “olefins”, are unsaturated hydrocarbon.

  4. H H H H C-H σ bond p orbital sp2 orbitals p orbital overlap πbond C-C σ bond 1）πbond have no axial symmetry, so does not permit rotate. 2) the pi bonds are more reactive than sigma bonds.

  5. 5.2 Elements of unsaturation (Degree of unsaturation)(不饱和度) (7-3) Elements of unsaturation (不饱和度): the presence of a pi bond of an alkene (or an alkyne) or the ring of a cyclic compound decreases the number of hydrogen atoms in a molecular formula. These structural features are called elements of unsaturation propane, C3H8 propene, C3H6 propyne, C3H4 cyclopropane, C3H6

  6. C4H10 , saturated C4H8 element of unsaturation = 1 Five constitutional isomers

  7. Elements of unsaturation with hydrocarbon: elements of unsaturation = (2C ＋2－H)/2 Elements of unsaturation with heteroatoms (杂原子): halogens: count halogens as hydrogen atoms. oxygen: ignore the oxygen atoms nitrogen (氮): count nitrogen as half a carbon atom. elements of unsaturation = (2C ＋N ＋2－H)/2 Calculate the number of elements of unsaturation for the following compounds C3H4 C4H5Br3 C4H6O C4H9N C4H6NOCl

  8. 5.3 Nomenclature of alkenes (7-4,5) Read pp 280 IUPAC name: ------alkene Common name: -------alkylene

  9. 2-Methyl-2-pentene 2-Methylpent-2-ene 2-甲基-2-戊烯 2,5-dimethyl-2-hexene 2,5-dimethylhex-2-ene 2,5-二甲基-2-己烯 1-Chloro-2-butene 1-Chlorobut-2-ene 1-氯-2-丁烯 5,5-Dimethyl-2-hexene 5,5-Dimethylhex-2-ene 5,5-二甲基-2-己烯 The double bond is given preference(优先选择) in numbering.

  10. two double bonds -diene three double bonds -triene four double bonds -tetraene …… 1,3-butadiene buta-1,3-diene 1,3-丁二烯 4-methyl-1,3-pentadiene 4-methylpenta-1,3-diene ４-甲基-1,3-戊二烯 6-methyl-1,3,5-heptatriene 6-methylhepta-1,3,5-triene 6-甲基-1,3,5-庚三烯

  11. 3,5-Dimethylcyclohexene (3,5-二甲基环己烯） (not 4,6-Dimethylcyclohexene) 1-Methylcyclopentene (1-甲基环戊烯） (not 2-methylcyclopentene) The vinyl group 乙烯基 The allyl group 烯丙基 The Propylene group 丙烯基 IUPAC: Bromoethene Common; vinyl bromide 乙烯基溴 3-Chloropropene allyl chloride 烯丙基氯 3-Bromopropene allyl bromide 烯丙基溴

  12. Nomenclature of cis-transisomers cis-2-butene 顺-2-丁烯 trans-2-butene 反-2-丁烯 The E-Znomenclature system (Z)-1-Bromo-1-chloropropene (Z)-1-氯-1-溴丙烯 (E)- 1-Bromo-1-chloropropene (E)- 1-氯-1-溴丙烯 Z: Zusammen, “together” E: entgegen, “opposite”

  13. (E)-3-Methyl-2-pentene (cis-3-Methyl-2-pentene) (Z)-3-Methyl-2-pentene (trans-3-Methyl-2-pentene) (3Z,5E)-3-bromo-3,5-octadiene (3Z,5E)-3-bromoocta-3,5-diene (3Z,5E)-3-溴-3,5-辛二烯

  14. 5.4 Stability of alkenes (7-7) See Table 7-1 less stable More stable More highly substituted double bonds are usually more stable. ——Saytzeff’s rule Trans isomers are generally more stable than corresponding cis isomers. How to explain by structure theory?

  15. unstable stable unstable stable For cycloalkene, when n>8, the trans isomers are stable. More stable stable

  16. 5.5 Physical properties of alkenes Boiling points and densities: Similar as alkane. bp -47℃ bp 4℃ bp 1℃ How to explain the facts? bp 60℃ bp 48℃

  17. 5.6 Synthesis of alkenes 1. Dehydrohalogenation of alkyl halides E2 reaction mechanism a) anti elimination b) stereospecific reactions c) The product follow Saytzeff’s rule

  18. An exception to Saytzeff’s Rule; Bulky base:

  19. = =

  20. Propose a mechanism for this reaction.

  21. 2. Alkenes by dehalogenation of vicinal dibromides anti elimination

  22. 3. Dehydration of alcohols Dehydration of alcohol-----Acid as a catalyst

  23. Alcohol dehydration: An E1 reaction

  24. 4. Hydrogenation of alkynes • Alkene synthesis by high-temperature • industrial methods Catalytic cracking of alkanes Dehydrogenation of alkanes

  25. Assignments: text 1: 7-32, 38, 45, 46, 54

  26. 5.7 Reactions of alkenes I. Additions （加成）to alkenes II. Oxidations （氧化） of alkenes III. Free-radical halogenation of α-H of alkenes

  27. 1. Electrophilic addition reactions (亲电加成反应) step 1 slow step 2 fast Electrophile (E+, 亲电试剂): I. Additions to alkenes proton acids: HX, H2SO4, HOH, Lewis acids: BH3, Mn+,C+, Br-Br

  28. 1) Addition of Hydrogen halides (HX) to alkenes (8-3) A mechanism (two steps;) alkyl halide haloalkane

  29. Orientation of addition ? major Markovnikov product minor anti Markovnikov product Markovnikov’s Rule (马氏规则） The addition of HX (proton acid) to the double bond of an alkenes results in a product with the acid proton bonds to the carbon atom that already has the greater number of hydrogen atoms. (H+加在含氢多的双键碳上)

  30. regioselective (区域选择性)reactions

  31. Theoretical explanation of Markovnikov’s Rule δ+ δ- • Electronic Effects (电子效应）or substituent effects (取代效应) • The Stabilities of carbocation

  32. Makovnikov’s rule (extented) (广义的马氏规则) In an electrophilic addition to an alkene, the electrophile adds in such a way as to generate the most stable intermediate. (亲电试剂的加成总是使中间体最稳定．) Predict the outcome of the addition reaction

  33. 2) Addition of sulfuric acid (硫酸) to alkenes Hydrogen Sulfate ion 硫酸氢根负离子 Alkyl hydrogen sulfate 烷基硫酸氢酯

  34. The addition of sulfuric acid is also regioselective (区域选择性), follow Mar’s rule Alcohol from alkyl hydrogen sulfates(从硫酸氢酯制醇） Alkyl hydrogen sulfates can be easily hydrolyzed to alcohols by heating them with water. Good leaving group

  35. 3) Addition of water to alkenes: Acid-catalyzed hydration (水合, 8-4) This reaction follow Markovnikov's Rule

  36. Mechanism for acid-catalyzed hydration

  37. Problem: ? product

  38. Hydration by oxymercuration-demercuration • (汞氧化-脱汞反应，汞羟化-脱汞反应, 8-5) Anti addition(反式加成), stereospecific, Follow Markovnikov’s rule(符合马氏规则)

  39. mercurinium ion 汞翁离子 Anti addition(反式加成), stereospecific, Follow Markovnikov’s rule(符合马氏规则)

  40. Advantages of alcohol synthesis by oxymercuration-demercuration • take place under milder conditions (条件温和) • better yields • no rearrangement(无重排) • stereospecific reaction (立体专一) • regieoselectivity (follow Mar’s rule) (区域选择性) Disdvantages Oranomercurial compounds are highly toxic.

  41. 5) Alkoxymercuration-demercuration (8-6)烷氧汞化-脱汞反应 ether (醚) Anti addition(反式加成), stereospecific, Follow Markovnikov rule(符合马氏规则)

  42. 6) Hydroboration (硼氢化) of alkene (8-7) syn addition anti Mar’s product

  43. alkyl borane TS alcohol trialkyl borane syn addition, stereospecific, anti Mar’s product

  44. Summary Synthesis of alcohols from alkenes a) Addition of sulfuric acid to alkenes to form Alkyl hydrogen sulfates , then hydrolyze to alcohols, follow Mar’s orientation b) Addition of water to alkenes: Acid-catalyzed hydration, follow Mar’s orientation. Poor yields. c) Hydration by oxymercuration-demercuration, anti addition, follow Mar’s orientation. Good yields. d) Hydroboration-oxidation of alkene, syn addition, anti Mar’s orientation. Good yields.

  45. 7) Addition of halogens to alkenes (8-10) Because of rapid decolorization (褪色) of Br2/CCl4, it is a useful test for carbon-carbon multiple bonds (检验多重键）， Distinguish alkanes with alkenes（区别烷烃和烯烃）。

  46. Addition reaction of chlorine to alkenes low temperature why ?

  47. Mechanism of halogen addition Bromonium ion(溴翁离子）intermediates anti addition (反式加成）; Stereospecific reactions (立体专一性反应)

  48. trans-1,2-Dibromocyclopentane

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