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Chapter 4 Alkyl halides ( 卤代烷 ): Nucleophilic substitution and elimination ( 亲核取代和消除 )

Chapter 4 Alkyl halides ( 卤代烷 ): Nucleophilic substitution and elimination ( 亲核取代和消除 ). Text 1: chapter 6 Text 2: 第九章. Contents. Classification and Nomenclature of Alkyl halides Physical properties of alkyl halides Preparation of Alkyl halides Reactions of alkyl halides

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Chapter 4 Alkyl halides ( 卤代烷 ): Nucleophilic substitution and elimination ( 亲核取代和消除 )

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  1. Chapter 4Alkyl halides (卤代烷):Nucleophilic substitution and elimination (亲核取代和消除) Text 1: chapter 6 Text 2: 第九章

  2. Contents • Classification and Nomenclature of Alkyl halides • Physical properties of alkyl halides • Preparation of Alkyl halides • Reactions of alkyl halides • Nucleophilic Substitution Reactions • Elimination Reactions

  3. 4.1 Classification and Nomenclature of Alkyl halides Halohydrocarbons (卤代烃) alkyl halides (卤代烷烃): vinyl halides (卤代烯烃): aryl halides (卤代芳烃): sp3 sp2 sp2

  4. Classification of alkyl halides Primary halides 伯卤代烷; 1° Secondary halides 仲卤代烷; 2° Tertiary halides 叔卤代烷; 3° a geminal dihalide 偕二卤代 a vicinal dihalide 邻（连）二卤代

  5. Nomenclature of alkyl halides Common name: “alkyl halide” (某基卤) IUPAC name: “haloalkane” (卤代烷), -X is treated as a substituent fluoride----------fluorine ------------ fluoro- chloride---------chlorine ----------- chloro- bromide---------bromine ----------- bromo- iodide------------iodine ------------- iodo-

  6. fluoroethane (氟乙烷) ethyl fluoride (乙基氟) 2-chloro-2-methylpropane t-butyl chloride (叔丁基氯) Bromocyclohexane (溴代环己烷) cyclohexyl bromide 环己基溴 2-bromobutane (2-溴 丁烷) sec-butyl bromide (仲丁基溴)

  7. (1R,3R)-1-chloro-3-methylcyclopentane trans-1-chloro-3-methylcyclopentane (1R,3R)-1-甲基-3-氯环戊烷 trans- 1-甲基-3-氯环戊烷 3-(chloromethyl)pentane 3-氯甲基戊烷 perchloromethane tetrachloromethane carbon tetrachloride (四氯化碳) dichloromethane (二氯甲烷) methylene dichloride trichloromethane chloroform (氯仿) perfluoropropane 全氟丙烷

  8. Common uses of alkyl halides reading material: text 1: 6-3 • solvents (溶剂): CHCl3, CH2Cl2, ClCH2CH2Cl • reagents (试剂): • anesthetics (麻醉剂): CF3CHClBr • refrigerants (致冷剂) : • CF2Cl2 (Freon-12，氟里昂-12) • CHClF2 (Freon-22) • pesticides (农药): DDT, • poly(vinyl chloride) (聚氯乙烯), Teflon（特氟隆，聚四氟乙烯）

  9. 4.2 Physical properties of alkyl halides • Solubilities（溶解性）: • very poor in water; • Miscible with each other and with other relatively nonpolar solvents.(彼此互溶, 与其它非极性溶剂互溶.) • Boiling point (bp, 沸点): • monohalides (一卤代烷): 规律与烷烃相似. • Density (d, 密度): • R-F, R-Cl, < 1; R-Br, R-I, > 1. • 多卤代烷大于1. • 随碳链增长, 同类型卤代烷密度降低. • 一卤代烷具有不愉快的气味, 有毒. • 在铜丝上燃烧时产生绿色火焰, 可用于鉴定卤素.

  10. 4.3 Preparation of alkyl halides (6-6) (Reading material: text 1 6-6, p 220-223) 1) Free-radical halogenation (自由基卤代) 6-6A

  11. Allylic halogenation (烯丙位卤化) 6-6B NBS N-bromosuccinimide N-溴代丁二酰亚胺 Allylic radical is resonance-stableilized. 烯丙基自由基稳定． Important reactions !!

  12. 2) From alkenes (烯烃加成) 3) From alcohol (醇的取代) 4) From other halides (halogen exchange, 主要用于制备碘代烷)

  13. 4.4 Reactions of alkyl halides • Nucleophilic substitution (亲核取代反应) • Elimination (消除反应): dehydrohalogenation (脱卤化氢) • Formation of organometallic compounds (形成有机金属化合物)

  14. Reactive intermediates (反应活性中间体)

  15. Nucleophilic substitution reactions(亲核取代反应): Structure of alkyl halides Carbon-halogen bond lengths In a nucleophilic sibstitution, a nucleophile(Nuc:- or Nu­ ) replaces a leaving group (X-) from a carbon, using its lone pair of electrons to form a new bond to the carbon atom.

  16. Nu: Nucleophiles 亲核试剂 X: Cl, Br, I，　 reactivity: I>Br>Cl For examples:

  17. 任何具有亲核性的试剂或中间体都可能与卤代烃反应，SH-, carbanion, etc

  18. 2) Elimination reactions (消除反应): B: base, OH－, CH3O－, CH3CH2O－ X: Cl, Br, I 1,2-elimination, β-elimination 81% 19%

  19. Positional Orientation of elimination 消去反应的取向 Saytzeff rule: (扎衣切夫规则) In elimination reactions, the most highly substituted alkene usually predominates. 在β-消去中，主要得到双键碳上取代基较多的烯烃，也称扎衣切夫烯烃。 21% 79% • Most Nu are also basic and can engage in either substitution or elimination, depending on the alkyl halide and the reaction conditions.

  20. Formation of organometallic compounds • (形成有机金属化合物) (T2: p234; T1 p 420 10-8, 9 ) M: Li, Na; Mg; B, Al, Ga; Si, Ge, Sn, Pb; P, As, Sb; etc. organomagnethium compound “Grignard reagent” (格利雅试剂, 格氏试剂) 活性顺序： Important reagents in organic synthesis　！！！

  21. organolithium compound 有机锂化合物 Wurtz reaction (武慈反应)

  22. 4) Reduction of alkyl halides to alkanes (还原反应) Reductive agents (还原剂)：LiAlH4, NaBH4 reactivity: ＞ ＞ ＞ ＞ primary secondary tertiary halides

  23. Testing of RX Physical methods: IR, 1H NMR, MS 在铜丝上燃烧时产生绿色火焰. Chemical methods: Reaction with AgNO3 / EtOH solution.

  24. 4.5 Nucleophilic substitution reactions (text 1: p 225-247; text 2: p 237-246) Experimental facts:

  25. SN2：second-order nucleophilic substitution (双分子亲核取代反应) SN1：first-order nucleophilic substitution (单分子亲核取代反应) 20世纪30年代英国伦敦大学教授英果(C. Ingold)

  26. δ– δ– slow • SN2：Second-order nucleophilic substitution • (双分子亲核取代反应) Mechanism nucleophile substrate (底物) transition state (过渡态, TS) fast product (产物)

  27. ： L Nu L Nu Nu L L Nu ： ： ： ：

  28. TS δ– δ– E Eact reaction coordination exothermic reaction 放热反应 The reaction-energy diagram (反应能线图)

  29. Characteristics of SN2 (1) second-order reaction (二级反应) ν= k[RX][Nu] (2) concerted reaction (协同进行，一步完成。) (3) having a transition state, no intermediate. (反应中经过一个过渡态，没中间体。) • stereochemistry: inversion of configuration (构型转化) • Walden inversion (瓦尔登转化)

  30. + + optically pure racemization (外消旋化) inversion of configuration (构型转化) Walden inversion (瓦尔登转化)

  31. + （S） （R） + （R） （R） 瓦尔登转变是指骨架构型转变，不是指R转为S, 或S转为R。

  32. Factors affecting SN2 reactions Nu, Substrate, Solvent, Temperature A. The effect of nucleophile • A base is always a stronger nucleophile than its conjugate acid.

  33. In a group of nucleophiles in which the nucleophilic atom is the same, nucleophilicities parallel basicities. (一般情况下, 含有相同亲核性原子的亲核试剂, 其碱性越强, 亲核能力越强.) Order of basicity and nucleophilicity: RO－> HO－>> RCOO－> ROH > HOH But: basicity ≠nucleophilicity

  34. Steric hindrance (空间位阻) basicity: > nucleophilicity: < Strong Nu:less electronegative, large valence shell, more polarizable, soft base,less steric hindrance (位阻小)

  35. strong weak strong strong R3P, HS-, I-, -CN, HO-, CH3O- , R2NH moderate Br-, Cl-, NH3, CH3OH, CH3SCH3, CH3COO- weak F-, H2O, CH3OH

  36. B. The effect of the structure of the substarate (底物结构的影响） R---X(L) General order of reactivity in SN2 reaction CH3- > 1° > 2°> 3 °

  37. R-: steric effects

  38. Leaving group effects A good leaving group must be electron withdrawing; stable after leaving; (usually weak base) polarizable, to stablize the TS I-, Br-, Cl-, HOH, ROH, NR3, etc.

  39. × poor leaving group

  40. C. Solvent effects on SN2 reactions Protic solvent (质子性溶剂) H2O, ROH, HCOOH, NH3, etc. Aprotic solvent (非质子性溶剂) Aprotic solvent (非质子性溶剂) hexane, acetone(丙酮), acetonitrile(乙腈), etc. Protic solvent Polar solvent (极性溶剂): H2O, ROH, acetone(丙酮), acetonitrile(乙腈), etc. Apolar solvent (非极性溶剂): Hexane, carbon tetrachloride

  41. Polar Aprotic Solvents （极性非质子溶剂） acetonitrile 乙腈 acetone 丙酮 Dimethyl sulfoxide (DMSO) 二甲亚砜 N,N-Dimethylformamide (DMF) N,N-二甲基甲酰胺

  42. Generally, polar solvents are required in the nucleophilic substitution reaction. in protic solvents: I- > Br- > Cl- > F- Relative Nucleophilicity in aprotic solvents: I- < Br- < Cl- < F- F-

  43. Factors affecting SN2 reactions Nu Strong Nucleophiles are needed. R less steric hindrance Substrate L good leaving groups are required. Solvent Wide variety, polar aprotic Temperature Higher temperature can increase the rate.

  44. SN1: First-order nucleophilic substitution (单分子亲核取代)

  45. Mechanism of SN1 step 1 rate-limiting step carbocation intermediate step 2 TS 1 TS 2

  46. E TS 1 TS 2 Eact2 Eact1 reaction coordination rate-limiting step The reaction-energy diagram (反应能线图)

  47. The structure of carbocations

  48. Stereochemistry racemization (外消旋化)

  49. Factors affecting SN1 reactions Nu No effects, weak ones are OK. Stability of carbocation, or Substituent effects R Substrate L Good leaving groups are required. Similar as SN1. Solvent Good ionizing solvents needed, protic solvents Temperature Higher temperature can increase the rate.

  50. The relative stabilities of carbocations How a methyl group helps stabilize the positive charge of a carbocation?

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