Chapter 6 lonic Reactions Nucleophilic substitution and elimination reactions of alkylhalides(卤代烃的亲核取代反 Because halogen 利消除 e more electronegative than carbon the carbon- halogen bond of alky halides is polarized; the carbon atom bears a partial positive charge, the halogen atom a partial negative charge 8X X=F. CI. Br
Chapter 6 Ionic Reactions--- Nucleophilic substitution and elimination reactions of alkylhalides (卤代烃的亲核取代反 Because halogen atoms are more electronegative 应和消除反应） than carbon, the carbon-halogen bond of alkyl halides is polarized; the carbon atom bears a partial positive charge, the halogen atom a partial negative charge. X X = F , Cl, Br, I + _
Table 6.1 Carbon-halogen bond lengths Bond Bond length(a) ChF 1.39 CH3CI 178 Ch2Br 193 CHaI 2.14
Table 6.1 Carbon-halogen bond lengths Bond Bond Length (A) CH3 F 1.39 CH3 Cl 1.78 CH3 Br 1.93 CH3 I 2.14
Vinyl halides(卤代乙烯)or phenyl halides(卤代苯基) X a vinylic halide A phenyl halide or ary halide ⅹ=F,C,Br,I Compounds in which a halogen atoms is bonded to an sp-hybridized carbon Carbon-halogen bond is difficult to break
Vinyl halides (卤代乙烯） or phenyl halides （卤代苯基） X X A vinylic halide A phenyl halide or ary halide X = F, Cl, Br, I Compounds in which a halogen atoms is bonded to an sp 2 -hybridized carbon Carbon-halogen bond is difficult to break
6.2 Physical properties of Very low s8rganessn balides They are miscible with each other and with other relatively nonpolar solvents CH,CI CHCI and cck are often used as solvents for nonpolar CHCI3 and CCl have a cumulative toxicity and are carcinogenIc Polyfluoroalkane癌铷质)的 boiling points, (Hexafluoroethane boils at -79o)
6.2 Physical properties of • Very low solubilities in water organic halides • They are miscible with each other and with other relatively nonpolar solvents • CH2Cl, CHCl3 and CCl4 are often used as solvents for nonpolar • CHCl3 and CCl4 have a cumulative toxicity and are carcinogenic. • Polyfluoroalkanes have low boiling points, • (Hexafluoroethane boils at –79o) (致癌物(质)的)
6.3 Reaction Mechanisms Mechanism of the reaction -The events that are postulated to take place at the molecular level as reactants become products If the reaction takes place in more than one step, then what are these steps, and what kinds of intermediates intervene between reactants and products?
6.3 Reaction Mechanisms Mechanism of the reaction—The events that are postulated to take place at the molecular level as reactants become products If the reaction takes place in more than one step, then what are these steps, and what kinds of intermediates intervene between reactants and products?
6. 3A Homolysis and heterolysis of covalent bonds(共价键的均裂和异 裂) Covent bond may break in three possible ways A·+B·Homs(均裂) A:: : B A: -+B Heteroly (异裂) a+: B
6.3A Homolysis and heterolysis of covalent bonds (共价键的均裂和异 裂） A : B A + B A: + B A + + :B - . . - + Homolysis（均 裂 ） Heterolysis （ 异 裂 ） Covent bond may break in three possible ways:
6.3b Reactive intermediates in organic chemistry Organic reactions that take place in more than one step involve the formation of an intermediate----one that results from either homolysis or heterolysis of a bond Homolysis of a bond to carbon leads to an intermediate known as a carbon radical (free adical) homolysis C Z C t Z 均裂 Carbon radical or free radical
6.3B Reactive intermediates in organic chemistry Organic reactions that take place in more than one step involve the formation of an intermediate----one that results from either homolysis or heterolysis of a bond. Homolysis of a bond to carbon leads to an intermediate known as a carbon radical (free radical) C : Z homolysis C . + Z . 均 裂 Carbon radical or free radical
Heterolysis of a bond can lead either to a trivalent carbon cation or carbon anion tZ Carbocation heterolysis (or carbenium ion) C Z (碳阳离子) 异裂 C Carbanion (碳阴离子 Carbanions are usually strong bases and strong nucleophiles Nucleophiles are lewis bases ley are electron-pair donors. Carbon radicals, carbocations, and carbonions are usually highly reactive species
Heterolysis of a bond can lead either to a trivalent carbon cation or carbon anion C : Z heterolysis C + Z 异 裂 Carbocation (or carbenium ion) （ 碳 阳 离 子 ） C + Z Carbanion （ 碳 阴 离 子 ） + : - : - + Carbanions are usually strong bases and strong nucleophiles. Nucleophiles are lewis bases they are electron-pair donors. Carbon radicals, carbocations, and carbonions are usually highly reactive species
6.3 c onic reactions(离子反应) and radical reactions(自由基反应 or游离基反应) In ionic reactions the bonds of the reacting molecules undergo heterolysis In radical reactions, they undergo homolysis(in detail in chapter 7) In this chapter we concern ourselves only with ionic reactions
6.3C Ionic reactions (离子反应） and radical reactions（自由基反应 or 游离基反应） • In ionic reactions the bonds of the reacting molecules undergo heterolysis; • In radical reactions, they undergo homolysis (in detail in chapter 7) • In this chapter we concern ourselves only with ionic reactions
6.4 Nucleophilic substitution reactions(亲核 取代反应) u R——X R—Nu+x Nucleophile Alkyl halide Product Halide ion 亲核试剂 Substrate底物) Ho CH CH3OH CI CH3O CH3-Br CH3OCH3 Br r+CH3CH2CH2-—c CH3CH, I Cl
6.4 Nucleophilic substitution reactions (亲核 取代反应） Nu : - + R X R Nu + : - X Nucleophile 亲 核 试 剂 Alkyl halide (Substrate 底物) Product Halide ion HO - + CH3 Cl CH3 OH + Cl - CH3 O - + CH3 Br CH3 OCH3 + Br - I - + CH3 CH2 CH2 Cl CH3 CH2 CH2 I + Cl -