卤烷的类型.PPT

Ch.32 – Halogeno-compounds Ch.32 –鹵代化合物 3. Polarity of the solvent溶劑的極性 A highly polar solvent will stabilize the carbonium ion, hence favours the SN1 mechanism. 高極性溶劑可穩定碳陽離子，所以偏向SN1機理。 Polarity of solvents: 溶劑極性： H2O C2H5OH 4. Nature of the leaving group離去基團的本質 Consider the following data: 考慮以下數據 Bond Bond enthalpy / kJ mol-1 C－F 484 C－Cl 338 C－Br 276 C－I 238 Hence, the rate of SN reaction (both 1 and 2) is in the order of 所以，SN反應(1及2)均次序如下 RF RCl RBr RI E. Reactivity of halobenzene鹵苯的活潑性 It has been found that substitution reactions for halobenzene, SN1 or SN2, is less reassctive than for haloalkanes. 我們發現鹵苯的取代反應，SN1或SN2，均比鹵烷不活潑。 NaOH 360oC, 150 atm HCl(aq) e.g. 1. Halobenzene can only be hydrolyzed to phenols under severe conditions: 鹵苯只在嚴苛條件下方水解成酚： 2. Halobenzene does not react with CN-. 鹵苯與CN--無反應。 3. Halobenzene does not react with NH3 in normal conditions. 鹵苯與NH 3在正常條件下無反應。 The lack of reactivity of halobenzene can be explain in this way: The p-orbital electrons of the halogen atom delocalize with the π-electron system of the benzene ring. 鹵苯的低活潑性可解釋如下：鹵素的p-軌態電子與苯環的π-電子側向重疊形成離域系統。 As a result, a)The C-X bond is stronger so that X is more difficult to leave. C-X鍵較，致使X更難離開。 b)The degree of polarity of the C-X bond is reduced. C-X鍵的極性降低了。 ● The rate of hydrolysis of haloalkanes can be compared by testing the amount of X- ions by silver nitrate solution: Cl- gives Br- gives I- gives 鹵烷水解速率可以以硝酸銀溶液來測試X-離子數量作比較： Cl-生成 Br-生成 I-生成 AgCl(s), white ppt AgBr(s), pale yellow ppt AgI(s), yellow ppt AgCl(s), 白色沉澱 AgBr(s),淺黃色沉澱 AgI(s),黃色沉澱 AgCl(s) AgBr(s) AgI(s) Ex. For the following pair of compounds, give one simple chemical test that would distinguish between them. Ex. 建議—化學測試以分辨以下兩化合物。 A B Heat the compound with ethanol and aqueous AgNO3(aq), the one gives white ppt is B. 將化合物在乙醇中與AgNO3(aq)共熱,生成白色沉澱的便是B. F. Elimination reaction消去反應 In the r