Edexcel A Level Chemistry:复习笔记3.4.4 Hydrolysis of Halogenoalkanes

Relative Rates of Hydrolysis

 

Measuring the rate of hydrolysis

  • Acidified aqueous silver nitrate can be used to measure the rate of hydrolysis of halogenoalkanes
  • The following method is used:
    • Set up three test tubes in a 50 oC water bath, with a mixture of ethanol and acidified silver nitrate
    • Add a few drops of a chloroalkane, bromoalkane and an iodoalkane to each test tube and start a stop watch
    • Time how long it takes for the precipitates to form
  • Reacting halogenoalkanes with aqueous silver nitrate solution will result in the formation of a precipitate
  • The rate of formation of these precipitates can also be used to determine the reactivity of the haloalkane

Haloalkane Precipitates Table

3.3-Halogen-Compounds-Table-2_Reactivity-of-Halogenoalkanes

 

  • The precipitates will form as the reaction progresses and the halide ions are formed
    • The yellow silver iodide precipitate is the fastest nucleophilic substitution reaction
      • This is because the C-I bond has the lowest bond enthalpy, so it is the easiest to break and will cause the I- ions to form the fastest
    • The white chloride precipitate is the slowest nucleophilic substitution reaction
      • This is because the C-Cl bond has the highest bond enthalpy, so it is the hardest to break and will cause the Cl- ions to form the slowest
    • Silver fluoride is soluble, so a precipitate will not be formed in this reaction
  • This confirms that fluoroalkanes are the least reactive and iodoalkanes are the most reactive halogenoalkanes
    • It can be predicted that the formation of silver astatide would be even quicker than silver iodide

3.3-Halogen-Compounds-Reactivity-of-Halogenoalkanes

The trend in reactivity of haloalkanes

 

 

 

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