Hydrolysis of Halogenoalkanes (Edexcel International A Level Chemistry)

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Halogenoalkanes - 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

Halogen Compounds Table 2_Reactivity of Halogenoalkanes, downloadable AS & A Level Chemistry revision notes

  • 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
        Halogen Compounds Reactivity of Halogenoalkanes, downloadable AS & A Level Chemistry revision notes

The trend in reactivity of haloalkanes

  • A similar experiment can be set up to compare the rate of hydrolysis in primary, secondary and tertiary halogenoalkanes
    • The halogen involved and the overall molecular formula of the halogenoalkane must remain the same
    • E.g. 1-chlorobutane (primary), 2-chlorobutane (secondary) and 2-chloro-2-methylpropane (tertiary)
  • Results would show that:
    • The tertiary halogenoalkane / 2-chloro-2-methylpropane reacts the quickest
    • The primary halogenoalkane / 1-chlorobutane reacts the slowest
    • Therefore, the secondary halogenoalkane / 2-chlorobutane is in the middle
      • Although, the explanation for this is not required at AS level, it is required at A-level
      • The rate of hydrolysis is determined by the use of either an SN1 or an SN2 reaction mechanism 

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Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.