Edexcel A Level Chemistry

Revision Notes

3.4.4 Hydrolysis of Halogenoalkanes

Test Yourself

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

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 

You've read 0 of your 0 free revision notes

Get unlimited access

to absolutely everything:

  • Downloadable PDFs
  • Unlimited Revision Notes
  • Topic Questions
  • Past Papers
  • Model Answers
  • Videos (Maths and Science)

Join the 80,663 Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Philippa

Author: Philippa

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.