Hydrolysis of Halogenoalkanes (Edexcel International A Level Chemistry)

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Core Practical 5: Investigating Halogenoalkane Hydrolysis

  • Halogenoalkanes can be hydrolysed with an aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH)
  • This reaction is very slow at room temperature, so the reaction mixture is warmed
  • The rate of this reaction depends on the type of halogen in the halogenoalkane
    • The stronger the C-X bond, the slower the rate of the reaction
    • In terms of bond enthalpy, C-F > C-Cl > C-Br > C-I
    • Fluoroalkanes do not react at all, but iodoalkanes have a very fast rate of reaction

Halogen Compounds Electrophilic Substitution by NaOH, downloadable AS & A Level Chemistry revision notes

The halogen is replaced by the nucleophile, OH- 

  • This reaction could also be done with water as the nucleophile, but it is very slow
    • The hydroxide ion is a better nucleophile than water as it carries a full negative charge
    • In water, the oxygen atom only carries a partial charge

Halogen Compounds Nucleophilicity, downloadable AS & A Level Chemistry revision notes

A hydroxide ion is a better nucleophile as it has a full formal negative charge whereas the oxygen atom in water only carries a partial negative charge; this causes the nucleophilic substitution reaction with water to be much slower than the aqueous alkali

Measuring the rate of hydrolysis

  • Acidified silver nitrate can be used to measure the rate of hydrolysis of halogenoalkanes
  • 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
  • The precipitate will form as the reaction progresses and the halide ions are formed
  • A white precipitate will form from the chloroalkane, a cream precipitate will form from the bromoalkane and a yellow precipitate will form from the iodoalkane
    • The yellow precipitate will form the fastest
    • 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 precipitate will form the slowest
    • 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

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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.