Edexcel A Level Chemistry

Revision Notes

4.2.1 Hydrolysis of Halogenoalkanes

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Core Practical 4: 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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