Edexcel A Level Chemistry:复习笔记4.2.1 Hydrolysis of Halogenoalkanes

• 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

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

   

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