2.8.4 Hydrolysis of Halogenoalkanes

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

• 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
     

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 S_N1 or an S_N2 reaction mechanism