Halogenoalkanes - 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
- 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
- The yellow silver iodide precipitate is the fastest nucleophilic substitution reaction
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- 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
- 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 SN1 or an SN2 reaction mechanism