Hydrolysis of Primary Haloalkanes
Bond Enthalpy
- The halogenoalkanes have different rates of substitution reactions
- Since substitution reactions involve breaking the carbon-halogen bond the bond energies can be used to explain their different reactivities
Haloalkane Bond Energy Table
- The table above shows that the C-I bond requires the least energy to break, and is therefore the weakest carbon-halogen bond
- During substitution reactions the C-I bond will therefore heterolytically break as follows:
R3C-I + OH- → R3C-OH + I-
halooalkane alcohol
- The C-F bond, on the other hand, requires the most energy to break and is, therefore, the strongest carbon-halogen bond
- Fluoroalkanes will therefore be less likely to undergo substitution reactions
Examiner Tip
- Bond energy / enthalpy values can vary in published literature despite theoretically being completed under the same conditions
- Another reason for variation in bond energy / enthalpy values is due to the published values being the mean average of the specific bond from different substances
- The key point is that you are able to use the data given to you and apply it in your answers
Aqueous silver nitrate
- Reacting haloalkanes 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 formation of the pale yellow silver iodide is the fastest (fastest nucleophilic substitution reaction) whereas the formation of the silver fluoride is the slowest (slowest nucleophilic substitution reaction)
- This confirms that fluoroalkanes are the least reactive and iodoalkanes are the most reactive haloalkanes
The trend in reactivity of haloalkanes
