3.3.5 Reactivity of Halogenoalkanes

• 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

Halogenoalkane 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^-

                                                              halogenoalkane          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

Aqueous silver nitrate

• 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 halogenoalkanes

Halogenoalkane 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 halogenoalkanes

The trend in reactivity of halogenoalkanes