Reactivity of the Carbon-Halogen Bond

Halogenoalkanes react when the carbon-halogen bond breaks. The rate of reaction is decided by the carbon-halogen bond enthalpy, not by the polarity of the bond.Bond enthalpy decreases down group 7 because the halogen atom gets larger and the bonding pair is further from the nuclei. The C-F bond is the strongest and the C-I bond is the weakest.This means iodoalkanes react fastest and fluoroalkanes are very unreactive. If polarity controlled the rate, fluoroalkanes would react fastest, which is not what is observed.The trend can be shown by warming halogenoalkanes with aqueous silver nitrate in ethanol. Water acts as a nucleophile and releases halide ions, which form a precipitate with silver ions.The yellow silver iodide precipitate appears first, then cream silver bromide, and white silver chloride appears most slowly.