The reactivity of alkyl halides toward nucleophilic substitution ($S_{N}1$ or $S_{N}2$) is determined by the Bond Dissociation Enthalpy of the $C-X$ bond.

• As the halogen increases in size $(F\to Cl\to Br\to I)$, the $C-X$ bond becomes longer and weaker.
• Bond Strength: $C-F>C-Cl>C-Br>C-I$
• Because the $C-I$ bond is the easiest to break, alkyl iodides release the leaving group most readily, making them the most reactive.