What is the order of ease of halogenation in alkanes?

The order of ease of halogenation in alkanes follows the trend F₂ > Cl₂ > Br₂ > I₂. Halogenation of alkanes is a free radical substitution reaction, where hydrogen atoms in an alkane are replaced by halogen atoms under the influence of ultraviolet light or heat. The rate of this reaction depends on the reactivity of the halogen involved.

Fluorine (F₂) is the most reactive halogen. It reacts explosively with alkanes even in the dark or at low temperatures. The reaction releases a large amount of energy, making it very difficult to control.
Chlorine (Cl₂) also reacts readily but less violently. It undergoes halogenation smoothly under UV light, producing alkyl chlorides through a chain mechanism involving initiation, propagation, and termination steps.
Bromine (Br₂) is less reactive compared to chlorine. Its reaction with alkanes requires higher temperature or light energy, and it proceeds more selectively, producing fewer side products.
Iodine (I₂) shows the least reactivity. The iodination of alkanes is thermodynamically unfavorable because the reaction absorbs energy rather than releasing it, and the equilibrium lies toward the reactants.

This decreasing order of reactivity — F₂ > Cl₂ > Br₂ > I₂ — is due to the differences in bond dissociation energy (BDE) and reaction enthalpy. Fluorine has the lowest bond energy, allowing it to form radicals easily, while iodine has the highest, making radical formation difficult.