Q.Arrange the halogens F2, Cl2, Br2, I2, in order of their increasing reactivity with alkanes.

Concept: Reactivity of halogens with alkanes (halogenation) depends on the balance between bond dissociation energy and the overall exothermicity of the free-radical substitution reaction.

Reasoning:

1. Halogenation of alkanes proceeds via a free-radical chain mechanism. The rate-determining step is typically the abstraction of hydrogen by the halogen radical: $\ce{X• + R-H → R• + HX}$.

2. Reactivity follows the trend of decreasing $\ce{X-X}$ bond strength and increasing exothermicity of $\ce{H-X}$ bond formation down the group. Fluorine forms the strongest $\ce{H-F}$ bond (568 kJ/mol), making fluorination explosively exothermic and highly reactive. Chlorine is moderately reactive, bromine is sluggish (requires heat/light), and iodine is essentially unreactive because $\ce{C-I}$ bond formation is endothermic.

3. The order of reactivity is: $\ce{F2}$ (most reactive, often explosive) $>$ $\ce{Cl2}$ (vigorous) $>$ $\ce{Br2}$ (slow, selective) $>$ $\ce{I2}$ (negligible reactivity).

4. Therefore, increasing reactivity: $\ce{I2 < Br2 < Cl2 < F2}$.

Why Halogens React Differently with Alkanes

Halogenation of alkanes is a free-radical substitution reaction. The reactivity of a halogen depends on two competing factors:

1. Bond dissociation energy – how easily the X–X bond breaks to form radicals
2. Overall thermodynamics – whether the net reaction releases or absorbs energy

The reaction proceeds through initiation (X₂ → 2X·), propagation (X· + R–H → R· + HX, then R· + X₂ → R–X + X·), and termination steps. The key is that even though fluorine has the strongest F–F bond, the formation of the very strong H–F and C–F bonds makes fluorination overwhelmingly exothermic. In contrast, iodination is endothermic overall, making it thermodynamically unfavourable.

Step-by-Step Analysis

1. Fluorine (F₂) – The F–F bond energy is relatively high (~158 kJ/mol), but the H–F bond formed is exceptionally strong (~570 kJ/mol). The overall reaction is so exothermic (ΔH ≈ −430 kJ/mol for CH₄ + F₂) that it is violent and uncontrollable, often leading to combustion rather than clean substitution. Fluorine reacts explosively with alkanes even in the dark and at low temperatures.

2. Chlorine (Cl₂) – The Cl–Cl bond (~243 kJ/mol) is weaker than F–F, and while H–Cl (~432 kJ/mol) is not as strong as H–F, chlorination is still exothermic (ΔH ≈ −100 kJ/mol). Chlorine reacts readily with alkanes in the presence of UV light or heat, giving good yields of chloroalkanes. This is the standard laboratory halogenation.

3. Bromine (Br₂) – The Br–Br bond (~193 kJ/mol) is weaker still, but H–Br (~366 kJ/mol) is also weaker. Bromination is only slightly exothermic or nearly thermoneutral (ΔH ≈ −30 kJ/mol). Bromine reacts with alkanes, but requires heat or strong light and proceeds more slowly than chlorination. It is selective, preferring tertiary > secondary > primary hydrogens.

4. Iodine (I₂) – The I–I bond is the weakest (~151 kJ/mol), but H–I is also very weak (~298 kJ/mol). The overall iodination reaction is endothermic (ΔH ≈ +50 kJ/mol), meaning it is thermodynamically unfavourable. Iodine does not react with alkanes under normal conditions; the reaction is reversible and the equilibrium lies far to the left.

Reactivity Order

Putting it all together, the increasing order of reactivity is:

Iodine is essentially unreactive, bromine reacts sluggishly, chlorine reacts well under standard conditions, and fluorine reacts explosively.