Concept:The reactivity of halogens in free-radical substitution of alkanes is dictated by the electronegativity of the halogen, the weakness of the X-X bond, and the high exothermicity of forming the H-X bond.
Solution:- Fluorine is incredibly reactive due to a weak F-F bond and the highly exothermic nature of the reaction. It often reacts explosively.
- Chlorine reacts rapidly under UV light.
- Bromine is less reactive and highly selective, reacting slower.
- Iodine is the least reactive; its reaction is endothermic, slow, and highly reversible.
- Therefore, the strictly descending order of reactivity follows the periodic group: \( \text{F}_2 > \text{Cl}_2 > \text{Br}_2 > \text{I}_2 \).
Why other options are incorrect:- Any other sequence breaks the periodic trend correlating reactivity with electronegativity and bond energies of the halogens.
Quality & Fidelity Assurance:
Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.