Fundamentals of Organic ChemistryChemistryHaloalkanes and Haloarenes
Arrange the following halogenating agents in decreasing order of their selectivity in free radical reactions: (i) NBS (ii) NCS (iii) Cl ₂ (iv) F ₂
Options
- A(iv) > (iii) > (ii) > (i)
- B(i) > (ii) > (iv) > (iii)
- C(ii) > (i) > (iii) > (iv)
- D(i) > (ii) > (iii) > (iv)
Correct answer
D. (i) > (ii) > (iii) > (iv)
Step-by-step solution
According to the reactivity-selectivity principle, a less reactive free radical is more selective in its reactions. The reactivity of halogen radicals follows the order: F ^ > Cl ^ > Br ^ . Consequently, the selectivity follows the reverse order: bromination is highly selective, chlorination is moderately selective, and fluorination is the least selective (almost unselective due to its explosive reactivity). NBS (N-bromosuccinimide) acts as a source of bromine radicals ( Br ^ ) in low concentrations, making it high