In the bromination of propane, the secondary bromide is fovoured by a 97 : 3 product ratio.

The 97 : 1 reactivity ratio for bromination is much larger than 4.5 : 1 ratio for chlorination. We say that bromination is more selective than chlorination because the major reaction is fovoured by a larger amount. The rate limiting step in bromination is the abstraction of a H atom by bromine radical. The energy required to break 1º C–H bond is + 98 kcal per mole and that of 2º C–H bond is + 95 kcal per mole and the energy required to break a H–Br bond is + 88 kcal/mole and that of H–Cl bond is +103 kcal/mole. The energy

differences between chlorination and bromination result from the differences in the bond dissociation energies of HCl and HBr.

Δ E1 > 2 kcal, Δ E2 = 3 Kcal

Δ E1 for chlorination of propane is less than 1 kcal.

(i) Consider the following steps –

(i) CH3–CH2–CH3 +

CH3–CH2– + HBr

(ii) CH3–CH2–CH3 +

CH3– –CH3 + HBr

Ea1 and Ea2 are activation energies choose the correct alternative –