Question

Which of the following statement is correct in relation to the halogenation of alkane?

• A. The reactivity of chlorine is more than bromine towards alkanes.
• B. The reactivity of chlorine is less than bromine towards alkanes.
• C. Free radicals are pryamidal intermediate, stabilised by hyperconjugation and resonance.
• D. Bromine has much higher regioslectivity than chlorine in abstracting $3^o$ hydrogen.

Answer 1

Answer: (A), (D)

Bromine has much higher regioslectivity than chlorine in abstracting $3^o$ hydrogen.

The reactivity of the halogens for free radical reaction decreases in the following order: $F_2>C{l}_2>B{r}_2>I_2$.
Thus, reactivity of chlorine is more than bromine toward alkanes.
Whereas, Br is more selective in its reactions and prefers to react with the weaker R-H bonds. The strength of R-H varies slightly depending on the hydrogen is primary, secondary or tertiary. Bromination is 1600 times more likely to occur at $3^o$ position than $1^o$. Chlorination is 5.0 times more likely to occur at $3^o$ position than $1^o$. This shows the selectivity of $Br$.
Free radicals are planar intermediate due to the $s{p}^2$ hybridised carbon.
Hence, (a) and (d) are correct.