\(\mathop {\underline {R - I\, > \,R\, - \,Br\, > \,R\, - \,Cl\, > \,R\, - \,F} }\limits_{\;{\text{ }}increasing{\text{ }}bond{\text{ }}energy} \)
decreasing halogen reactivity.
This order depends on the carbon-halogen bond energy; the carbon-fluorine bond energy is maximum and thus fluorides are least reactive while carbon - iodide bond energy is maximum hence iodides are most reactive.
$(a)$ $CH_3CH_2CH_2CH_2Br$
$(b)$ $\begin{matrix}
\begin{matrix}
\,\,\,\,\,\,C{{H}_{3}} \\
| \\
\end{matrix} \\
{{H}_{3}}C-C-C{{H}_{3}} \\
| \\
\,\,Br \\
\end{matrix}$
$(c)$
$(A)$ પ્રક્રિયાઓમાં સમાન નીપજ $A$ શેની રચના કરશે.