Question

The reactivity order of halogenation of alkanes is $F_2>C{l}_2>B{r}_2>I_2$ . Choose the correct statements.
(I) Lower the activation energy for the chain initiation step, more reactive is the halogen.
(II) Lower the activation energy for the first chain-propagation step, more reactive is the halogen.
(III) Lower the activation energy for the second chain-propagation step, more reactive is the halogen.
(IV) More negative is the overall heat of the reaction ( $\Delta H_r^o$ of the halogenation of alkane) more reactive is the halogen.

• A. I and II
• B. I, II and III
• C. II and IV
• D. II, III and IV

Answer 1

The correct option is C II and IV

There are two factors that determine the reactivity of the halogenation of alkanes.
(i) Lower the positive value of $E_{act}$ for the chain propagation step, more reactive is the halogen.
(ii) Higher the negative value of the overall heat of reaction ( $\Delta H_r^o$), more reactive is the halogen.
This explains high reactivity and explosive violence with which $F_2$ reacts with $C{H}_4$.