# Reactivity and selectivity in free radical halogenation of alkanes

I read about the free radical halogenation of alkanes with $$\ce{X2}$$ and I have the following questions:

1. Given that the first step is the homolytic breaking of the $$\ce{X-X}$$ bond, and the homolytic bond dissociation enthalpies of halogens (page 23, 31, and 34 of the PDF - I think it does not have data on $$\ce{I-I}$$ - but I'll use values from my textbook) are in the order $$\ce{I_2 < F_2 < Br_2 < Cl_2}$$, why is the order of reactivity of halogens $$\ce{F_2 > Cl_2 > Br_2 > I_2}$$?
2. My book said that bromine is very selective when reacting with alkanes - it will prefer a tertiary carbon greatly over primary or secondary ones. Chlorine is less selective and fluorine is not selective at all - it's happy to react with all types of carbons. Why is this so?

PS: I am using the textbook 'Organic Chemistry For JEE' by Solomons, Fryhle, and Snyder.

• – Loong May 14 '18 at 7:44
• Your order for bond dissociation energies correct (DOI: 10.1039/TF9625800633) – Gaurang Tandon May 14 '18 at 9:15
• – user55119 Oct 19 '18 at 23:51