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.


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