Regiochemistry of Free Radical Halogenation

With alkanes that have more than one kind of hydrogen, free radical halogenation will usually lead to a mixture of constitutional isomers. Which isomer dominates will depend upon the relative reactivity and number of different hydrogens that can be abstracted. Whi le the structure of the substrate determines which sites are the more reactive, the quantitative selectivity of a reaction will depend upon the reactivity of the halogen radical atom. 

The relative reactivities of various radicals for abstracting hydrogens attached to 1°, 2°, and 3° carbons are given in Table 11.10. These numbers have been corrected for statistical factors, meaning that to use these numbers to predict product ratios, you need to multiply each number by the actual number of 1° 2°, and 3° hydrogens present in your reactant. 

As seen in Table 11.10, abstraction of 3° hydrogens is always more favorable than 2° hydrogens, which is more favorable than 1° hydrogens. Recall that the order of stability of radicals is 3° 2° 1°. Correspondingly, the order of C-H bond strengths for various substituted 

Relative Rates of Hydrogen Abstraction by Various Radicals  

Quantitatively, though, the relative reactivities of the C-H bonds is very different depending upon the nature of the radical that is doing the abstraction. The more reactive radicals are less selective. More reactive in this case refers to a more exothermic hydrogen atom abstraction step. The more exothermic this step, the more the transition state resembles the reactants, with a correspondingly lower character of the carbon-centered radical product. This makes the reaction less sensitive to the stability of the carbon-centered radical. This is a classic example of the reactivity-selectivity principle, and was examined in a Connections highlight in Section 7.3.2. 

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