The Reactivity vs. Selectivity Principle

It is often stated, The more reactive a compound is, the less selective it will be . In order to understand such statements, we must first determine what is meant by more (or less) reactive and by selective . Both terms imply a comparison. Concerning selectivity, the com pa ri son is the yield of two or more products as a function of the reactivity of two or more reactants. A reaction is selective if one product is formed in a higher yield than another. 

Comparing Reactivity to Selectivity in Free Radical Halogenation 

A classic example of how reactivity is related to selectivity is concerned with the free radical halogenation of alkanes by CI2 and Br2. In this free radical chain reaction, the step that sets the position of the halogen in the alkane is a hydrogen atom abstraction step. The carbon based radical created in this first propagation step then abstracts a halogen atom from the CI2 or Br2, giving the alkyl halide (see below). In free radical halogenation by either CI2 or Br2, tertiary alkyl halides are created preferentially to secondary, which in turn are formed preferentially to primary alkyl halides. This reflects the fact that the order of radical stability decreases from tertiary to secondary to primary. Yet, the extent of the selectivity for tertiary over secondary over primary is quite different for chlorination and bromination. 

Diagram demonstrating the Curtin-Hammett principle, where the product ratio is determined by the relative heights of the barriers and not the relative stabilities of the intermediates. 

Our analysis above is a descriptive one. However, we can solve for the product ratio by using rate laws. The product ratio is given by Eq. 7.24 (you are asked to derive this expres- 

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