The Distribution of Products Depends On Probability and Reactivity

MO diagrams showing the stabilization achieved when the electrons of an orbital of a C—H or C—C o- bond overlap with 

Two different alkyl halides are obtained from the monochlorination of butane. Substitution of a hydrogen bonded to one of the primary carbons produces 1-chlorobutane, whereas substitution of a hydrogen bonded to one of the secondary carbons forms 2-chlorobutane. 

The expected (statistical) distribution of products is 60% 1-chlorobutane and 40% 2-chlorobutane because six of butane s 10 hydrogens can be substituted to form 1-chlorobutane, whereas only four can be substituted to form 2-chlorobutane. 

When the reaction is carried out in the laboratory, however, the products are found to be 29% 1-chlorobutane and 71% 2-chlorobutane. In other words, the product distribution does not depend solely on the probability of a chlorine radical colliding with a primary or a secondary hydrogen. 

Because more 2-chlorobutane is obtained than expected and the rate-determining step of the overall reaction is removal of the hydrogen atom, we can conclude that it is easier to remove a hydrogen atom from a secondary carbon to form a secondary radical than it is to remove a hydrogen atom from a primary carbon to form a primary radical. 

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