The Free-Radical Chain Mechanism of Halogenation

PROBLEM 2.20 Write the structures of all possible products of monochlori-nation of pentane. Note the complexity of the product mixture, compared to that from the corresponding reaction with cyc/opentane (eq. 2.14). 

PROBLEM 2.21 How many organic products can be obtained from the monobromination of heptane Of cycloheptane  

PROBLEM 2.22 Do you think that the bromination of 2,2-dimethylpropane might be synthetically useful  

One may well ask how halogenation occurs. Why is light or heat necessary Equations 2.9 and 2.10 express the overall reaction for halogenation. They describe the structures of the reactants and the products, and they show necessary reaction conditions or catalysts over the arrow. But they do not tell us exactly how the products are formed from the reactants. 

A reaction mechanism is a step-by-step description of the bond-breaking and bond-making processes that occur when reagents react to form products. In the case of halogenation, various experiments show that this reaction occurs in several steps, and not in one magical step. Indeed, halogenation occurs via a free-radical chain of reactions. 

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