Generation and Characterization of Free Radicals

Two early studies have special historical significance in the development of the concept of free radical reactions. The work of Gomberg around 1900 provided evidence that when triphenylmethyl chloride was treated with silver metal, the resulting solution contained PhjC in equilibrium with a less reactive molecule. Eventually it was shown that the dimeric product is a cyclohexadiene derivative.  

The dissociation constant is small, only about 2 x 10 M at room temperature. The presence of the small amount of the radical at equilibrium was deduced from observation of reactions that could not reasonably be attributed to a normal hydrocarbon. 

The second set of experiments was carried out in 1929 by Paneth. The decomposition of tetramethyllead was accomplished in such a way that the products were carried by an inert gas over a film of lead metal. The lead was observed to disappear with re-formation of tetramethyllead. The conclusion reached was that methyl radicals must exist long enough in the gas phase to be transported from the point of decomposition to the lead film, where they are reconverted to tetramethyllead. 

Since these early experiments, a great deal of additional information about the structure and properties of free radical intermediates has been developed. In this chapter, we discuss the structure of free radicals and some of the special features associated with free radical reactions. We also consider some of the key chemical reactions that involve free radical intermediates. 

A free-radical reaction is a chemical process which involves molecules having unpaired electrons. The radical species could be a starting compound or a product, but the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates and/or transition states in which all electrons remained paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur. The generalized reactions shown below illustrate the formation of alkyl, vinyl, and aryl free radicals by hypothetical homolytic processes. 

The extent of dissociation is small, with K = 2 x.lQr M in benzene at room temperature. Gomberg deduced the existence of the radical from the fact that the solution exhibited reactions that could not be acceptably explained in terms of the properties expected for a normal organic molecule. 

In 1929, Paneth studied the decomposition of tetramethyllead and came to the conclusion that methyl radicals generated in one area of an apparatus could move with inert carrier gas to another region of the system, where their presence was indicated by disappearance of a metal film by the reverse of the cleavage reaction  

Since these early experiments, much additional evidence for the existence of radical intermediates and of their importance in many reactions has accumulated. In the first section of this chapter, we will discuss some of the radicals that have been studied directly and indicate the structural conclusions that have been reached. Free radicals have some special properties that are associated with the existence of an unpaired electron. Because species with an unpaired electron are attracted by a magnetic field, they are said to be paramagnetic. This property itself is not used extensively in the study of organic free radicals however, the existence of the unpaired electron does give rise to special spectral properties for free-radical species, which will be discussed more fully in subsection 12.1.3. 

The extent of dissociation is small, with K = M in benzene at room 

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