Some initial considerations of radical mechanisms and chapter overview

2 Some initial considerations of radical mechanisms and chapter overview 

These radical-neutral molecule reactions tend to be chain reactions, which is an important facet of a large percentage of radical mechanisms. As with all chain mechanisms, there are initiation, propagation and termination steps. The efficiency of the chain is defined by the number of propagation cycles (chain length) and, in many reactions, termination becomes unimportant so is not commonly considered, except for conventional radical polymerisations or reactions in viscous solvents. Rate constants for propagation help to determine the synthetic utility and mechanism of radical reactions. Initiation is covered in Section 10.2. 

In the following sections, we deal with the main aspects of radical reactions and, in each section, examples of mechanistic determination are included. The selection is representative rather than exhaustive, and texts on radical chemistry should be consulted for greater detail. Key examples rather than detailed descriptions of mechanistic determination are presented. 

Initiation is the generation of the primary radical or initiator radical. The formation of propagating radicals in polymerisations by addition of initiator radicals to double bonds, so-called primary radical reactions, is discussed in Section 10.3. Initiation is essential for most radical reactions and therefore becomes a key but simple diagnostic tool in determining mechanism. The most basic test for a radical mechanism is to carry out blank reactions under identical conditions with all the reactants present except the initiator. A zero or very low yield of product in these blank reactions represents excellent evidence for a radical reaction. 

The Polymer Handbook [ 1 ] lists kdec for a variety of initiators in different solvents at specified temperatures and, where the Arrhenius parameters (A jec and E(icc) are included, Aqec may be calculated at any other temperature using the expression in Equation 10.2  

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