Types of Radical Reactions and Kinetics

Free radicals are highly reactive species that can react with other radicals or molecules, or undergo spontaneous decomposition (fragmentation) with the formation of secondary radicals. Radical-to-radical reactions typically involve combination reactions of radical R( with another radical R( (radical recombination)  

Since each radical has one unpaired electron, the products have even number of electrons and typically are molecules or ions. An example of radical recombination is the combination of two hydroxyl radicals to form hydrogen peroxide  

An example of the combination of two different radicals is the reaction of nitric oxide and superoxide radicals that results in the formation of peroxynitrite (4.6). Radical recombination reactions follow second-order kinetics  

The kinetics of (4.11) are more complex because the initial concentrations of R and R2 radicals can be different, and the decay of radicals can also occur by other, competitive recombination reactions. 

Reactions of free radicals with molecules (or ions) can occur via an addition reaction, for instance the addition of a C-centered radical to oxygen (4.2), hydrogen atom abstraction (4.3), or electron transfer mechanism (e.g., oxidation of CO by SO ). Since the total number of electrons is odd, one of the products is a new radical (e.g., (4.2) and (4.3)), except for the reactions with transition metal ions, such as the oxidation of superoxide radicals by Fe3+ ions  

---