Destruction of Radicals Termination

Radicals show a very important difference from anionic and cationic intermediates. Because radicals are uncharged and possess an unpaired electron in an orbital which is capable of forming a covalent bond, they normally react with each other on every collision with little or no activation energy. The bimolecular rate constants are large, in the region of 1010 M-1 s-1. Combination, results in a new covalent bond, as in reaction (6.12). If more than one type of radical is present in the system, 

Simple free radicals are destroyed rapidly by combination or disproportionation. This means that only very low concentrations of free radicals can be built up 

Hall-headed arrows (fish-hooks, are used to show the movement of individual electrons in free radical reactions, i-contrast to the curved arrows used in polar and molecular reactions to show the movement of electron pairs. 

The rapid rate of radical termination reactions for simple organic radicals means that only very small steady-state concentrations can be maintained, typically 10-8 M or lower. However, some larger stabilized radicals do not dimerize readily and therefore larger concentrations can be built up. These stabilized radicals will be considered later in the chapter. 

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Draw termination steps showing the recombination or destruction of radicals. Termination steps are side reactions rather than part of the product-forming mechanism. Reaction of any two free radicals to give a stable molecule is a termination step, as is a collision of a free radical with the container. 

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