Radical Substitution Reactions EOB

Radical substitutions in paraffin derivatives do not take place by attack on carbon as in the 5 and reactions. Instead, they involve abstraction of an atom, usually hydrogen, by a radical reagent to give an alkyl radical which then reacts in some way to form the product. This secondary reaction also usually involves transfer of an atom to the alkyl radical from some normal molecule which is thus in turn converted to a radical. The result is a radical chain reaction in which radicals effectively catalyze substitution. A good example is the photochemical bromination of paraffins, which takes place in the following way  

The two propagation reactions have the overall effect of converting RH -h Br2 into RBr -f HBr. Bromine atoms catalyze this reaction since the bromine atom used up in the first propagation step [equation (5.172)] is regenerated in the second [equation (5.173)]. Bromine atoms are removed by combination [equation (5.174)] but are generated by photochemical dissociation of Br2 [equation (5.171)]. 

In most of these reactions, the slowest step in the propagation cycle is the abstraction of an atom from the substrate, e.g., equation (5.172). If the rate of initiation is held constant, the overall rate is then determined by this step. Moreover the resulting radicals R- are removed very rapidly [e.g., equation (5.173)] so their concentration is very small. The termination reaction is therefore almost confined to radicals derived from the substituting agent [e.g., Br atoms, equation (5.174)]. The relative reactivities of different 

Reactions of this kind are clearly analogous to 5 2 and reactions except that the reaction center involves a single atom (e.g., H) instead of an alkyl group thus, 

In equation (5.176), the fishhook arrows denote formal displacements of single electrons. Thus the unpaired electron of Y- and one of the H—R bond electrons combine to form the Y—H bond in the product, while the other H—R bond electron remains unpaired in the radical R.  

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