Further examples of radical reactions

As indicated earlier, heat may be used to form radicals derived from the fission of weak bonds. A very common example of this is the thermal decomposition of metal alkyls. For example, tetraethyl lead, Pb(C2H5)4, readily gives ethyl 

In contrast to light and heat, it is possible to initiate a reaction using a compound that spontaneously forms radicals. For example, a very useful synthetic radical reaction is the bromination of an allylic carbon using N-bromosuccinimide, NBS. This reaction exemplifies the third manner in which radical reactions may be initiated. This reaction will not start unless there is a trace of a radical initiator, In This may be the result of some impurity, such as a small amount of a peroxide, which readily decomposes to form a radical. This initial radical, In, then reacts with traces of bromine or HBr molecules to form bromine atoms. These then abstract the allylic hydrogen from the alkane, R-H. Write down the steps that have been considered so far. 

So far, the NBS has not been involved in the reaction. We know that HBr will react by adding across the double bond that is present in the allylic compound, but in this case that would be an unwanted side reaction. This is where the NBS comes into play. It reacts with the HBr to produce Br2, which 

the NBS acts so as to produce a very low, steady concentration of bromine molecules. The very low concentration accounts for the fact that there is very little addition of the bromine molecule across the double bond. The addition reaction of HBr is also kept to a minimum, because the HBr is consumed by the NBS. This allylic bromination is called the Wohl-Ziegler bromination. 

Earlier in this section, it was mentioned that Pb(C2H5)4 was used to ensure the smooth 

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