General Features of Radical Reactions

Radicals are formed from covalent bonds by adding energy in the form of heat (A) or light hv). Some radical reactions are carried out in the presence of a radical initiator, a compound that contains an especially weak bond that serves as a source of radicals. Peroxides, compounds with the general structure RO-OR, are the most commonly used radical initiators. Heating a peroxide readily causes homolysis of the weak 0-0 bond, forming two RO- radicals. 

Radicals undergo two main types of reactions they react with a bonds, and they add to 7t bonds, in both cases achieving an octet of electrons. 

A radical X abstracts a hydrogen atom from a C-H (T bond to form H-X and a carbon 

A radical X- abstracts a hydrogen atom from a C-H 5 bond to form H-X and a carbon radical. One electron from the C-H bond is used to form the new H-X bond, and the other 

This radical reaction is typically seen with the nonpolar C—H bonds of alkanes, which cannot react with polar or ionic electrophiles and nucleophiles. 

Whenever a radical reacts with a stable single or double bond, a new radical is formed in the products. 

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Although alternative ionic mechanisms have been formulated," the essential feature of the reaction, namely the generation of free hydroxyl radicals, is now generally accepted." ... 

The bulk polymerization of acrylonitrile has been studied by numerous workers (for a literature survey on the problem see ref. j O and JL1J. The kinetic features of this reaction at room temperature are summarized in Table I. It is one of the typical examples of polymerization under heterogeneous conditions in which the anomalies are generally assumed to arise as a result of non-stationary conditions caused by the "occlusion" of growing chains in the precipitated polymer (10). The presence of occluded radicals was indeed demonstrated by 5R measurements (12) and by... 

There can be little doubt that the summarized work of Dainton and and his collaborators has pointed to a new important feature of the reactions of chlorine atoms with olefins. The incorporation of the reactions of hot radicals into the general olefin photochlorination mechanism brings these reactions into closer analogy with the other atomic addition reactions discussed in this article. It may be anticipated that further work on such effects will be forthcoming. It would be in particular desirable to obtain further verification of the postulated collisional deactivation of the excited AClf radicals by carrying out... 

AIBN, the radical 15 can attack the aryl ring to give an intermediate cyclohexadienyl radical 16 this is then converted to the fully aromatic product 17. The feature of this reaction that is particularly noteworthy is that the conversion of 16 to 17 is formally an oxidation reaction, yet it takes place in the presence of BusSnH, a reducing agent various mechanistic possibilities could explain this anomaly. The approaches applied to investigate this reaction further illustrate some of the methods used to elucidate radical reactions generally and are applicable to other specific examples. 

As we just argued, for reactions of electrophiles and nucleophiles/increases as the nucleophile becomes more delocalized. Thus, the series of delocalized nucleophiles, in Fig. 6.10, is more selective to changes (of any kind) that affect the gap, G, compared with the series of localized nucleophiles. This would be general for other processes as well delocalization of the single electrons in the R states of the diagram results in higher/values, and vice versa. Such trends abound in electrophile—nucleophile combinations they were analyzed also for radical addition to olefins (40), and are likely to be a general feature of reactivity. 

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