Radical Reactions and How They Occur

Radical reactions are not as common as polar reactions, but they re nevertheless important in organic chemistry, particularly in some industrial processes. Let s see how they occur. 

Radicals are highly reactive because they contain an atom with an odd number of electrona (usually seven) in its valence shell, rather than a stable noble-gas octet. A radical can achieve a valence-shell octet in several ways. For example, a radical might abstract an atom from another molecule, leaving behind a new radical. The net result is a radical substitution reaction  

Alternatively, a reactant radical might add to an alkene, taking one electron from the alkene double bond and yielding a new radical. The net result is a radical addition reaction  

Let s look at a specific example of a radical reaction—the chlorination of methane—to see its characteristics. A more detailed discussion of this radical substitution reaction is given in Chapter 10. For the present, it s only necessary to know that methane chlorination is a multistep process. 

Radical substitution reactions normally require three kinds of steps initiation, propagation, and termination. 

Reactant Alkene Addition product radical radical 

---

Under normal circumstances, this occurs by collisions with a third-body species and the reaction rate therefore depends on total pressure. Such a mechanism is impossible in the super-rarified environment of interstellar space. However, the kinetics of such reactions are of indirect interest to astrochemists on two counts. First, treatments of radiative association [22], which is implicated in the formation of molecular species in interstellar clouds, have much in common with those of three-body association [23]. Second, the rate constants for radical association in the limit of high pressure correspond to those for formation of the energised associated molecule, since all such species are collisionally stabilised in the limit of high pressure. Consequently, the values of kggg and how they vary with temperature provide an important test of theories of reactions occurring over attractive potential energy surfaces [6]. 

Radicals add to unsaturated bonds to form new radicals, which then undergo addition to other unsaturated bonds to generate further radicals. This reaction sequence, when it occurs iteratively, ultimately leads to the production of polymers. Yet the typical radical polymerization sequence also features the essence of radical-induced multicomponent assembling reactions, assuming, of course, that the individual steps occur in a controlled manner with respect to the sequence and the number of components. The key question then becomes how does one control radical addition reactions such that they can be useful multicomponent reactions Among the possibilities are kinetics, radical polar effects, quenching of the radicals by a one-electron transfer and an efficient radical chain system based on the judicious choice of a radical mediator. This chapter presents a variety of different answers to the question. Each example supports the view that a multicomponent coupling reaction is preferable to uncontrolled radical polymerization reactions, which can decrease the overall efficiency of the process. 

To achieve stability, these reactive entities may undergo any number of reactions. The purpose here is to enumerate what these reactions are and the mechanisms by which they occur. Moreover, an attempt is made to show how the organometallic free radicals are analogous to organic radicals, which are in general more familiar. The 17e organometallic species are isolobal with alkyl radicals which are 7e spedes, also one electron shy of a closed shell. 

The thermal decompositions (pyrolyses) of hydrocarbons other than the cyclic ones invariably occur by complex mechanisms involving the participation of free radicals the processes are usually chain reactions. In spite of this, many of the decompositions show simple kinetics with integral reaction orders, and this led to the conclusion by the earlier workers that the mechanisms are simple. Ethane, for example, under the usual conditions of a pyrolysis experiment, decomposes by a first-order reaction mainly into ethylene and hydrogen, and the mechanism was thought to involve the direct split of the ethane molecule. Rice et however, showed that free radicals are certainly involved in this and other reactions, and this conclusion has been supported by much later work. An important advance was made in 1934 when Rice and Herzfeld showed how complex mechanisms can lead to simple overall kinetics. They proposed specific mechanisms in a number of cases most of these have required modification on the basis of more recent work, but the principles suggested by Rice and Herzfeld are still very useful. 

The differential reactivity of allyltributyltin towards electron-rich versus electron-poor radicals means that it is possible to carry out reaction sequences in which multiple carbon-carbon bonds are formed in a single transformation. The first example of such a sequence was reported by Mizuno and Otsuji [24], They showed that reaction of alkyl iodides with electron-deficient alkenes such as l,l-dicyano-2-phenylethene 33 and allyltributyltin gives good yields of three-component coupling products 34 (Scheme 6). In this process, an electron-rich alkyl radical 35 generated either by photolysis or by AIBN-mediated initiation undergoes selective addition to the electron-deficient alkene. Addition to the alkene occurs selectively since this process is much faster than addition of the alkyl radical to allyltributyltin. How-the resulting adduct radical 36 is now electron deficient, so it adds to allyl-... 

The vast majority of stoichiometric reactions do not occur by transformation of the reactants to the products in a single step rearrangement of the constituent atoms. They occur via a series of reactive interactions at the atomic and molecular levels, and they involve reactive chemical species that are formed and then entirely consumed, so they do not appear in the stoichiometric equation. These molecular level interactions are called elementary chemical reactions. The reactants and products in an elementary reaction may be atoms, molecules, free radicals, ions, excited states, etc. An elementary chemical reaction is an isolated interaction between such species in which the transformation from reactants to products occurs by rearrangement of the constituent atoms. Elementary reactions are fundamental descriptions of how chemical transformations occur. The list of elementary reactions that take place during the course of a stoichiometric reaction is called the mechanism of the reaction. The mechanism thus embodies the detailed atomic and molecular level chemistry that accounts for the overall chemistry that is observed in a stoichiometric reaction. 

---