Radical Path Initiation

Since almost all stable species are even electron and spin paired (molecular oxygen is the obvious exception), radical processes must start either by single electron transfer or by homolytic cleavage. It is important to recognize these initiation steps in order to know when to use the one-electron paths from this chapter rather than the two-electron paths described previously. Half-headed arrows are used to symbolize the movement of one electron. Homolytic cleavage is the simple extension of a bond-stretching vibration (Fig. 11.4). The process is always endothermic, for the barrier must be at least equal to the strength of the bond cleaved. 

The following are some common radical initiators and their cleavage into radicals. 

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The halo quinones (363) undergo photochemical acylation to afford the acyl (364) and the quinol derivatives (365). - The sunlight irradiation of acetic anhydride solutions of the quinone (366) affords the triquinone product (367). Irradiation of (366) in acetone also affords (367) but in addition the diquinone (368) is formed by a free radical path initiated by the abstraction of hydrogen from the amino group by excited state acetone. 

Addition. Chlorine adds to vinyl chloride to form 1,1,2-trichloroethane [79-00-5] (44—46). Chlorination can proceed by either an ionic or a radical path. In the Hquid phase and in the dark, 1,1,2-trichloroethane forms by an ionic path when a transition-metal catalyst such as ferric chloride [7705-08-0], FeCl, is used. The same product forms in radical reactions up to 250°C. Photochernically initiated chlorination also produces... 

Historically, the first color-forming reaction to be discovered which involves electron transfer is probably the photoinitiated reaction of diphenylamine with carbon tetrabromide, which forms blue colors [42]. In fact, the major path for color formation is due to radical reactions, initiated by photolysis of C—Br bonds to produce Br3C and bromine atoms. An alternative mechanistic path, possible when the light is absorbed by the diphenylamine, would involve electron transfer. MacLachlan has shown that such processes do occur durin> photolysis of aminotriarylmethanes in the presence of electron acceptors such as CBr4 and CC14 [43]. Other electron deficient species (quinones, nitroaromatics) were also demonstrated to be effective. 

The photoreactions of ethyl acetoacetate in water have been studied. " Irradiation of the butanethioates (249) yields products via various free radical paths.Electron-transfer-induced coupling has been reported in the reactions of tertiary amines (tri-n-propyl or tri-n-butyl) with tris(pentane-2,4-dionate)cobalt(iii). ° The products obtained in moderate to good yields from the reaction were identified as the dihydrofurans (250). Analogous products were obtained using the enamines (251), thereby indicating the possible involvement of such species in the initial reaction. 

Ashmore and Levitt and later Ashmore and Burnett found an anomalously fast rate early in the reaction with the rate falling to a value in agreement with the earlier workers after about 10 % decomposition. The fast initial rate was eliminated on adding NO. These effects were explained by concurrent molecular and free radical paths to the decomposition, viz. 

This result argued for the alternate view of the EEC Cp mechanism sequence, involving initial face-selective attack on the chiral radical-cation intermediate by methoxyl radical (path A), followed by acetal cyclization and proton loss. A rationale for the increased chemical yields observed for the systems studied in Eqs. (44) and (45) was not addressed in this work, although intramolecular solvation and stabilization of the intermediate radical cation by the appended hydroxyether side chain was suggested as one of the possible factors involved in the observed stereodifferentiation [101,102]. 

It must also be pointed out, however, that initiation of acid-catalyzed alkane transformations under oxidative conditions (chemical or electrochemical) can also involve radical cations or radical paths leading to the initial carbenium ions. In the context of our present discussion, we shall not elaborate on this interesting chemistry further and limit our treatment to purely protolytic reactions. ... 

Figure 8. Time resolved absorption changes in AO following reaction with pulse-radiolytically produced C02 radicals (48). (a) Absorption changes at 610-nm monitoring reoxidation of a 5.5- xAf solution of the protein by C02 radicals. The initial, fast bimolecular reduction of T1 Cu° has a half-life of < 100 ps and is thus not resolved on the time scales shown here, (b) Absorption changes at 330 nm following intramolecular reduction of the T3 copper by TlCu. T = 286K pH 5.5 0.1 M formate lOmAf phosphate N2O saturated pulse width 1.0 ps optical path 12.3 cm. Time is in seconds the left panel shows the faster phase, while the right one shows the reaction taking place at the slower phase. The lower panels show residuals of the fits to the data.

Other approaches to free-radical polymers Include capping with a peroxy end group or using a dual path initiator. The former approach was used (65) to convert a,(o-hydroxy polyethers (Eth) to esters to peroxycarbamate end groups (Reaction 19) ... 

There is no sense or profit in talking about theories of catalytic reactions in general. The theory of catalysis is the theory of chemical reaction velocity, and the methods of operation of catalysts are as diverse as the modes of chemical change. Normally the catalyst adds a new path of reaction of lowered activation energy, but sometimes it is the non-exponential factor for the new mechanism which is more favourable, as for example in a chain reaction. Anything, such as an extraneous source of radicals, which initiates a chain reaction is of course a catalyst. 

Inference of a radical path can also be made on the basis of structure and reactivity arguments. For example, two-electron Sn2 processes generally show a maximum rate in halide displacements from alkyls of Me > Et > /-Pr > r-Bu. The reverse reactivity order is generally characteristic of radicals. R > RBr > RC reactivity is also expected for radical reactions. Initiation of polymerization of styrene or acrylonitrile is also an indicator of the presence of radicals. 

Much of modem synthetic organic chemistry involves trapping short-lived reaction intermediates in known chemical reactions in order to divert reactions into other paths producing different products. Electrochemistry is ideally suited for this purpose because single electrons are generally transferred to or from the working electrode. The initial intermediate in most organic electrode reactions is an ion radical the initial reduction step affords an anion radical and... 

It has to be emphasized that the experimental results accumulated on the oxidation processes of mechanoradicals are very important in mechanochemical degradations of polymer materials. Polymer materials used for practical purposes suffer from mechanical damage such as scratches, usually in the atmosphere. The mechanical damage produces mechanoradicals in the atmosphere and the mechanoradicals are easily converted into peroxy radicals, which initiate the autoxidation. Mechanoradicals also start the self-degradation reaction described above. Both paths of the reactions initiated by mechanical damage contribute to the self-degradation of the polymer materials. 

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