Radical paths

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... 

Figure 1.31 Electrophilic and radical paths in direct-fluorination chemistry leading to substitutions, additions and polymerizations with the example of toluene as substrate.

Besides on-line UV analysis, off-line HPLC analysis was performed after about 48 h dark reaction [72, 74]. In this period, the photoinduced reaction proceeds by radical paths in the dark. 

OS 87] ]R 35] ]P 67/The longer the residence time, the higher is the conversion, as expected [72, 74]. This trend is seen in the on-line UV- and off-line HPLC spectra. Whereas on-line UV absorption showed zero conversion at too short a residence time (flow rate 10 pi min ), a level of about 50% was found in the HPLC analysis. This clearly proves that the reaction proceeds by a radical path in the dark, if sufficient time is given. 

Fishburne et al.22 have recalculated Hiraoka and Hardwick s data223 erroneously. f Rate coefficients for the overall decomposition, including the free radical path. 

The disappearance of nitroxides may also occur by unimolecular reactions. A good example is found with t-butoxy t-butyl nitroxide, known to be a relatively short-lived species, for which three fragmentation pathways are possible (1 la,b,c). Loss of butoxyl radicals (path a) was shown to be reversible (Perkins and Roberts, 1974), and, in the absence of any substrate which... 

An innovative way to introduce trifluoroethanol and trifluoroethylamine moiety by the radical path has been described starting from xanthates of (0,0)- and (<9,A0-hemiacetals of fluoral. ° This powerful method allows cascade reactions, yielding elaborated molecules (Figure 2.52). ... 

Saccharidic Difluorophosphonates Difluoromethylene phospho-nates have been the focus of numerous works. Indeed, these compounds are able to mimic the phosphate bond in the synthesis of enzyme inhibitors. This interest is obvious for the furanose series in this case, they are non scissible analogues of 5-phosphate nucleosides (cf. Chapter 7). Difluoromethylene phosphonates can be prepared via a radical path starting from compounds that have the difluoromethylene moiety in the pseudo-anomeric position. Nevertheless, methods based on metal derivatives of difluorophosphonates are generally easier and broadly applicable. 

The mechanism operating in these reactions seems to follow a radical path that could be related to the classic Haber-Weiss radical-chain sequence [32] based on the couple [Rh2 H jt-OAc)4]/[Rh2 (p.-OAc)4j. 

The interesting "dimerization of lithiated N-nitrosamines to cyclic tetrazene N-oxides has been described (U8) and provides another route to the novel cyclic tetrazenes (U9, 0) A radical path has been suggested for this reaction [Scheme h] ( ). 

Taking into account the contribution from the radical path, Ashmore and Spencer13 used the value of k calculated by Ashmore and Chanmugam to recompute the value of kx. They found the data in the temperature range 431-523°K very well represented by the Arrhenius form log Ai = 5.1 — 6/0. This equation extrapolated to 298°K gives a rate constant of 5 M 2 sec-1. 

Addition. Vinyl chloride undergoes a wide variety of addition reactions. Chlorine adds to vinyl chloride to form 1,1.2-tnchloroethane by either an ionic or a radical path. Hydrogen halides add to vinyl chloride, usually to yield the 1.1-adduct. Many other vinyl chlonde adducts can be formed under acid-catalyzed Fnedel-Crafts conditions. Vinyl chloride can be hydrogenated to ethyl chloride and ethane over a platinum on alumina catalyst. 

The much studied photochemistry of aryldisilanes carried out in earlier years has been reviewed51,52. Cleavage of the silicon-silicon bond of the disilyl moiety is always involved, but various other reactions have been observed depending on the structure of the disilane and the conditions employed. Thus cleavage to a pair of silyl radicals, path a of Scheme 15, is normally observed, and their subsequent disproportionation to a silene and silane, path b, is often observed. There is evidence that the formation of this latter pair of compounds may also occur by a concerted process directly from the photoex-cited aryldisilane (path c). Probably the most common photoreaction is a 1,3-silyl shift onto the aromatic ring to form a silatriene, 105, path d, which may proceed via radical recombination52. A very minor process, observed occasionally, is the extrusion of a silylene from the molecule (path e), as shown in Scheme 15. 

In the peroxidase reaction, Compound I is reduced to the Fe111 resting state via Compound II in two consecutive one-electron transfer steps which results in the formation of two substrate radicals (path B in Scheme 2.11). With respect to the whole catalytic cycle, this behavior can be described by the following equation ... 

NaN03, or -Et4NBr 1 96K Both reactions presumably occur via the radical path (89). 

The mode of cation formation in electrochemical decarboxylation appears not to be uniform. Skcll 204 found two discrete 1 e-steps for oxidation of car-boxylates by chronopotentiometry. He attributed the second electron transfer to oxidation of the alkyl radical (path b, Eq. (94) ) as the carboxylate radical RC02 is to shortlived (r 10 10 sec) to survive for the second oxidation step. 

Alkoxylation of enol ethers to cr-alkoxy acetals 35 certainly involves radical cations as intermediates as tail-to-tail dimers, which exclude the radical path, (Eq. (126) ) are formed as side 3S or main products 36>268). 

The radical path is most consistent with the existing experimental data. A correlation of half-wave potentials with Taft polar (a ) and steric (Es) constants 478)indicates that a parallel orientation of the C-X dipole with regard to the electrode suiface is most favourable for an electron transfer to the antibonding o orbital of the C-X bond. Thereby a radical anion 162) is formed 471 which rapidly dissociates to halide ion and radical. The more facile reduction of exo-2-... 

In contrast to the cyclooctene [30-32] and cycloheptene cases [58], the plot of the logarithm of the relative rate constant did not give a straight line, particularly at high temperatures, as a result of the major contribution of the radical path (Fig. 7). Good linear plots were obtained, however, at lower temperatures, and the ee of 79 reached the highest value of 68% ee at — 78°C [60]. The differential activation parameters obtained are much larger than those for cyclooctene [30-32] and cycloheptene [58]. 

Ozonation of saturated compounds appears to proceed by hydride transfer to give the ion pair [R+H03], which decomposes by a radical path giving high yields of singlet oxygen other paths give H202. 

The pathways differ in outcome with substrates of defined configuration racemization for the radical path, inversion for the Sn2, and retention for the concerted route. 

X3E)RN— N=N—R], Such triazenes can thus decompose only by the free radical path of Thermolysis Pathway I and are, therefore, thermally as stable as triorganyltriazenes (RN=N—NR2). 

PVC degrades around 250-300°C, giving HCl. The mechanism of PVC degradation is highly complex in nature the free radical path seems to play a major role. The probable mechanism is that a free radical R, produced either by impurity or by reaching a desired temperature, attaches to a methylene group and subtracts H with simultaneous transfer of the free radical site onto the chain. The labile chlorine atom which is at a P-position with respect to the free radical chlorine atom is now released as a free radical and stabilizes the structure. 

The sensitivity of this procedure to errors in the predicted yields of Table IV was checked. An overestimation of all three (C8H16, C9H18> and C10H2o) yields from the prediction would have no effect on the estimated retro-ene contribution, and any changes in the free radical path contributions would be equal and opposite and proportional to the size of the error in prediction. An error in decene alone would affect all three. For example, a 10% overestimate of decene in Table IV changes the relative importances of the retro-ene, abstraction, and addition paths by + 0.004 ( 3.5% ), — 0.05 (10% ), and -f 0.05 (10% ), respectively. Thus, the predicted contribution of the retro-ene path is relatively insensitive to errors in the Rice-Kossiakoffff predictions. 

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