Allyl-type radicals

On the other hand, radicals are undoubtedly involved in the photodegradation of PVC under some experimental conditions. Recent ESR studies have provided evidence for the formation of alkyl and allyl-type radicals during the low-temperature UV irradiation of the polymer (43,72,87). Peroxy radicals were also observed when molecular oxygen was present (43,87). Other ESR work has shown convincingly that the radical -CHCI-CH2-CH-CH2-CHCI- results from the irradiation of PVC at liquid-nitrogen temperature (61,93) and is converted into a -CH2-CC1-CH2- radical at -110°C (93). 

Addition of silyl radicals to cumulenes and their isoelectronic derivatives has mainly been studied by EPR spectroscopy. The adducts of MesSi radical with the two substituted allenes 54 and 55 have been recorded [73,74]. The attack occurs at the central atom affording unconjugated allyl-type radicals. In particular the adduct radical with 55 has been described as a very persistent perpendicular allyl radical [74]. 

A convenient route to triquinanes is based on a strategy of silyl radical addition to conjugated dienes to form allylic type radicals and their subsequent intramolecular addition to C=C double bonds. By exposure of 10 to (TMS)3SiH and AIBN at 80 °C (Reaction 7.16) the triquinane 11 is obtained with an unoptimized 51 %> yield [26]. 

The actual species responsible for cationic polymerizations initiated by ionizing radiation is not established. The most frequently described mechanism postulates reaction between radical-cation and monomer to form separate cationic and radical species subsequently, the cationic species propagates rapidly while the radical species propagates very slowly. The proposed mechanism for isobutylene involves transfer of a hydrogen radical from monomer to the radical-cation to form the r-butyl carbocation and an unreactive allyl-type radical ... 

Problem 6.42 Use the concepts of (a) resonance and b) extended tt orbital overlap (delocalization) to account for the extraordinary stability of the allyl-type radical. 

Each of the three electron loss products result from deprotonation at one of the nitrogens, radical 3 from N7, radical 4 from N3 and radical 5 from Nl. Radical 3 is present at 100 K but its spectral lines decay slowly at 150 K radical 4 decays at temperatures above 200 K. The third of the deprotonated electron-loss product, radical 5, is thought to be an allylic type radical with considerable spin density on Nl. It is present at 100 K the temperature at which it decays is unknown. 

A considerable difference has been observed between the spectrum of cyclohexyl and that of the cyclopentyl radical, the former exhibiting a pronounced shoulder at 250 nm with e = 920 m -1 cm-1. Cyclohexenyl and cyclopentenyl radicals show a much stronger absorption with definite maxima at 240 nm. These are allyl type radicals and like the allyl radical itself they show extinction coefficients of 7000-9000 M -1 cm-1. The optical spectrum of the allyl radical is greatly affected by unsaturated substituents which conjugate with the allylic 1 and 3 positions. These positions bear all the spin density and their interaction with carboxyl groups, for example, shifts max to 270 nm with extinction coefficients of 20,000-40,000 M 1 cm 1 (Neta and Schuler, 1975). A carboxyl group attached to the central carbon of allyl has only a minimal effect on the absorption. 

At high temperatures, both simplifications and complications of the above mechanism occur. Bimolecular initiation processes (involving at least one unsaturated molecule) cannot be excluded (see, for example, ref. 15). Transfer processes must be included since chains are no longer long. H abstraction from alkenes generates not only allylic type radicals, but also vinylic type radicals. As the temperature increases, allylic type radicals become thermally unstable. As the activation energy of unimolecular fissions of radicals is much higher than that of bimolecular processes such as metatheses, when the temperature increases the relative concentration of the p- radicals, compared with that of the thermally stable / and Y- radicals, decreases. Therefore, termination processes involve mainly / radicals (except for H- radicals, because they are very reactive and recombine in a third-order process) and Y-radicals. Finally, the addition of the most concentrated / and Y- radicals to unsaturated molecules can play a role, because this process is followed by a very fast unimolecular fission. For reasons of size limitation, the addition of radicals (e.g. H- and CH3-) will mainly be considered. Of course, the above a priori hypotheses about relative radical concentrations or reaction rates must be checked a posteriori, when numerical calculations have been carried out. 

Their spectrum contains 6 lines, corresponding to the interaction of an unpaired electron with one a-proton and four /J-protons. When the temperature is raised (or if polyethylene is irradiated at room temperature), the spectrum of the radical (I) disappears and another EPR spectrum is observed, having an odd number of lines. This spectrum belongs probably to an allyl type radical ... 

However, allyl-type radicals are thought to be important in the au-toxidation of membrane lipids etc. (Chapter 5). These are more stable because the unpaired electron is delocalized on the two outer carbon atoms. 

This should be compared to the slowest possible chain path for the isopropyl alcohol decomposition, which (like /-butyl alcohol) is one propagated by allyl type radicals. A calculation, similar to that for /-butyl alcohol (shown above), where the major chain process is... 

Irradiation of pure PHTP produces saturated hydrocarbon radicals which are stable for weeks and months the spectral complexity is probably due to the presence of a mixture of different radicals. After monomer inclusion, the spectrum changes in a few minutes and converts to that of an allyl type radical. As an exEunple, in the case of butadiene the spectrum recorded at room temperature consists of six lines, spaced 14 gauss, with the intensity ratio 1 5 10 10 5 1, in agreement with structure 1) ... 

From the above consideration it can be deduced that the initiation of cyclo-alkanes then turns into the initiation reactions of alkenes. In this situation, the weakest C-C bonds in the alkene skeleton are those forming allyl type radicals. 

Selectivity of formation of methylcyclopentene decreases rapidly with conversion of propylene. In the thermal reaction of ethylene this compound was not identified. Formation of five-membered ring compounds—i.e., methylcyclopentene, cyclopentene, and cyclopentadiene— may be attributed to allyl type radicals (3,14). 

As a result, some chain reactions occur simultaneously, the decomposition of allyl type radicals competing with reactions like Reaction 1. Hence, the cracking of 1-pentene is described by Voevodsky in the following way ... 

We think that it proceeds in the following way. The active radicals (H, CH3 , C2H5 ) enter into the substitution reaction of an olefin molecule at the weakest C—H bonds and cause the formation of allyl type radicals, which have an activation energy of 6-8 kcal/mole (25). In addition, the active radicals enter into the addition to the double bonds, resulting in alkyl radicals ... 

M = olefins with the C—C bond conjugated with the double bond R2 = allyl-type radicals R = active radicals A = starting paraffin From the scheme it follows ... 

An allyl-type radical is also obtained by irradiation of thiourea adduct of cycloheptane, o-values also presented for 4-methyl cyclohexenyl. 

On heating to 130 K ring opening to an allyl-type radical is observed. Not observed owing to inclusion in line width. ... 

The decay behavior of allenes (1,2-propadienes) is quite different from that of the conjugated 1,3-dienes. Figure 8 shows the decay of ArS in cyclohexane for the reaction with methyl-substituted allenes [46]. By adding allene, the decay of ArS is accelerated even in the degassed solution, suggesting that the reaction proceeds irreversibly. Such irreversibility occurs when the incipient C atom-centered radical becomes a resonance stable allyl-type radical by rotation of the C-C bond, as shown in Scheme 9. In the aerated solution, the decay of ArS is further accelerated, indicating that the irreversibility due to the rotation is not completely established the addition of O2 further shifts the equilibrium to the peroxy radical side by trapping the incipient short-lived C atom-centered radicals. 

From the high reactivity of allenes, it is also presumed that the localized perpendicular C atom-centered radical is easily transformed to a planar allyl-type radical with delocalized unpaired electron by rotation of the C-C bond (Scheme 9). 

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