Free radicals, occurrence processes

The occurrence of free radical chain processes involving electron transfer is, of course, most likely to occur when easily oxidized or reduced substrates are involved, with radical anion formation being favoured by strongly basic or reductive conditions and radical cations by acidic or oxidative environments. 

Free Electron Molecular Orbital method colour and constitution, 1, 342 Freelingyne occurrence, 4, 706 Free radical processes in photography, 1, 387-389 Friedlander synthesis quinolines, 2, 443 thioindigo dyes, 4, 910 Fries rearrangement chroman-4-one synthesis from, 3, 850 Fructose, 1-deoxy- C NMR, 4, 575 Frusemide as diuretic, 1, 174 metabolism, 1, 245 FS-32 — see 1/f-Indazole, l-[3-... 

In reality, many other chemical and photochemical processes take place leading to a sort of steady-state concentration of O3 which is a sensitive function of height. To be accurate, it is necessary to include the reactions of nitrogen oxides, chlorine- and hydrogen-containing free radicals (molecules containing an unpaired electron). However, occurrence of a layer due to the altitude dependence of the photochemical processes is of fundamental geochemical importance and can be demonstrated simply by the approach of Chapman (1930). 

The first attempts at ROP have been mainly based on anionic and cationic processes [4,5]. In most cases, polyesters of low molecular weight were recovered and no control on the polymerization course was reported due to the occurrence of side intra- and intermolecular transesterification reactions responsible for a mixture of linear and cyclic molecules. In addition, aliphatic polyesters have been prepared by free radical, active hydrogen, zwitterionic, and coordination polymerization as summarized in Table 2. The mechanistic considerations of the above-mentioned processes are outside the scope of this work and have been extensively discussed in a recent review by some of us [2 ]. In addition, the enzyme-catalyzed ROP of (di)lactones in organic media has recently been reported however, even though this new polymerization procedure appears very promising, no real control of the polyesters chains, or rather oligomers, has been observed so far [6]. 

However, it was subsequently shown9 that these observations could be more satisfactorily accounted for by the occurrence of free radical chain reactions. The free radicals might arise from ionic or excited species. The radiation chemistry of liquid water was claimed to be explicable10 in terms of the reactions of the hydrogen atom and the hydroxyl free radical. Ionic mechanisms consequently fell into disrepute, but it is interesting that in recent years ionic processes have been recognised as of increasing importance. The wheel has indeed turned full circle ... 

It has already been pointed out (p. 73) that the postulate of free radical chain reactions provided a reasonable explanation for early results of the radiolyses of gases. It was later suggested10 that the radiation chemistry of aqueous solutions could best be explained by the production of H atoms and OH radicals. Subsequently, the results of a large variety of radiolyses were explained in terms of radical reactions occurring therein. Although such experiments did not provide conclusive evidence for the existence of free radicals in the systems, the results obtained, e.g. product analysis, rate coefficients, were not inconsistent with the occurrence of free radical processes. 

Free radicals formed in polymers due to thermomechanical stress appear not only during the polymer use but also during the polymer processing and shaping to final products [46], The kind of initiation which prevails in a certain polymer depends not only on initial conditions of oxidation but also on the extent of a previous oxidation as well as on the occurrence of additional interactions among oxidation products. Increasing extent of oxidation is usually characterized by higher concentration of hydroperoxides which are secondary sources of initiation. The products of oxidation formed may alter the kinetics and mechanism of hydroperoxide decomposition so that the rate of initiation is the result of several mutually coupled processes. 

The photochemistry of these aliphatic ketones is distinguished from that of other aliphatic ketones by the occurrence of an intramolecular primary process which gives an olefin and a methyl ketone. This process is classified as a Norrish Type II process to distinguish it from the primary process which leads to free radicals. It occurs in the photolysis of many high aliphatic ketones and a similar process also occurs in the photolysis of many aldehydes 101 and esters.4 Primary processes which give rise to free radicals also occur. 

Primary process I. (i) The participation of free radicals in the formation of carbon monoxide and propane is indicated by the strong temperature dependence of (f>co and ciHa in the uninhibited reaction, and by the considerable inhibition of CO and C3H8 formation by iodine, (ii) The occurrence of C3H6 and in the uninhibited reaction and their absence in the inhibited one are evidence for the formation of the n-C3H7 radical. Hi) Propyl iodide is a product in the presence of iodine the values of level off above about 2 torr iodine pressure. This limit is independent of the temperature. Accordingly, (j>i = in ih presence... 

It has already been mentioned that free radicals are produced almost exclusively in the primary process. This is supported by the observation that the amounts of methane and ethane, formed in the experiments made in the presence of iodine or nitric oxide are the same within the limits of experimental error. However, the fact that a small amount of acetone was also formed in the presence of iodine led Bell and Blacet to assume the occurrence of primary process... 

Living polymerizations are limited to the realm of chain-growth polymerizations, in which a monomer is transformed to a polymer by a reactive species (an initiator, I) via a kinetic chain reaction (Scheme 15.1). An intrinsic limitation of a typical chain-growth process, such as free-radical polymerization, is the occurrence of termination reactions that lead to the formation of dead chains, chains that are incapable of further growth. 

---