Radical Chain Pathways

HBr is the only one of the four hydrogen halides that will add readily to alkenes via a radical pathway. The reason for this is reflected in the AH values—in kJ (kcal) mol-1—below for the two steps of the chain reaction for addition of HX to CH2=CH2, for example ... 

The first paints were based upon linseed oil (obtained from flax). This is an unsaturated long-chain triglyceride, which, with metal activators, crosslinks via radical pathways to form a continuous film on the substrate. Modern paints use synthetic polymers together with either a solvent or suspending medium (e.g., water), which evaporates leaving the deposited film. Exceptions are powder coatings, which require heat for completion. 

The surface carbide is hydrogenated in subsequent steps, which leads to an appreciable coverage of the surface by CHx(ads) intermediates. Chain growth processes are assumed to occur by radical pathways such as Eqs. V and VI. 

Spin traps can act as one-electron oxidizers. This property is even more pronounced in the interactions of traps with anion-radicals. Traps can block the ion-radical pathway. In other words, they inhibit the whole reaction, including the ion-radical step. This can be explained by both the oxidation of substrate anion-radical and chain termination due to oxidation of product anion-radical. An illustrative example is the inhibition of nucleophilic substitution of 2-chloroquinoxaline by the radical trap bis(tcrt-butyl)nitrone (Carver et al. 1982). 

Eollowing a radical process, radiation induced chain addition of allylbenzene to 1,4-dioxane 16. Efficiency of the addition depends on the concentration of the monomer <1999JRN953>. Alcohols also react, albeit in low yields (10%), with 16 in the presence of (diacetoxyiodo)benzene, probably via a radical pathway, to afford 2-alkoxy-l,4-dioxanes <2004SL2291 >. Free radicals have also been generated by decarboxylation of dimethoxydioxanecarboxylic acids 101 and added to some maleimides and acrylates with high stereoselectivity (Scheme 9) <20020L2035>. 

It has also been reported that treatment of the alkyl iodide 42 with activated zinc led to the spirobicyclic ketone 43. Due to the presence of an activated carbon—carbon bond, THF was a suitable solvent and kinetic studies strongly supported an anionic carbozincation process arising from an open-chain organozinc, although a small part of the cyclic product may derive from an initial radical pathway (equation 15)32. 

The addition of hydrogen halides to simple olefins, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule.116 When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 751).137 It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCI only rarely. In the rare cases where free-radical addition of HCI was noted, the orientation was still Markovnikov, presumably because the more stable product was formed.,3B Free-radical addition of HF, HI, and HCI is energetically unfavorable (see the discussions on pp. 683, 693). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides (4-9). Markovnikov addition can be ensured by rigorous purification of the substrate, but in practice this is not easy to achieve, and it is more common to add inhibitors, e.g., phenols or quinones, which suppress the free-radical pathway. The presence of free-radical precursors such as peroxides does not inhibit the ionic mechanism, but the radical reaction, being a chain process, is much more rapid than the electrophilic reaction. In most cases it is possible to control the mechanism (and hence the orientation) by adding peroxides... 

The addition of thiols to C—C multiple bonds may proceed via an electrophilic pathway involving ionic processes or a free radical chain pathway. The main emphasis in the literature has been on the free radical pathway, and little work exists on electrophilic processes.534-537 The normal mode of addition of the relatively weakly acidic thiols is by the electrophilic pathway in accordance with Markovnikov s rule (equation 299). However, it is established that even the smallest traces of peroxide impurities, oxygen or the presence of light will initiate the free radical mode of addition leading to anti-Markovnikov products. Fortunately, the electrophilic addition of thiols is catalyzed by protic acids, such as sulfuric acid538 and p-toluenesulfonic acid,539 and Lewis acids, such as aluminum chloride,540 boron trifluoride,536 titanium tetrachloride,540 tin(IV) chloride,536 540 zinc chloride536 and sulfur dioxide.541... 

On the other hand, the Gif-tert-butyl hydroperoxide (TBHP) systems seem to be much less complicated. These have been extensively studied by many groups and all workers agree that the reaction proceeds via radical pathways based on the reactivity of tert-butylperoxy and tert-butyloxy radicals [22-24]. Minisci et al. [22] suggested a Haber-Weiss radical chain mechanism [25] that accounts for the observed selectivities. 

If one assumes that there is no radical-induced chain decomposition of the hydroperoxide and small amounts of epoxide formed via a radical pathway are neglected, then the selectivity is given by... 

Radical reactions are looked upon by many chemists as uncontrollable processes which yield intractable mixtures of products. Even so, the ingenuity of synthetic chemists has resulted in many useful synthetic schemes via radical pathways. These include chain lengthening, ring formation and ring expansion processes. This synthetic work has seen dramatic steps forward over the last few decades and has been the subject of several general reviews recently664"669. Other notable reviews have also been written10,11,61,670 which cover more specific aspects of radical reactions and the reader is referred to all these sources for further and in-depth information. 

The autoxidation of organic substrates catalyzed by transition-metal salts has been used widely in the petrochemical industry for many years (32), but the oxidations are frequently nonselective since they operate by free-radical pathways that are sometimes initiated by the transition-metal ion. An example is the chain reaction of Reactions 2, 3, 4, and 5 (4). Propagation is maintained via Reactions 3 and 4, and any interaction between the metal ion and 02 would be incidental. 

As noted above, dioxygen reacts with organic molecules, e.g. hydrocarbons, via a free radical pathway. The corresponding hydroperoxide is formed in a free radical chain process (Fig. 4.3). The reaction is autocatalytic, i.e. the alkyl hydroperoxide accelerates the reaction by undergoing homolysis to chain initiating radicals, and such processes are referred to as autoxidations [1]. 

These results clearly indicate that in the tellurium tetrachloride addition to alkenes the main reaction pathway does not occur via a telluronium ion intermediate a radical addition pathway could explain the poor selectivity observed in many of these additions (see Tables 15 and 16). The strong effect of 4-benzoquinone on the syn/anti ratio supports the involvement of a radical pathway in chloro-telluration with tellurium tetrachloride. A possible radical chain reaction is shown87. 

Thiophosphates (110) derived from benzyl and vinylogous alcohols in CH3CN were conveniently isomerised into the corresponding thiolophosphates (111) under photochemical conditions through a non-chain radical pathway (Scheme 22). ... 

Richards, A.K Anhydrous Conversion of Methane and Other Alkanes into Methanol and Other Derivatives Using Radical Pathways and Chain Reactions with Minimal Waste Products. World Patent Application 2004/041399A2, May 21, 2004. 

A closer look at the optimum reaction conditions and at the byproduct spectrum reveals that the observed reactivity can most probably be attributed to a chain reaction involving electrochemically generated and regenerated perfluoroalkyl radicals [6] (Scheme 2.97). In contrast to alkyl radicals, perfluoroalkyl radicals are rather electrophilic in nature. Therefore, the radical pathway sometimes mimics the outcome of a nucleophilic substitution on a perfluoroalkyl bromide or iodide. 

The efficient decomposition of hydroperoxides by a non-radical pathway can greatly increase the stabilizing efficiency of a chain-breaking antioxidant. This generally occurs by an ionic reaction mechanism. Typical additives are sulfur compounds and phosphite esters. These are able to compete with the decomposition reactions (either unimolecular or bimolecular) that produce the reactive alkoxy, hydroxy and peroxy radicals and reduce the peroxide to the alcohol. This is shown in the first reaction in Scheme 1.69 for the behaviour of a triaryl phosphite, P(OAr)3 in reducing ROOH to ROH while itself being oxidized to the phosphate. 

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