Displacement reactions radical

Replacement of Labile Chlorines. When PVC is manufactured, competing reactions to the normal head-to-tail free-radical polymerization can sometimes take place. These side reactions are few ia number yet their presence ia the finished resin can be devastating. These abnormal stmctures have weakened carbon—chlorine bonds and are more susceptible to certain displacement reactions than are the normal PVC carbon—chlorine bonds. Carboxylate and mercaptide salts of certain metals, particularly organotin, zinc, cadmium, and antimony, attack these labile chlorine sites and replace them with a more thermally stable C—O or C—S bound ligand. These electrophilic metal centers can readily coordinate with the electronegative polarized chlorine atoms found at sites similar to stmctures (3—6). 

Halogen Displacement. Poly(phenylene oxide)s can also be prepared from 4-halo-2,6-disubstituted phenols by displacement of the halogen to form the ether linkage (48). A trace of an oxidizing agent or free radical initiates the displacement reaction. With 4-bromo-2,6-dimethylphenol, the reaction can be represented as in equation 10 ... 

Although Otsu et al. [12] have studied the BPO-DMA system by electron spin resonance (ESR) technique and trapped the aminomethyl radical, there is still a lack of direct proof of the above second step, particularly concerning the behavior of the aminium radical salt. We [13] have proposed the aminium radical salt with purple color through this reaction of DMT with CCI4 in the presence of O2 following the displacement reaction as ... 

The nucleophilic displacement reactions with azide, primary amines, thiols and carboxylatc salts arc reported to be highly efficient giving high (>95%) yields of the displacement product (Table 9.25). The latter two reactions are carried out in the presence of a base (DBU, DABCO). Radical-induced reduction with tin hydrides is quantitative. The displacement reaction with phenolates,61j phosphines,6M and potassium phthalimide608 gives elimination of HBr as a side reaction. 

The photolysis of chlorinated aromatic compounds occurs by several processes which follow predictable routes 13). They frequently undergo photochemical loss of chlorine by dissociation of the excited molecule to free radicals or, alternatively, through a nucleophilic displacement reaction with a solvent or substrate molecule. Either mechanism is plausible, and the operation of one or the other may be influenced by the reaction medium and the presence of other reagents. 

For a simple displacement reaction involving atoms or radicals, such as A + BC -> AB + C, where the reaction is written in the exothermic direction, the activation energy is 5.5% of the dissociation energy of the bond which is broken. For the reverse endothermic reaction, the standard energy change of reaction must be added to this quantity in order to obtain the activation energy. 

Syntheses are limited to mercuric salts of weak acids (2,110). Generally, increasing the length of the straight alkyl chain decreases the extent of decarboxylation (e.g., Ref. 133). Electron-withdrawing substituents suppress decarboxylation. For example, mercurials are not formed with Me02C, Cl, and Me(CH2)nO substituents on the a carbon (137,148,149), but some decarboxylation occurs with these on the j8 carbon (135-137). Chain decarboxylation predominated in reactions in benzene, butyric acid [R = Me(CH2)2] (150), or acetic acid (R = Me) (124). The chain reaction was also observed for R = Me(CH2)2 in the absence of solvent and in ethylacetate or heptane solution, but in these media the radical displacement reaction was dominant (2,150). When benzene was used as solvent... 

Catalysis by radicals will usually be due to a radical addition or displacement reaction, hydrogen and halogen being the atoms on which the displacement most often occurs. It is usually a chain reaction once the substrate is converted into a radical it carries the reaction to many molecules of substrate. Examples are polymerization and autoxidation. 

Similarly, exposure of 180 to trifluoroacetic acid also promoted an internal SN2 displacement reaction to form 181 (Scheme 20.37) [68], The Myers-Saito cyclization generated the biradical 182 and, subsequently, 183. As in the case of 55, the benzylic radical center in 182 is a stabilized triarylmethyl radical. Several related transformations to produce enyne-allenes have also been reported [69, 70]. 

We note that hydrogen as well as methane, ethane and toluene have been reported [25], but do not currently have good explanations in the schemes shown above. For the methane and ethane, one would have to resort to either radical displacement reactions of the methyl ester shown in Scheme 18.3 in the middle... 

Several very stable radicals (e.g. those derived from AIBN) react with tetraphenyl-biphosphine in a homolytic displacement reaction.39 Phosphoranyl radicals, e.g. (36), were postulated as being intermediates. The reaction of the triplet state of olefin (37) is thought to occur via a similar mechanism. 

In the above-described displacement reactions (Sh2) of sulfur- and selenium-containing compounds, the mechanism could either be a synchronous or a stepwise process (Scheme 4.2). Thus, in the Sh2 stepwise mechanism an intermediate sulfuranyl or selenyl radical is formed, followed by a-cleavage [56]. Based on competitive studies, a stepwise process was suggested to occur in the reaction of (TMS)3Si radical with -decylphenylselenide [43]. On the other hand, ab initio calculations supported the synchronous process and the same observations were explained in terms of an overall reversible reaction [57]. 

The transfer constants for various compounds are shown in Table 3-6. These data are useful for the information they yield regarding the relationship between structure and reactivity in radical displacement reactions. Aliphatic hydrocarbons such as cyclohexane with strong C—H bonds show low transfer constants. Benzene has an even lower Cs value because of... 

This method is by far the most successful and most widely used in the synthesis of four-membered heterocycles with two heteroatoms. In principle these formal additions can be envisioned to occur by (a) two displacements, (b) thermal concerted [2 + 2] cycloadditions, (c) photoinduced [2 + 2] additions, (d) stepwise zwitterionic intermediates or dipolar reactants, and (e) radical intermediates. In some cases the mechanisms have been elucidated but in many others the actual mode of addition is not known. For this reason, no attempt has been made to formally divide the latter four (b-e) into categories and only the first [(a), two displacement reactions] is covered separately. 

Several mechanisms have been proposed for the intriguing interconversions of sulfur (or selenium) rings. These include the formation of (i) radicals by homolytic S-S bond cleavage, (ii) thiosulfoxides of the type S =S via ring contraction (an intramolecular process) or (iii) spirocyclic sulfuranes (or sele-nanes) via an intermolecular process. A fourth alternative (iv) invokes nucleophilic displacement reactions. Generic examples of mechanisms (ii)-(iv) for homoatomic sulfur or selenium rings are depicted in Scheme 12.1. 

The cathodic pinacolisation of 2- and 4-acetylpyridine, which had been investigated by one of the present authors (231-233), offered the chance for a complete kinetic analysis as the respective current voltage curves are of reversible character. They allow for evaluation of the kinetics of consecutive reactions, and one can show that at low pH reaction, Eq. (45c) is only possible if strong surfactants are absent. Such surfactants, by occupying the electrode surface, displace ketyl radicals, RiR2(OH)C , from the electrode surface because the latter are relatively weakly adsorbed and cannot compete with strong surfactants in adsorption. Ketyl radicals dissolved in aqueous or organic solvents of low pH are protonated in a fast almost diffusion-controlled reaction. After protonation they are further immediately reduced to form the monomeric carbinol instead of the hydrodimer—the pinacol ... 

The Barton-McCombie deoxygenation reaction was invented for use in the manipulation of aminoglycoside antibiotics. It has become a popular method because of the mild conditions employed. Radical reactions have advantages over ionic reactions for carbohydrate chemistry. In this context, there is little neighboring group interference in cationic reactions and little elimination compared with normal nucleophilic displacement reactions. 

In 6/ -hydroxy-19-iodo steroids (Figure 12-2) the orientation of the reacting centers O—CH2—I resembles the arrangement in the transition state of an Sn2 displacement reaction and consequently the activation energy for the transition (3) - (4) is low. It has been suggested that the conformation indicated (Figure 12-2) also faciliates an analogous radical displacement reaction [SH2, (5) - (4)] of the iodine atom at C-19 by a 6/ -oxy radical formed by homolysis of the O—I bond in (5). In such cases only ethers (4) can be expected. 

The direct substitution steps are analogous to the Sw2 or S 2 displacements of heterolytic chemistry and are termed SH2 reactions radical substitutions that are most reasonably formulated as being initiated by addition of a radical to an unsaturated system (Equation 9.65) (analogous to addition-elimination sequences in heterolytic reactions) are considered in Section 9.4. 

Directive effects, in aromatic substitution, a quantitative treatment of, 1, 35 Directive effects, in gas-phase radical addition reactions, 16, 51 Discovery of mechanisms of enzyme action 1947-1963, 21, 1 Displacement reactions, gas-phase nucleophilic, 21, 197 Donor/acceptor organizations, 35, 193... 

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