Reaction mechanisms free-radical mechanism

CHsSNa in an aprotic solvent or by BusSnH. Both reactions have free-radical mechanisms.Tertiary nitro compounds can be reduced to RH by NaHTe. The compound Bu3SnH also reduces isocyanides (RNC, prepared from RNH2 by formylation followed by 17-33) to a reaction that can also be... 

Aromatic compounds can also be arylated by aryllead tricarboxylates. Best yields ( 70-85%) are obtained when the substrate contains alkyl groups an electrophilic mechanism is likely. Phenols are phenylated ortho to the OH group (and enols are a phenylated) by triphenylbismuth dichloride or by certain other Bi(V) reagents. O-Phenylation is a possible side reaction. As with the aryllead tricarboxylate reactions, a free-radical mechanism is unlikely. ... 

Common photopolymerization reactions by free radical mechanism follow the same laws of chemistry as do the thermal polymerizations. The differences are primarily in the formations of the initiating radicals. Typical chain growth polymerization reactions are initiated by Ifee radicals that come from thermal decomposition of the initiators. The initiating Ifee radicals in photo polymerizations, on the other hand, come If om photolyses of the photoinitiators. 

The mechanism of Freund reaction is more likely as same as the Wurtz reaction, a free-radical mechanism. In the presence of iodide ions, the pathways might be a combination of substitution (SnI or Sn2) with a free-radical mechanism. ... 

The free radical mechanism is confirmed by the fact that if a substituted aromatic hydrocarbon is used in this reaction, the incoming group (derived from the diazotate) may not necessarily occupy the position in the benzene ring normally determined by the substituent present—a characteristic of free radical reactions. 

Under CO pressure in alcohol, the reaction of alkenes and CCI4 proceeds to give branched esters. No carbonylation of CCI4 itself to give triichloroacetate under similar conditions is observed. The ester formation is e.xplained by a free radical mechanism. The carbonylation of l-octene and CCI4 in ethanol affords ethyl 2-(2,2,2-trichloroethyl)decanoate (924) as a main product and the simple addition product 925(774]. ... 

FIGURE 4 21 The initiation and propagation steps in the free radical mechanism for the chlorination of methane Together the two propaga tion steps give the overall equation for the reaction... 

The second mechanism is the one followed when addition occurs opposite to Markovmkov s rule Unlike electrophilic addition via a carbocation intermediate this alternative mechanism is a chain reaction involving free radical intermediates It is pre sented m Figure 6 7... 

Hydrogen bromide (but not hydrogen chloride or hydrogen iodide) adds to alkynes by a free radical mechanism when peroxides are present m the reaction mixture As m the free radical addition of hydrogen bromide to alkenes (Section 6 8) a regioselectiv ity opposite to Markovmkov s rule is observed... 

Alkenes react with N bromosuccimmide (NBS) to give allylic bromides NBS serves as a source of Br2 and substitution occurs by a free radical mechanism The reaction is used for synthetic purposes only when the two resonance forms of the allylic radical are equivalent Otherwise a mixture of isomeric allylic bromides is produced... 

The reaction follows a free radical mechanism and gives a hydroperoxide a compound of the type ROOH Hydroperoxides tend to be unstable and shock sensitive On stand mg they form related peroxidic derivatives which are also prone to violent decomposi tion Air oxidation leads to peroxides within a few days if ethers are even briefly exposed to atmospheric oxygen For this reason one should never use old bottles of dialkyl ethers and extreme care must be exercised m their disposal... 

Among other possible reactions, these free radicals can initiate ordinary free-radical polymerization. The Ziegler-Natta systems are thus seen to encompass several mechanisms for the initiation of polymerization. Neither ionic nor free-radical mechanisms account for stereoregularity, however, so we must look further for the mechanism whereby the Ziegler-Natta systems produce this interesting effect. 

Metal Catalysis. Aqueous solutions of amine oxides are unstable in the presence of mild steel and thermal decomposition to secondary amines and aldehydes under acidic conditions occurs (24,25). The reaction proceeds by a free-radical mechanism (26). The decomposition is also cataly2ed by V(III) and Cu(I). 

In the radiant section, the hydrocarbon mixture undergoes reactions involving free radicals (51). These mechanisms have been generalized to include the molecular reactions shown below ... 

Hydrosdylation can also be initiated by a free-radical mechanism (227—229). A photochemical route uses photosensitizers such as peresters to generate radicals in the system. Unfortunately, the reaction is quite sluggish. In several apphcations, radiation is used in combination with platinum and an inhibitor to cure via hydro sdylation (230—232). The inhibitor is either destroyed or deactivated by uv radiation. 

In tbe first attempt to prepare a two-dimensional crystalline polymer (45), Co y-radiation was used to initiate polymerization in monolayers of vinyl stearate (7). Polymerization at the air—water interface was possible but gave a rigid film. The monomeric monolayer was deposited to give X-type layers that could be polymerized in situ This polymerization reaction, quenched by oxygen, proceeds via a free-radical mechanism. 

This reaction proceeds through a chain mechanism. Free-radical additions to 1-butene, as in the case of HBr, RSH, and H2S to other olefins (19—21), can be expected to yield terminally substituted derivatives. Some polymerization reactions are also free-radical reactions. 

Addition to the Double Bond. Chlorine, bromine, and iodine react with aHyl chloride at temperatures below the inception of the substitution reaction to produce the 1,2,3-trihaLides. High temperature halogenation by a free-radical mechanism leads to unsaturated dihalides CH2=CHCHC1X. Hypochlorous and hypobromous acids add to form glycerol dihalohydrins, principally the 2,3-dihalo isomer. Dehydrohalogenation with alkah to epicbl orobydrin [106-89-8] is ofgreat industrial importance. 

Hydrogen hahdes normally add to form 1,2-dihaLides, though an abnormal addition of hydrogen bromide is known, leading to 3-bromo-l-chloropropane [109-70-6], the reaction is beUeved to proceed by a free-radical mechanism. Water can be added by treatment with sulfuric acid at ambient or lower temperatures, followed by dilution with water. The product is l-chloro-2-propanol [127-00-4]. 

Homogeneous Oxidation Catalysts. Cobalt(II) carboxylates, such as the oleate, acetate, and naphthenate, are used in the Hquid-phase oxidations of -xylene to terephthaUc acid, cyclohexane to adipic acid, acetaldehyde (qv) to acetic acid, and cumene (qv) to cumene hydroperoxide. These reactions each involve a free-radical mechanism that for the cyclohexane oxidation can be written as... 

Resin-modified glass—ionomer lining and restorative materials add a multifunctional acidic monomer to the poly(acryhc acid) [9003-01 Hquid component of the system. Once the glass powder and Hquid are mixed, setting can proceed by the acid—glass—ionomer reaction or the added monomer can be polymerized by a free-radical mechanism to rapidly fix the material in place (74,75). The cured material stiH retains the fluoride releasing capabiHties of a glass—ionomer. 

Mechanism. The thermal cracking of hydrocarbons proceeds via a free-radical mechanism (20). Siace that discovery, many reaction schemes have been proposed for various hydrocarbon feeds (21—24). Siace radicals are neutral species with a short life, their concentrations under reaction conditions are extremely small. Therefore, the iategration of continuity equations involving radical and molecular species requires special iategration algorithms (25). An approximate method known as pseudo steady-state approximation has been used ia chemical kinetics for many years (26,27). The errors associated with various approximations ia predicting the product distribution have been given (28). 

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

It has recently been suggested that a free radical mechanism i.e., homo-lytic cleavage of the oxygen-oxygen bond rather than the heterolytic cleavage pictured) may be involved in the reaction of some substituted benzophenones and peroxyacetic acid. 

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