Radical chain reactions initiation

The reaction with fluorine occurs spontaneously and explosively, even in the dark at low temperatures. This hydrogen—fluorine reaction is of interest in rocket propellant systems (99—102) (see Explosives and propellants, propellants). The reactions with chlorine and bromine are radical-chain reactions initiated by heat or radiation (103—105). The hydrogen-iodine reaction can be carried out thermally or catalyticaHy (106). 

Rate laws for radical chain reactions initiated by thermolysis are 1.5 order, first order in the component reacting in the rate-controlling step, and 0.5 order in the initiator. When the initiator is the same component as that reacting in the ratecontrolling step, the reaction will be 1.5 order in this reagent. When chain reactions are initiated by photolysis instead of thermolysis, the rate constant for initiation, the... 

Deoxygenation of primary and secondary alcohols.1 This deoxygenation has been effected by reduction of the thiocarbonyl esters with tributyltin hydride and AIBN as the radial initiator (11, 550). A newer, milder method uses diphenylsilane in a radical chain reaction initiated by triethylborane and air. Even secondary thiono-carbonates, particularly those derived from 4-fluorophenol, are deoxygenated at 25°. 

Pyrolysis of acetylene to a mixture of aromatic hydrocarbons has been the subject of many studies, commencing with the work of Berthelot in 1866 (1866a, 1866b). The proposed mechanisms have ranged from formation of CH fragments by fission of acetylene (Bone and Coward, 1908) to free-radical chain reactions initiated by excitation of acetylene to its lowest-lying triplet state (Palmer and Dormisch, 1964 Palmer et al., 1966) and polymerization of monomeric or dimeric acetylene biradicals (Minkoff, 1959 see also Cullis et al., 1962). Photosensitized polymerization of acetylene and acetylene-d2 and isotopic analysis of the benzene produced indicated involvement of both free-radical and excited state mechanisms (Tsukuda and Shida, 1966). 

In early attempts to produce an iron-oxo species (20) from typical porphyrins like chloro-a,/3,y,8-tetraphenylporphinatoiron(III) [Fe(III)TPP-Cl] and chloroferriprotoporphyrin(IX)[Fe(III)PPIX-Cl], we examined the reaction of t-butyl hydroperoxide and peroxy-acids with alkanes and olefins in the presence of these catalysts. With peroxyacids, decomposition of the porphyrin ring was observed, while with the f-butyl hydroperoxides, product distributions were indistinguishable from free-radical chain reactions initiated photochem-ically in the absence of any metals. 

In a free-radical chain reaction, initiation steps generally create new free radicals. Propagation steps usually combine a free radical and a reactant to give a product and another free radical.Termination steps generally decrease the number of free radicals. 

Fig. 8-4. Examples of radical chain reaction initiated by oxygen and decomposition of hydroperoxides catalyzed by transition metal ions. R denotes an organic residue.

A radical chain reaction initiated by visible light irradiation occurs with l,2-dihydro-2-thioxo-1-pyridinyl 4-alkenylcarbamates (PTOC carbamates) 1 in the presence of a carboxylic acid and a hydrogen-atom donor, preferably tert-butyl mercaptan after homolytic cleavage of the weak N—O bond and a decarboxylation step, the aminyl radical is protonated by the carboxylic acid and cyclizes to give a carbon radical. After reaction with tert-butyl mercaptan and a base the pyrrolidine 2 is obtained. In competition with trapping by tert-butyl mercaptan, or in the absence of this, the carbon radical can react with the PTOC carbamate 1 in a chain-propagating step to give the sulfide 3 (Section 7.2.5.9.)127. 

Most phenol nowadays is obtained from isopropylbenzene (cumene), which is oxidized by air in the cumene proces.s (Scheme 4.1). Acetone (propanone) is a valuable by-product of the process and this route is a major source of this important solvent. The formation of cumene hydroperoxide proceeds by a free radical chain reaction initiated by the ready generation of the tertiary benzylic cumyl radical, which is a further illustration of the ease of attack at the benzylic position, especially by radicals (see Chapter 3). 

Xanthate 446 undergoes cyclization in the presence of camphorsulfonic acid via a radical chain reaction initiated by a small amount of lauroyl peroxide to give pyrroloimidazoles 449 in 56% yield. The use of an acid and anhydrous medium inhibits nucleophilic attack of the basic heterocycles at the xanthate moiety and allows radical reactions to occur. Fused heteroaromatic compounds can also be prepared directly from benzimidazole carrying an N-alkenyl substituent and xanthates by a tandem radical addition/cyclization to provide, for example, pyrrolobenzimidazole 453 in 57% yield (Scheme 106) <2002OL4345>. 

It has been reported that xanthane derivatives of tetraalkylmethylene-1, 1-bisphosphonate (442) are smoothly added to different olefins to give various functionalised geminal bisphosphonates (443) via a radical chain reaction initiated by lauroyl peroxide (Figure 81). ... 

Many reduced gases and vapors released by both human activities and natural ecosystems are eventually oxidized in the atmosphere by free-radical chain reactions initiated by OH. Rate constants for the reaction of OH with several organic vapors are shown in Table 4-12. 

Give a mechanism for this radical chain reaction (initiated by light). 

The sulphochlorination is a radical chain reaction, initiated by vis./UV light (A. < 480 nm, quantum yield of -2000). The reagents must contain O2 and H2O in very limited quantities. As a result, complex mixtures of isomeric alkanesulphonates are formed comprising di- and polysulphonates, paraffins, chloroparaffins and salts as well. Other stages allow for alkanesulphonate manufacturing of a sufficiently high quality. 

Spin-trapping Experiments. As mentioned earlier, some of the radical chain reactions initiated by light-activated DPE s can terminate with the subsequent formation of PUFA polymers... 

The main products of the sulfochlorination process are the alkane sulfochlorides, which in turn are saponified to the desired alkane sulfonates. This process was first discovered in the 1930s, and industrially realized shortly afterwards. Sulfochlorination is a radical chain reaction, initiated by irradiation with short-wavelength light. 

The electrochemical properties of 27 have been studied with a view to learn more of the mechanism of oxidation [50].The results rule out a simple free radical chain reaction initiated by interaction between the cobalt(II) complex and dioxygen. 

The mechanism for anti-Markovnikov addition of hydrogen bromide is a radical chain reaction initiated by peroxides. 

Possibly an oxidative addition of Cl—CCI3, insertion of C=C and reductive elimination, but this could also be a radical chain reaction initiated by the metal. In this case CCl3 would add to the free alkene to give RCH CH2CCl3, which would abstract Cl from another mole of CCI4. If the latter were true, however, we would see crossover, so we can rule out the radical pathway. 

Thermal chlorination is preferred, but photochemical or catalytic methods are also employed. The thermal chlorination is a radical chain reaction, initiated by chlorine atoms at temperatures of 350-550 °C... 

The reduction of alkyl hahdes has been important in many syntheses. Sodium cyanoborohydride in HMPA will reduce alkyl iodides, bromides, and tosylates selectively in the presence of ester, amide, nitro, chloro, cyano, alkene, epoxide, and aldehyde groups [118]. Tri-n-butyltin hydride will replace chloro, bromo, or iodo groups with hydrogen via a free radical chain reaction initiated by thermal decomposition of AIBN [119]. Other functionality such as ketones, esters, amides, ethers, and alcohols survive unchanged. The less toxic tris(trimethylsilyl) silane can be used similarly [120]. 

It is generally accepted that the elimination of HCl occurs by way of a free-radical chain reaction initiated by chromophore impurities. Whilst chlorine atoms may function as the propagating species, as shown in Scheme 3.21, unsaturated... 

Model 8 Peroxides as Radical Chain Reaction Initiators... 

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