Radical reactions chains

Almost all useful radical reactions are chain reactions. The major products are determined by the propagation steps. Only minor amounts of by-products come Irom initiation or termination reactions. 

Radical transfer reactions involve abstraction of an atom or group B by a radical A from a molecule B-C (reaction 6.24). B is nearly always an atom transfer of a group, which would correspond to substitution at a polyvalent atom, though important in nucleophilic and electrophilic reactions, is very uncommon in radical reactions. The atom transferred is almost always a hydrogen or a halogen atom. 

Reactions such as (6.25) which involve transfer of an oxygen atom are important in atmospheric chemistry. They are thought to involve addition of the hydroxyl radical to form an intermediate 40, which then loses a hydrogen atom. 

Reactions such as (6.26) involve transfer of a hydrogen atom from a radical to a multiply bonded atom, and can be seen as hopping of a hydrogen atom between a C=0 and an 0=0 double bond. 

Radical transfer reactions involved in chain reactions are almost always exothermic. Endothermic reactions would be faster in the reverse direction. This principle can sometimes be used to generate a more stable radical from a less stable radical. Thus the non-stabilized t-butoxyl radical 41 reacts with toluene to give r-butanol and the benzyl radical 42 (reaction 6.27). 

Diesters such as dialkyl glutarates were able to chelate the Lewis acid and form the dialkyl glutarate-Lewis acid complex, thus allowing the detection of Lewis acid-ester complexes by ESI-MS. For example, the complexwith Sc(OTf)3 dissociates to form a chelate complex cation, and a triflate anion was intercepted. In addition to monomeric complex ions, dimeric complex ions [82-Sc2(OTf)5] were also observed, giving evidence of the respective dimeric complexes in solution [44]. 

Two main and characteristic fragmentations of this ion are the dissociation by the loss of neutral radical complex 11-Sc(OTf)3 (—803 u) that gives the substrate complex ion 9 Sc(OTf)2] (m/z 597), and the loss of neutral substrate complex 9 Sc(OTf)3 (—746 u) results in the radical complex ion [ll-Sc(OTf)2] (m/z 654). Additionally, a fragmentation to complex ion [9-ll-Sc(OTf)2] (m/z 908) by loss of Sc(OTf)3 (—492u) was observed. The elemental composition of the ions [ll Sc(OTf)2], [911Sc2(OTf)5], and [10-ll-Sc2(OTf)s] were confirmed via their accurate masses determined by Q-TOF measurements. 

The radical chain allylation of diethyl 2-iododiadipate with allyltributyltin in the presence of Sc(OTf)3 to give diethyl 2-allyladipate via a radical intermediate was studied quite analogously. The transient diester radical was detected and characterized by MS/MS [28]. 

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W. B. Motherwell, D. Crich Free Radical Chain Reactions in Organic Synthesis (Academic Press 1992)... 

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Termination steps (Section 4 17) Reactions that halt a chain reaction In a free radical chain reaction termination steps consume free radicals without generating new radicals to continue the chain... 

Like most other engineering thermoplastics, acetal resins are susceptible to photooxidation by oxidative radical chain reactions. Carbon—hydrogen bonds in the methylene groups are principal sites for initial attack. Photooxidative degradation is typically first manifested as chalking on the surfaces of parts. 

The alkanes have low reactivities as compared to other hydrocarbons. Much alkane chemistry involves free-radical chain reactions that occur under vigorous conditions, eg, combustion and pyrolysis. Isobutane exhibits a different chemical behavior than / -butane, owing in part to the presence of a tertiary carbon atom and to the stability of the associated free radical. 

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). 

E. S. Huyser, Free Radical Chain Reactions, Wiley-Interscience, New York, 1970. 

The main industrial use of alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. Decomposition of unsymmetrical diperoxyesters, in which the two peroxyester functions decompose at different rates, results in the formation of polymers of enhanced molecular weights, presumably due to chain extension by sequential initiation (204). 

Ethylene Dichloride Pyrolysis to Vinyl Chloride. Thermal pyrolysis or cracking of EDC to vinyl chloride and HCl occurs as a homogenous, first-order, free-radical chain reaction. The accepted general mechanism involves the four steps shown in equations 10—13 ... 

The mechanism by which an oiganic material (RH) undergoes autoxidation involves a free-radical chain reaction (3—5) ... 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

Chloroform reacts readily with halogens or halogenating agents. Chlorination of the irradiated vapor is beUeved to occur by a free-radical chain reaction (7). 

Thermal chlorination of ethane is generally carried out at 250—500°C. At ca 400°C, a free-radical chain reaction takes place ... 

The ultraviolet lamps used in the photochlorination process serve to dissociate the chlorine into free radicals and start the radical-chain reaction. Other radical sources, such as 2,2 -a2obisisobutyronitrile, have been used (63,64). Primary by-products of the photochlorination process include 1,1,2-trichloroethane (15—20%), tetrachloroethanes, and pentachloroethane. Selectivity to 1,1,1-trichloroethane is higher in vapor-phase chlorination. Various additives, most containing iodine or an aromatic ring in the molecule, have been used to increase the selectivity of the reaction to... 

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

The present method offers several advantages over earlier methods. The use of carbon tetrachloride instead of diethyl ether as solvent avoids the intrusion of certain radical-chain reactions with solvent which are observed with bromine and to a lesser degree with chlorine. In addition, the potassium bromide has a reduced solubility in carbon tetrachloride compared to diethyl ether, thus providing additional driving force for the reaction and ease of purification of product. The selection of bro-... 

The oxidation of hydrocarbons, including hydrocarbon polymers, takes the form of a free-radical chain reaction. As a result of mechanical shearing, exposure of ultraviolet radiation, attack by metal ions such as those of copper and manganese as well as other possible mechanisms, a hydrocarbon molecule breaks down into two radicals... 

The result of the steady-state condition is that the overall rate of initiation must equal the total rate of termination. The application of the steady-state approximation and the resulting equality of the initiation and termination rates permits formulation of a rate law for the reaction mechanism above. The overall stoichiometry of a free-radical chain reaction is independent of the initiating and termination steps because the reactants are consumed and products formed almost entirely in the propagation steps. 

The presence of oxygen can modify the course of a fiee-radical chain reaction if a radical intermediate is diverted by reaction with molecular oxygen. The oxygen molecule, with its two unpaired electrons, is extremely reactive toward most free-radical intermediates. The product which is formed is a reactive peroxyl radical, which can propagate a chain reaction leading to oxygen-containing products. 

E. S. Huyser, Free Radical Chain Reactions, Wiley-Interscience, New York, 1970, Chapter 4 G. A. Russell, in Free Radicals, Vol. 1, J. Kochi, ed., John Wiley Sons, New York, 1973, Chapter 7. 

The kinetics of reaction of free radical chain reactions are complicated compared to the second-order kinetics of epoxy and urethane adhesives. Many of these complications offer practical advantages to the process of using acrylic adhesives. 

Free radical chain reactions depend on an easily generated free radical to initiate the chain. One way to generate this radical is to irradiate halogens, such as Ch and Brj. Another way is to add a small amount of an initiator molecule to the reaction mixture, such as AIBN. This molecule, when heated, decomposes into free radicals that react with other molecules to initiate a chain reaction. 

Wawzonek et al. first investigated the mechanism of the cyclization of A-haloamines and correctly proposed the free radical chain reaction pathway that was substantiated by experimental data. "" Subsequently, Corey and Hertler examined the stereochemistry, hydrogen isotope effect, initiation, catalysis, intermediates, and selectivity of hydrogen transfer. Their results pointed conclusively to a free radical chain mechanism involving intramolecular hydrogen transfer as one of the propagation steps. Accordingly, the... 

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