The Free-Radical Chain Reaction

A chain reaction mechanism has been proposed to explain the chlorination of methane. A chain reaction consists of three kinds of steps  

The initiation step, which generates a reactive intermediate. 

Propagation steps, in which the reactive intermediate reacts with a stable molecule to form another reactive intermediate, allowing the chain to continue until the supply of reactants is exhausted or the reactive intermediate is destroyed. 

Termination steps, side reactions that destroy reactive intermediates and tend to slow or stop the reaction. 

In studying the chlorination of methane, we will consider just the first reaction to form chloromethane (common name methyl chloride). This reaction is a substitution Chlorine does not add to methane, but a chlorine atom substitutes for one of the hydrogen atoms, which becomes part of the HCl by-product. 

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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. 

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... 

The free-radical chain reaction may also be terminated by coupling of two carbon-radical species. As solvent carbon tetrachloride is commonly used, where the A-bromosuccinimide is badly soluble. Progress of reaction is then indicated by the decrease of the amount of precipitated NBS and the formation of the succinimide that floats on the surface of the organic liquid layer. 

In order to induce the free-radical chain reaction, a starter compound such as dibenzoyl diperoxide, azo-Zj -(isobutyronitrile) or tcrt-butyl hydroperoxide or UV-light is used. The commercially available, technical grade N-bromosuccinimide contains traces of bromine, and therefore is of slight red-brown color. Since a small amount of elemental bromine is necessary for the radical... 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

Thus, the enhancements in chlorine removal from W diads compared to EV diads and from m-W diads compared to r-W diads observed in the (n-Bu)3SnH reduction of DCP, TCH, and PVC are consistent with the free-radical chain reaction mechanism. Inductive effects produced by neighboring 7-Cl s tend to favor the reduction of W diads relative to EV diads and steric interactions resulting from different preferred conformations in each isomer favor the removal of Cl from m-W diads relative to r-W diads. 

Hydrogen halides will easily add to unsaturated compounds under radiolysis or photolysis. The free-radical chain reaction process is initiated by the dissociation of the halide or by the radiolytic production of radicals from the halide or the organic compound. Thus, for the radiolysis of a mixture of HBr and ethene the postulated initiation is... 

Over time, oxygen will react with fuel components to degrade the fuel into a viscous, sludgelike mass. Fuel components most susceptible to oxidation are olefins and alicyclic naphthenes. Paraffins and aromatics are less susceptible to attack but can eventually be consumed by the free-radical chain reaction process. 

In the thermal-catalytic method a peroxide catalyst is usually used to initiate the free radical chain reaction. The main disadvantages are the higher temperatures required for carrying out the polymerizations, the potential hazard of explosion on addition of catalyst to the monomer, and disposal of excess catalyzed monomer after impregnating. Combinations of heat, radiation, and catalyst have been experimented with to reduce the radiation and catalyst requirements and to increase the rate of polymerization. In thermal polymerization a muffle furnace, infrared heating, and microwave heating can be used to provide the thermal energy. 

For a mechanistic interpretation of the EPR data, one must keep the possibility in mind that the reducing C(5)-adduct radical may have been oxidized by peroxo-diphosphate in the free-radical chain reaction (see below) and that only the oxidizing C(6)-adduct radical remains in this sequence of reactions, although formed in small amounts, and is eventually detected by EPR as the only radi-... 

The foods can be protected against lipid oxidation either by the addition of antioxidants or by packaging in vacuum or inert gases to exclude oxygen. The antioxidants can be of various types. They can work as "chain-breakers" that interfere with the free radical chain reaction, as "metal inactivators", that bind otherwise pro-oxidative metals, or as "peroxide destroyers", which react with hydroperoxides to give stable products by nonradical processes (1). 

Write all steps in the free-radical chain reaction between a halogen and an alkane, and identify the initiation, propagation, and termination steps. 

Many students wonder why the reaction with Markovnikov orientation does not take place in the presence of peroxides, together with the free-radical chain reaction. It actually does take place, but the peroxide-catalyzed reaction is faster. If just a tiny bit of peroxide is present, a mixture of Markovnikov and anti-Markovnikov products results. If an appreciable amount of peroxide is present, the radical chain reaction is so much faster than the uncatalyzed ionic reaction that only the anti-Markovnikov product is observed. 

Similarly, the peroxide effect is not observed with HI because the reaction of an iodine atom with an alkene is strongly endothermic. Only HBr has just the right reactivity for each step of the free-radical chain reaction to take place. 

The free radical chain reaction of 0-acyl thiohydroxamates with a tertiary thiol (r-butyl, triethylmethyl or more recently t-dodecyl) is by far the most wide-ranging reductive decarboxylation method describe to date7 A wide variety of functional groups, including aldehydes, ketones, esters, amides, isolated and conjugated double bonds, are tolerated. Representative examples are given in equations (10) and (11). ... 

Fire is a chain process. It can be stopped by breaking this chain. Scavengers are used to stop the free radical chain reactions and subsequently fire is extinguished. Halogenated compounds are usually good chain-reaction inhibitors. 

Propagation is the second step in the free radical chain reaction. The free radicals generated by p-chain scission can eliminate the monomer by scission of another p-link and shorten the macromolecular radical chain by the reaction ... 

Propagation is the second step in the free radical chain reaction. This step leads to new radicals but also to the formation of small stable molecules. Because the stability of a long chain free radical is usually higher than that of a small free radical, if small radicals are formed in the first step, they usually react with the polymer, forming polymeric chain radicals and small molecules. This type of propagation is known as radical transfer reaction. Using the notation Pn for the polymer and Rn for the polymeric radical, the radical transfer reactions can be indicated as follows ... 

In addition polymerization, monomers react to form a polymer chain without net loss of atoms. The most common type of addition polymerization involves the free-radical chain reaction of molecules that have C = C bonds. As in the chain reactions considered in Section 18.4, the overall process consists of three steps initiation, propagation (repeated many times to build up a long chain), and termination. As an example, consider the polymerization of vinyl chloride (chloro-ethene, CH2 = CHC1) to polyvinyl chloride (Fig. 23.1). This process can be initiated by a small concentration of molecules that have bonds weak enough to be broken by the action of light or heat, giving radicals. An example of such an initiator is a peroxide, which can be represented as R—O—O—R, where R and R represent alkyl groups. The weak 0—0 bonds break... 

The free radical chain reaction between PhCOCHjHgCl and 1-morpholinocyclohex-cne has been reported to involve addition of the acceptor radical PhCOCHj- to the jS-position of the enamine followed by electron transfer to regenerate the attacking radical (Scheme 19). Photostimulated reactions of simple alkylmercury halides failed since an electrophilic radical is required. Photolysis of p-nitrobenzyl chloride in the presence of enamines gave the -/>-nitrobenzyl ketone on hydrolysis . Radical mediated reductive alkylation of acyclic-enamines has also been reported with radical precursors such as PhSCH2CN, PhS02CH2Cl and Me3CS02CH2SePh . Reductive alkylation also occurred with chloromethyl p-tolyl sulphone in the presence of tributyltin hydride and azobis(isobutyronitrile) (AIBN) (Scheme 20). 

The maleimide group in BMI can undergo a wide range of possible reactions, either in the neat resin or copolymerized, with other monomers. The predominant reaction is the free radical chain reaction of the double bond ([20, 21] and references therein), which, due to the difunctionality of BMI monomers, results in a crosslinked three-dimensional network. Maleimides have been shown to undergo copolymerization with a number of monomers including methyl methacrylate [22, 23], styrene [22-24], acrylonitrile [22] and... 

Some solvents are able to filter parts of the optical spectrum — an effect that may provide some protection of the dissolved or suspended drug. However, absorption of photons may lead to formation of free radicals from solvent molecules, resulting in chain reactions that are well documented for unsaturated fats and oils (Florence and Attwood, 1998). Drugs dissolved or suspended in such vehicles may participate in the free radical chain reactions, leading to photosensitized decomposition of the drug induced by the solvent. 

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