Chain reactions free radical addition

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. 

Free-Radical Chain Reactions. Free-radical chain reactions are sensitive to two types of catalysis a catalyst may increase the rate of initiation of chains by introducing an additional initiation pathway or may lead to new chain propagation steps. For example a chain reaction involving S(IV) and 02 can be initiated by a number of alternative pathways ... 

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... 

Free radical additions to mono-olefins are quite common and can frequently be employed to advantage on a synthetic scale. Formamide, for example, on exposure to sunlight or UV radiation adds to olefins in an anti-Markovnikov sense giving 1 1 adducts that are readily isolated and crystallized. Moreover, since alkyl formamides may be conveniently converted to carboxylic acids by conventional means, the reaction represents a general method of chain extension. 

The free radical additions of sulfonyl halides to alkenes, catalyzed by light or typical chemical radical initiators (In), were first investigated in the 1950s69. The products which are / -halo sulfones (22) were obtained via a chain reaction in which RSO j acts as the chain carrier, namely61-62,70,71... 

Still another, and chains, long or short, may be built up. This is the mechanism of free-radical polymerization. Short polymeric molecules (called telomers), formed in this manner, are often troublesome side products in free-radical addition reactions. 

Polymers can be formed from compounds containing a c=c double bond. Alkenes, such as ethene, can undergo addition polymerisation to form a polymer. A polymer is a compound consisting of very long chain molecules built up from smaller molecular units, called monomers. The polymerisation of ethene, to form poly(ethene), is a free radical addition reaction. 

Intermolecular free-radical addition reactions almost always proceed by chain mechanisms. Here light photoexcites acetone, and O then abstracts H- from the a-position of another molecule of acetone to complete the initiation. 

The addition of hydrogen halides to simple olefins, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule.116 When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 751).137 It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCI only rarely. In the rare cases where free-radical addition of HCI was noted, the orientation was still Markovnikov, presumably because the more stable product was formed.,3B Free-radical addition of HF, HI, and HCI is energetically unfavorable (see the discussions on pp. 683, 693). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides (4-9). Markovnikov addition can be ensured by rigorous purification of the substrate, but in practice this is not easy to achieve, and it is more common to add inhibitors, e.g., phenols or quinones, which suppress the free-radical pathway. The presence of free-radical precursors such as peroxides does not inhibit the ionic mechanism, but the radical reaction, being a chain process, is much more rapid than the electrophilic reaction. In most cases it is possible to control the mechanism (and hence the orientation) by adding peroxides... 

An alkyl group (primary, secondary, or tertiary) can be added to the oxime ether CHr=NOCH2Ph by treatment with the appropriate alkyl halide and an equimolar amount of bis(trimethylstannyl)benzopicolinate.483 This reaction, which is a free radical addition, is another way to extend a chain by one carbon. 

There are some important differences between anionic and free-radical addition. First, unlike free-radical initiators, which decompose and start chains randomly throughout the course of the reaction, anionic initiators ionize readily in fairly polar organic solvents or at low concentrations in hydrocarbons, and chains are started immediately, one for each molecule of initiator. Second, in the absence of impurities, there is no termination,... 

Essentially, TFE in gaseous state is polymerized via a free radical addition mechanism in aqueous medium with water-soluble free radical initiators, such as peroxy-disulfates, organic peroxides, or reduction-activation systems.15 The additives have to be selected very carefully since they may interfere with the polymerization. They may either inhibit the process or cause chain transfer that leads to inferior products. When producing aqueous dispersions, highly halogenated emulsifiers, such as fully fluorinated acids,16 are used. If the process requires normal emulsifiers, these have to be injected only after the polymerization has started.17 TFE polymerizes readily at moderate temperatures (40 to 80°C) (104 to 176°F) and moderate pressures (0.7 to 2.8 MPa) (102 to 406 psi). The reaction is extremely exothermic (the heat of polymerization is 41 kcal/mol). 

The simplest way to catalyze the polymerization reaction that leads to an addition polymer is to add a source of a free radical to the monomer. The term free radical is used to describe a family of very reactive, short-lived components of a reaction that contain one or more unpaired electrons. In the presence of a free radical, addition polymers form by a chain-reaction mechanism that contains chain-initiation, chain-propagation, and chain- termination steps. 

Tihe theory of free-radical addition polymerization, described in numer-ous publications (2, 3, 4, 17, 21), makes it clear that radical chain-growth reactions of polymers are regulated by statistical laws. Because of their statistical character the products from these reactions must be heterodisperse. The ranges extend from a single unit upward, depending upon kinetic details of the reactions. 

A method of incorporating between 5% to 45% maleic anhydride into polypropylene without chain scission or viscosity increase is described. The method entails an initial thermally induced ene reaction followed by the free radical addition of the anhydride to the polymer backbone. 

If the reaction is too rapid, as it could be with most olefins, it might be faster than the reaction of Br H- H2 -> HBr + H and thus utterly suppress that chain. Olefins would not be too interesting a suppressor for the additional reason that they set up their own chains. A free radical trap which added Br directly without creating new radicals would be the ideal reagent. 

Figure 5.9. Reactions involved in free-radical addition polymerization. Shown are (a) (i)-(iii) generation of free radicals from a variety of initiators, (b) initiation of polymer chain growth through the combination of a free radical and unsaturated monomer, (c) propagation of the polymer chain through the combination of growing radical chains, (d) chain-transfer of free radicals between the primary and neighboring chains, and (e) termination of the polymer growth through either combination (i) or disproportionation (ii) routes.

Under certain conditions, benzene can react with halogens by addition rather than by substitution. In the presence of sunlight, a free-radicaL reaction takes place with chlorine that leads to addition products in which the aromatic character has been lost. The final product is hexa-chlorocyclohexane (benzene hexachloride), which can exist in eight possible stereoisomeric forms. The process starts with the photolytic dissociation of chlorine. Free-radical addition to the 7i-electron system of the aromatic ring follows and a chain reaction ensues (Scheme 9.1). 

Free radical polymerization offers a convenient approach toward the design and synthesis of special polymers for almost every area. In a free radical addition polymerization, the growing chain end bears an unpaired electron. A free radical is usually formed by the decomposition of a relatively unstable material called initiator. The free radical is capable of reacting to open the double bond of a vinyl monomer and add to it, with an electron remaining unpaired. The energy of activation for the propagation is 2-5 kcal/mol that indicates an extremely fast reaction (for condensation reaction this is 30 to 60 kcal/mol). Thus, in a very short time (usually a few seconds or less) many more monomers add successively... 

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