Solvents radical reactions

Once the radicals diffuse out of the solvent cage, reaction with monomer is the most probable reaction in bulk polymerizations, since monomers are the species most likely to be encountered. Reaction with polymer radicals or initiator molecules cannot be ruled out, but these are less important because of the lower concentration of the latter species. In the presence of solvent, reactions between the initiator radical and the solvent may effectively compete with polymer initiation. This depends very much on the specific chemicals involved. For example, carbon tetrachloride is quite reactive toward radicals because of the resonance stabilization of the solvent radical produced [1] ... 

Liquid-phase chlorination of butadiene in hydroxyhc or other polar solvents can be quite compHcated in kinetics and lead to extensive formation of by-products that involve the solvent. In nonpolar solvents the reaction can be either free radical or polar in nature (20). The free-radical process results in excessive losses to tetrachlorobutanes if near-stoichiometric ratios of reactants ate used or polymer if excess of butadiene is used. The "ionic" reaction, if a small amount of air is used to inhibit free radicals, can be quite slow in a highly purified system but is accelerated by small traces of practically any polar impurity. Pyridine, dipolar aptotic solvents, and oil-soluble ammonium chlorides have been used to improve the reaction (21). As a commercial process, the use of a solvent requites that the products must be separated from solvent as well as from each other and the excess butadiene which is used, but high yields of the desired products can be obtained without formation of polymer at higher butadiene to chlorine ratio. 

The refined grade s fastest growing use is as a commercial extraction solvent and reaction medium. Other uses are as a solvent for radical-free copolymerization of maleic anhydride and an alkyl vinyl ether, and as a solvent for the polymerization of butadiene and isoprene usiag lithium alkyls as catalyst. Other laboratory appHcations include use as a solvent for Grignard reagents, and also for phase-transfer catalysts. 

It has already been mentioned that some radical reactions can occur as side reactions by irradiation of pyridazine derivatives, especially in hydroxylic solvents. 

The regioselectivity of addition of Itydrogen bromide to alkenes can be complicated if a free-radical chain addition occurs in competition with the ionic addition. The free-radical reaction is readily initiated by peroxidic impurities or by light and leads to the anti-Markownikoff addition product. The mechanism of this reaction will be considered more fully in Chapter 12. Conditions that minimize the competing radical addition include use of high-purity alkene and solvent, exclusion of light, and addition of free-radical inhibitors. ... 

As an example of an industrially useful radical reaction, look at the chlorination of methane to yield chloromethane. This substitution reaction is the first step in the preparation of the solvents dichloromethane (CHoCl ) and chloroform (CHCI3). 

A radical polymerization involves free radical ends which of course do not associate and which interact only weakly with solvents. Consequently, the early investigators assumed that the course of propagation of radical polymerization is independent of the environment (see, for example, the recent monograph by Walling60). Actually, more recent studies, notably by Russell,36 showed that the nature of the solvent sometimes might considerably affect even the course of radical reactions. Therefore, unusual behavior of the propagation step might be expected in certain solvents. 

Significant, though smaller, solvent effects have also been reported for alkoxy radical reactions (Section 3.4.2.1).133 137... 

Solvent effects on the reactions of small radicals have been discussed in general terms in Chapter 2 (see 2.3.6.2 2.4.5). Small, yet easily discernible, solvent effects have been reported for many reactions involving neutral radicals. These effects on the rates of radical reactions often appear insignificant when... 

The diazonium group can be replaced by a number of groups. Some of these are nucleophilic substitutions, with SnI mechanisms (p. 853), but others are free-radical reactions and are treated in Chapter 14. The solvent in all these reactions is usually... 

The monomer must also be considered a potential transfer agent. It may perform the function of the solvent in reaction (30), an atom being transferred from the monomer to saturate the radical. Possibly a proton may be transferred from the carbon atom of the radical chain to the unsaturated monomer as follows ... 

Aromatic diazo compounds can be reduced in water via a radical process (Scheme 11.5).108 The reduction mechanism of arenediazo-nium salts by hydroquinone was studied in detail.109 Arenediazonium tetrafluoroborate salts undergo facile electron-transfer reactions with hydroquinone in aqueous phosphate-buffered solution containing the hydrogen donor solvent acetonitrile. Reaction rates are first order in a... 

Other preparative snags also occur in the addition of HHal to alkenes. Thus in solution in H20, or in other hydroxylic solvents, acid-catalysed hydration (p. 187) or solvation may constitute a competing reaction while in less polar solvents radical formation may be encouraged, resulting in anti-Markownikov addition to give 1-bromopropane (MeCH2CH2Br), via the preferentially formed radical intermediate, MeCHCH2Br. This is discussed in detail below (p. 316). 

The participation of surface radical species is, then, strongly suspected in the Kotbe reaction, but there are other radical reactions, such as the reductive dimerisation of C02, that are thought to be homogeneous, particularly in non-aqueous solvents. The basic mechanism in this latter case is thought to be ... 

An improved synthesis of 3,4-dihydro-2,l-benzothiazine 2,2-dioxide was reported by Togo and co-workers using photochemical conditions . Treatment of A-alkyl 2-(aryl)ethanesulfonamides 18 with (diacetoxyiodo)arenes under irradiation with a tungsten lamp at 20-30 °C afforded 2,1-benzothiazines 19 and 20. Chemical yields and selectivities were dependent upon the choice of solvents and the reactant s substituents 18 (Table 1). When THF and EtOH were used as solvents, the reactions failed to give the cyclized products, since their a-hydrogen was abstracted by the intermediate sulfonamidyl radical. Compound 20 was obtained as a major product when 1,2-dichloroethane was employed as a solvent. In contrast, in the case of EtOAc as solvent, compound 19 was obtained as the major product. 

Rate Constants of Haloidalkylperoxyl Radicals Reactions with Hydrocarbons (348 K, Solvent is Oxidized Hydrocarbon)... 

Ketone and the formed a-ketoperoxyl radical are polar molecules. Hence the polar effect influences the reactivity of the ketones and the peroxyl radicals. Polar solvents also influence the reactions of peroxyl radicals with ketones as well as other free radical reactions. 

The observed rate constant is kobs = kkn(k + vD)-1. For the fast reactions with k vD the rate constant is kobs = kI). In the case of a slow reaction with k vD the rate constant is k0bs = kx KAb, where KAB = k y vn is the equilibrium constant of formation of cage pairs A and B in the solvent or solid polymer matrix. The equilibrium constant KAB should not depend on the molecular mobility. According to this scheme, the rate constant of a slow bimolecular reaction kobs = kKAB(kobs kD) should be the same in a hydrocarbon solution and the nonpolar polymer matrix. However, it was found experimentally that several slow free radical reactions occur more slowly in the polymer matrix than in the solvent. A few examples are given in Table 19.1. 

For example, in the decomposition of phenylazotriphenylmethane in benzene, triphenylmethyl peroxide is formed if oxygen is present, indicating a radical reaction. The unstable phenyl radical does not persist long enough to dimerize but reacts instead with the solvent. 

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