Free radicals, Table

Dissociation energies D values) of R—H bonds provide a measure of the relative inherent stability of free radicals Table 5.4 lists such values. The higher the D value, the less stable the radical. Bond dissociation energies have also been reported for the C—H bond of alkenes and dienes and for the C—H bond in radical precursors XYC—H, where X,Y can be H, alkyl, COOR, COR, SR, CN, NO2, and so... 

Free radicals, R, are formed in these reactions only if R could be trapped before any isomerization occurs would the stereochemistry of the organic halide be retained. The equilibration of radicals proceeds at a faster rate than reaction of R with [ArH]"Li to give RLi. The ratios of organo-Li products reflect the equilibrium ratios of the intermediate free radicals. Table 1 provides further examples. 

While no real labels meet all of these needs, the properties of some of the more recently introduced labelling systems are approaching the ideal. Radioisotopes, once the only type of label used for immunoassays, have clearly been overwhelmed by current applications of fluorescent labeling methods, enzyme labels, and even coenzyme and prosthetic group labels. A variety of alternative labels has also been investigated, including red blood cells, latex particles, viruses, metals, and free radicals. Table 6.1 shows a representative listing of labels used in modem immunoassays.1... 

Several criteria, including hydride affinities of carbocations (R+—H heterolytic BDE) and adiabatic ionization potentials of the corresponding free radicals (Table 1.30), indicate the order of decreasing stability of fluoromethyl carbocations to be CHF2+ > CH2F+ > CF3+ > CH3+ (Scheme 1.54) [1],... 

Rate constants kp increase only slightly with the viscosity of the solvent. The cause of this effect is unknown. Rate constants are higher for less resonance-stabilized polymer free radicals (Table 20-4). On the other hand, all the activation energies lie in the same region of 21-29 kJ/mol. 

INITIATORS, FREE-RADICAL Table 2. Commercial Organic Peroxide Classes ... 

ESR measurements provide direct evidence for the formation of different types of free radicals (Table 3.2) [331]. 

Quite recently Hodgson et al (1963) have reported that sulfur solutions in certain amine solvents contain free radicals (Table XVI) as detected by electron spin resonance. Sulfur in ethylene diamine, piperidine, and tetramethylguanidine forms at least 10 moles of free radicals per mole of octatomic sulfur. 

In agreement with the theory of polarized radicals, the presence of substituents on heteroaromatic free radicals can slightly affect their polarity. Both 4- and 5-substituted thiazol-2-yl radicals have been generated in aromatic solvents by thermal decomposition of the diazoamino derivative resulting from the reaction of isoamyl nitrite on the corresponding 2-aminothiazole (250,416-418). Introduction in 5-position of electron-withdrawing substituents slightly enhances the electrophilic character of thiazol-2-yl radicals (Table 1-57). 

TABLE 1-57. REACTTVITY OF S-SUBSTITU-TED TH1AZOLYL-2-YL FREE RADICALS TO TOLUENE AND NITROBENZENE RELATIVE TO BENZENE (250i... 

In the intermediate complexe of free radical arylation, it is necessary to oxidize the reaction intermediate to avoid dimerization and disporportio-nation (190-193, 346) In this case isomer yield and reactivity will be highest with radical sources producing very oxidative radicals or in solvents playing the role of oxidants in the reaction. The results are summarized in Tables III-29 and III-30. 

TABLE m-33. FREE-RADICAL METHYLATION IN ACIDIC MEDIUM OF AL-KYLTHIAZOLE (197)... 

Sections Free radical halogenation and oxidation involve reactions at the benzylic 11 12-11 13 carbon See Table 112... 

All of the reactions listed in Table 6.1 produce free radicals, so we are presented with a number of alternatives for initiating a polymerization reaction. Our next concern is in the fate of these radicals or, stated in terms of our interest in polymers, the efficiency with which these radicals initiate polymerization. Since these free radicals are relatively reactive species, there are a variety of... 

Table 6.5 Some Free Radical Combination Reactions Which Yield n-mers and Their Rate Laws...

Note that this inquiry into copolymer propagation rates also increases our understanding of the differences in free-radical homopolymerization rates. It will be recalled that in Sec. 6.1 a discussion of this aspect of homopolymerization was deferred until copolymerization was introduced. The trends under consideration enable us to make some sense out of the rate constants for propagation in free-radical homopolymerization as well. For example, in Table 6.4 we see that kp values at 60°C for vinyl acetate and styrene are 2300 and 165 liter mol sec respectively. The relative magnitude of these constants can be understod in terms of the sequence above. 

The sample labeled atactic in Fig. 7.10 was prepared by a free-radical mechanism and, hence, is expected to follow zero-order Markov statistics. As a test of this, we examine Fig. 7.9 to see whether the values of p, P, and Pj, which are given by the fractions in Table 7.9, agree with a single set of p values. When this is done, it is apparent that these proportions are consistent with this type... 

Because the decomposition is first order, the rate of free-radical formation can be controlled by regulating the temperature equations relating half-life to temperature are provided in Table 7. These decomposition rates ate essentially independent of the solvent (73). 

Usually, free-radical initiators such as azo compounds or peroxides are used to initiate the polymerization of acrylic monomers. Photochemical (72—74) and radiation-initiated (75) polymerizations are also well known. At a constant temperature, the initial rate of the bulk or solution radical polymerization of acrylic monomers is first order with respect to monomer concentration and one-half order with respect to the initiator concentration. Rate data for polymerization of several common acrylic monomers initiated with 2,2 -azobisisobutyronittile (AIBN) [78-67-1] have been determined and are shown in Table 6. The table also includes heats of polymerization and volume percent shrinkage data. 

In general, acryUc ester monomers copolymerize readily with each other or with most other types of vinyl monomers by free-radical processes. The relative ease of copolymerization for 1 1 mixtures of acrylate monomers with other common monomers is presented in Table 7. Values above 25 indicate that good copolymerization is expected. Low values can often be offset by a suitable adjustment in the proportion of comonomers or in the method of their introduction into the polymerization reaction (86). 

The two possible initiations for the free-radical reaction are step lb or the combination of steps la and 2a from Table 1. The role of the initiation step lb in the reaction scheme is an important consideration in minimising the concentration of atomic fluorine (27). As indicated in Table 1, this process is spontaneous at room temperature [AG25 = —24.4 kJ/mol (—5.84 kcal/mol) ] although the enthalpy is slightly positive. The validity of this step has not yet been conclusively estabUshed by spectroscopic methods which makes it an unsolved problem of prime importance. Furthermore, the fact that fluorine reacts at a significant rate with some hydrocarbons in the dark at temperatures below —78° C indicates that step lb is important and may have Httie or no activation energy at RT. At extremely low temperatures (ca 10 K) there is no reaction between gaseous fluorine and CH or 2 6... 

Each isomer has its individual set of physical and chemical properties however, these properties are similar (Table 6). The fundamental chemical reactions for pentanes are sulfonation to form sulfonic acids, chlorination to form chlorides, nitration to form nitropentanes, oxidation to form various compounds, and cracking to form free radicals. Many of these reactions are used to produce intermediates for the manufacture of industrial chemicals. Generally the reactivity increases from a primary to a secondary to a tertiary hydrogen (37). Other properties available but not Hsted are given in equations for heat capacity and viscosity (34), and saturated Hquid density (36). 

The degree of polymerization is controlled by the rate of addition of the initiator. Reaction in the presence of an initiator proceeds in two steps. First, the rate-determining decomposition of initiator to free radicals. Secondly, the addition of a monomer unit to form a chain radical, the propagation step (Fig. 2) (9). Such regeneration of the radical is characteristic of chain reactions. Some of the mote common initiators and their half-life values are Hsted in Table 3 (10). 

Thermal decomposition of hydroxyalkyl hydroperoxyalkyl peroxides produces mixtures of starting carbonyl compounds, mono- and dicarboxyHc acids, cycHc diperoxides, carbon dioxide, and water. One specific hydroxyalkyl hydroperoxyalkyl peroxide from cyclohexanone (2, X = OH, Y = OOH) is a soHd that is produced commercially as a free-radical initiator and bleaching agent (see Table 5). On controlled decomposition, it forms 1,12-dodecanedioic acid (150). 

The instabihty of tert-huty areneperoxysulfonates is increased by the presence of electron-withdrawing substituents on the aromatic ring and decreased by electron-donating substituents. However, even the most stable members decompose violently on warming, as indicated in Table 14. These peroxyesters appear to decompose heterolyticaHy without the formation of free radicals (44). 

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