Reactions of Alkyl Radicals

Isomerization Reactions of Alkyl Radicals.—For a quantitative interpretation of the products of hydrocarbon oxidation, it is necessary to calculate the proportion of each species of alkyl radical formed from radical attack on the alkane [reaction (2)]. Arrhenius parameters are available for the l,4sp and l,5sp isomerizations, (36) and (37), and at about 750 K their rate is comparable with that of alternative reactions of alkyl radicals under normal conditions, so that the proportions of alkyl radicals produced may be perturbed. 

Benson and H. E. O Neal, Kinetic Data on Oas-Phase Unimolecular Reactions , NSRDS-NBS21, U.S. Government Printing Office, Washington, D.C. 1970. 

Kerr and M. J. Parsonage, Evaluated Kinetic Data on Oas-Phase Addition Reactions, Butterworths, London, 1972. 

Critical energies for transfo of H atoms in alkyl radicals have been determined by studies with diemically activated pentyl and pentenyl radicals formed by addition of H atoms to pentenes and pentynes. For the 1,4 transfer 

MeC= HCHiCH - MeCH=CHCH2CH3 the critical taetgy is 79 5, and fm the 1,3 transfo  

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On the basis of the reaction of alkyl radicals with a number of polycyclic aromatics, Szwarc and Binks calculated the relative selectivities of several radicals methyl, 1 (by definition) ethyl, 1.0 n-propyl, 1.0 trichloromethyl, 1.8. The relative reactivities of the three alkyl radicals toward aromatics therefore appears to be the same. On the other hand, quinoline (the only heterocyclic compound so far examined in reactions with alkyl radicals other than methyl) shows a steady increase in its reactivity toward methyl, ethyl, and n-propyl radicals. This would suggest that the nucleophilic character of the alkyl radicals increases in the order Me < Et < n-Pr, and that the selectivity of the radical as defined by Szwarc is not necessarily a measure of its polar character. 

Dialkyldiazenes (15, R—alkyl) are sources of alkyl radicals. While there is dear evidence for the transient existence of diazcnyl radicals (17 Scheme 3.18) during the decomposition of certain unsymmetrieal diazenes49 51 and of cis-diazenes,54 all isolable products formed in thermolysis or photolysis of dialkyldiazenes (15) are attributable to the reactions of alkyl radicals. 

Metal hydride trapping agents have been used extensively in studying the reaction of alkyl radicals with monomers.489 400... 

With ft2 and ft3 known, k could thus be determined. In a related example, a number of reactions of alkyl radicals (CH, C2H, etc.) have been studied by the application of similar principles. An intensely colored probe that reacts with R was added with (or without) the substrate of interest. The probe is a species like the methyl viologen radical ion, MV,+. If S represents the substrate, the scheme is... 

Consider now a series of compounds A, that react with two reagents, Bi and B2. A good example is the reactions of alkyl radicals (the A, s are R") with BrCCl3 (B ) and CCI4 (B2). The radicals considered are planar, tt radicals that are primary, secondary, and tertiary, but not bridgehead.21 The scheme is... 

The isoselective relationship is illustrated by the reactions of alkyl radicals with bromotrichloromethane and tetrachloromethane as in Eq. (10-45). The lines intersect at T- = 50 10 °C. Data are from Ref. 21. 

The radiolytic technique has also been applied to the reaction of alkyl radicals R with Ni1 porphyrins anions.279 In analogy with the postulated reaction of NiIF43o to form CH3NiinF430, short lived R-Ni111 products have been detected. 

In solution the reaction of alkyl radical with dioxygen occurs extremely rapidly with a diffusion rate constant (see Table 2.4). The data on solubility of dioxygen in different organic solvents are collected in Table 2.2. 

Rate Constants of Chain Termination in Oxidized Hydrocarbons by the Reaction of Alkyl Radicals with Peroxyl Radicals at 293 K in RH Solution... 

Practically one reaction of alkyl radicals with dioxygen is known, namely addition reaction (see Chapter 2). Ketyl radicals having a free valence on the carbon atom add dioxygen also. However, they possess high reducing activity and easily react with dioxygen by the abstraction reaction. 

The data described above proved that isomerization of alkyl and peroxyl radicals plays a very important role in polymer oxidation. They influence the composition of products of polymer oxidation including the structure of hydroperoxy groups. The competition between reactions of alkyl radical isomerization and addition of dioxygen appeared to be very important for the self-initiation and, hence, autoxidation of PP (see later). 

It can be seen that the steric effect is profound in radical reactions of Ar2OH with peroxyl and methyl radicals. It will be shown later that the steric effect exists in other free radical reactions of Ar2OH. The AES values of the reactions of alkyl radicals with Ar2OH are considerably higher than those for phenols reacting with oxygen-centered radicals. The steric effect can also manifest itself in the inverse reactions of sterically hindered phenoxyl radicals Ar20 with various molecules (see later). 

TABLE 3. Activation energies for the reaction of alkyl radicals R- with Bu3GeH and Bu3SnH... 

Few kinetic studies of reactions of alkyl radicals with tin hydrides other than Bu3SnH have been reported. Studies of the reactions of the tert-butyl radical with Me3SnH and Ph3SnH were performed by the rotating sector method,80 but an error in absolute values exists in that method as judged by differences in rate constants for reactions of Bu3SnH with alkyl radicals... 

The deuterium KIEs for reactions of alkyl radicals with Bu3SnH at 27°C... 

While a large number of studies have been reported for conjugate addition and Sn2 alkylation reactions, the mechanisms of many important organocopper-promoted reactions have not been discussed. These include substitution on sp carbons, acylation with acyl halides [168], additions to carbonyl compounds, oxidative couplings [169], nucleophilic opening of electrophilic cyclopropanes [170], and the Kocienski reaction [171]. The chemistry of organocopper(II) species has rarely been studied experimentally [172-174], nor theoretically, save for some trapping experiments on the reaction of alkyl radicals with Cu(I) species in aqueous solution [175]. 

The rate constant for reaction (48) has been reported as (5.7 0.4) X 10-12 cm2 molecule-1 s-1 at 1 atm (Wallington et al., 1993) and somewhat smaller at lower pressures (Butkovskaya and Le Bras, 1994), consistent with similar reactions of alkyl radicals (see Chapter 6.D.1). 

The rate constants for reactions of alkyl radicals with various organic halides demonstrate a clear dependence on the thermodynamics of the reactions. Iodides react faster than bromides, which react faster than chlorides, and the rate constants for reactions for a series of bromides or iodides correlate with the stability of the radical product. The reactions of a primary alkyl radical with an iodomalonate and with a bromomalonate are quite fast (k = 2x 10 M s and k=lx 10 M s, respectively, at 50 °C). To a good approximation, the rate constants for reactions of RSePh are the same as those for reactions of RBr, and the rate constants for reactions of RTePh are about the same as those for reactions of RI. Dichalcogenides are useful for radical functionalization reactions they react with primary alkyl radicals at ambient temperature with the following rate constants MeSSMe, 6 x 10 M s PhSSPh, 2 x lO M s PhSeSePh, 2.6 x 10 M s PhTeTePh, 1.1 x 10 M s... 

If we assiime that there is no activation energy for the disproportionation or recombination, then fcj 109-5 liter/mole-sec. (see Table III). This is about a factor of 10 higher than the values to be expected of H-abstraction reactions of alkyl radicals. It is furthermore anomalous in having a negligible activation energy compared to the expected 8 3 kcal. Note that if we assign 1 kcal. of activation energy to the disproportionation then Ad 101 U liter/mole-sec. 

Somewhere in the temperature range 450° to 600°C. pyrolysis must compete on nearly equal terms with oxidation of alkyl radicals. The work of Baldwin is therefore particularly important since the rate constants for pyrolysis of alkyl radicals are reasonably well established. There is therefore the strong possibility that we shall soon possess rate constants for oxidation reactions of alkyl radicals at high temperatures. Examination of the oxidation products of the higher alkanes by the Baldwin method should go far toward resolving the problem of the source of fragmentation products at lower temperatures. 

Carlier fundamental studies of autoxidations of hydrocarbons have concentrated on liquid-phase oxidations below 100 °C., gas-phase oxidations above 200°C., and reactions of alkyl radicals with oxygen in the gas phase at 25°C. To investigate the transitions between these three regions, we have studied the oxidation of isobutane (2-methylpropane) between 50° and 155°C., emphasizing the kinetics and products. Isobutane was chosen because its oxidation has been studied in both the gas and liquid phases (9, 34, 36), and both the products and intermediate radicals are simple and known. Its physical properties make both gas- and liquid -phase studies feasible at 100°C. where primary oxidation products are stable and initiation and oxidation rates are convenient. 

Small radicals such as tert-butylperoxy and ethylperoxy can, however, react via 1,4 H-transfer only the strain energy involved in O-heterocycle formation is 28 kcal. per mole. In this case, k.4(x — 106 sec."1 whereas krta = 10r> 4 sec. 1 and when [02] = 200 mm. of Hg, ko[02] = 105,3 sec. 1, so that k.4ct < < (tkr,a + k [02]). The result is that in the oxidation of small alkyl radicals, the route via alkylperoxy radicals will be blocked because reverse Reaction —4 competes successfully with Reaction 5. Reaction 2 will thus be a more effective mode of reaction of alkyl radicals with oxygen and the conjugate alkene will be a major product. 

Effectively, we must compare the symmetrical and crossed terminating or nonterminating reactions of alkyl radicals with alkylperoxy radicals. 

The major product of this chain is the alkyl hydroperoxide. The secondary products which are observed are the results of the reactions of alkyl radicals in the system with the hydroperoxide or of the secondary spontaneous breakdown of the hydroperoxide if the temperature is sufficiently high. This chain mechanism predominates in the temperature regime from about 30° to about 250°C., for the gas phase or in relatively inert solvents. 

Similar hypervalent iodine radicals (9-1-2) are formed in the reaction of alkyl radicals with alkyliodides (R + RI — R2I ), and as an intramolecular complex they are stable enough that a reaction with 02 is only low (Miranda et al. 2000). Such 9-X-2 radicals have also been postulated as intermediates in the reduction of alkylhalides by a-hydroxyalkyl radicals (Lemmes and von Sonntag 1982). 

It is important to note that the rate of reaction of alkyl radicals with thiols does not simply correlate with the exothermicity of the reaction, i.e., with the BDE of the C-H bond to be formed. For example, the tertiary 2-hydroxypropyl radical reacts more readily with thiols than the primary hydroxymethyl radical, and this reacts even faster than the methyl radical (Table 6.4). The reason for this surprising behavior has been discussed in terms of the charge and... 

In the y-radiolysis, similar reactions are expected to occur as discussed above for the Ura system. Here, however, about equal amounts of OH and 02 -are formed initially and thus the latter will play an even larger role on the way to the products, and thus it is even more difficult to come up with a well-substantiated reaction scheme. An interesting product is 5, 6-cyclo-5-hydroxy-5,6-dihydro-2 -deoxyuridine. Its formation is discussed below in the context of the reactions of alkyl radicals. 

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