Simple Alkyl Radicals

The free radicals that we usually see in carbon chemistry are much less stable than these Simple alkyl radicals for example require special procedures for their isolation and study We will encounter them here only as reactive intermediates formed m one step of a reaction mechanism and consumed m the next Alkyl radicals are classified as primary secondary or tertiary according to the number of carbon atoms directly attached to the carbon that bears the unpaired electron... 

The degree to which allylic radicals are stabilized by delocalization of the unpaired electron causes reactions that generate them to proceed more readily than those that give simple alkyl radicals Compare for example the bond dissociation energies of the pri mary C—H bonds of propane and propene... 

Simple alkyl radicals such as methyl are considered to be nonnucleophilic. Methyl radicals are somewhat more reactive toward alkenes bearing electron-withdrawing substituents than towards those with electron-releasing substituents. However, much of this effect can be attributed to the stabilizing effect that these substiments have on the product radical. There is a strong correlation of reaction rate with the overall exothermicity of the reaction. Hydroxymethyl and 2-hydroxy-2-propyl radicals show nucleophilic character. The hydroxymethyl radical shows a slightly enhanced reactivity toward acrylonitrile and acrolein, but a sharply decreased reactivity toward ethyl vinyl ether. Table 12.9 gives some of the reactivity data. 

B. Products of Alkylation 1. Reactions of Methyl and Simple Alkyl Radicals... 

The traditional means of assessment of the sensitivity of radical reactions to polar factors and establishing the electrophilicity or nucleophilieity of radicals is by way of a Hammett op correlation. Thus, the reactions of radicals with substituted styrene derivatives have been examined to demonstrate that simple alkyl radicals have nucleophilic character38,39 while haloalkyl radicals40 and oxygcn-ccntcrcd radicals " have electrophilic character (Tabic 1.4). It is anticipated that electron-withdrawing substituents (e.g. Cl, F, C02R, CN) will enhance overall reactivity towards nucleophilic radicals and reduce reactivity towards electrophilic radicals. Electron-donating substituents (alkyl) will have the opposite effect. 

For simple alkyl radicals, the product distribution appears to be predictable using statistical arguments. 

In the absence of heteroatom containing substituents (e.g. halo-, cyano-), at or conjugated with the radical center, carbon-centered radicals have nucleophilic character. Thus, simple alkyl radicals generally show higher reactivity toward electron-deficient monomers (eg. acrylic monomers) than towards electron-rich monomers (e.g, VAc, S) - Table 3.6. 

Simple alkyl radicals thus seem ideal as initiating species ... 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

There are two possible structures for simple alkyl radicals. They might have sp bonding, in which case the structure would be planar, with the odd electron in ap orbital, or the bonding might be sp, which would make the structure pyramidal and place the odd electron in an sp orbital. The ESR spectra of CHs and other simple alkyl radicals as well as other evidence indicate that these radicals have planar structures.This is in accord with the known loss of optical activity when a free radical is generated at a chiral carbon. In addition, electronic spectra of the CH3 and CD3 radicals (generated by flash photolysis) in the gas phase have definitely established that under these conditions the radicals are planar or near planar. The IR spectra of CH3 trapped in solid argon led to a similar conclusion. " °... 

Ayscough and coworkers studied the ESR spectra of y-irradiated sulfones and polysulfones. Irradiation at 77 K leads mainly to alkyl radicals R formed by rupture of the C—S bonds. Small amounts of RS02 were also observed in some cases as well as radicals formed by loss of an a-hydrogen from the parent molecule—R". On warming the irradiated samples the simple alkyl radicals disappear first. At room temperature only the RS02 radicals have any appreciable stability. In some cases the yield of RS02 radicals increases on warming of the sample. 

The new phenomenon discovered in these experiments consists in different chemical activity revealed by one and the same kind of adsorbed particles in contact with one and the same kind of molecules of the medium, but at different nature of the interface either interface of a solid (ZnO film) with a polar liquid or interface of the solid with vapours of the polar liquid. This difference is caused by the fact that in the case of contact of the film with an adsorbed layer (oxygen, alkyl radicals) with a polar liquid, the solvated ion-radicals O2 chemically interact with molecules of the solvent (see Chapter 3, Section 3.4). In the case where alkyl radicals are adsorbed on ZnO film, one can assume, by analogy with the case of adsorbed oxygen, that in the process of adsorption on ZnO, simple alkyl radicals from metalloorganic complexes of the type... 

In general, the rate ratio of the disproportionation k and dimerisation k. increases vith the bulk or size of the radicals concerned117. For simple alkyl radicals even a... 

Simple alkyl radicals are very much more reactive, and were first studied systematically only in 1929. The radicals were generated by the thermal decomposition of organometallic compounds, such as PbMe4,... 

The relative stability of simple alkyl radicals is found to follow the sequence ... 

Radicals of allylic, RCH=CHCH2- (47), and benzylic, PhCHR (48), type are more stable, and less reactive, than simple alkyl radicals, because of delocalisation of the unpaired electron over the n orbital system in each case ... 

A majority of radical addition occurs with electron-poor alkenes using alkyl halides in the presence of BusSnH. These reactions are feasible due to a proper matching between the radical acceptor and the donor. However, when the alkene is electron-rich and since simple alkyl radicals are considered as nucleophilic, the reaction is not a practical method for carbon-carbon bond formation. By applying the concept of polarity-reversal catalysis, an additional reagent is introduced which alleviates the mismatch between the partners and makes the reaction feasible. A few examples illustrating this concept have been described in this review. 

The different behavior of the alcohols probably arises from differences in bond dissociation energies. Experiments show that radical attack on methanol (4) and ethanol (27) leads to rupture of the C—H rather than the O—H bond. There appear to be no direct measurements of C—H bond energies in alcohols. However, D(R—OH) has been determined as 102 kcal. and does not appear to vary greatly with changes in R, provided R is a simple alkyl radical (16). Moreover, the heat of rearrangement of alkoxy radicals to hydroxyalkyl radicals has been determined from electron impact data (12). Considering, for example, 2-propanol and the following reactions... 

In principle one can expect that the formation of transient complexes of the type LmM + 1-R might result in the rearrangement of the carbon-skeleton of R in analogy to B-12 catalyzed processes (151). However, only one such reaction was observed till now (89), probably due to the fact that very simple alkyl radicals are used in most studies. This rearrangement is discussed in Section IX. 

With resonance possibilities, the stability of free radicals increases 149 some can be kept indefinitely.150 Benzylic and allylic151 radicals for which canonical forms can be drawn similar to those shown for the corresponding cations (pp. 168, 169) and anions (p. 177) are more stable than simple alkyl radicals but still have only a transient existence under ordinary conditions. However, the triphenylmethyl and similar radicals152 are stable enough to exist in solution at room temperature, though in equilibrium with a dimeric form. The concen-... 

The HO-toluene adduct corresponds to the methyl-substituted o-, m- and p-hydroxycyclohexadienyl radical isomers. The kinetics of formation and of unimolecular dissociation of HO-aromatic adducts have been studied extensively [30], The benzyl radical -CH2 is converted to the aldehydic product 0-CHO in the presence of NO via a series of reactions analogous to those involving simple alkyl radicals. Bandow et al. [144-146] have determined the yields of aromatic aldehydes to be < 12% of the overall reactions of toluene, xylenes, and trimethylbenzenes, and thus, the H-atom abstraction channel is relatively small but significant. In the case of 0-CHO, the... 

Early structural information about the simple alkyl radicals was obtained by Herzberg, who observed the ultraviolet spectra of CH3 and CD3 in the gas phase by flash photolysis.9 Analysis of the fine structure led to the conclusion that the methyl radical is planar, a deviation from planarity of not more than 10° being consistent with the data.10 The infrared spectrum of GH3 has also been recorded it does not yield conclusive structural information.11 Radicals trapped in a matrix of solid adamantane can be formed by X-irradiation of suitable precursors. In this way, spectra can be obtained at room temperature.12... 

Because of the problem of maintaining even the low concentrations (about 5 x 10"7 M) required for recording epr spectra, simple alkyl radicals can be observed only if the special methods mentioned earlier are used.18... 

Rates of recombination and disproportionation of simple alkyl radicals are, as we have indicated earlier, very high, and special techniques are required to measure them. These methods measure total termination rate, recombination... 

Not all radicals terminate as rapidly as do the simple alkyl radicals. There are, for example, those relatively rare, highly stabilized radicals mentioned in Section 9.1 (p. 463) for which termination rates are comparable to or less than the dissociation rates. There is also a middle ground of radicals that still terminate rapidly enough to preclude obtaining stable solutions, but that nevertheless react with each other more slowly, often by several powers of ten, than the diffusion-controlled rate. Phenoxy radicals with an unsubstituted ortho or para position,... 

A large number of accurate rate constants are known for addition of simple alkyl radicals to alkenes.33-33 Table 2 summarizes some substituent effects in the addition of the cyclohexyl radical to a series of monosubstituted alkenes.36 The resonance stabilization of the adduct radical is relatively unimportant (because of the early transition state) and the rate constants for additions roughly parallel the LUMO energy of the alkene. Styrene is selected as a convenient reference because it is experimentally difficult to conduct additions of nucleophilic radicals to alkenes that are much poorer acceptors than styrene. Thus, high yield additions of alkyl radicals to acceptors, such as vinyl chloride and vinyl acetate, are difficult to accomplish and it is not possible to add alkyl radicals to simple alkyl-substituted alkenes. Alkynes are slightly poorer acceptors than similarly activated alkenes but are still useful.37... 

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