Radical-molecule reactions

As described in the experimental section single-step radical-molecule reactions can be studied in isolation and in a very direct way by using the rotating cryostat. The identity of the initial radical is known and the product radical can usually be identified unambiguously by e.s.r. Also the relative amounts of the primary and product radicals can be obtained by analysis of the composite e.s.r. spectrum and thus the extent of reaction can be determined directly. When there is more than one site in a molecule at which reaction can occui, the resulting e.s.r. spectrum, which consists of the spectra of the different product radicals, can often be analysed to give the relative degree of attack at the different sites in the molecule (e.g. as for H-atom addition to an unsymmetric olefin). 

Many of the problems in the study of free radical reactions are associated with the assumptions that have to be made about the radicals present in a system and about the complex sequence of reaction steps that result in the transformation of the initial reactants into products. These problems are clearly minimized in the studies using the rotating cryostat, but there are inherent limitations and difficulties in the use of the cryostat to study radical reactions. Thus, only very efficient reactions can be studied and also the exact conditions under which the reactions occur are not always clearly defined. 

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Most radicals react in self- or cross-termination reactions with another radical in diffusion-limited processes. Therefore, most of the useful radical-molecule reactions are very fast. The lifetimes of the radicals are typically in the microsecond range, and photochemical generation of radicals is... 

These reactions belong to the most thoroughly studied ones in the field of radical- molecule reactions in aqueous solution. The interest in this area is to a large part due to its relevance to the understanding of the mechanism of action of nitroaromatics as sensitizers in the radiotherapy of cancer [12]. 

The reaction coordinate again is simple it is the H-Si-H bond angle. In this case, however, the reverse reaction is a radical-molecule reaction, and we cannot make the a priori assumption that its activation energy would be zero. In fact, the literature is full of examples of radical-molecule reactions with large activation energies (Benson, 1976). As a result, we cannot also make the assumption that for the forward reaction E = AH as we did in the case of the Si-H bond fission reaction. At this point, we must resort to either quantum chemical calculations or experiments to resolve this issue. 

If the picture is correct then we see that the observed order of activation energies which is largest for molecular reactions, small for radical-molecule reactions and nearly zero for radical-radical reactions, falls into a 1 1 relation with the acid-base model. The radical-radical reactions have the open orbital and the electron donor, hence little promotion energy to form an attractive pair. The radical-molecule reactions have one open orbital but require polarization of the molecule in order to form the complimentary acid or base. For the molecule-molecule addition type reaction, complimentary polarization of both species must take place for an attractive transition state to form and the activation energy is the highest. 

Direct, time-resolved investigation of radical-radical and atom-radical rate coefficients present more experimental difficulty than radical-molecule reactions, for both species of interest must be generated simultaneously and their time dependence must be accurately followed. Furthermore, in contrast with radical-molecule reactions studied by pseudo-first-order kinetics, where relative radical concentrations combined with straightforward measurement of the molecule concentration suffice, the concentration of one radical (when it is in excess), or both radicals, must be known. The FPTRMS method is readily adaptable to these reactions when species concentrations are suitably calibrated. 

Flash photolysis greatly increases the number of intermediate reactions able to be studied. These are generally radical-radical reactions, in contrast to the radical-molecule reactions which predominate in low intensity photolysis. 

Fig. 2. Arrangement of jets for deposition of independent matrix or for study of radical-molecule reactions.

The product radicals together with any of the original radicals which have not reacted are trapped by the next layer of halohydrocarbon. Further reactants may be admitted through additional jets to permit a sequence of single step reactions to occur. An independent matrix is often used in the study of radical-molecule reactions, in which case four jets are used for a single step reaction, one each for the matrix, halohydrocarbon, sodium and molecular reactant. 

Table 6 summarizes the results obtained for a large number of radical-molecule reactions. The range of radical-molecule reactions examined illustrates the versatility of the technique, by which virtually any combination of hydrocarbon radicals and molecules can be studied. From the results it is evident that the most reactive radicals are the u-type, such as phenyl, vinyl and cyclopropyl. This high reactivity is probably due to two factors, (i) the very strong bonds that this type of radical forms, and (ii) the highly directional nature of the orbital of the unpaired electron (discussed earlier (IIIB) for the phenyl radical). 

Many radical-molecule reaction mechanisms have been shown to be complex and to involve a fast equilibrium between the reactants and the pre-reactive complex, followed by the irreversible formation of products. An example of this mechanism for toluene is shown in Reactions Ibi and Ibii, respectively,... 

Since reaction (32) is a radical—molecule reaction and reaction (36) is a molecule—molecule reaction, is less than El, (E3 2 = ca. > 7 and E36... 

In the important reactions discussed above we have three free radicals HCO, CjHs, and CH3CH2CO. We have already considered all radical-molecule reactions. Now we must consider all radical-radical reactions. Such reactions are of two kinds, recombination and disproportionation. The reactions of radical recombination are ... 

On the other extreme, the rate of radical-radical reactions may be extremely fast compared to radical-molecule reactions, and then the half-life of reactant foUows the familiar law for a first-order reaction... 

At the boundary between chain and non-chain regions, the rate of radical-radical reactions is just equal to that for radical-molecule reactions and thus at this boundary condition, eq. (31) reduces to... 

Logarithm of Ratio p, Rate of Radical-Molecule Reactions and Radical-Radical Reactions, as Function of Rate of Light Absorption J and Product of Rate CJonstant and Reactant Molecule Concentration, kA ... 

The steady-state concentration of all radicals X does not depend on the rate of radical-molecule reactions, and these concentrations are... 

Pig. 1. Ratio p of rate of radical-molecule reactions to radical-radical reactions as function of reactant pressure and temperature for weak light source. 

Other findings which show the difficulty in forming the cyclopropyl radical by some radical molecule reactions are the failure of chlorine atoms to abstract the tertiary ring hydrogen from methylcyclopropane and the failure of r-butoxy radicals to abstract the... 

Reaction Mechanisms for Stable Products. Hydrogen Formation. Molecular hydrogen may result from the dissociative ionization (Reaction II) or dissociation (Reaction III) of CH4 as a primary product. At low pressures (10"° torr range) where ion-molecule and radical-molecule reactions are repressed all H2 appears to be primary. The data in Table I shows that the summation of H2 and H2+ equals 9.65% of the primary products. The summation of other probable fragments—CH2+, CH+, 2(C+), -CH2, CH, and 2( -C)—also totals 9.65%. 

Propane Formation. Previous workers who report mechanisms for the formation of C3H8 (20, 28, 30, 44) invoke neutral-neutral reactions. From our results (Table III) these reactions account for only one-third of the C3H8 while two-thirds are caused by reactions involving CH/, CH3 and CH2+ (Table V). Recent work (2, 15, 37, 45, 47) in simple HC systems has demonstrated that the contribution of excited states of reactant ions to ion-molecule reactions cannot be neglected. Similar considerations are true for radical-radical and radical-molecule reactions (16). We postulate the following ion-molecule reaction involving CH4+. ... 

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