Methyl radicals addition

Figure 1.1 Transition state for methyl radical addition to ethylene. Geometric parameters are from ab initio calculation with QCISD(T)/6-31GT(d) basis set.29...

As an illustration of the performance of various levels of theory in determining such barriers, we examine the addition of radicals to alkenes, beginning with methyl radical addition to ethylene,... 

The data compiled in Tables 6.15 and 6.16 indicate how a selection of methods perform in determining reaction barriers for methyl radical additions to a series of substituted alkenes. The experimental values with which comparisons are made in Tables 6.15 - 6.20 come from experiments in solution [40, 42, 45, 46] so there is the possibility of non-negligible solvent effects in some instances. 

At the UB3LYP/6-31G(d) and CBS-RAD levels of theory, barriers for the limited number of hydroxymethyl radical additions exhibit larger MADs from experiment (8.5 and 7.3 kJ/mol) than those observed for the methyl radical additions (Tables 6.15 and 6.16). In contrast, barriers obtained at the UB3LYP/6-311+G(3df,2p) and G3(MP2)-RAD levels... 

Table 6.23 presents calculated barriers for the cyclization of the but-3-enyl radical [i.e. the reverse of reaction (7.2)]. This reaction is an example of an intramolecular radical addition. A number of the features observed in the barriers for the intermolecular radical additions (e.g. methyl radical addition to ethylene, Table 6.14) are also seen here. 

Very few directly measured experimental enthalpies are available for methyl radical additions to substituted ethylenes. Reaction enthalpies are therefore normally estimated from other known thermochemical quantities (e.g. C-H BDEs), which often have considerable uncertainties [3], and the derivation generally involves the use of additivity approximations [42, 45], Therefore, theory may be able to provide more accurate values for these enthalpies. Tables 6.25 and 6.26 present reaction enthalpies determined at several levels of theory and compared with the experimental estimates. 

In the case of methyl radical addition to double bonds, L. Herk, A. Stefani, and M. Szwarc [J. Am. Chem. Soc., 83, 3008 (1961)] have drawn attention to the importance of the electron-withdrawing power of the conjugated substituent in determining the reactivity of olefins. More recently, to explain a somewhat similar phenomenon, F. Minisci and R. Galli (Tetrahedron Letters, 1962, 533) have invoked the concept that CHS is nucleophilic in character. 

The methyl radical produced in primary Reaction 2 can abstract a hydrogen atom from an unreacted butene molecule in a chain propagation step or it can add to a butene molecule to provide a pentyl radical, which is the precursor for the observed Cr, products (see Reactions 7 and 8). Methyl radical addition (Reaction 8) is favored at low conversion... 

The methylvinyl radical (IV) can abstract a hydrogen atom from a feed or product molecule to form propylene or it can lose a hydrogen atom to form allene or propadiene as products. For the 2-butenes, steric factors inhibit methyl radical addition thus C5 products are formed to a far lesser extent than from 1-butene. While ethylene may be formed by a sequential decomposition of propylene, this cannot be the only path for its formation, as the yield of ethylene in the high conversion region increases about twice as rapidly as does the methane yield. An additional source of ethylene is the symmetrical cleavage of butadiene to vinyl radicals. 

Quantitative data are available in the literature only for methyl radical addition to conjugated dienes relative to the hydrogen abstraction reaction. According to the results obtained by Szwarc et al. methyl substitution on the terminal carbon atom of the diene system decreases reactivity, whereas methyl substitution elsewhere... 

Bakken, G.A. and Jurs, PC (1999b). Prediction of Methyl Radical Addition Rate Constants from Molecular Structure. J.Chem.lnf.Comput.ScL, 39,508-514. 

Bakken, G.A. and Jurs, P.C. (1999b) Prediction of methyl radical addition rate constants from molecular structure./. Chem. Inf. Comput. Sci., 39, 508-514. 

Again, in the case of a dissociation reaction in which the transition state was not loose (e.g., the reverse reaction of methyl radical addition to an olefin had a steric factor), PST did not adequately describe the translational energy distribution of the products, and resort was made to a more general treatment that made some allowance for hindered motions.9... 

With the more complex olefins, the rate of methyl radical addition does not vary a great deal with structure (Table 49). 

Relative rates for methyl radical addition to various olefins [163]... 

Methyl radical additions to some miscellaneous compounds... 

This radical decomposes to give 2-methyl-l-butene plus a methyl radical. The resulting methyl radical can then enter into the noimial chain propagation sequence, or add to the parent molecule. As in the case of hydrogen atom addition, methyl radical addition gives predominantly tertiary radical adducts, 2,3-dimethyl-... 

Methyl radical addition to 2-methyl-2-pentene leads to a tertiary radical system which is capable of decomposing to give additional ethyl radicals which also contribute to undesirable decomposition reactions ... 

A similar argument can be put forward for isoprene yield effects arising from methyl radical addition to 2-methyl-l-pentene ... 

The trends in the other bimolecular examples are reasonably understandable. There are fewer orientational constraints in H-atom addition to ethylene than in methyl radical addition the values of ASq are in accord with this interpretation. The hydrogen-transfer reactions of methyl with ethane and ethylene have the same entropy of activation, indicative of similar orientational requirements. The addition of CH3 to CO has a large negative ASq, reflecting the severe geometric constraints necessary for this reaction to be possible. 

Figure 8-28 Rate data for methyl radical addition are plotted against the maximum free valence found in each molecule. The original methyl affinity (Levy and Szwarc [27]) has been multiplied by 6/m, where m is the number of sites having maximum free valence. (The asterisks in the figure identify these sites.)...

Dimethylmercury is then believed to be synthesized also by methyl radical addition to metallic mercury. Indeed it seems feasible in certain organisms that Hg(II) is transported across cell membranes, reduced to metallic mercury, and then methylated. Dimethylmercury, being volatile, would readily... 

A finer distinction can be drawn in the case of methyl radical addition to styrene, studied by Matsuoka and Szwarc (103) ... 

---