Methyl radicals, combination

Step through the sequence of structures corresponding to combination of two methyl radicals to give ethane (methyl radical combination). 

In words, we describe the process as initiated by the decomposition of acetaldehyde to form the methyl radical CH3 and the formyl radical CHO. Then methyl attacks the parent molecule acetaldehyde and abstracts an H atom to form methane and leave the acetyl radical CH3CO, which dissociates to form another methyl radical and CO. Finally, two methyl radicals combine to form the stable molecule ethane. 

Ayscough56 also studied the recombination of trifluoromethyl radicals by the rotating sector technique and found a rate constant of 2.34 X 1013 (mol./cc.)-1sec.-1, which is close to that for methyl radical combination obtained in an analogous manner from acetone. 

In the second propagation step, the methyl radical reacts with a molecule of chlorine to form chloromethane. The odd electron of the methyl radical combines with one of the two electrons in the Cl—Cl bond to give the Cl—CH3 bond, and the chlorine atom is left with the odd electron. 

The radiolysis of methane in the solid phase has been examined in several studies. ESR studies by Smaller and Matheson and by Wall eta/. have shown that methyl radicals and hydrogen atoms are formed in nearly equal quantities and that Gchj = 0.9. A product analysis in a study by Ausloos et shows that hydrogen and ethane are almost the only products of solid methane radiolysis at 20 or 77 °K. It seems that ethane is formed by both methylene insertion and by methyl radical combination, while hydrogen is formed by direct elimination and by bimolecular processes. A small quantity of ethylene formed at 20 °K is absent at 77 °K this has been attributed to the reaction of hydrogen atoms with ethylene at 77 °K (but not at 20 °K) to give ethyl radicals and finally higher products which are observed in increased yield at 77 °K. 

When methyl radicals combine to form ethane,... 

Just as two methyl radicals combine to form ethane (H3C-CH3), two Mn(CO)5 radicals form the Mn-Mn single bond in Mn2(CO)io (see chapter 7). When Mn2(CO)io is chemically reduced, the anion Mn(CO)5 reacts with CH3I to form the stable mixed compound CH3-Mn(CO)5 analogous to reaction of CH3 with CH3I to form ethane. The CH2 firagment is a carbene and is isolobal with Fe(CO)4. The corresponding dimer H2C=CH2 is well known, although (CO)4Fe=Fe(CO)4 is not very stable and has been observed only at very low temperature. Mixed compounds, such as H2C=Fe(CO)4, are metallocarbenes (see chapter 7) known to play vital roles in olefin metathesis (see the later section in this chapter). 

PROBLEM 6.3 Draw an interaction diagram to show the stabilization when two methyl radicals combine. How great is that stabilization For an example of an interaction diagram see Rgure 1.39, p. 33. 

Combination of a methyl radical with a chlorine atom... 

Termination steps are m general less likely to occur than the propagation steps Each of the termination steps requires two free radicals to encounter each other m a medium that contains far greater quantities of other materials (methane and chlorine mol ecules) with which they can react Although some chloromethane undoubtedly arises via direct combination of methyl radicals with chlorine atoms most of it is formed by the propagation sequence shown m Figure 4 21... 

The oxidation of hydrocarbons involves the sequential formation of a number of similar reactions in which various intermediate radicals which are combinations of carbon, hydrogen and oxygen are formed. In the simplest case, the oxidation of medrane, the methyl radical CH3 plays an important part both in direct oxidation to CO(g) and in indirect oxidation duough the formation of higher hydrocarbons such as CaHe before CO is formed. The chain reactions include... 

The preparation of semiconductors by thermal decomposition would appear to be impossible because of the high amount of energy required to break all of the metal-carbon bonds before the atomic species could be formed. However, the thermal method is successful because the reaction to form free methyl radicals, which combine to form ethane, lowers the energetic requirements for the formahon of gallium, for example, according to the equation... 

Methyl radical Chlorine atom Combination of two methyl radicals ... 

An elementary reaction is a molecular event. Thus, its rate is proportional to the concentrations of the species entering the reaction itself. Consider the combination of two methyl radicals, Eq. (1-7). This elementary reaction, occurs at a rate that is proportional to [CH3]2. Given the elementary reaction in Eq. (1-7), its rate can be written as a particular derivative, Eq. (1-8). 

Activation energies for chain termination are smaller than for chain propagation, but they are significantly greater than zero. This might not have been anticipated inasmuch as methyl radicals seem to combine in the gas phase without measurable activation energy. ... 

Cyclopentadienyl radical can combine with methyl radicals to form fulvene, along with a subsequent loss of H atom. Fulvene may also be formed by reactions of the iso isomer of C4H5 with aeetylene. Once fulvene adds an H atom, subsequent rearrangement to benzene has been shown to be thermodynamically favorable using quantum chemieal methods.Cyelopentadienyl radicals may also combine with eaeh other and then rearrange to form naphthalene. 

The possibility of predicting solid state reactivity from calculated thermochemical data was first addressed with ketodiesters 65a-e, which were substituted with methyl groups to vary the extent of the RSE in the radicals 65-BRl - 65-BR3 involved along the photodecarbonylation pathway (Scheme 7.19). " All ketones reacted in solution to give complex product mixtures from radical combination (66a-e) and disproportionation processes. Calculations revealed RSEs of 8.9 kcal/mol, 15.1 kcal/mol, and 19.8 kcal/mol for radicals 65-BRl (primary enol radical), 65-BR2 (secondary enol radical), and 65-BR3 (tertiary enol radical), respectively. In the... 

In the methyl radical, the reaction takes place in the direction of SO (2pn of central carbon) extension, that is to say, the direction perpendicular to the molecular plane. Walsh 76> correlated the remarkable localization of SO at the nitrogen atom in NO 2 to the experimental results indicating that NO 2 abstracts hydrogen from other molecules to form HNO2 rather than HONO, combines with NO to form ON—NO2, dimerizes to produce O2N—NO2, and so forth. Also he pointed out that the SO MO of C1CO is highly localized at the carbon atom, which is connected with the production of CI2CO in the reaction with CI2. The SO extension of NO 2 is schematically shown below 103>. 

Figure 10.5 Potential energy diagram for the combination of two methyl radicals to form a molecule of ethane.

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