Ethyl and methyl radicals

Termination reactions include other reactions in addition to Reaction (5) of Table IV. There are numerous free radicals present in addition to ethyl and methyl radicals. These other free radicals can also combine or couple. The coupling reactions in the gas phase, including Reaction (5), are highly exothermic. To promote such coupling in the gas phase, a relatively heavy molecule (or third body) is likely needed to help dissipate the exothermic heat of reaction. 

The flash-photolysis study revealed that the vinyl radicals produced in process (3) can be stabilized at higher pressures. Butene-1 and propylene, which are attributed to the combination of vinyl radicals with ethyl and methyl radicals respectively, were found to be absent at low ethylene pressure but became significant at 10-20 torr. Furthermore the addition of 600 torr N2 sharply increased the butene-1... 

Via this mechanism, n-butyl radicals decompose directly to form ethylene and propylene, with ethyl and methyl radicals, respectively. The successive dehydrogenation reaction of ethyl radical forms ethylene and H radicals,... 

Let us assume that the combustion takes place at a temperature where reaction (1) is feasible, but not (2) or (3). Since about 88 kcal/mol is needed to break the CC bond, there will be also enough energy to overcome the barriers of reactions (6) and (7). Therefore, once there are enough methyl radicals in the system, an equilibrium between the n-propyl, i-propyl, ethyl and methyl radicals will exist. 

The mass spectrum of an equimolar mixture of Pb(C2H5)4 and Pb(CH3)4 at different ionization voltages was studied [15]. Concentrations of ethyl and methyl radicals and of their reaction products, formed in the pyrolysis of a mixture of Pb(C2Hs)4 and Pb(CH3)4 at 10"" Torr and between 0 and 500 C, were measured in a field ion mass spectrometer the pyrolysis of Pb(C2Hs)4 [11], of mixtures of Pb(C2H5)4 and 1,2-dibromoethane, and of Pb(C2Hs)4, Pb(CH3)4, and 1,2-dibromoethane were studied similarly [10]. 

The results obtained show that radicals can be divided into two classes depending on the signs of a and p. The methyl, ethyl, and cyclopropyl radicals... 

Microstmcmral changes on irradiation have been observed by IR and UV spectroscopy. Changes in absorption bands due to vinyhdene double bonds [356,357], substituted double bonds, and ethyl and methyl groups give a measure of modifications in the presence of radiation. The ratio of the double bonds (located mainly at the end of a polymer chain) and scission is reported by some investigators [356-358] and found to be independent of temperature and dose. This is beheved to be due to the reaction of the methyl radical side group with hydrogen atoms on the backbone of the parent chain. 

Methyl, ethyl, and phenyl radicals have been detected by mirror removal downstream from an organic halide sodium flame. 

The methyl radical adds to the terminal carbon of propadiene (la) with a rate constant fc= 1 x 104 M-1 s-1 [27]. This elementary reaction requires an activation energy of 34 kj mol-1 based on an Arrhenius analysis of data recorded in the temperature range 100-210 °C. Comparable results were obtained for ethyl and isopropyl radical addition to substrate la (Table 11.3) [27]. 

On the other hand, high 4-selectivities are observed when smaller alkyl radicals such as cyclohexyl, ethyl, and methyl are used (Equation (13)). 

We are not attempting to compare the absolute energy contents of, say, methyl and ethyl radicals we are simply saying that the difference in energy between methane and methyl radicals is greater than the difference between ethane and ethyl radicals. When we compare stabilities of free radicals, it must be understood that our standard for each radical is the alkane from which it is formed. As we shall see, this is precisely the kind of stability that we are interested in. 

Polanyi et have identified ethyl, phenyl and methyl radicals by allow-... 

P Cleavage of these radicals gives more ethylene and the methyl, ethyl, and propyl radicals ... 

Figure 1 shows the percentage of backward character of the KI angular distributions obtained in the molecular beam study oftheK + RI->KI + R reactions, where R was the methyl, ethyl and propyl radicals. The increase in this backward character as the radical size increases was interpreted as an effect of a larger steric factor as one goes from the methyl to the propyl radical (see below). This reduction in the reaction cross-section, as the radical size increases, would confine the reactive trajectories to the coUinear approach, leading to a more repulsive interaction, and so to an increase in the backward distributions of the product. 

It was also found (for those compounds for which a direct onnparison could be made) that there is an excdlent correspondence between the E, Ep values calculated from gas phase ethyl rascal data at subatmos-pheric pressures and the data in Table 7, indicating that the ethyl radical is a good model for the polyethylene radical. Hydrogen atoms and methyl radicals are not good models for polyethylene radicals, and chain-transfer data obtained for the former two correlate only roughly with the latter (7,68, 71). 

Flowers M C and Rabinovitch B S 1985 Localization of excitation energy in chemically activated systems. 3-ethyl-2-methyl-2-pentyl radicals J. Rhys. Chem. 89 563-5... 

As the table indicates C—H bond dissociation energies m alkanes are approxi mately 375 to 435 kJ/mol (90-105 kcal/mol) Homolysis of the H—CH3 bond m methane gives methyl radical and requires 435 kJ/mol (104 kcal/mol) The dissociation energy of the H—CH2CH3 bond m ethane which gives a primary radical is somewhat less (410 kJ/mol or 98 kcal/mol) and is consistent with the notion that ethyl radical (primary) is more stable than methyl... 

Cleavage of the carbon-carbon bond in ethane yields two methyl radicals whereas propane yields an ethyl radical and one methyl radical Ethyl radical is more stable than methyl and so less energy is required to break the carbon-carbon bond in propane than in ethane The measured carbon-carbon bond dissociation energy in ethane is 368 kJ/mol (88 kcal/mol) and that in propane is 355 kJ/mol (85 kcal/mol)... 

Because high temperatures are required to decompose diaLkyl peroxides at useful rates, P-scission of the resulting alkoxy radicals is more rapid and more extensive than for most other peroxide types. When methyl radicals are produced from alkoxy radicals, the diaLkyl peroxide precursors are very good initiators for cross-linking, grafting, and degradation reactions. When higher alkyl radicals such as ethyl radicals are produced, the diaLkyl peroxides are useful in vinyl monomer polymerizations. 

The reaction rate of fumarate polyester polymers with styrene is 20 times that of similar maleate polymers. Commercial phthaHc and isophthaHc resins usually have fumarate levels in excess of 95% and demonstrate full hardness and property development when catalyzed and cured. The addition polymerization reaction between the fumarate polyester polymer and styrene monomer is initiated by free-radical catalysts, commercially usually benzoyl peroxide (BPO) and methyl ethyl ketone peroxide (MEKP), which can be dissociated by heat or redox metal activators into peroxy and hydroperoxy free radicals. 

---