Methyl radicals, from decomposition

For compound (151 R = Me or Ph), the base peak arises from an [M-HI]t ion. The molecular ion from the parent compound is not observed. Metastable ion peaks aided the elucidation of the fragmentation pathways which are outlined in Scheme 25. The [M—HI] ion (151a) may possess either a methylene pyran or an oxepin structure. Further decomposition of this ion occurs by loss of a hydrogen radical. Expulsion of a methyl radical from (151a) generates (151b) which decomposes as shown (Scheme 25). 

The loss of a methyl radical from the 4- -butylpyridine ion has been shown to exhibit an isotope effect of 1.1 on decomposition in the El source and an isotope effect in the range 1.3—1.6 for metastable ion decomposition [642] [see discussion of t-butylbenzene in Sect. 7.5.4(a)]. 

Methyl radicals, from the thermal decomposition of azomethane, initiate the chain decomposition of acetaldehyde at around 250-350 the chain length... 

The occurrence of chains at temperatures slightly above 100 °C was proved by Volman and Brinton ° . They found that methyl radicals from the pyrolysis of di-/-butyl peroxide induce the chain decomposition of propionaldehyde in the temperature range 122-156 °C. 

The decomposition of azomethane-tfe has been investigated by Chang and Rice . The mechanism established is similar to that of azomethane except that owing to the deuterium isotope effect on the hydrogen abstraction reaction of methyl radicals from the parent molecule, the chains are shorter here. It has been suggested that for this reason the NO inhibited reaction of this molecule is a better approximation... 

Kinetics of decomposition at short times (methyl radical from the n-butane [825], 2, 2-dimethylbutane [240] and 2, 2-dimethylpentane [240] ions, loss of ethyl from n-heptane, n-hexane and n-octane ions [522, 825], loss of methane from the neopentane ion [825], and loss of ethane from the 3-ethylpentane ion... 

In 1929, F. Paneth and W. Hofeditz detected the presence of free methyl radicals from the thermal decomposition of lead tetramethyl. The apparatus they used is shown in Fig. 32.4. Lead tetramethyl is a volatile liquid. After evacuating the apparatus, a stream of H2 under about 100 Pa pressure is passed over the liquid where it entrains the vapor of Pb(CH3)4 and carries it through the tube. The gases are removed by a high-speed vacuum pump at the other end. The furnace is at position M. After a short period, a lead mirror deposits in the tube at M, f ormed by the decomposition of the Pb(CH3)4. If the furnace is moved upstream to position M, a new mirror forms at M, while the original mirror at M slowly disappears. 

The second set of experiments was carried out in 1929 by Paneth. The decomposition of tetramethyllead was carried out in such a way that the decomposition products were carried by a flow of inert gas over a film of lead metal. The lead was observed to disappear with re-formation of tetramethyllead. The conclusion was reached that methyl radicals must exist long enough in the gas phase to be transported from the point of decomposition to the lead film, where they were reconverted to tetramethyllead. 

The radical is generated by photolytic decomposition of di-/-butyl peroxide in methylcy-clopropane, a process that leads to selective abstraction of a methyl hydrogen from methylcyclopropane ... 

Both symmetrical and unsymmetrical azo compounds can be made, so that a single radical or two different ones may be generated. The energy for the decomposition can be either thermal or photochemical. In the thermal decomposition, it has been established that the temperature at which decomposition occurs depends on the nature of the substituent groups. Azomethane does not decompose to methyl radicals and nitrogen until temperatures above 400°C are reached. Azo compounds that generate relatively stable radicals decompose at much lower temperatures. Azo compounds derived from allyl groups decompose somewhat above 100°C for example ... 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

Pyrolysis of the phosphorodichloridothioate (59) at 550 °C gives mainly dibenzothiophen and a smaller amount of the cyclic phosphonochlorido-thioate (60). Thermal decomposition of di-t-butyl peroxide in triethyl phosphate gives rise to diethyl methyl phosphate in a reaction which may be interpreted as resulting from attack of methyl radical on the phosphoryl oxygen. An extension of this mechanism accounts for the formation of (61) from tri-isopropyl phosphate under the same conditions. 

The kinetics of the various reactions have been explored in detail using large-volume chambers that can be used to simulate reactions in the troposphere. They have frequently used hydroxyl radicals formed by photolysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis of NO2. This would result in the formation of 0( P) atoms, and subsequent reaction with Oj would produce ozone, and hence NO3 radicals from NOj. Nitrate radicals are produced by the thermal decomposition of NjOj, and in experiments with O3, a scavenger for hydroxyl radicals is added. Details of the different experimental procedures for the measurement of absolute and relative rates have been summarized, and attention drawn to the often considerable spread of values for experiments carried out at room temperature (-298 K) (Atkinson 1986). It should be emphasized that in the real troposphere, both the rates—and possibly the products—of transformation will be determined by seasonal differences both in temperature and the intensity of solar radiation. These are determined both by latitude and altitude. 

Several other methods have been employed to access the conditions of bubble collapse. Misik et al. studied H20—D20 mixtures and through measurements with the use of spin traps, were able to determine the temperature from the relative rates of O—H and O—D cleavage [21]. They reported temperatures ranging from 2,000 to 4,000 K. Hart et al. developed a method based on the gas phase recombination of methyl radicals (MRR method), formed from the decomposition of methane [22]. They calculated temperatures of 2,000-2,800 K depending on the methane concentration. 

Free radicals formed from an initiator in the gas phase take part in other reactions and recombine with a very low probability (0.1-2%). The decomposition of the initiator in the liquid phase leads to the formation of radical pairs, and the probability of recombination of formed radicals in the liquid phase is high. For example, the photolysis of azomethane in the gas phase in the presence of propane (RH) gives the ratio [C2H6]/[N2] = 0.015 [76]. This ratio is low due to the fast reactions of the formed methyl radicals with propane ... 

The overall process shown as reaction (14) is a necessary consequence of the observed CH3D/CH4 ratio. To be consistent with a ratio of unity, this reaction must proceed without the liberation of free methyl radicals and must account quantitatively for the fate of the methyl zinc. The exact nature of reaction (14) is unknown but several important observations have been made. Decomposition of Zn(CD3)2 with C6H12 in a vessel conditioned using Zn(CH3)2 produced the expected yield of CD4 indicating that the additional hydrogen needed for reaction (14) does not come from the coating on the conditioned vessel. Since reaction (15) cannot compete successfully under the experimental conditions used, it is doubtful if the reaction... 

The overall process given by reaction (1) and reaction (2) is common to all mechanisms proposed for the decomposition of this alkyl. Whether this process occurs as written or by the simultaneous release of both methyl radicals is uncertain. Gowenlock et al.66 tried to resolve this problem and the similar problem that arises with other mercury alkyls by determining D(RHg-R) [R = CH3, C2H5, (CH3)2CH, CH3CH2CH2CH2] from appearance potential measurements. 

Eltenton141 studied the thermal decomposition of a very dilute stream of tetramethyl lead vapour in He (total pressure = 0.4 torr) in a fast flow system (contact time 0.1-0.001 sec) over the temperature range 400-700 °C. The decomposition was essentially complete at 600 °C. A small portion of the effluent from the reaction zone passed directly into the ionization chamber of a mass spectrometer. The reaction was followed by observing the methyl radical concentration. The rate-controlling step observed under these conditions is probably the loss of the first CH3 group by the reaction... 

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