Methyl free radicals

Methyl free radicals, generated either by thermolysis of lead tetracetate in acetic acid solution (401) or by radical cleavage of dimethylsulfoxide by H2O2 and iron (II) salts (408), afford 2- and 5-methylthiazole in the proportion of 86 and 14%, respectively, in agreement with the nucleophilic character of alkyl free radicals and the positive charge of the 2-carbon atom of the thiazole (6). 

Chlorine atoms obtained from the dissociation of chlorine molecules by thermal, photochemical, or chemically initiated processes react with a methane molecule to form hydrogen chloride and a methyl-free radical. The methyl radical reacts with an undissociated chlorine molecule to give methyl chloride and a new chlorine radical necessary to continue the reaction. Other more highly chlorinated products are formed in a similar manner. Chain terrnination may proceed by way of several of the examples cited in equations 6, 7, and 8. The initial radical-producing catalytic process is inhibited by oxygen to an extent that only a few ppm of oxygen can drastically decrease the reaction rate. In some commercial processes, small amounts of air are dehberately added to inhibit chlorination beyond the monochloro stage. 

Hypoxanthine, 7-ethyl-synthesis, 5, 584 Hypoxanthine, 1-methyl-deuterium-hydrogen exchange, 5, 527 synthesis, 5, 594 Hypoxanthine, 2-methyl-methylation, 5, 532 synthesis, 5, 587 Hypoxanthine, 3-methyl-free radical methylation, 5, 544 irradiation, 5, 543 synthesis, 5, 584 Hypoxanthine, 8-methyl-synthesis, 5, 584 Hypoxanthine, 9-methyl-methylation, 5, 532... 

The Cl atom attacks methane and forms a methyl free radical plus HCI. The methyl radical reacts in a subsequent step with a chlorine molecule, forming methyl chloride and a Cl atom ... 

Triphenylmethyl reacts with atomic hydrogen and with methyl free radicals when a solution of it is exposed to a gas stream containing the other radicals.82... 

Porter has recently shown that decomposing tetramethylam-monium amalgam is a source of trimethylamine and methyl free radicals.148 When a current of nitrogen is passed over the amalgam, tellurium mirrors downstream are removed.148... 

Methyl Radicals. Yoshida and Ranby (41) have recently shown by ESR measurements that the methyl free radical is produced in polypropylene by irradiation with ultraviolet light at liquid nitrogen temperature but decays at that temperature. The reaction is slow at —196°C., which suggests that all other hydrocarbon free radicals of larger molecular weight than the methyl free radical can be frozen in and immobilized at that temperature. 

The mechanism of the decay reaction of the methyl free radicals at —196° is not known however, the y-ray irradiation of polypropylene at — 196°C. produces only methane and no ethane (36), as demonstrated by gas analysis after warming to room temperature after irradiation. It may be that the methyl free radicals abstract hydrogen atoms on warming to room temperature or that hot methyl radicals are produced during the radiolysis with sufficient excess energy to abstract hydrogen atoms at liquid nitrogen temperature. 

The past decade has been an extremely fruitful one in the field of quantitative free radical kinetics. Two papers can be identified as the starting point of much of this work. The first of these is the acetone photolysis study by Noyes and Dorfman1 which gave confidence to the use of acetone as a reproducible source of methyl free radicals in a fairly simple kinetic environment. The second is the study of Gomer and Kistiakow-sky2 of the absolute rate of recombination of CH3 radicals. The latter study made it possible to give absolute values for the Arrhenius parameters for the reactions of alkyl free radicals with stable molecules. It also opened the way for putting the reactions of methyl radicals with other alkyl radicals on an absolute basis. 

From the preceding calculation we see that the hypothetical ion pair for ethane lies about 57 kcal. above the energy of two adjacent methyl free radicals. While this is large it still permits us to consider that the ground state of two adjacent CHs may contain a small amount of partial ionic character. If we had made our calculation for a separation of 2.0 A. between the two C atoms of the methyl groupsf then Qc would... 

Thus, among the sulfur amino acids, hydrogen sulfide is a likely derivative of cysteine, and dimethyl disulfide is a likely derivative of cystine. The formation of dimethyl sulfide and dimethyl disulfide from methionine is readily deduced from expected recombinations of thiomethyl and methyl free radicals. Dimethyl disulfide is the major product. 

The chlorine atoms then react further with methane molecules, and a hydrogen chloride molecule is produced along with a methyl free radical. 

Now it s time to learn more about the chemistry of chain-growth reactions. To begin, let s consider the most prevalent type of chain reaction, free radical polymerization. A free radical is defined as a species having one unpaired electron. For example, the methyl free radical would look like this ... 

The interesting behavior own mainly by (V)-EPTM was not confirmed by terpolymers containing 5,6-dimethylene-24ioibomene (X) or 5- 2 -vinyl-3 -butenyl)-2-norbomene (XI), both being model compounds of triene (V). In fact, the first EPTM contains two conju ted double bonds in obligatory dsoid conformation which were found very reactive toward methyl free radicals. The second EPTM has one bis-allylic hydrogen atom in the unsaturated system ... 

We have spent a good part of this chapter discussing the formation and reactions of the methyl free radical CH3. Just what is this molecule like What is its shape How are the electrons distributed and, in particular, where is the odd electron ... 

Differences in stability between carbonium ions are much larger than between free radicals. The /er/-butyl free radical, for example, is 13 kcal more stable than the methyl free radical the / r/-butyl cation is 71 kcal more stable than the methyl cation. 

So far a hydrogen chloride molecule and a methyl free radical have been formed. Notice that the carbon radical was not formed originally, but only after the reaction with the more reactive chlorine radical, which in turn had been formed by the photoinduced homolytic fission. The hydrogen chloride takes no further part in the reaction. The methyl radical may react further with, for example, a chlorine molecule. Write down the equation and mechanism for this step. 

The methyl free radical CH3 has bond angles of about 120°, whereas the methyl carbanion... 

Togo, Yokoyama, and their co-workers prepared the lepidin-2-yl homo C-nucleoside 254 by C—C bond formation between the o-ribofuranosyl-methyl free radical 253, generated by photolytic or thermolytic decarboxylation of the D-ribofuranosylacetic acid derivative 252, in the presence of HVIC, with lepidinium trifluoroacetate (91TL6559) (Scheme 75). 

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