Fluorine 18 atom hydrogen abstraction

Of the two chain-propagating steps, then, step (2) is more difficult than step (3) (see Fig. 2.8). Once formed, methyl radicals react easily with any of the halogens it is how fast methyl radicals are formed that limits the rate of overall reaction. Fluorination is fast because fluorine atoms rapidly abstract hydrogen atoms from methane E cx. is only 1 kcal. lodination does not take place because iodine atoms. And it virtually impossible to abstract hydrogen from methane tLCt is more than 33 kcal. 

The fluorination reaction is best described as a radical-chain process involving fluorine atoms (19) and hydrogen abstraction as the initiation step. If the molecule contains unsaturation, addition of fluorine also takes place (17). Gomplete fluorination of complex molecules can be conducted using this method (see Fluorine compounds, organic-direct fluorination). 

Consider abstraction of a hydrogen atom from propan( by fluorine atom. This can generate either of two propy radicals, depending on which hydrogen is attacked. 

The reactions of fluorine atoms with hydrocarbons are similar to those of active nitrogen in that they provide an essentially universal response. Fluorine atoms abstract H atoms from hydrocarbons at near-collisional reaction rates. Reactions with fluorine are highly exothermic, forming strong H—F (=570 kJ mol-1) and C—F (=485 kJ mol-1) bonds while breaking much weaker C—H (=414 kJ mor1) and C—C (=368 kJ mol-1) bonds. The hydrogen abstraction reaction... 

It is believed that polymer surface fluorination proceeds via a free radical mechanism, where fluorine abstracts hydrogen atoms from the hydrocarbon, and fluorine atoms are substituted.11 Of course, the precise conditions depend on the nature of the polymer in question and the surface properties required. 

To probe the effects of HCFC structure on toxicity the metabolism of three penta-haloethanes, HCFC-123, HCFC-124, and HCFC-125 were studied. The three compounds differ one from the other by the number of fluorine atoms present in the /3-carbon (Fig. 4.64). It was found that the enthalpies of activation, AHact, for hydrogen atom abstraction paralleled the rate of trifluoroacetic acid excretion suggesting that the more difficult it was... 

There are no abstractable hydrogen atoms on CF30, so that reaction with 02 is negligible (k < 4 X 10-17 cm3 molecule-1 s-1 at 373 K Turnipseed et al., 1994 Niki et al., 1994). In addition, the C-F bond is too strong for decomposition to give a fluorine atom and CFzO to take place. However, CF3Q reacts with NO,... 

This has been found to be the case with valproic acid (2-propylpentanoic acid) (12), which is hepatotoxic, and 2-fluorovalproic add (22), which is much less hepatotoxic, discussed in Sedion 4.3.1. The hepatoxidty of 12 involves cytochrome P450 abstraction of its C-2 hydrogen atom. The C-2 fluorine atom of 22 cannot be removed by cytochrome P450 metabolism. It would be interesting to observe if the same isosteric replacement would reduce the hepatoxidty of other carboxylic acids, such as the widely used 2-ethylhexanoic add. 

The methyl and trifluoromethyl radicals formed in the photolysis are assumed to abstract hydrogen, but not fluorine, atoms from the parent ketone. 

Hydrogen abstraction from CHXO may also occur by reaction with another halogen atom (equation 38). The barrier for abstraction of hydrogen from CHFO by fluorine (Y = F) is 1.6 kcalmol"1, whereas that for abstraction of chlorine is 3.2 kcal mol"1. Addition (equation 39) of halogen Y may also occur as a competing process, followed by either a-elimination of HX (equation 40) or dissciation (equation 41) of a weaker CX bond54,55. 

The upper limit for the adiabatic first IE of GeH2 has been obtained by means of photoionization mass spectrometry in the course of photoionization studies of GeH molecules, generated by abstraction of hydrogen atoms from GeH4 by fluorine atoms . It was found to be 9.25 eV . On the basis of simple MO considerations it can be concluded that this ionization comes from HOMO, which is of the germanium atom lone-pair character. This is the only study of molecules of this simplest type. 

The free radicals CH3 and OH can be produced by different methods. A wide group of radicals is formed in the hydrogen abstraction reactions of hydrogen-containing molecules with fluorine atoms. Fluorine atoms can be produced by pulse radiolysis of the three-component mixtures SF6/CH4/H2O in the reactions... 

The NH2 and OH radicals were produced by pulse radiolysis of the gas mixture SF6/NH3/H20, with SF6 always in great excess. The fluorine atoms formed initiate the hydrogen atom abstraction reactions... 

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