Atomic fluorine abstraction

The experimental results which we will describe are primarily those obtained in this laboratory but a few experimental data exist which have been collected elsewhere. Our experimental program in fluorine atom chemistry has been motivated primarily by two facts which have also been important to studies performed by other methods (1) atomic fluorine abstraction of hydrogen atoms from appropriate molecules has been demonstrated to be an important class of reactions for chemical lasers W. In particular, the reactions of F + H2 HF + H and F + D DF + D have been investigated in great detail by various theoretical and experimental approaches (3-11) the latter reaction provides us with an example from the general class of reactions of fluorine atoms with diatomic molecules. (2) Substitution reactions of fluorine atoms with unsaturated hydrocarbons Involving the formation of C-F bonds frequently are observed to proceed through a "complex" which... 

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. 

The major source for atmospheric FCO is expected to be photolysis of CF20 and CFCIO, but the advent of hydrofluorocarbons, such as CF3CFH2 (HFC-134a), as CFC-alternatives now provides an additional source by means of hydrogen-atom abstraction from CHFO by OH, F or Cl. Yet another source is addition of atomic fluorine to carbon monoxide. 

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 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... 

Limitations of the Moderated Nuclear Recoil Technique for Investigating Thermal Hydrogen Abstraction Reactions by Atomic Fluorine... 

Table VII. Absolute Rate Constants for Selected Thermal Hydrogen Abstraction and Olefinic Addition Reactions by Atomic Fluorine"...

A comprehensive listing of absolute thermal rate coefficients for hydrogen abstraction and olefinic addition reactions by atomic fluorine has been given in Table X. 

Atomic fluorine can be generated by microwave discharges in F2, CF4, or SFg. Chemiluminescence from the reaction of fluorine atoms with hydrocarbons is almost universal and results from the production of vibrationally excited HF via hydrogen abstraction. Although the HF overtone band at 880 nm can be used to detect many species that contain hydrogen, this reaction also generates chemiluminescence from C2 and CH at 470 and 431 nm, respectively, which provides selective detection of hydrocarbons. Other classes of compounds can be selectively monitored. For example, iodo compounds react to produce excited IF, which has been monitored at 580 nm, with reported detection limits of 1 pg. 

Inasmuch as the F2 molecule is only bound by about 155 kj/ mole, fluorine abstraction by F atoms requires a reagent with a very weeikly bound fluorine. One possibility is the reaction... 

The measured terminal to central ratio of 1.4 contrasts sharply with the ratio of about 15 found for the thermal hydrogen atom addition to propylene (38). The value of 1.4 indicates only a moderate preference for reaction at the CH2 end of propylene, indicating that atomic fluorine is a rather indiscriminate, highly reactive species. Both the F recoil and N2F4 Aotolysis e 5)eriments show substantial H atom abstraction from propylene... 

Recent comprehensive literature reviews Include those by Foon and Kaufman (l8) and Jones and Skolnik (15). Hydrogen abstraction reactions by atomic fluorine exhibit unusually large exothermicities with activation energies in the range 0-2500 cal mole Since the values for many substances are less than 5OO cal mole exceptional experimental sensitivity is required. Severe kinetic complications further increase the difficulty of obtaining precise and accurate thermochemical kinetics results for these reactions (18-21). 

In the present work we have sought to resolve this discrepancy based upon new experimental results and to provide a critique of the nuclear recoil technique for the investigation of thermal hydrogen abstraction reactions by atomic fluorine. 

Tatle Absolute Rate Constants for 300 K Thermal Hydrogen Abstraction Reactions hy Atomic Fluorine. 

Relative Rate Constants. A variety of new techniques have been developed for obtaining accurate measurements of relative rate constants for hydrogen abstraction reactions by atomic fluorine (t.i6.18.19.22.2i -27. 5-8q.87.88). Of principal present interest ai4 the HF chemiluminescence (81 85). H F product analysis (2 -26). and Indirect atom loss (87.88) procedures. 

Rydrogen Abstraction and Olefin Addition Reactions By Atomic Fluorine. ... 

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. 

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