REGULAR PAPERS

Thermal Gas-Phase Oxidation Of Trifluorobromoethene, CF₂CFBr, Initiated By NO₂

Arce, V.^a; dos Santos Afonso, M.^b; Romano, R.M.^a; Czarnowski, J.^c

^aCEQUINOR (CONICET, UNLP) Departamento de Química, Facultad de Ciencias Exactas, Universidad de La Plata, 47 y 115, 1900 La Plata, Argentina.

^bINQUIMAE, Departamento de Química Inorgánica, Analítica y Química, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón II, 3er Piso, (1428) Buenos Aires, Argentina.

^cInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas, INIFTA, Sucursal 4, Casilla de Correo 16, (B1906ZAA) La Plata, Argentina
Fax: +54 221 425 4642, Email: bdq782@infovia.com.ar

Received July 26th, 2005. In final form September 2^nd, 2005
Dedicated to the memory of the late Prof. Hans J. Schumacher on the occasion of his 100^th birthday

Abstract
The oxidation of CF₂CFBr by molecular O₂ initiated by the addition of NO₂ to the double bond of the alkene was studied at 313.4 K, using a conventional static system. The initial pressure of CF₂CFBr was varied between 18.8 and 43.9 torr, that of NO₂ between 0.9 and 4.8 torr and that of O₂ between 96.9 and 402.9 torr. The following products were formed: CF₂BrC(O)F, as the main product, C(O)F₂ and C(O)FBr and small amounts of peroxynitrate, CF₂BrCFBrO₂NO₂, and trifluorobromoethene epoxide.
CF₂BrC(O)F was characterized by its IR spectrum consistent with both the proposed structure and the calculations carried out using ab initio and Density Functional Theory methods. In presence of CF₂CFBr the reaction proceeded with a pressure decrease. After the alkene was consumed an increase of the pressure and formation of bromine was observed. The oxidation is a chain reaction of pseudo-zero order with respect to O₂ as reactant at the pressure of oxygen used in this work. Its basic steps are: chain initiation by addition of NO₂ to the double bond leading, through reaction sequence in presence of O₂, to generation of bromine atoms and chain propagation by reaction of Br^• with alkene, originating CF₂BrCFBrO₂^• and CF₂BrCFBrO^• radicals. The predominant fate of the latter is the bromine atoms extrusion, with C-C bond cleavage playing only a minor role. A full mechanism is postulated. The value of (2.2 ±1) x 10^{-5 s-1} was obtained for the room temperature rate constant for the unimolecular decomposition of CF₂BrCFBrO₂NO₂.

Resumen
Se estudió la oxidación de CF₂CFBr por O₂ molecular iniciada por la adición de NO₂ al doble enlace del alqueno a 313,4 K en un sistema convencional estático. Se varió la presión inicial de CF₂CFBr entre 18,8 y 43,9 torr, la de NO₂ entre 0,9 y 4,8 torr y la de O₂ entre 96,9 y 402,9 torr. Se formaron los siguientes productos: CF₂BrC(O)F, como producto principal, C(O)F2 y C(O)FBr y pequeñas cantidades de peroxinitrato, CF₂BrCFBrO₂NO₂ y epóxido de trifluorobromoeteno. Se caracterizó el CF₂BrC(O)F por su espectro infrarrojo comparándolo con el calculado usando métodos ab initio y la teoría de los funcionales de la densidad. En presencia de CF₂CFBr la reacción procede con disminución de la presión. Consumido el alqueno se observó aumento de la presión y la formación de bromo. A las presiones de oxígeno usadas en este trabajo, la oxidación es una reacción en cadena de seudo-cero orden con respecto a O₂ como reactivo. Sus pasos básicos son: iniciación de cadena por adición de NO₂ al doble enlace, que en presencia de O₂, mediante una secuencia de reacciones, genera los átomos de bromo y la propagación de la cadena por reacción de Br^• con el alqueno, originando radicales CF₂BrCFBrO₂^• y CF₂BrCFBrO^•. Este último se descompone principalmente por eliminación de átomo de bromo y en menor grado por la ruptura de enlace C-C. Se postuló el mecanismo completo de la reacción. Se obtuvo el valor de (2,2 ±1) x 10^-5 s^-1 para la constante de descomposición unimolecular de CF₂BrCFBrO₂NO₂ a temperatura ambiente.

Introduction

   In the reactions between NO₂ and halogenated olefins, NO₂ can act as nitrating or oxidating agent. The products of these reactions are nitrohaloacetyl halides and XNO, where X = Cl or F, vicinal dinitro products and nitrohaloketones [1-8]. These works were generally executed for preparative purposes.
   The detailed kinetic and mechanistic studies were reported for the thermal gas-phase reactions of NO₂ with CF₂CF₂ [9], CF₂CFCl, [10], CF₂CCl₂ [11], CClHCCl₂ [12], perfluoropropene, C₃F₆, [13] and CF₂CFBr [14]. In these works a basic common reaction mechanism for nitration or oxidation of CX₂CX₂ by NO₂, where X = Cl, F or Br, can be postulated as follows:

1.	NO2 + CX2CX2 → CX2(NO2)CX2•
2.	CX2(NO2)CX2• + NO2 + M → CX2(NO2)CX2NO2 + M
3.	CX2(NO2)CX2• + NO2 → CX2(NO2)C(O)X + XNO

   The products of nitration of CF₂CF₂ were CF₂(NO₂)C(O)F and FNO, those of CF₂CFCl were CF₂(NO₂)CFClNO₂, CF₂(NO₂)C(O)F and ClNO and those of CF₂CCl₂ were CF₂(NO₂)CCl₂NO₂, CF₂(NO₂)C(O)Cl and ClNO. In the reaction of NO₂ with trichloroethene, NO₂ oxidizes CClHCCl₂, giving HC(O)C(O)Cl and ClNO as the only products [15]. The main products of the reaction between NO₂ and C₃F₆ were the oxidation product perfluoropropene oxide (PFPO) and nitric oxide, NO, produced in equivalent amounts. The dinitro-compound CF₃CF(NO₂)CF₂NO₂ and nitroperfluoroacetone CF₃C(O)CF₂NO₂ were also formed in minor amounts. The relation R= perfluoropropene oxide]/ ([CF₃CF(NO₂)CF₂NO₂]+[CF₃C(O)CF₂NO₂]) increased with temperature. In the reaction of NO₂ with CF₂CFBr the observed products were CF₂(NO₂)C(O)F, CF₂(NO₂)CFBrNO₂, BrNO, Br2 and NO.
   The addition of O₂ to the reaction system NO₂ + 1,1-dichlorodifluoroethene, changed the reaction course [16]. The oxidation of alkene, initiated by the addition of NO₂ to the double bond of the olefin, occurred, leading in presence of O₂ to the formation of nitroperoxy-radicals, CF₂(NO₂)CCl₂O₂• and nitrooxy- radicals, CF₂(NO₂)CCl₂O•. The CF₂(NO₂)CCl₂O• radicals released chlorine atoms, which add to 1,1-dichlorodifluoroethene originating radicals CF₂ClCCl₂O₂• and CF₂ClCCl₂O•. The free chlorine atoms generated from CF₂ClCCl₂O• radicals initiate a chain reaction with chlorine atoms as chain carriers, giving haloacetyl chloride, CF₂ClC(O)Cl as the main product. Some 15% of the CF₂ClCCl₂O• radicals decomposed by the scission of their C-C bond leading to the formation of C(O)F₂ and C(O)Cl₂ and reforming chlorine atoms. Also small amounts of peroxynitrate CF₂ClCCl₂O₂NO₂ and 1,1- dichlorodifluoroethene epoxide were formed.
   The NO₂-initiated oxidation of tetramethylene was reported in literature [17]. The use of chemical initiators in absence of light permits a better control of the reaction course in organic synthesis. The stable and easily handled trifluoromethyl hypofluorite, CF₃OF, containing a weak O-F bond (43.5 kcal/mol) [18-20] is also an effective initiator of oxidation of haloalkenes (E). CF₃OF adds to the double bond giving CF₃O(E) radicals, where E = CF₂CCl₂ [21], CHClCCl₂ [22] and CCl₂CCl₂ [23]. The reaction of these radicals with O₂ is very fast leading to the formation of CF₃O(E)O, which principally decomposes by the detachment of the Cl from the CCl₂O group of the alkene. The chlorine atoms react rapidly with CX₂CCl₂, where X = H, Cl or F, initiating in presence of O₂, a chain reaction with Cl• as chain carrier and giving CX₂ClC(O)Cl as the main product. Minor amounts of C(O)X₂, where X = H, Cl and F, are also formed. In the case of C(O)HCl, it decomposes rapidly to CO and HCl.
   The direct Cl- and Br-atom initiated oxidation of CHClCCl₂ [24,25] and CCl₂CCl₂ [25] and Br-atom oxidation of CHClCCl₂ and CCl₂CCl₂ [26] were reported. These studies were made to investigate the atmospheric fate of trichloro- and tetrachloroethenes, used as solvents, dry cleaning agents and degreasing agents and released to the atmosphere in 90-100 % of their anthropogenic production.
   The basic common chain reaction mechanism for the oxidation of haloalkenes, initiated by the addition of NO₂ or CF₃OF to the double bond of olefins in presence of O₂, and that initiated by ultraviolet or visible photolysis of Cl₂ and Br₂, can be resumed as:

   In this work the investigation of the reaction of NO₂ with bromotrifluoroethene, CF₂CFBr, in presence of molecular oxygen was undertaken to elucidate the elementary steps involved and to characterize the products. Comparing with the analogue reaction between NO₂ and chloroperfluoroalkene, CF₂CFCl in presence of oxygen [11], where free chlorine atoms are produced, it is reasonable to postulate that the free bromine atoms are formed in the reaction system NO₂ + CF₂CFBr + O₂. We have found no data on the addition of bromine atoms to fluorinated olefins.

Experimental

   All reactants were purchased commercially. NO was eliminated from NO₂ (Matheson 99.5 %) by a series of freeze-pump-thaw cycles in presence of O₂ until disappearance of the blue colour of N₂O₃. Finally, the degassed NO₂ was purified by fractional condensation using the fraction that distilled between 213 and 243 K. The CF₂CFBr (PCR, 97-98%) contained CF₄, and CF₃CF₃ as impurities. These impurities are more volatile than CF₂CFBr, but could not be separated by fractional condensation, distilling together. The CF₂CFBr was purified by intermittent brief evacuation cycles at 153 K, opening and closing the trap valve. This procedure was repeated several times, until the disappearance of the respective very strong absorption bands of CF₄ [27] and CF₃CF₃ [28] at 1279 and 1250 cm^-1 in the IR spectrum of CF₂CFBr. Oxygen was bubbled through 98 % analytical-grade H₂SO₄ and passed slowly through a Pyrex coil at 153 K.
   The experiments were performed in a grease-free static system, allowing pressure measurements at constant volume and temperature. A spherical quartz bulb of 270 cm³ (S/V = 0.75 cm^-1) was used as reaction vessel. The pressure was measured with a quartz spiral gauge and the temperature maintained within ±0.1 K using a Lauda thermostat.
   The reaction was followed measuring the pressure change as a function of time. 21 experiments were made at 313.4 K. The initial pressure of CF₂CFBr was varied between 18.8 and 43.9 torr, that of NO₂ between 0.9 and 4.8 torr and that O₂ between 96.9 and 402.9 torr. Initially the reaction proceeded with pressure decrease. After all CF₂CFBr was consumed a slow pressure increase was observed.
   Infrared spectra of the reaction mixtures were recorded on a Shimadzu IR-435 spectrometer and a Perkin-Elmer 1600 Series FTIR spectrometer, using a 10 cm cell provided with NaCl and KBr windows, respectively. The gas FTIR spectra of the products, separated by fractional condensation after all alkene was consumed, were recorded on a Nexus Nicolet instrument equipped with a cryogenic MCTB detector between 4000 and 400 cm^-1 at room temperature, using a 10 cm cell provided with KBr windows. The UV-visible spectra of the products in the gas phase were recorded on a Hewlett-Packard Model 8452A spectrometer, using a 10 cm quartz cell.

Results

   In the presence of CF₂CFBr the reaction proceeded with a pressure decrease, -∆p/∆t. After all the alkene was consumed the reaction continued with a slow pressure increase, ∆p/∆t, as a function of time. No reaction between CF₂CFBr and O₂ was observed after several hours in the absence of NO₂.
   The following products were formed: CF₂BrC(O)F as the main product, minor quantities of C(O)F₂ and C(O)FBr, and small amounts of CF₂BrCFBrO₂NO₂ and trifluorobromoethene epoxide (TFBrEO). The compound CF₂BrC(O)F was characterized by its IR spectrum consistent with the gauche structure and with the calculations for this molecule using ab initio and Density Functional Theory methods [29]. The identification was confirmed by comparison with the experimental IR spectrum of CF₂ClC(O)F [30], given that infrared spectra of analogous molecules, where the bromine atom is substituted by chlorine are similar. The most intense absorptions for this molecule are found to appear at 1887, 1195, 1102 and 937 cm^-1, and are assigned to the ν(CO), ν_as(CF₂), ν_s(CF₂), and ν(CF), respectively. The compounds C(O)F₂ [31] and C(O)FBr [32,33] were identified by their respective IR spectra and the formation of C(O)FBr was also detected by its UV spectrum at the range of 200-220 nm [33]. The product CF₂BrCFBrO₂NO₂ was identified by its infrared absorption band at 1758 cm^-1, characteristic to the NO₂ group of the haloalkylperoxynitrates and haloalkoxylperoxynitrate. This band appears at 1754 cm^-1 for CCl₃O₂NO₂ [34-36], at 1757 cm^-1 for CCl₂FO₂NO₂ [34-36], at 1761 cm^-1 for CClF₂O₂NO₂ [34,35], at 1762 cm^-1 for CF₃O₂NO₂ [37], at 1761 cm^-1 for CF₃C(O)O₂NO₂ [38], at 1759 cm^-1 for CClF₂C(O)O₂NO₂ [38], at 1755 cm^-1 for CCl₂FC(O)O₂NO₂ [38], at 1755 cm^-1 for CCl₃C(O)O₂NO₂ [38] and 1758 cm^-1 for CF₂ClCCl₂O₂NO₂ [16]. The trifluorobromoethene epoxide (TFBrEO), was identified in the reaction mixture by its infrared band at 1540 cm^-1, assigned to the ring-breathing mode, characteristic of fluoroepoxides. This band appears at 1500 cm^-1 for 1,1-dichloro-2.2-difluoroethene epoxide and chlorotrifluoroethene epoxide [39] and at 1551 cm^-1 for perfluoropropene epoxide [13].
   After the complete consumption of CF₂CFBr, the reaction continued with pressure increase, ∆p/∆t, producing Br₂ and reforming NO₂ in addition to CF₂BrC(O)F, C(O)F₂ and C(O)FBr. The bromine was identified by its UV spectrum [40].
   All experiments were carried out to the complete consumption of CF₂CFBr. For analyzing the reaction mixtures the reaction vessel was rapidly cooled to liquid air temperature and the mixture separated by fractional condensation. When the reaction was interrupted at the inflection point, when -∆p/∆t changed to ∆p/∆t, the first fraction Fr₁, volatile at 158 K, consisted of C(O)F₂ and C(O)FBr, the second fraction Fr₂, volatile at 183 K, was CF₂BrC(O)F and the third fraction Fr₃ remaining as a residue at 158 K consisted of NO₂ and CF₂BrCFBrO₂NO₂. In the infrared spectrum of this fraction, weak absorption bands at 1321, 1025 and 821 cm^-1 were observed. These bands are characteristic of the symmetric stretching of NO₂ in the C-NO₂ group and of the C-F and C-N groups, respectively, and were attributed to the presence of traces of CF₂BrCFBrNO₂. The very strong band at 1618 cm^-1 corresponding to the asymmetric stretching of NO₂ in the C-NO₂ group was overlapped by the broad band of NO₂ at 1635- 1590 cm^-1, suggesting that CF₂BrCFBrNO₂ was formed in very small amounts. When the reaction was interrupted while it proceeded with the pressure increase, the fractions Fr₁ and Fr₂ remained unchanged, but the fraction Fr₃ contained Br₂ in addition to CF₂BrCFBrO₂NO₂, NO₂ and CF₂BrCFBrNO₂.
   The absence or presence of Br₂ in the reaction mixtures at different reaction time intervals corresponding to the pressure decrease and pressure increase, respectively, was monitored by UV spectroscopy.
   It was observed in the successive infrared spectra of the same fraction Fr₂, that intensity of the absorption band of CF₂BrCFBrO₂NO₂ at 1758 cm^-1 decreases as a function of time, appearing bands of CF₂BrC(O)F and very small bands of C(O)F₂ and C(O)FBr, and increasing the intensity of the band of NO₂. In addition, the formation of Br₂ was detected by its UV spectra.
   Considering that at low pressure the infrared absorption is proportional to the concentration of the corresponding compound, the value of (2.2±1) x 10^-5 s^-1 was obtained for k, the effective room temperature rate constant for the unimolecular decomposition of CF₂BrCFBrO₂NO₂, measuring the absorbance A at 1758 cm^-1 as a function of time t and using k= ln (A_m / A_n) / (t_m - t_n).
   The reaction is a homogeneous chain reaction whose rate depends on the relation between NO₂, CF₂CFBr and O₂ and the total pressure. At the pressures of O₂ used in this work, the reaction is of the pseudo-zero order with respect to O₂ as reactant, indicating the third body character of O₂. The initial reaction rates were proportional to the respective pressures of NO₂ and CF₂CFBr and also were influenced in a specific way by the total pressure. At pressure of NO₂ and the temperature of our work, the equilibrium constant for N₂O₄ ↔ 2 NO₂ [41] indicated that N₂O₄ was practically dissociated into NO₂.
   The behavior of the reaction rate, -∆p/∆t, as a function of time, corresponding to the pressure decrease interval, is illustrated in Figs. 1-3. The initial pressures of NO₂, CF₂CFBr and O₂ in torr for each run presented are given inside the graphs. The analytical data of 10 experiments are summarized in the Table 1, where indices i and f signify initial and final, respectively.

Fig. 1. Reaction rates, - ∆p/ ∆t as a function of time, corresponding to a pressure decrease interval. Initial pressures of NO₂, CF₂CFBr and O₂ in torr for each run are given inside the graph.

Fig. 2. Reaction rates, - ∆p/ ∆t as a function of time corresponding to a pressure decrease interval. Initial pressures of NO₂, CF₂CFBr and O₂ in torr for each run are given inside the graph.

Fig. 3. Reaction rates, - ∆p/ ∆ as a function of time t, corresponding to a pressure decrease interval. Initial pressures of NO₂, CF₂CFBr and O₂ in torr for each run are given inside the graph.

Table 1. Analytical data of 10 experiments carried out to the complete consumption of CF₂CFBr.

Discussion

   In order to propose the reaction mechanism, in addition to products analysis, the following reactions were considered: the reactions between NO₂ and CF₂CFCl [10], CF₂CCl₂ [11], CHClCCl₂ [12], and CF₂CFBr [14] in absence of O₂, the reaction of NO₂ with CF₂CCl₂ in presence of O₂ [16], the oxidation of CF₂CCl₂ [21], CHClCCl₂ [22] and CCl₂CCl₂ [23] initiated by CF₃OF addition to the double bond, the bromine and chlorine atoms initiated oxidation of CHClCCl₂ [24,25] and CCl₂CCl₂ [25] and the respective oxidations of CHClCCl₂ and CCl₂CCl₂ [26] initiated by the bromine atoms.
   The following mechanism appears to be consistent with the experimental results obtained in our work:

   Other steps were also considered: CF₂(NO₂)CFBr• + NO₂ + M → CF₂(NO₂)CFBrNO₂ + M and CF₂(NO₂)CFBrO₂• + NO₂ + M → CF₂(NO₂)CFBrO₂NO₂ + M but, as their products have not been detected, these reaction can be neglected in comparison with the steps of the chain reaction. Besides, they did not appear necessary to explain the results.
   The primary path is the thermal addition of NO₂ to CF₂CFBr, giving CF₂(NO₂)CFBr• radicals. The corresponding rate constant for this reaction, k₁ = (1.51±0.45) x 10⁶ exp(-(10.88±1) kcal mol^-1/RT) dm³ mol^-1 s^-1, was determined in a previous work [14]. The oxidation of CF₂(NO₂)CFBr• radicals leads through reactions (2) and (3) to the formation of CF₂(NO₂)CFBrO• radicals, which decompose giving bromine atoms. The bromine atoms initiate a chain reaction by their addition to the double bond of CF₂CFBr.
   The formation of CF₂BrC(O)F as a principal product indicates that Br• atoms add to the CF₂ group of the alkene. We have found no published data on the reaction rate constants for the addition of bromine atoms to the double bond of fluorinated olefins. As in the presence of CF₂CFBr, bromine did not form, then the reaction (5) under the conditions of our work, must be more rapid, than the reaction (18) [42]. It indicates that the addition of Br• atoms to the fluorinated alkenes is faster that their reaction with chlorinated olefins. The reported room temperature rate constants for reactions of Br• with CHClCH₂ [43] and CH₂CCl₂ [43] are (4.4 ±1.1) x 10⁸ and (2.2±0.4) x 10⁸ dm³ mol^-1 s^-1, respectively, those with CHClCCl₂ are (6.6±2.4) x 10⁷ [24,25], 9.4 x 10⁷ [26] and (5.4±1.1) x 10⁷ dm³ mol^-1 s^-1 [43] and those with CCl₂CCl₂ are (5.4±01) x 10⁴ [25],< 7 x10⁴ [26] and < 6 x10⁶ dm³ mol^-1 s^-1 [43]. The room temperature rate constant for the reaction of Br• with CH₂CH₂ is 7.4 x 10⁷ dm³ mol^-1 s^-1 [26]. The addition of chlorine atoms to chloroethenes at 298 K is fast, of order of 1010 dm³ mol^-1 s^-1[44].
   Only very small amounts of the termination product, CF₂BrCFBrNO₂, was formed through reaction (16), corroborating the almost complete elimination of CF₂BrCFBr• radicals by O₂. The equilibrium studies of CCl3• + O₂ ↔ CCl3O₂• [45] and of R• + O₂ ↔ RO₂• [46-48], where R• are alkyl radicals, suggest that the elimination of radicals CF₂BrCFBr• should be almost complete at the oxygen pressure used in our work. It was reported by other authors, that the fraction of ethyl radicals that escape oxidation is <0.1% at 2 torr of O₂ [49].
   Under conditions of our work some peroxy radicals react with NO₂ to give CF₂BrCFBrO₂NO₂. The calculated rate constants for CX₃O₂• + NO₂ → CX₃O₂NO₂, where X = H, F, Cl, are of order of 10⁹ dm³ mol^-1 s^-1 [36] and that reported for CF₃CFClO₂• + NO₂ + M→ CF₃CFClO₂NO₂ + M is (3.5±0.3) x 10⁹ dm³ mol^-1 s^-1 [50]. The formation of small amounts of trifluorobromoethene epoxide, indicates that a few radicals CF₂BrCFBrO₂• add to the double bond of alkene, reactions (8) and (9), regenerating CF₂BrCFBrO•. The epoxidation of alkenes by addition of peroxyl radicals to the double bond, producing RO• radical and epoxide was reported [51]. The expression for k_a, the rate constant for the addition of peroxy radicals RO₂• to CF₂CCl₂, obtained in our previous work [21], is k_a = (1.9±1) x 10⁸ exp(-4.8±1.4 kcal mol^-1 /RT) dm³ mol^-1 s^-1.
   The formation of CF₂BrC(O)F as the major product, indicates that the main via of disappearance of peroxyradicals CF₂BrCFBrO₂• is reaction (7) producing oxyradicals CF₂BrCFBrO•. The rate constant for the reaction RO₂• + RO₂• → 2 RO• + O₂ are of order of 10⁹ dm³ mol^-1 s^-1. It was reported that rate constant for the self-reaction of CF₃CFCO₂• to give CF₃CFCO• is (1.57±0.3) x 10⁹ dm³ mol^-1 s^-1 [50].
   The predominant fate of perhalogenated RO• is the halogen atom detachment, with C-C bond cleavage playing only a minor role. The elimination of Br• from the radical CF₂BrCFBrO• can be explained in terms of concomitant weakening of the C-Br bond, when C-O bond is forming and the lower bond dissociation energy of the C-Br bond (~70 kcal/mol) as compared with the C-F bond (~110 kcal/mol). The extrusion of fluorine atom from perhalofluoromethoxy radicals is not expected as this process is highly endothermic [52]. The elimination of Br• from CF₂BrCFBrO• predominates over C-C bond cleavage. The bond scission produces C(O)FBr and C(O)F2 and bromine atoms through the reaction sequence (11)-(14).
   The pre-exponential factors, A, and activation energies, E, obtained for the decomposition of radicals CF₂ClCCl₂O• [21], CHCl₂CCl₂O• [22] and CCl₃CCl₂O• [23] were A = (7±3) x 10^₁₃ s^-1 and E = 9.4±1.4 kcal mol^-1 , A = (1.1±1) x 10¹⁴ s^-1 and E = 9.4±2.4 kcal mol^-1, and A = (3.0±1.4) x 10¹³ s^-1 and E = 9.7±1 kcal mol^-1, respectively. In the theoretical study of the decomposition of halogenated alkoxy radicals [53] the following kinetic parameters were calculated for temperature range 240-260 K: A = (4.06-4.49) x 10¹³ s^-1 and E = 9.7 kcal/mol for CCl₃O•, A = (2.85-3.1) x 10¹³ s^-1 and E = 10.6 kcal mol^-1 for CFCl₂O•, and A = (2.85- 3.13) x 10¹³ s^-1and E = 12.4 kcal mol^-1 for CF₂ClO•.
   Molecular bromine is produced through reactions (17), (7), (8), (18) and the fast reaction (19), when all the alkene is consumed. Evidence was reported that the magnitude of the rate constant, k₁₉, falls between 10⁹ and 10¹⁰ dm³ mol^-1 s^-1 [42].
   As the rate constant k₁₇ = (2.2±1) x 10^-5 s^-1, for the decomposition of CF₂BrCFBrO₂NO₂, is slow in comparison with those for the chain reaction steps, then in the time interval in which the alkene is present, the reaction (15) may be taken as a chain termination step together with the reaction (16). The reported room temperature rate constants for decomposition of CCl₃O₂NO₂[54], CCl₂FO₂NO₂ [54], CClF₂O₂NO₂ [54], CF₃O₂NO₂ [55], CCl₃C(O) O₂NO₂ [54], CF₃O₂C(O)NO₂ [54], CF₂ClCCl₂O₂NO₂ [16] and CH₃C(O)O₂NO₂, PAN [56] are 0.19, 6.6 x 10^-2, 4.0 x 10^-2, 6.1 x 10^-2, 1.2 x 10^-4, 8.0 x 10^-5, 1.3 x 10^-4 and 3.2 x 10^-4 s^-1, respectively.
   Applying the steady-state approximation to the mechanism in presence of alkene, the following expressions for the respective consumption of CF₂CFBr and NO₂ were obtained:

-d[CF2CFBr]/dt = k1[NO2][ CF2CFBr] {1 + (k5/k1)[Br•]/[NO2] + (k8/k1)[CF2BrCFBrO2•]/[NO2]}

I

-d[NO2]/dt = k1[NO2][CF2CFBr]{1 + (k15/k1) [CF2BrCFBrO2•] [M]/ [CF2CFBr] + (k16/ k1)[CF2BrCFBr•][M] / [CF2CFBr]}

II

   Neglecting the contribution of the reactions of (4), (8) and (9) and assuming that [CF₂BrCFBr•] ≈ [CF₂BrCFBrO₂•], as practically all the radicals CF₂BrCFBr• react with O₂ to give peroxyradicals CF₂BrCFBrO₂•, the following expression was deduced:

k5 [Br•][[CF2CFBr] = (k10 + k11) [CF2BrCFBrO•] = k6 [CF2BrCFBr•][O2][M] {1 - (k15/k6) [NO2]/[O2]}

III

   At the pressures of oxygen used in this work, O₂ has a third body character, as the reaction is of pseudo-zero order with respect to O₂ as reactant.
   The chain velocity depends on the generation rate of the radicals CF₂BrCFBrO• by reactions (5)-(7). Each oxy radical releases one bromine atom through reactions sequence (10)-(14), reforming the CF₂BrCFBr• radicals. Decreasing the pressure of NO₂, increases the generation of the chain carriers CF₂BrCFBrO₂• and CF₂BrCFBrO•. With increasing generation of the radicals CF₂BrCFBrO₂• the rate of the reaction (7) raises more rapidly that the rates of the reactions (8) and (15), favoring the formation of CF₂BrC(O)F and the release of the bromine atom.

Conclusion

   This investigation provides evidence, not reported until now, that the free bromine atoms can be released in the absence of light as a consequence of oxidation of CF₂CFBr initiated by addition of NO₂ to the double bond of trifluorobromoethene in the presence of molecular oxygen. The formation of bromodifluoroacetyl fluoride CF₂BrC(O)F indicates that the bromine atoms add to the CF₂ group of the alkene, forming radicals CF₂BrCFBrO₂• and CF₂BrCFBrO• in the presence of oxygen. There are no data on the addition of Br• atoms to the fluorinated ethenes. The lack of formation of bromine in presence CF₂CFBr indicates that the addition of Br• atoms to the double bond competes successfully with the fast reactions Br• + NO₂ → BrNO₂ and Br•+ BrNO₂ → Br₂ + NO₂ and suggests that the reactions of bromine atoms with fluorinated alkenes are more rapid than those with the chlorinated counterpart. A new and simple way of preparation of CF₂BrC(O)F is reported here. Previously it was prepared by catalytic reduction by lithium aluminum hydride of bromodifluoroacetate, obtained treating 1,2-dibromo-1- chlorotrifluoroethane with fuming H₂SO₄ and HgO [57]. The room temperature rate constant, k₁₇ = (2.2±1) x 10^-5 s^-1 for the decomposition of peroxynitrate, CF₂BrCFBrO₂NO₂ is presented for the first time.

Acknowledgment

   The authors thank Mr. Z. Czarnowski for helpful comments. This work was supported by CONICET (National Science Research Council), Argentina. J.C. thanks financial support by the Max Planck Institute for Biophysical Chemistry Goettingen through the Partner Group for Chlorofluorocarbons in the Atmosphere (C. J. Cobos grant). R.M.R. thanks Fundación Antorchas (14022-3), CONICET (PEI-6177) and Volkswagen Foundation (Professors Oberhammer and Della Védova grant) for the purchase of the FTIR instrument.

References

[1] Dyatkin, B.L.; Mochalina, E.P.; Knunyants, I.L., Russian Chem. Rev., 1966, 35, 417.         [ Links ]

[2] Barr, D.A.; Haszeldine, R.N., J. Chem. Soc., 1960, 1151.         [ Links ]

[3] Fokin, A.V.; Uzun, A.T., Zh. Obshchei Khimii, 1966, 36, 117.         [ Links ]

[4] Coffman, D.D.; Raasch, G.W.; Rigby, G.W.; Barrick, P.L; Hanford, W.E., J. Org. Chem., 1949, 14, 747.         [ Links ]

[5] Haszeldine, R.N., J. Chem. Soc., 1953, 2075.         [ Links ]

[6] Bissel, E., J. Org. Chem., 1961, 26, 5100.         [ Links ]

[7] Knunyants, I.L.; Fokin, A.V.; Komarov, V.A., Zh. Vses. Khim. Obshchestvam. D. I. Mendeleeva, 1962, 7, 709.         [ Links ]

[8] Knunyants, I.L.; Fokin, A.V.; Komarov, V.A., Izv. Akad. Nauk SSSR, Ser. Khim., 1966, 466.         [ Links ]

[9] Spicer, Ch.W.; Heicklen, J., Int. J. of Chem. Kinet, 1972, 4, 575.         [ Links ]

[10] Romano, R.M.; Della Védova, C.O.; Czarnowski, J.,Z. Phys. Chem., 2002, 216,1203.         [ Links ]

[11] Czarnowski, J.; Schumacher, H.J., Int. J. Chem. Kinet., 1986, 18, 907.         [ Links ]

[12] Czarnowski, J.,Int. J. Chem. Kinet., 1992, 24, 679.         [ Links ]

[13] Romano, R.M.; Czarnowski, J., Z. Phys. Chem., 2004, 218, 575.         [ Links ]

[14] Romano, R.M.; Czarnowski, J., Z. Phys. Chem., 2005, 219, 849.         [ Links ]

[15] Czarnowski, J., J. Chem. Soc. Perkins Trans. 2, 1991, 1459.         [ Links ]

[16] Czarnowski, J., Bull. Polish Acad. Science Chem., 1991, 39, 49.         [ Links ]

[17] Niki, H.; Maker, P.D; Savage, C.M.; Preitenbach, L.P.; Hurley, M.D., Int. J. Chem. Kinet., 1986, 18, 1235.         [ Links ]

[18] Czarnowski, J.; Castellano, E.; Schumacher, H.J.Z, Phys. Chem. NF, 1969, 65, 225.         [ Links ]

[19] Czarnowski, J.; Schumacher, H.J.Z., Z. Phys. Chem. NF, 1970, 73, 68-76.         [ Links ]

[20] Kennedy, R.C.; Levy, J.B., J. Phys. Chem., 1972, 76, 3480.         [ Links ]

[21] Czarnowski, J.,J. Chem. Soc. Faraday Trans., 1989, 85, 1425.         [ Links ]

[22] Czarnowski, J., Z. Phys. Chem., 1995, 191, 103,         [ Links ]

[23] Czarnowski, J., Z. Phys. Chem., 1998, 203, 183        [ Links ]

[24] Catoire, V.; Ariya, P.A.; Niki, H.; Harris, G.W., Int. J. Chem. Kinet., 1997, 29, 695.         [ Links ]

[25] Ariya, P.A.; Catoire, V.; Sander, R.; Niki, H.; Harris, G.W., Tellus, 1997, 49B, 583.         [ Links ]

[26] Ramacher, B.; Orlando, J.J., G. S. Tyndall, Int. J. Chem. Kinet., 2001, 33, 198.         [ Links ]

[27] Park, J.D.; Snow, C.M.; Lecher, J.R., J. Am. Chem. Soc., 1951, 73, 2342.         [ Links ]

[28] Nielsen, J.R.; Richard, C.M.; Mc Murry, H.L., J. Chem. Phys., 1948, 16, 67.         [ Links ]

[29] Arce, V.; Czarnowski, J.; Romano, R.M., to be published.         [ Links ]

[30] Drew, B.R.; Gounev, T.K.; Guirguis, G.A.; Durig, J.B., J. Raman Spectrosc., 1998, 29, 205.         [ Links ]

[31] Nielsen, A.H.; Burke, T.G.; Woltz, P.J.W., E. A. Jones, J. Phys. Chem., 1952, 20, 596.         [ Links ]

[32] Parkington, M.J.; Ryan, T.A.; Seddon, K.R., J. Chem. Soc. Dalton Trans., 1997, 251.         [ Links ]

[33] García, P.; Willner, H.; Oberhammer, H.; Francisco, J.S., J. Chem. Phys., 2004, 121, 11900.         [ Links ]

[34] Niki, H.; Maker, P.D.; Savage, C.M.; Breitenbach, L.P., Chem. Phys. Lett., 1979, 61, 100.         [ Links ]

[35] Köppenkastrop, D.; Zabel, F., Int. J. Chem. Kinet., 1991, 23, 1.         [ Links ]

[36] Forst, W.; Caralp, F., J. Phys. Chem., 1992, 96, 6291.         [ Links ]

[37] Kopitzky, R.; Willner, H.; Mack, H.; Pfeiffer, A.; Oberhammer, H., Inorg. Chem., 1998, 37, 6208.         [ Links ]

[38] Zabel, F.; Kirchner, F.; Becker, K.H., Int. J. Chem. Kinet., 1994, 26, 827.         [ Links ]

[39] Chow, D.; Jones, M.H.; Thorne, M.P.; Wong, E.C., Can. J. Chem., 1969, 47, 2491.         [ Links ]

[40] Hubinger, S.; Nee, J.B., J. Photochem. Photobiol., A: Chem., 1995, 86, 1.         [ Links ]

[41] Blend, H., J. Chem. Phys., 1970, 53, 4497        [ Links ]

[42] Orlando, J.J.; Burkholder, J., J. Phys. Chem., 2000, 104, 2048.         [ Links ]

[43] Bierbach, A.; Barnes, I.; Becker, K.H., Int. J. Chem. Kinet., 1996, 28, 565.         [ Links ]

[44] Atkinson, R.; Aschmann, S.M., Int. J.Chem. Kinet., 1987, 19, 1097.         [ Links ]

[45] Russel, J.J.; Seetula, J.A.; Gutman, D.; Danis, F.; Caralp, F.; Lightfoot, P.D.; Lesclaux, R.; Melius, C.F.; Senkan, S.M., J. Phys. Chem., 1990, 94, 3277.         [ Links ]

[46] Slage, I.R.; Ratajczak, E.; Heaven, M.C.; Gutman, D.; Wagner, A.F., J. Am. Chem. Soc., 1985, 107, 1838.         [ Links ]

[47] Slage, I.R.; Gutman, D., J. Am. Chem. Soc., 1985, 107, 5342.         [ Links ]

[48] Slage, I.R.; Ratajczak, E.; Gutman, D., J. Phys. Chem., 1986, 90, 402.         [ Links ]

[49] Adachi, H.; Basco, N.; James, D.G.L., Int. J. Chem. Kinet., 1979, 11, 121.         [ Links ]

[50] Sehested, J., Int. J. Chem. Kinet., 1994, 26, 1023.         [ Links ]

[51] Stark, M.S., J. Phys. Chem., 1997, 101, 8296.         [ Links ]

[52] Rayez, J.C.; Rayez, M.T.; Halvick, P.; Duguay, B.; Danneberg, J.J., Chem. Phys., 1987, 118, 265.         [ Links ]

[53] Rayez, J.C.; Rayez, M.T.; Halvick, P.; Duguay, B.; Lesclaux, R.; J. J. Danneberg, J.J.,Chem. Phys., 1987, 116, 203.         [ Links ]

[54] Zabel, F., Z. Phys. Chem., 1995, 188, 119.         [ Links ]

[55] Mayer-Figge, A.; Zabel, F.; Becker, K.H., J. Phys. Chem., 1996, 100, 6587.         [ Links ]

[56] Roberts, J.M.; Bertman, S.B., Int. J. Chem. Kinet., 1992, 24, 297.         [ Links ]

[57] Cambell, R.W.; Vogl, O., Die Macrom. Chem., 1979, 180, 633.         [ Links ]