The kinetics of the addition of alkyl radicals to carbonyl groups. I. The kinetics of the gas phase decomposition reactions of the trifluoro-t-butoxy radical

Trifluoro-t-butoxy radicals have been generated by reacting fluorine with 2-trifluoromethyl propan-2-ol: Over the temperature range 361-600 K the trifluoro-t-butoxy radical decomposes exclusively by loss of the ? CF₃ group [reaction (?2)] rather than by loss of ? CH₃ group [reaction (?1)]: The limits of detectability of the product CF₃COCH₃, by gas-chromatographic analysis, place a lower limit on the ratio k_?2/k_?1 of ca. 75. The implications of these results in relation to the reverse radical addition reactions to the carbonyl group are discussed along with the thermochemistry of the reactions.     

The kinetics of the addition of alkyl radicals to carbonyl groups. Part I. The addition of methyl radicals to hexafluoroacetone in the gas phase. The formation of the hexafluoro-t-butoxy radical

By pyrolyzing di-t-butyl peroxide over the temperature range of 405–450 K in the presence of hexafluoroacetone the kinetics of the addition reaction (1), CH₃ + (CF₃)₂CO→; (CF₃)₂C(?)CH₃, have been studied. Detailed analyses have shown that the principal product of the adduct radical, (CF₃)₂C(?)CH₃, is CF₃COCH₃ from reaction (2), (CF₃)₂C(?)CH₃ → CF₃COCH₃ + CF₃. The rate constant of the addition reaction was determined to be k₁(dm³/mol·s) = (1.1 ± 4.0) + 10⁹ exp(-(3680 ± 480)/T) over the temperature range 405–450 K, based on the value k₃ = 2.2 × 10¹⁰ dm³/mol·s for reaction (3), 2CH₃ → C₂H₆. The results are discussed in relation to existing data for radical additions to carbonyl groups.     

The addition of methyl radicals to hexafluoroacetone

The reaction of methyl radicals (Me) with hexafluoroacetone (HFA), generated from ditertiary butyl peroxide (dtBP), was studied over the temperature range of 402–433 K and the pressure range of 38–111 torr. The reaction resulted in the following displacement process taking place: where TFA refers to trifluoroacetone. The trifluoromethyl radicals that were generated abstract a hydrogen atom from the peroxide: such that k_6a is given by: where θ = 2.303RT kcal/mol. The interaction of methyl and trifluoromethyl radicals results in the following steps: Product analysis shows that k₁₇/kk = 2.0 ± 0.2 such that k₁₇ = 10^10.4±0.5M^?1 · s^?1. The rate constant k₅ is given by: It is concluded that the preexponential factor for the addition of methyl radicals to ketones is lower than that for the addition of methyl radicals to olefins.     

An evaluation of the kinetic parameters for the reaction of trifluoromethyl radicals with CH3Cl in the gas phase in the temperature range from 416 to 636 K

The hydrogen and chlorine atom abstraction reactions from CH₃Cl by CF₃ radicals produced by the photolysis of hexafluoroacetone (HFA) and CF₃I were studied relative to the recombination of CF3 radicals (I) The Arrhenius parameters obtained in the temperature range 416 to 578 K are: where Θ = 2,303.RT cal mol^?1 and k₂ is the recombination rate constant for the CF₃ radicals. The factors that influence the transfer processes of chlorine and hydrogen are analyzed in a series of reactions of halomethanes with CF₃ and CH₃ radicals. © 1993 John Wiley & Sons, Inc.     

The reactions of CF3 and CD3 radicals with boron trimethyl

Hydrogen abstraction from boron trimethyl has been studied using the abstracting radicals CF₃ and CD₃, from the photolysis of the corresponding ketones over the temperature range of 150° to 300°C. The following Arrhenius parameters were obtained: The difference E – E in the case of BMe₃ is considered due, in part, to polar effects. An exchange reaction is proposed for both CF₃ and CD₃ in collisions with BMe₃: Radical combination of CF₃ and CH₂BMe₂ leads to a hot molecule which undergoes a β-fluoro rearrangement elimination process, or a stabilized molecule which can thermally decompose:      

Reactions of alkoxy radicals with O2. I. C2H5O radicals

C₂H₅ONO was photolyzed with 366 nm radiation at ?48, ?22, ?2.5, 23, 55, 88, and 120°C in a static system in the presence of NO, O₂, and N₂. The quantum yield of CH₃CHO, Φ{CH₃CHO}, was measured as a function of reaction conditions. The primary photochemical act is and it proceeds with a quantum yield ?_1a = 0.29 ± 0.03 independent of temperature. The C₂H₅O radicals can react with NO by two routes The C₂H₅O radical can also react with O₂ via Values of k₆/k₂ were determined at each temperature. They fit the Arrhenius expression: Log(k₆/k₂) = ?2.17 ± 0.14 ? (924 ± 94)/2.303 T. For k₂ ? 4.4 × 10^?11 cm³/s, k₆ becomes (3.0 ± 1.0) × 10^?13 exp{?(924 ± 94)/T} cm³/s. The reaction scheme also provides k_8a/k₈ = 0.43 ± 0.13, where      

The gas phase reaction of CF3 radicals with C6F5H in the temperature range 373–658 K

CF₃ radicals were generated by the photolysis of perfluoroacetic anhydride. In the presence of pentafluorobenzene, the CF₃ radicals react according to the following mechanism: It was found that the addition reaction (3) becomes reversible above ca. 453 K. The addition rate parameters have been revised and they satisfactorily agree with those reported previously. At temperatures higher than 593 K, only true H-abstraction occurs. The rate constant k_H for reaction (5) is given by: where θ = 2.303 RT kJmol^?1 and k_c is the rate constant for combination of CF₃ radicals. The reactions of CF₃ with benzene and pentafluorobenzene are compared.     

Arrhenius parameters for the hydrogen abstraction from ammonia by CF3 radicals

The reaction of CF₃ radicals with NH₃ has been studied over a wide temperature range 298–673 K, using the photolysis and the thermal decomposition of CF₃I as the free radical source. It was found that the reaction could not be explained in terms of a simple mechanism in the whole temperature range because a marked pressure dependence on the rate of products formation and the presence of a dark reaction complicate the system at low temperatures. Thus, Arrhenius parameters for reaction (1) have been calculated relative to the CF₃ recombination from data in the range 523–673 K where pure hydrogen transfer occurs. The rate constant expression is given by where k_H/k is in units of cm^3/2/mol^1/2 s^1/2 and θ = 2.303 RT/kJ/mol.     

Reaction of CF3 radicals with H2S

Hydrogen abstration from H₂S by CF₃ radicals, generated by the photolysis of both CF₃COCF₃ and CF₃I, has been studied in the temperature range 314–434 K. The rate constant, based on the value of 10^13.36 cm³/mol · s for the recombination of CF₃ radicals, is given by with CF₃COCF₃ as the radical source, and with CF₃I as the radical source, where k₂ is in cm³/mol · s and E is in J/mol. These results resolve a previously existing controversy concerning the values of the rate constants for this reaction. They show that CF₃ radicals are less reactive than CH₃ radicals in attacking H₂S, and this behavior indicates that polar effects play a significant role in the hydrogen transfer reactions of CF₃ radicals.     

Gas phase reaction of heptafluoroisopropyl radicals with carbon tetrachloride

The reaction was investigated in the gas phase over the range 80–225°C using the photolysis of heptafluoroisopropyl iodide as the source of radicals. The rate constant, based on the value of 10^13.36 cm³ mol^?1 s^?1 for the recombination of i-C₃F₇ radicals, is given by where θ = 2.303 RT/cal mol^?1. Arrhenius parameters for chlorine abstraction from CCl₄ by CF₃, C₂F₅, n-C₃F₇, and some hydrogenated radicals are compared.     

The addition of methyl radicals to tetrafluoroethylene

The addition of methyl radicals to tetrafluoroethylene in the gas phase has been studied over the temperature range 80–180°C, using a material balance method. Arrhenius parameters of 10^11.95±0.23 (mole^?1 cm³ sec^?1) and 5.7 ± 0.4 (kcal/mole) have been measured for the addition reaction. Electrophilic reagents such as O or CF₃ appear to react almost equally readily with ethylene and tetrafluoroethylene but methyl radicals add much more rapidly to tetrafluoroethylene than to ethylene, the difference in reactivity being principally due to an activation energy difference of ～2 kcal/mole.     

Rotating sector study of the gas phase photolysis of the 1,1,1-trichloro-2,2,2-trifluoroethane–cyclohexane system

The rate constant for the combination of 1,1-dichloro-2,2,2-trifluoroethyl radicals in the gas phase has been measured by applying the rotating sector technique to the 1,1,1-trichloro-2,2,2-trifluoroethane–cyclohexane photochemical system. The combination rate constant, k₅, was found to be 6.6 × 10¹² cc mole^?1 sec^?1. Arrhenius parameters for the reaction, are given by the expression log k₄ = 11.81 – (9700/2.3RT).     

Hydrogen abstraction from organosilicon compounds. The reactions of fluoromethyl radicals with tetramethylsilane. Polar effects in gas phase reactions

The photolysis of 1,1,3,3-tetrafluoroacetone has been reinvestigated as a source of CHF₂ radicals at temperatures up to 578°K, and the following rate constant ratio was determined for the reactions

1 θ= 2.303 RT in kcal/mole.

1,1-Difluoro- and 1,1,3,3-tetrafluoroacetone were photolyzed in the presence of tetramethylsilane, and Arrhenius parameters were measured for the hydrogen abstraction reactions: R + Me₄Si → RH + Me₃SiCH₂

The kinetics of the interaction of peroxy radicals. I. The Tertiary Peroxy Radicals

Existing data on the self-reactions of tertiary peroxy radicals RO₂ has been reanalyzed and corrected to deduce Arrhenius parameters for both termination and nontermination paths. For R = t-Butyl, these are logk_t(M^?1sec^?1) = 7.1 - (7.0/θ) and logk_nt(M^?1sec^?1) = 9.4 - (9.0/θ), respectively, different from those recommended by other authors. The higher magnitudes observed for termination processes of tertiary peroxy radicals like those of cumyl and 1,1-diphenylethyl have been discussed in terms of a much greater cage recombination of cumyloxy radicals as contrasted with t-butoxy radicals. It is shown that for benzyl peroxy radicals, the R—O bond dissociation energy is sufficiently low (18–20 kcal) that reversible dissociation into R^˙ + O₂ opens a competing second-order path to fast recombination R^˙ + RO → ROOR. This path is probably not important for cumyl peroxy radicals under usual experimental conditions but can become important for 1,1-diphenyl ethyl peroxy radicals at (O₂) < 10^?3M. At very low RO concentrations (<10^?5M), in the absence of added O₂, an apparent first-order disappearance of RO can occur reflecting the rate determining breaking of the cumyl—O bond followed by the second step above. The thermochemistry of RO is used to show that the reaction of R₂O₄ → 2RO + O₂ must be concerted and cannot proceed via RO which is too unstable and cannot form even from RO^˙ + O₂.     

Rate constants for the reactions of OH radicals with CH3OCF2CF3, CH3OCF2CF2CF3, and CH3OCF(CF3)2

The rate constants for the reactions of OH radicals with CH₃OCF₂CF₃, CH₃OCF₂CF₂CF₃, and CH₃OCF(CF₃)₂ have been measured over the temperature range 250–430 K. Kinetic measurements have been carried out using the flash photolysis, laser photolysis, and discharge flow methods combined respectively with the laser induced fluorescence technique. The influence of impurities in the samples was investigated by using gas‐chromatography. The following Arrhenius expressions were determined: k(CH₃OCF₂CF₃) = (1.90) × 10⁻¹² exp[−(1510 ± 120)/T], k(CH₃OCF₂CF₂CF₃) = (2.06) × 10⁻¹² exp[−(1540 ± 80)/T], and k(CH₃OCF(CF₃)₂) = (1.94) × 10⁻¹² exp[−(1450 ± 70)/T] cm³ molecule⁻¹ s⁻¹. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 846–853, 1999     

Reaction of CF3 radicals with H2O and D2O

The reaction of CF₃ radicals with H₂O (D₂O) has been studied over the range of 533–723 K using the photolysis and the pyrolysis of CF₃I as the free radical source. Arrhenius parameters for the reactions where X = H or D, relative to CF₃ radical recombination are given by where k/k is in cm^3/2/mol^1/2·s^1/2 and θ = 2.303RT/cal/mol. The activation energy and the primary kinetic isotope effect have been compared with those derived from the BEBO method.     

Kinetic study of gas-phase reactions with CF2Cl2 and CFCl3. Analysis using the transition state theory (TST) of the chlorine atom transfer reactions by CF3 and CH3 radicals from halomethanes

The following gas-phase reactions: were studied by the competitive method with CF₃I as the source of radicals. The kinetic parameters obtained in the temperature range 533–613 K and 503–613 K respectively for chlorine atom transfer reactions are given by: where θ = 2.303 RT (cal mol^?1). The Arrhenius A values were calculated for seven chlorine atom transfer reactions (CF₂Cl₂, CFCl₃, CCl₄ with CF₃ radicals; CF₃Cl, CF₂Cl₂, CFCl₃ and CCl₄ with CH₃ radicals) by using the thermochemical kinetic version of the Transition State Theory (TST).     

UV absorption spectra,kinetics, and mechanisms of the self reaction of CF3O2 radicals in the gas phase at 295 K

The ultraviolet absorption spectrum and the self reaction kinetics of CF₃O₂ radicals have been studied in the gas phase at 298 K using the pulse radiolysis technique. Long pathlength Fourier transform infrared (FTIR) spectroscopy was used to identify and quantify reaction products. Absorption cross sections were quantified over the wavelength range 215–270 nm. The measured cross section at 230 nm was; Errors represent statistical (2σ) together with our estimate of potential systematic errors. The absorption cross section data were then used to derive the observed self reaction rate constant for reaction (1), defined as ?d[CF₃O₂]/dt = 2k _obs[CF₃O₂]² k_lobs = (3.6 ± 0.9) × 10^?12 cm³ molecule^?1 s^?1. The only carbon containing product observed by FTIR spectroscopy was CF₃OOOCF₃. Consideration of the loss of CF₃O₂ radicals to form the trioxide CF₃OOOCF₃ allows derivation of the true bimolecular rate constant for reaction (1); k₁ = (1.8 ± 0.5) × 10^?12 cm³ molecule^?1 s^?1. These results are discussed with respect to previous studies of the absorption spectra of peroxy radicals, the kinetics, and mechanisms of their self reaction. © John Wiley & Sons, Inc.     

The oxidation of diethylhydroxylamine by nitrogen dioxide

Diethylhydroxylamine, (C₂H₅)₂NOH, was oxidized by NO₂ at 25°C in a long-path-length infrared gas cell. The measured products of the reaction were HONO and CH₃CHO. The reaction scheme which explains the reaction is was oxidized by NO₂, and the reaction was found to be very rapid with k₁ > 10^?16 cm³/s. The products of the reaction were verified by both infrared absorption (CH₃CHO, C₂H₅NO) and gas chromatography (CH₃CHO, NO).     

Kinetics of the reactions of 2,4,6-tri-t-butylphenoxyl with cumene hydroperoxide,cumylperoxyl radicals,and molecular oxygen

Reactions of 2,4,6-tri-t-butylphenoxyl (TBP) with cumene hydroperoxide (ROOH), cumylperoxyl radicals (RO₂), and molecular oxygen in benzene solution have been investigated kinetically by the ESR method. The rate constant of the reaction TBP + ROOH has been estimated in the temperature range 27°-75°C: log₁₀(k_?7/M^?1sec^?1) = (7.1 ± 0.4) - (10.9 ± 0.6 kcal mole^?1)/θ The ratio of the rate constants of reactions TBPH + RO₂ products has been determined from the experimental dependence of the rate constant of reaction TBP with ROOH on [TBPH]₀/[TBP]₀. Putting k₇ = 4.0 × 10³M^?1sec^?1, we obtain k₈ = (2.0 ± 0.2) × 10⁸M^?1sec^?1 at 30°C. The reaction of TBP with O₂ obeys the kinetic law ?d[TBP]/dt = k′[O₂][TBP]². This is in accordance with scheme TBP + O₂ ← TBP ?O₂ [I]; TBP ?O₂ + TBP · products, log₁₀ (k′/M^?2sec^?1) = (?14.5 ± 0.9) + (27.2 ± 1.4)/θ at 66°?78°C, where ° = 2.303RT.