The free-radical reaction of tri-n-butyltin hydride with disulfides

The kinetics and absolute rate constants for the free-radical chain reaction of tri-n-butyltin hydride with di-t-butyl disulfide have been measured in cyclohexane at 30°. The rate controlling step for chain propagation involves the cleavage of the disulfide bond by an attacking tributyltin radical. The rate constant for this bimolecular homolytic substitution at sulfur is ～8 × 10⁴ Mole^?1 sec^?1. Chain termination involves the self-reaction of two tributyltin radicals. The rate constants for attack of tributyltin radicals on some other disulfides and on elemental sulfur have also been measured. The results are compared with literature data for homolytic substitutions on these compounds by a variety of radicals which have their unpaired electron centered on carbon.     

Very low-pressure pyrolysis. VIII. The decomposition of Di-t-amyl peroxide

The very low-pressure pyrolysis (VLPP) technique has been applied to the pyrolysis of di-t-amyl peroxide (DTAP) over the temperature range 523-633°K. VLPP yields a low-pressure rate constant, k_uni The conversion of k_uni to k_∞ which must be made to calculate the Arrhenius parameters, is accomplished via the RRKM theory. The transition state model used in the RRKM calculations was based on a transition state model which accurately reproduced the VLPP data for di-t-butyl peroxide for which the Arrhenius parameters are well known. For the decomposition of DTAP it was found that log k_∞(300°K) = 15.8 - 36.4/θ, where θ = 2.303RT, in kcal/mole, and the units of k_∞, are sec^?1.     

Kinetics and rate constants for the reaction of tri-n-butylgermanium hydride with methyl iodide and carbon tetrachloride. The combination of methyl and trichloromethyl radicals in solution.

The kinetics of the photoinitiated reductions of methyl iodide and carbon tetrachloride by tri-n-butylgermanium hydride in cyclohexane at 25°C have been studied and absolute rate constants have been measured. Rate constants for the combination of CH₃? and CCl₃? radicals are equal within experimental error and are also equal to the values found for the self-reactions of most non-polymeric radicals in low viscosity solvents, i.e. ～1–3 × 10⁹ M^?1 sec^?1. Rate constants for hydrogen atom abstraction by CH₃? and CCl₃? radicals are both ～1?2 × 10⁵ M^?1 sec^?1. Tri-n-butyltin hydride is about 10–20 times as good a hydrogen donor to alkyl radicals as is tri-n-butylgermanium hydride. The strength of the germanium–hydrogen bond, D(n-Bu₃Ge–H) is estimated to be approximately 84 kcal/mole.     

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.     

The gas-phase decomposition of alkoxy radicals

It is shown that it is possible to obtain good data for the rate constant for the decomposition of alkoxy radicals [RO] by using nitric oxide as a radical trap. Two experimental systems have been used. The first system involves the use of dialkyl peroxides [(RO)₂] as thermal sources of alkoxy radicals. The peroxide concentration was ～10^?4M, nitric oxide ～2 × 10^?4M, and the extent of reaction was ～10%. The total pressure was altered using carbon tetrafluoride as an inert gas. The mechanism is Hence R₂/R₃ = k₂[N O]/k₃. Our previous studies show that k₂ lies in the range 10^10.3±0.2M^?1·sec^?1. The second system employs alkyl nitrites [RONO] as a thermal source of alkoxy radicals. The experimental conditions are very similar, except that we chose to use an atmosphere of nitric oxide for initial experiments. If anything nitric oxide appears to be superior to carbon tetrafluoride as an energy transfer agent. The mechanism is Hence R₃ = k₁'k₃[RO NO]/(k₃ + k₂ + k₆ [N O]). Results are given for R = t-Am, s-Bu, t-Bu, i-Pr, Et, and Me. In addition the first unequivocal evidence is given for the pressure dependence of k₃ when R = t-Bu. The implications for atmospheric chemistry and combustion are also discussed.     

Absolute rate constants for the reactions between amino and alkyl radicals at 298 K

The absolute rate constants for the reactions of NH₂ radicals with ethyl, isopropyl, and t-butyl radicals have been measured at 298 K, using a flash photolysis–laser resonance absorption method. Radicals were generated by flashing ammonia in the presence of an olefin. A new measurement of the NH₂ extinction coefficient and oscillator strength at 597.73 nm was performed. The decay curves were simulated by adjusting the rate constants of both the reaction of NH₂ with the alkyl radical and the mutual interactions of alkyl radicals. The results are k(NH₂ + alkyl) = 2.5 (±0.5), 2.0 (±0.4), and 2.5 (±0.5) × 10¹⁰ M^?1·s^?1 for ethyl, isopropyl, and t-butyl radicals, respectively. The best simulations were obtained when taking k(alkyl + alkyl) = 1.2, 0.6, and 0.65 × 10¹⁰M^?1·s^?1 for ethyl, isopropyl, and t-butyl radicals, respectively, in good agreement with literature values.     

Kinetics and mechanisms of substitution reactions with sulfur-containing nucleophiles in aqueous acid solutions. I—rates of reactions of some tetrahalopalladate (II) anions with thiourea and selenourea

The activation energy parameters for the reaction of PdX (X=Cl^?, Br^?) in aqueous halide acid solution with thiourea (tu) and selenourea (seu) have been determined. High rates of reaction parallel low enthalpies and appreciable negative entropy of activation. The rate law in each case simplifies to k_obs=k[L] where L=tu or seu, and only ligand-dependent rate constants are observed at 25°C. The ligand-dependent rate constants for the first identifiable step in the PdCl + X system is (9.1±0.1) × 10³ M^?1 sec^?1 and (4.5±0.1) × 10⁴ M^?1 sec^?1 for X=tu and seu, respectively, while for the PdBr + X system it is (2.0±0.1) × 10⁴ M^?1 sec^?1 and (9.0±0.1) × 10⁴ M^?1 sec^?1 for X=tu and seu, respectively.     

Effect of thiourea on the radical polymerization of vinyl monomers. Part III. Effect of diphenyl thiourea on the polymerization of methyl methacrylate with various organic peroxides as initiator

The effect of diphenyl thiourea (DPTU) on the radical polymerization of methyl methacrylate (MMA) has been studied in benzene solution at 50°C. with the use of cumene hydroperoxide (CHP), p-menthane hydroperoxide (PMHP), tert-butyl perbenzoate (tBPBz), di-tert-butyl peroxide (DBP), and dicumyl peroxide (DCP) as initiators. In the CHP-initiated polymerization, the rate of polymerization increased appreciably on addition of DPTU with a linear dependence on the square root of DPTU concentration up to a maximum which was observed when the ratio of the concentration of CHP to DPTU was 2.5. Then the rate decreased gradually with increasing DPTU concentration in the range greater than the above ratio. It was found from kinetic studied that the overall polymerization rate R_p was expressed by the equation: R_p = K[peroxide]^1/2 [DPTU]^1/2[MMA], where K is the rate constant, α = 1.2 for CHP and α = 1.0 for tBPBz. It was thought that the acceleration effect observed was due to a redox reaction caused by the interaction of a peroxide–monomer and/or a peroxide–solvent complex with DPTU, and the decrease in the polymerization rate which was observed over a certain concentration of DPTU was due to the action of the oxidized product of DPTU as a transfer agent. The effect of substituents was studied by using para and meta-substituted DPTU. It was found that the polymerization rate increased as electron-donating substituents are added to the benzene ring of DPTU with considerable dependence on Hammett's equation (p = ?0.36). The acceleration effect is also observed for PMPH-and tBPBz-initiated polymerizations, whereas the DCP- and DBP-initiated systems show no effects on the polymerization rate.     

Thermal decomposition of perfluoro-di-t-butyl peroxide

The thermal decomposition of perfluoro-di-t-butyl peroxide has been studied for the first time. The reaction was carried out in the gas phase between 5 and 600 torr in the 108-149°C temperature region. The products consisted solely of C₂F₆ and CF₃COCF₃. The decomposition was found to be first order and homogeneous. The rate constant is given by log k_decomp(sec^?1) = (16.2 ± 1.2) - (148.7 4.4)/2.3RT where R is 0.008314kJ/mol · °K. These Arrhenius parameters are consistent with those determined for the decomposition of di-t-butyl peroxide.     

Laser-induced kinetics: Arrhenius parameters for t-C4H9 + XI → i-C4H9X + I (X = H,D) and the Heat of Formation of the t-butyl radical

The metathesis reaction of DI with t-C₄H₉ generated by 351-nm photolysis of 2,2′-azoisopropane was studied in a low-pressure reactor (VLP? Knudsen cell) in the temperature range of 302–411 K. The data obeyed the following Arrhenius relation when combined with recent data by Rossi and Golden gathered by the same technique (t-C₄H₉ by thermal decomposition of 2,2′-azoisobutane): log k₂^D(M^?1s^?1) = 9.60 – 1.90/θ, where θ = 2.303RT kcal/mol for 302 K < T > 722 K. The metathesis reaction of HI with t-C₄H₉ was studied at 301 K and resulted in k₂^H(M^?1·s^?1) = (3.20 ± 0.62) × 10⁸. An analogous Arrhenius relation was calculated for the protiated system if the small primary isotope effect k₂^H/k₂^D was assumed to be √2 at 700 K. It was of the following form: log k₂^H(M^?1·s^?1) = 9.73 – 1.68/θ. Preliminary data of Bracey and Walsh indicate that earlier Arrhenius parameters determined for the reverse reaction are somewhat in error. Their value of log k₁(M^?1·s^?1) = 11.5 – 23.8/θ yields 7delta;H_f,300⁰(t-butyl) = 9.2 kcal/mol and S₃₀₀⁰(t-butyl) = 74.2 cal/mol7°K when taken in conjuction with this study.     

Rates of recombination of free radicals. V. The tert-butyl radical

Analysis of the products from a vapor-phase mixture of EtI and t-BuI, in which free radicals were generated thermally, has yielded a rate constant for the recombination of 2t-Bu? of 10^5.4 M^?1 sec^?1 at 100°C. The close agreement between this and other recent estimations from vapor-phase work is noted, as is the disparity between these values and those on the order of 10⁹ M^?1 sec^?1 obtained in solution.     

Primary alkyl radicals: Can they be persistent?

The structure of t-Bu₃Si?H₂, (I), suggested that it might be a persistent primary alkyl radical since it has (i) a bulky group to protect the radical center; (ii) no β-hydrogens, so that a radical-radical disproportionation reaction is impossible; (iii) a β-silicon atom, which should prevent β-scission of tert-butyl as a unimolecular decomposition pathway. However, the self-reaction of (I) in isooctane was found to be a diffusion controlled process with log(A/M^?1 sec^?1) = 10.7 ± 0.3 and E = 2.5 ± 0.2 kcal mole^?1. Hence (I) is not persistent and it is concluded that a persistent primary alkyl will only be observed when the—?H₂ moiety is deeply buried among sterically protecting groups.     

Equilibrium,kinetics, and mechanism of the malonic acid–iodine reaction

The equilibrium constant for the reaction CH₂(COOH)₂ + I₃^? ? CHI(COOH)₂ + 2I^? + H⁺, measured spectrophotometrically at 25°C and ionic strength 1.00M (NaClO₄), is (2.79 ± 0.48) × 10^?4M². Stopped-flow kinetic measurements at 25°C and ionic strength 1.00M with [H⁺] = (2.09-95.0) × 10^?3M and [I^?] = (1.23-26.1) × 10^?3M indicate that the rate of the forward reaction is given by (k₁[I₂] + k₃[I₃^?]) [HOOCCH₂COO^?] + (k₂[I₂] + k₄[I₃^?]) [CH(COOH)₂] + k₅[H⁺] [I₃^?] [CH₂(COOH)₂]. The values of the rate constants k₁-k₅ are (1.21 ± 0.31) × 10², (2.41 ± 0.15) × 10¹, (1.16 ± 0.33) × 10¹, (8.7 ± 4.5) × 10^?1M^?1·sec^?1, and (3.20 ± 0.56) × 10¹M^?2·sec^?1, respectively. The rate of enolization of malonic acid, measured by the bromine scavenging technique, is given by k_en[CH₂(COOH)₂], with k_en = 2.0 × 10^?3 + 1.0 × 10^?2 [CH₂(COOH)₂]. An intramolecular mechanism, featuring a six-member cyclic transition state, is postulated to account for the results on the enolization of malonic acid. The reactions of the enol, enolate ion, and protonated enol with iodine and/or triodide ion are proposed to account for the various rate terms.     

The very-low-pressure pyrolysis (VLPP) of n-propyl nitrate,tert-butyl nitrite,and methyl nitrite. Rate constants for some alkoxy radical reactions

The pyrolysis of n-propyl nitrate and tert-butyl nitrite at very low pressures (VLPP technique) is reported. For the reaction the high-pressure rate expression at 300°K, log k₁ (sec^?1) = 16.5 ? 40.0 kcal/mole/2.3 RT, is derived. The reaction was studied and the high-pressure parameters at 300°K are log k₂(sec^?1) = 15.8 ? 39.3 kcal/mole/2.3 RT. From ΔS_1,?1⁰ and ΔS_2,?2⁰ and the assumption E_?1 and E_?2 ? 0, we derive log k_?1(M^?1·sec^?1) (300°K) = 9.5 and log k_?2 (M^?1·sec^?1) (300°K) = 9.8. In contrast, the pyrolysis of methyl nitrite and methyl d₃ nitrite afford NO and HNO and DNO, respectively, in what appears to be a heterogeneous process. The values of k_?1 and k_?2 in conjunction with independent measurements imply a value at 300°K for of 3.5 × 10⁵ M^?1·sec^?1, which is two orders of magnitude greater than currently accepted values. In the high-pressure static pyrolysis of dimethyl peroxide in the presence of NO₂, the yield of methyl nitrate indicates that the combination of methoxy radicals with NO₂ is in the high-pressure limit at atmospheric pressure.     

Thermal decompositions of dialkyl peroxides studied by DSC

Studies on the thermal decompositions of diamyl peroxide (DAPO), dicumyl peroxide (DCPO), and tert-butyl cumyl peroxide (TBCP) were conducted by DSC. Heat of decomposition, exothermic onset point, and chemical kinetics were determined and compared to those data of di-tert-butyl peroxide (DTBP), a model compound for studying thermokinetics of organic peroxide and standardization of a calorimeter. Similarities and differences of decomposition mechanisms between these organic peroxides were proposed and verified. Kinetics on decomposition of uni-molecular reaction via these similar alkoxyl radials accompanying β C–C bond scission were discussed and compared to the results from ab initio calculations. The ranking of thermal stability on dialkyl peroxides is determined to be in the following sequence: DCPO < TBCP < DAPO < DTBP. This rate-determining step in thermal decomposition of dialkyl peroxides possessed an average eigenvalue of log A at about 13.1 ± 1.2. Activation energy on the thermal decomposition of these peroxides was calculated to be 139.5 ± 14.4 kJ mol^?1.     

Highly reactive μ-carbido diiron tetraphenylporphine oxo-species: chemical generation and the oxidation ability

Abstract

The multistep interaction of a μ-carbido diiron tetraphenylporphine complex and its imidazole derivative with tert-butylhydroperoxide ^tBuOOH were investigated in benzene at 298 K by UV–visible, IR spectroscopy and electrochemical study. The kinetics of the stepwise processes were studied and their rate constants k were obtained. The reaction of both non-ligated [Fe^IVTPP]₂C (1) and [ImFe^IVTPP]₂C (2) with ^tBuOOH led to high-oxidized species generation following the coordination of a peroxide molecule on the metal center (k₁?=?2.8 M^?1s^?1) and homolytic cleavage of O–O bond in attached ^tBuOOH (k₂?=?3.1?s^?1 for 1, k₂?=?12.8?s^?1 for 2). [Fe^IVTPP]₂C underwent one-electron ring oxidation to form a π-cation radical (k_ox?=?3.9?s^?1) detected by UV–visible and IR spectroscopy. Imidazole-derivative formed a powerful oxidizing species that is able to oxidize the organic peroxide employed. The oxidant-intermediate reacting rapidly when produced in solution was proposed to be double-oxidized species due to its high reactivity (k_red?=?2660 M^?1s^?1). The results of UV–visible, IR, ESI-MS study supported the final products with Fe?=?C=Fe unit.     

Radical-initiated homo- and copolymerization of methoxymethyl methacrylate

The kinetics of methoxymethyl methacrylate (MOMA) homopolymerization has been investigated in benzene, using azobis(isobutyronitrile) as an initiator. The rate of polymerization (R_p) could be expressed by R_p = k[AIBN]^0.5 [MOMA]^1.19. The overall activation energy was calculated to be 73.2 kJ/mol. Kinetic constants for MOMA polymerization were obtained as follows: k_p/k_t^1/2 = 0.091 L^1/2 · mol^?1/2 · s^?1/2; 2fk_d = 1.37 × 10^?5 s^?1. The values of K and a in the Mark–Houwink equation, [η] = KM^a, where K = 5.89 × 10^?5 and a = 0.82 when M = M _n and the solvent was benzene. The relative reactivity ratios of MOMA (M₂) copolymerizations with styrene (r₁ = 0.40, r₂ = 0.58) were obtained. Applying the Q-e scheme led to Q = 0.78 and e = 0.67. The glass transition temperature (T_g) of poly(MOMA) was observed to be 64°C by DSC. Thermogravimetry of poly(MOMA) showed a 10% weight loss at 230°C in air.     

An absolute measurement of the rate constant for t-butyl radical combination

The rate constant for tert-butyl radical recombination has been measured near 700°K by the very-low-pressure pyrolysis (VLPP) technique and was found to be 10^8.8±0.3 M^?1·sec^?1 with neglibible temperature dependence. The thermochemical parameters for tert? butyl radicals were varied within reasonable limits to bring into agreement the data for the decomposition of 2,2,3,3-tetramethyl butane and the recombination of tert-butyl radicals. The revised thermochemistry also makes the gas-phase results and liquid-phase results compatible.     

The gas-phase pyrolysis of alkyl nitrites VI. t-Amyl nitrite

The rate of decomposition of tert-amyl nitrite (t-AmONO) has been studied in the absence (120°–155°C) and presence (160°–190°C) of nitric oxide. In the absence of nitric oxide for low concentrations of tert-amyl nitrite (～10^?4M) and small extents of reaction (～1%), the first-order homogeneous rates of acetone formation are a direct measure of reaction (1) since k_3a ? k₂(NO): The rate of acetone formation is unaffected by the addition of large amounts of carbon tetrafluoride or isobutane (～1 atm) but is completely suppressed by large amounts of nitric oxide (1 atm 120°–155°C). The rate of reaction (1) is given by k₁ = 10^16.3±0.1 10^?40.3±0.1/θ sec^?1. Since (E₁ + RT) and ΔH°₁ are identical, both may be equated with D(t-AmO – NO) = 40.9 ± 0.1 kcal/mol and E₂ = 0 ± 0.1 kcal/mol. The thermochemistry leads to the result that ΔH°_f (t-AmO) = ?26.6 ± 1 kcal/mol. From ΔS°₁ and A₁, k₂ is calculated to be 10^10.5±^0.2 M^?1·sec^?1. Although the addition of nitric oxide completely suppresses acetone formation at lower temperatures, it reappears at higher temperatures. This is a result of reaction (3a) now competing with reaction (2), thus allowing k_3a to be determined. The rate constant for reaction (3a) is given by k_3a = 10^{14.7 ± 0.2} 10^{?14.3 ± 1}/θ sec^?1. There are two possible routes for the decomposition of the tert-amyloxyl radical: The dominating process is (3a). From the result at 160°C that k_3a/k_3b = 80, we arrive at the result k_3b = 10^15.0–18.7/θ sec^?1. In addition to the products accounted for by the radical split (1), methyl-2-but-1-ene and methyl-2-but-2-ene are produced as a result of the six-centre elimination of nitrous acid (5): The ratio k_5a/k_5b was 0.35. Unlike tert-butyl where the rates of the two paths were comparable [(l) and (5)], here the total rate of the elimination process was only 0.5% that of the radical split (1). The reason for this is not clear.