Kinetics of Gas Phase Halogenation Reactions

Benson, Sidney W.; Buss, Jerry H.

doi:10.1063/1.1744111

Cited by 14 publications

(4 citation statements)

References 22 publications

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“…The conditions under which such a n equilibrium is established have been closely examined [4] for I, , C1, , and other systems. It is clear from this study that, since in practice no induction period is ever observed in I, + R H systems, the dissociation and association of I, takes place, at least partly, on the surface by some heterogeneous mechanism.…”

Section: A Ic1 As a Thermal Source Of Chlorine Atommentioning

confidence: 99%

Iodine monochloride as a thermal source of chlorine atoms: The reaction of chlorine atoms with hydrogen

Benson

Cruickshank

Shaw

1969

Int J of Chemical Kinetics

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The use of iodine monochloride (IC1) as a thermal source of chlorine atoms in known concentration is discussed with particular reference to the suppression, by large excesses of iodine, of the chain processes normally associated with chlorine atom reactions. The kinetics and mechanism of the reaction of IC1 with hydrogen are presented in a study covering the temperature range 205-337"C, and the pressure ranges: ICI, 6-20 torr; I,, 3-13 torr; and H, , 9-520 torr. The reaction, followed spectrophotometrically in a static system, is shown to be homogeneous, first order in IC1 and in H, , and inverse halforder in I, , over several half-lifetimes of the ICl, yielding HCl as the sole product. The rate data obtained in this work for the reaction C1. + H, & HC1 + H. are combined with the critically evaluated results of other workers in an Arrhenius plot covering the temperature range 286-730°C, and three orders-of-magnitude in the rate constant, yielding the result, log k,/( l/mole sec) = 10.68-5.26/8, where 8 = 2.303RT in kcal/mole. This value of k, is lower by a factor of about two than that proposed in a recent review by Fettis and Knox, and is clearly at variance by a factor of two or more with the most recent data of Clyne and Stedman.

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Section: A Ic1 As a Thermal Source Of Chlorine Atommentioning

confidence: 99%

Iodine monochloride as a thermal source of chlorine atoms: The reaction of chlorine atoms with hydrogen

Benson

Cruickshank

Shaw

1969

Int J of Chemical Kinetics

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“…Reactions of small hydrocarbon free radicals (such as CH 3 , C 2 H 5 , C 3 H 7 , C 4 H 9 ) with hydrogen halides (HI, HBr, and HCl) have been a subject of research during the last 50 years. − Beyond their importance for the fundamental chemical kinetics, these kinetic measurements were used as a source of the C−H bond energies . Combination of the rate constant of reaction of a free radical with hydrogen halide with the rate constant of the reverse reaction, reaction of the halogen atom with the corresponding hydrocarbon molecule, yields a temperature-dependent equilibrium constant.…”

Section: Introductionmentioning

confidence: 99%

Kinetic Study of CH₃+ HBr and CH₃+ Br Reactions by Laser Photolysis−Transient Absorption over 1−100 Bar Pressure Range

and

Mehta

1999

J. Phys. Chem. A

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Reactions of methyl radicals with hydrogen bromide CH3 + HBr → CH4 + Br (1) and bromine atoms CH3 + Br → CH3Br (2) were studied using excimer laser photolysis−transient UV spectroscopy at 297 ± 3 K over the 1−100 bar buffer gas (He) pressure range. Methyl radicals were produced by 193 nm (ArF) laser photolysis of acetone, (CH3)2CO, and methyl bromide, CH3Br. Temporal profiles of methyl radicals were monitored by UV absorption at 216.51 nm (copper hollow cathode lamp with current boosting). The yield of acetyl radicals in photolysis of acetone at 193 nm was found to be less than 5% at 100 bar He based on the transient absorptions at 222.57 and 224.42 nm. The measured rate constants for reaction 1 are k 1 = (2.9 ± 0.7) × 10-12, (3.8 ± 1.5) × 10-12, and (3.4 ± 1.3) × 10-12 cm3 molecule-1 s-1 at the buffer gas (He) pressures of 1.05, 11.2, and 101 bar, respectively. The rate data obtained in this study confirmed high values of the previous (low pressure) measurements and ruled out the possibility of interference of excited species. The measured rate constant is independent of pressure within the experimental error. The rate constant of reaction of methyl radicals with bromine atoms (2) was determined relative to the rate constant of methyl radical self-reaction, CH3 + CH3 → C2H6 (3) in experiments with photolysis of CH3Br: k 2/k 3 = 0.92 ± 0.32, 1.15 ± 0.30, and 1.65 ± 0.26 at 1.05, 11.2, and 101 bar He, respectively. On the basis of the literature data for reaction 3, this yields k 2 = (5.8 ± 2.2) × 10-11, (7.4 ± 2.2) × 10-11, (10.7 ± 2.3) × 10-11, and (11.9 ± 2.5) × 10-11 cm3 molecule-1 s-1 at 1.05, 11.2, 101 bar (He), and in the high-pressure limit, respectively.

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“…Bromine reactions chains are shorter because propagation step 2 has a larger activation energy than the same step for chlorine reactions.17 Similar long chains occur in other photobrominations of various gaseous hydrocarbons. 18 A quantum yield of 30-40 was reported by Jost19 in 1931 for the gas phase photobromination of cyclohexane. Except for this last example, chain lengths larger than 103 Br2 consumed per photon absorbed are evidence of low activation energies for the chain carrying steps.…”

Section: Resultsmentioning

confidence: 92%

Photobromination of liquid 3-methylpentane

Arce¹,

Bartolome²

1976

J. Phys. Chem.

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Publication costs assisfed by the University of Puerto RicoThe photochemical bromination of liquid 3-methylpentane was studied in the temperature range between 210 and 298 K to compare the liquid phase reactions with those in the gas phase. The quantum yield of Brz consumption varies from 0.06 a t 210 K at an absorbed light intensity of 5.82 X 1015 photons cm-3 s-l and an initial bromine concentration of 1.8 X M to 524 at 298 K at an absorbed light intensity of 2.50 X 1014 photons cm-3 s-1 and initial bromine concentration of 1.7 X 10-3 M. The activation energy for the overall reaction is found to be 11 kcal/mol. Quantum yields have been determined for the formation of HBr and 3methylpentyl bromides and are of the same order of magnitude as the yield for bromine consumption a t 298 K. Kinetic studies show that the initial rate of bromine disappearance depends on the 0.6 power of the absorbed light intensity and on the 0.6 power of the initial bromine concentration. These results and the high quantum yields indicate that the bromination goes through an atom-radical chain mechanism. Hydrogen bromide and oxygen inhibit the reaction. The relative rate of substitition for tertiary relative to secondary hydrogen atoms is 42, and that for tertiary relative to primary is larger than lo4. A comparison is made with previous photochlorination studies in 3-methylpentane.

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Kinetics of Gas Phase Halogenation Reactions

Cited by 14 publications

References 22 publications

Iodine monochloride as a thermal source of chlorine atoms: The reaction of chlorine atoms with hydrogen

Iodine monochloride as a thermal source of chlorine atoms: The reaction of chlorine atoms with hydrogen

Kinetic Study of CH₃+ HBr and CH₃+ Br Reactions by Laser Photolysis−Transient Absorption over 1−100 Bar Pressure Range

Photobromination of liquid 3-methylpentane

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Kinetics of Gas Phase Halogenation Reactions

Cited by 14 publications

References 22 publications

Iodine monochloride as a thermal source of chlorine atoms: The reaction of chlorine atoms with hydrogen

Iodine monochloride as a thermal source of chlorine atoms: The reaction of chlorine atoms with hydrogen

Kinetic Study of CH3+ HBr and CH3+ Br Reactions by Laser Photolysis−Transient Absorption over 1−100 Bar Pressure Range

Photobromination of liquid 3-methylpentane

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Kinetic Study of CH₃+ HBr and CH₃+ Br Reactions by Laser Photolysis−Transient Absorption over 1−100 Bar Pressure Range