The reinvestigation of the kinetics of the metathesis reactions t-C4H9• + HBr (HI) → i-C4H10 + Br• (I•) and of the t-C4H9• free radical thermochemistry (original) (raw)
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Journal of the American Chemical Society, 1988
The kinetics of both the forward (4) and the reverse (-4) reactions involved in the equilibrium r-C4H9 + HBr i-C4HI0 + Br were studied as a function of temperature and pressure. Reactions 4 (296-532 K) and -4 (533-710 K) were isolated for direct investigation in a tubular reactor coupled to a photoionization mass spectrometer. Reaction -4 was also investigated (298-478 K) by using a temperature-controlled flash photolysis apparatus equipped for time-resolved detection of Br using atomic resonance fluorescence. The temperature dependencies of the rate constants measured below 532 K were used to determine AH01(298) and SO(298) for the t-C4H9 radical. The Arrhenius expressions obtained from these measured rate constants are as follows (units are cm3 molecule-' S I ) : k4(296-532 K) = 9.86 (i1.3) X exd5.8 (i0.9) kJ mol-'/R7); k4(298-478 K) = 1.83 (f0.18) X lo-'' exd-28.7 (f0.8) kJ mol-llR7). The values of two thermodynamic variables for t-C4H9 obtained directly from these results are AiYOX298) = 48.6 i 1.7 kJ mol-' and SO(298) = 316 f 7 J mol-' K-'. The tertiary C-H bond energy (298 K) in i-C4HI0 is DH(t-C4H9-H) = 401.2 i 1.7 kJ mol-I. The difference between the observed negative activation energy for the t-C4H9 + HBr reaction and its presumed value accounts for the disparity between heats of formation of the t-C4H9 radical obtained in prior studies of this bromination equilibrium and from kinetic studies of dissociation-recombination equilibria. A mechanism is proposed to account for the kinetic behavior of the t-C,H9 + HBr reaction which includes the formation of a bound adduct that may either dissociate to re-form the original reactants or rearrange to produce the final products.
The kinetics and thermodynamics of free radical reactions
Pure and Applied Chemistry, 1992
Three experimental studies are described which emphasise the inter-relastionship of kinetics and thermodynamics. The first involves AH7(t-C4HJ, where descrepancies in experimental values have led to extensive kinetic studies of the reactions t-C4€& + HX (X = Br, I) which surprisingly have negative activation energies. The second study relates to forward and reverse reactions in the system H + HBr + H2 + Br where use of the equilibrium constant enables rate parameters for both reactions to be defined over a very wide temperature range. Finally, recent direct measurements are reported on the rate constant for the neopentyl peroxy + hydroperoxy radical isomerisation. The results demonstrate an overestimate of the rate constant in previous indirect studies, which can be ascribed to the use of inaccurate thermodynamic data.
Journal of Molecular Structure, 2002
tive for systematic gaining of unknown so far values of the enthalpies of formation of free radicals. The following new or corrected values of the free radicals' enthalpies of formation were obtained (kcal mol 21 ): 2bicyclo[2.2.2]octyl z (18), PhCH 2 CH 2 z (54.5), BrCH 2 CH 2 z (33.5), ICH 2 CH 2 z (46.5), CH 2 yCHOCH 2 z (17), z CH 2 O-COCH 3 (254), z CH 2 CH 2 COOEt (263), z CH 2 OCOPh (224.5), z CH 2 OPh (26), H 2 NNHCH 2 z (65), NCCH 2 CH 2 z (60.5), O 2 NCH 2 CH 2 z (24.5), F 2 NCH 2 CH 2 z (19), 3-cyanocyclobutyl z (83) HSCH 2 CH 2 z (36), EtS(O)CH 2 CH 2 z (21.5), EtSO 2 CH 2 CH 2 z (254), z CH 2 PH 2 (41.2), z CH 2 At (68), H 3 ECH 2 z (39.5, 57.5, 77.5, 97) (E Si, Ge, Sn, Pb, respectively). The DH f 0 values for z CH 2 SH and Me 3 CCH 2 z free radicals 42 and 5 kcal mol 21 , respectively, were derived by using all three procedures as compared with earlier found values of 36.3 and 8.7 kcal mol 21 , respectively. From the known enthalpic shift DDH f 0 7.2 kcal mol 21 for RCHyCH 2 ! RPh replacement and the regularity in thermochemistry of RCH 2 z /RCHyCH 2 species, the new or corrected values of the molecules' enthalpies of formation were found (kcal mol 21 ): BrCHyCH 2
J Am Chem Soc, 1989
. W e a r e grateful for the support of the National Science Foundation and the Petroleum Research Fund, administered by the American Chemical Society, as well as the contributions of D. B. Palladino and J. Feilong to early and related studies, respectively. The support of the DAAD and the hospitality of Prof. Dr. W. Siebert a t Heidelberg where the seeds of these ideas were planted a r e also gratefully acknowledged. W e thank Prof. T. A. Albright for information in advance of publication and appreciate the comments of a referee. Supplementary Material Available: Listing of thermal parameters and hydrogen atom positions and the packing diagram (6 pages); listing of observed and calculated structure factor amplitudes (29 pages). Ordering information is given on any current masthead page. (46) Cromer, D. T.; Waber, Abstract: Absolute values for the rate constants of the metathesis reactions of DX (X = Br, I) with tert-butyl, generated by 351-nm photolysis of 2,2'-azoisobutane, were determined in a low-pressure Knudsen cell reactor using the VLPQ (very low pressure photolysis) technique. For X = Br, the values are 10-8k (M-' s-') = 0.9 and 2.3, and for X = I the values are 2.1 and 3.1 at 295 and 384 K. The latter are in good agreement with earlier measurements from this laboratory.
Journal of Physical Chemistry A, 2010
The rate coefficient for the reaction of the ethynyl radical (C 2 H) with 1-butyne (H-C≡C-CH 2 -CH 3 ) is measured in a pulsed Laval nozzle apparatus. Ethynyl radicals are formed by laser photolysis of acetylene (C 2 H 2 ) at 193 nm and detected via chemiluminescence (C 2 H + O 2 → CH (A 2 Δ) + CO 2 ). The rate coefficients are measured over the temperature range of 74-295 K. The C 2 H + 1-butyne reaction exhibits no barrier and occurs with rate constants close to the collision limit. The temperature dependent rate coefficients can be fit within experimental uncertainties by the expression k = (2.4 ± 0.5) × 10 -10 (T/295 K) -(0.04 ± 0.03) cm 3 molecule -1 s -1 . Reaction products are detected at room temperature (295 K) and 533 Pa using a Multiplexed Photoionization Mass Spectrometer (MPIMS) coupled to the tunable VUV synchrotron radiation from the Advanced Light Source at the Lawrence Berkeley National Laboratory. Two product channels are identified for this reaction: m/z = 64 (C 5 H 4 ) and m/z = 78 (C 6 H 6 ) corresponding to the CH 3 -and H-loss channels, respectively. Photoionization efficiency (PIE) curves are used to analyze the isomeric composition of both product channels. The C 5 H 4 products are found to be exclusively linear isomers composed of ethynylallene and methyldiacetylene in a 4:1 ratio. In contrast, the C 6 H 6 product channel includes two cyclic isomers, fulvene 18(±5)% and 3,4-dimethylenecyclobut-1ene 32(±8)%, as well as three linear isomers, 2-ethynyl-1,3-butadiene 8(±5)%, 3,4-hexadiene-1yne 28(±8)% and 1,3-hexadiyne 14(±5)%. Within experimental uncertainties, we do not see appreciable amounts of benzene and an upper limit of 10 % is estimated. Diacetylene (C 4 H 2 )
Atmospheric Environment, 1995
The structure-reactivity approach proposed by Atkinson (1986, Chem. Rev. 86, 69-201) and extended by Atkinson (1987, Inc. J. Chem. Kinet. 19,799~828) for the calculation of rate constants for the gas-phase reactions of the OH radical with organic compounds has been re-investigated using the presently available database. Substituent group factors for several new groups are derived, including those for fluorinated ethers. Using a large fraction of the available database to derive the parameters needed to calculate the OH radical reaction rate constants, the 298 K rate constants of-90% of approximately 485 organic compounds are predicted to within a factor of 2 of the experimental values. Disagreements between calculated an'd experimental rate constants most commonly occur for halogen-containing compounds, and in particular for haloalkanes, haloalkenes and halogenated ethers. Disagreements also arise for ethers, especially for polyethers and cycloethers. The present estimation technique is reasonably reliable when used within the database used in its derivation, but extrapolation to organic compounds outside of this database results in a lack of assurance of its reliability, and its use for organic compounds which belong to classes other than those used in its development is discouraged.
Kinetics and Thermochemistry of the Reaction of 1-Chloroethyl Radical with Molecular Oxygen
The Journal of Physical Chemistry, 1995
The kinetics of the reaction CH3CHC1+ 0 2 F?. CH3CHC102products (1) has been studied at temperatures 296-839 K and He densities of (3-49) x 10l6 molecule cm-3 by laser photolysis/photoionization mass spectrometry. Rate constants were determined in time-resolved experiments as a function of temperature and bath gas density. At low temperatures (298-400 K) the rate constants are in the falloff region under the conditions of the experiments. Relaxation to equilibrium in the addition step of the reaction was monitored within the temperature range 520-590 K. Equilibrium constants were determined as a function of temperature and used to obtain the enthalpy and entropy of the addition step of the reaction (1). At high temperatures (750-839 K) the reaction rate constant is independent of both pressure and temperature within the uncertainty of the experimental data and equal to (1.2 f 0.4) x cm3 molecule-' s-'. Vinyl chloride (C2H3C1) was detected as a major product of reaction 1 at T = 800 K. The rate constant of the reaction CH3CHC1 + C12 products (6) was determined at room temperature and He densities of (9-36) x 10l6 molecule cm-3 using the same technique. The value obtained is k6 = (4.37 f 0.69) x cm3 molecule-' s-'. An estimate of the high-pressure limit for reaction 1 was determined using this measured k6 and the kl/k6 ratio obtained by Kaiser et al.:l k"1 (T=298K) = (1.04 f 0.22) x lo-" cm3 molecule-' s-'. In a theoretical part of the study, structure, vibrational frequencies, and energies of nine conformations of CH3CHC102 were calculated using ab initio UHF/6-31G* and MP2/6-31G** methods. The theoretical results are used to calculate the entropy change of the addition reaction As0298 =-152.3 f 3.3 J mol-' K-'. Th~s entropy change combined with the experimentally determined equilibrium constants resulted in a CH3CHC1-02 bond energy m 2 9 8 =-131.2 f 1.8 kJ mol-l. The rooq-temperature entropy (S O 2 9 8 = 341.0 f 3.3 J mol-' K-') and the heat of formation (A H f o~9 8 =-54.7 f 3.7 kJ mol-') of the CH3CHC102 adduct were obtained.
The fate of thermally produced cyclohexyloxyl radicals as determined by mass spectrometry
Journal of the American Chemical Society, 1988
Tandem mass spectrometry combined with in-source radical trapping was used to follow in detail the fate of gas-phase cyclohexyloxyl-Z-d, radicals (lb) produced by the thermal decomposition of dicyclohexyl-a,a'-dz peroxide. Under the conditions of these experiments l b undergoes @-scission to produce w-formyl hexyl radicals. The w-formyl radicals are involved in reversible isomerization reactions except that isomerization to produce acyl radicals through a seven-membered transition state results in irreversible loss of carbon monoxide and production of primary pentyl radicals. Primary-to-primary and primary-to-secondary isomerizations were the lowest energy processes available to the pentyl radicals. Fragmentation of secondary pentyl radicals occurred at lower energy than fragmentation of the primary radicals.
Journal of the American Chemical Society, 1996
Two methods are described and illustrated for the measurement of organo-cobalt bond homolysis energies through reactions of tetra(p-anisyl)porphyrinato cobalt(II), (TAP)Co II• , with organic radicals of the form • C(CH 3 )(R)-CN in the presence of olefins. Thermodynamic values for bond homolysis have been determined directly for (TAP)-Co-C(CH 3 ) 2 CN (∆H°) 17.8(0.5 kcal mol -1 , ∆S°) 23.1 ( 1.0 cal K -1 mol -1 ) and (TAP)Co-CH(CH 3 )C 6 H 5 (∆H°) 19.5 ( 0.6 kcal mol -1 , ∆S°) 24.5 ( 1.1 cal K -1 mol -1 ) from evaluation of the equilibrium constants for the dissociation process (Co-R h Co II• + R • ) in chloroform. The bond homolysis enthalpy for (TAP)Co-C 5 H 9 (∆H°) 30.9 kcal mol -1 ) was determined indirectly by measuring the thermodynamic values for the competition reaction (TAP)Co-C(CH 3 ) 2 CN + C 5 H 8 h (TAP)Co-C 5 H 9 + CH 2 dC(CH 3 )CN (∆H°) 0.9 ( 0.3 kcal mol -1 ) in conjunction with a thermochemical cycle. This indirect approach was also used to evaluate (TAP)Co-CH(CH 3 )C 6 H 5 BDE (20.5 kcal mol -1 ) which agrees favorably with the value determined directly. When the Co-R bond homolysis enthalpies are known from independent evaluation, these equilibrium measurements provide a method for evaluating relative heats of formation of organic radicals. Application of this approach gives 40.8 kcal mol -1 for the heat of formation of • C(CH 3 ) 2 CN in chloroform. Success of these methods is dependent on fast abstraction of H • from the organic radicals by (TAP)Co II• to form (TAP)Co-H and rapid addition of (TAP)Co-H with olefins to form organocobalt complexes. Kinetic-equilibrium simulations utilizing reaction schemes for these processes provide an accurate description of the kinetic profiles and the equilibrium concentrations of solution species when the organic radical species achieve steady state.