RESEARCH
Shock tube yields reliable kinetic data Systematic deviations reduced when hydrocarbon decomposition rates are measured against a standard in the same reflected shock Kinetic data of considerable reliability and accuracy are emerging from a new way to measure unimolecular decomposition rates in a single-pulse shock tube. The method, devised by Dr. Wing Tsang of the National Bureau of Standards, Gaithersburg, Md., involves measuring the relative decomposition rates of two compounds—one a "standard" and the other the "unknown"—in the same reflected shock. (For more on shock tubes and gas dynamics, see page 76.) The NBS chemist has used this approach to study the kinetics of the thermal decomposition of several alky 1 halides. More recently, he has used the technique to obtain rate parameters for the thermal decomposition of hexamethylethane, 2,2,3-triethylbutane, and neopentane [/. Chem. Phys., 44, 4283 (1966)]. The single-pulse shock tube technique offers definite advantages in
APPARATUS. Dr. Wing Tsang adjusts shock tube before collecting data on decomposition of tertiary butyl hydrocarbons at about 1100° K. Since a run takes about 0.001 second, side reactions caused by molecules colliding with tube walls are minimized 40 C&EN JULY 18, 1966
studying the kinetics of unimolecular thermal decompositions, Dr. Tsang says. In a shock tube, the homogeneous heating and short reaction times eliminate the wall as a factor. Thus heterogeneous effects are not present. Also, the short residence time (about one millisecond) and the high dilution of the samples used (less than 1% by volume in argon) should minimize chain reactions. In the past, Dr. Tsang points out, use of the single-pulse shock tube to obtain accurate kinetic data was hampered by uncertainties in measuring reaction temperature and reaction time. Rate constants determined by the single-pulse shock tube technique were sometimes off by as much as a factor of two or three. Reduced deviations. In Dr. Tsang's standard compound approach, however, systematic deviations are greatly reduced. Errors in determining the reaction temperature are no longer significant since the temperature is not used to treat the experimental data. Also, the same dwell time is used in determining rate constants for the two compounds in the reaction mixture, so there is large-scale cancellation of errors. The NBS chemist's apparatus is patterned after that used at Cornell University by Dr. A. Lifshitz, Dr. S. H. Bauer, and Dr. E. L. Resler in their shock tube study of the cis-trans isomerization reaction of 2-butene. For analysis of reaction products, Dr. Tsang uses gas chromatography (GC) with flame ionization detection. Dr. Tsang's shock tube apparatus consists principally of a brass tube 2.54 cm. in diameter and 3.00 meters long. A cellophane diaphragm divides the tube's high-pressure section from its low-pressure section. A 30liter dump tank is connected to the low-pressure section. Function of the dump tank is to damp out the numerous reflected shock waves that are bounced successively off the end walls during an experiment. If unchecked, they would continually reheat the test sample. The damping action, together with the large volume of the dump tank, helps ensure that the sample used for analysis suffers a minimum of contamination from mixing of sample and
driver gas. Dr. Tsang uses helium as the driver gas for the low-temperature runs. For higher temperatures, he uses hydrogen. In a typical run, the dump tank is filled with argon (carrier gas) to the pressure at which the experiment is to be carried out. The test section (low-pressure section) is then evacuated to less than 1 micron of Hg. After filling the low-pressure section with the test gas ( 1 % or less of the standard and compound to be studied in argon), the ball valve joining it with the dump tank is opened. The high-pressure section is filled with the driver gas, the diaphragm ruptured, and a sample of about 10 cc. (room temperature and 1 atm.) of the test gas is taken for GC analysis. The 10 cc. used for analysis represents from 2 to 8% of the total test gas. In an early study, Dr. Tsang compared rates of decomposition of tertbutyl chloride and tert-butyl bromide with the decomposition rate of isopropyl bromide as a standard. The mechanism of thermal decomposition of these alkyl halides is unimolecular dehydrohalogenation. The rate constant for the decomposition of isopropyl bromide to propylene and hydrogen bromide has been determined by two groups of scientists. Dr. Arthur T. Blades and Dr. George W. Murphy of the University of Wisconsin use the toluene carrier method. Dr. Allan Maccoll and Dr. P. J. Thomas of University College, London, England, use a static method and work at about 150° K. lower than does the Wisconsin group. Both groups arrived at nearly the same Arrhenius parameters for the thermal decomposition. Thus, these parameters for isopropyl bromide can be considered well established, Dr. Tsang says. Two series. The NBS scientist carried out two series of experiments, one with dilute mixtures of isopropyl bromide and tert-butyl chloride and the other with mixtures of isopropyl and tert-butyl bromides. In each run, the GC analysis gave the percentage reaction for each of the components present. The dwell time was obtained from the pressure history. The rate constants were calculated from the expression k = 1/t In Ci/c f , where
!•«••••• •HHHMBH
flHHflHI
Illliilli^^
I^B^^^^^HlB(IHHIIHIillllllil
BBB^BIIIIBIIIII^BHll^BHI
IGHHHHB SERIES. Constants for the rate-determining steps in thermal decompositions of a homologous series of tertiary butyl hydrocarbons indicate that increased methyl substitution lowers the activation energy, but has little influence on the frequency factor. Rate constants for thermal decomposition of tertiary butyl halides show a rise in activation energy going from iodide to bromide to chloride. The activation energy for a butyl halide is about 2 8 % of its ion dissociation energy, according to Dr. Wing Tsang of the National Bureau of Standards
t is the dwell time, q and cf are the initial and final concentrations, and k is in sec. - 1 The logarithms of the pairs of rate constants were plotted against each other, giving straight lines. An unweighted least-squares analysis of these lines gave expressions relating log 10 k of isobutyl chloride and log 10 k of isobutyl bromide with log 10 k of isopropyl bromide. Assuming that the rate expression for the unimolecular dehydrohalogenation of isopropyl bromide is k = 10 13 - 62 exp ( - 4 7 , 8 0 0 / RT) sec. - 1 (determined by Dr. Maccoll and Dr. Thomas), the straightline plots obtained by Dr. Tsang show that the tertiary butyl compounds also decompose unimolecularly. The rate constant for the chloride is 10 13 - 74
e x p ( - 4 4 , 6 9 0 / R T ) sec.- 1 and that for the bromide is 10 13 - 87 exp ( - 4 1 , 4 9 0 / RT) sec. - 1 These results generally agree with those obtained by earlier workers who used static experimental methods at much lower temperatures, Dr. Tsang notes. In later work, Dr. Tsang studied the decomposition of several other alkyl halides by the relative rate method. He compared rate constants for the elimination of hydrogen halide from the parent molecule with the rate of elimination of hydrogen bromide from isopropyl bromide. Using the same values for the Arrhenius expression for the isopropyl bromide as in the work with ter£-butyl chloride and ter£-butyl bromide, Dr. Tsang obtained rate expressions for
the decomposition of ethyl bromide at 740° to 940° K.; ethyl chloride at 820° to 1000° K.; isopropyl chloride at 750° to 950° K.; isopropyl iodide at 680° to 850° K.; and tert-butyl iodide at 650° to 760° K. The experiments on the decomposition of alkyl halides generally confirm the results of earlier workers. Increasing alpha methylation causes a large increase in decomposition rates, he finds. This is due largely to the decrease in activation energy of about 6 kcal. per methyl group added. The NBS work confirms the existence of a rough linear relationship between the activation energy and the ion dissociation energy (RX—>R+-lX-). For example, the activation energies and the ion dissociation energies increase linearly going from ter£-butyl iodide to ter£-butyl bromide to tert-butyl chloride. Dr. Tsang's results indicate that the activation energy is about 2 8 % of the ion dissociation energy. Bond breaking. In a shock tube study of the pyrolysis of 2,3-dimethylbutane, Dr. Tsang found that the main decomposition reaction involves the breaking of the weakest carbon-carbon bond. In his most recent work he has made a systematic study of the thermal decomposition at about 1100° K. of a series of structurally similar compounds-hexamethylethane, 2,2,3trimethylbutane, and neopentane. In this homologous series, one tertiary butyl group was "held fixed" while the number of methyl groups on the neighboring carbon atom was reduced from three to two to none. One aim of this study was to find trends in rate parameters that would give insight into the nature of reaction mechanisms and permit empirical prediction of decomposition rates of other aliphatic hydrocarbons. The procedure in the hydrocarbon decomposition studies was the same as that in the halide studies except that 1% toluene was added to the argon carrier gas to act as a scavenger for free radicals. This minimized chain reactions produced by the radicals. Each compound was pyrolyzed in the presence of a standard. The standard was cyclohexene in most experiments; in a few it was 2,3-dimethylbutane (standardized with cyclohexene). Decomposition rates of both these standards had been previously determined by Dr. Tsang. He found that the main decomposition products of hexamethylethane are isobutene and smaller amounts of propylene and 2,3-dimethylbutene-2. In a given run, more than 90% of the decomposition product is isobutene. Since no alkane forms, Dr. Tsang concludes that the initial pyrolytic process (rate-determining step) inJULY 18, 1966 C&EN
41
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volves rupture of a carbon-carbon bond. This is followed by degradation of the newly formed radicals into the appropriate olefins—isobutene and a small amount of propylene. His calculations for the rate constant for the initial bond-breaking step give kx = 10 16 - 3 exp ( - 6 8 , 5 0 0 / R T ) s e c 1 In the decomposition of 2,2,3-trimethylbutane, the NBS scientist found that the concentrations of propylene and isobutene produced are about equal. His results therefore indicate that in the initial bond-breaking step, a tertiary butyl and an isopropyl free radical form. The tertiary butyl radicals can then form isobutylene or propylene. The isopropyl radicals can form propylene or ethylene. Rate constant for the compound's initial bond-breaking step is k 9 = 10 16 - 2 e x p ( - 7 3 , 0 0 0 / R T ) sec.- 1 " Less clear-cut. Neopentane's decomposition is less clear-cut, Dr. Tsang finds. The main reaction product is isobutene. Methane also forms, but its concentration is only two thirds that of isobutene. This indicates to Dr. Tsang i that the initial decomposition step is a bond-breaking one, producing tertiary butyl and methyl radicals. The butyl radicals form either isobutene plus a hydrogen atom or propylene plus a methyl radical. The rate constant for neopentane's bond-braking step is k 3 = 10 10 - 1 e x p ( - 7 8 , 2 0 0 / R T ) sec." 1 Dr. Tsang's results on the hydrocarbon series give a good picture of the effect on the rate constants for bond rupture when methyl groups are present in the alpha position. His data indicate that increased methyl substitution lowers the activation energy as he expected, but has little influence on the frequency factor. For all the hydrocarbons he has studied, Dr. Tsang has found that the frequency factors are about 10 16 .
Two new anthelmintics show promise Disclosure of Pfizer's new anthelmintic-pyrantel tartrate—and further details on a Belgian compound—tetramisole, which was revealed earlier this year—drew attention at the 10th National Medicinal Chemistry Symposium, held in sweltering Bloomington, Ind. Anthelmintics—drugs used to treat intestinal worms—are a growing segment of the veterinary chemical market. In the U.S. alone in 1965, value of producers' shipments of anthelmintics totaled about $11 million. Losses in livestock due to intestinal worms are between a quarter and a half billion dollars a year. Worldwide, the
potential market for anthelmintics is huge. Pfizer's compound, trade named Banminth, was developed in a joint effort by scientists at Pfizer Medical Research Laboratories at Groton, Conn.; Pfizer, Ltd., Sandwich, England; and the company's agricultural research and development department in Terre Haute, Ind. The Belgian compound (R 8299) was developed at Janssen Pharmaceutica, N.V., Beerse. Laboratory and extensive field trials in sheep show that Banminth has broad-spectrum anthelmintic activity, W. C. Austin of Pfizer, Ltd., told the symposium. Its action compares favorably with other anthelmintics now in use, he adds. Some additional preliminary testing with Banminth indicates possible usefulness in treating intestinal infections caused by roundworms in dogs, swine, cattle, and possibly man. Chemically, Banminth is trans-1,4,-
Pfizer's pyrantel (as the tartrate)
•C4W Janssen's tetramisbfe (as the hydrochloride)
5,6-tetrahydro-l-methyl-2-[2 - (2-thienyl) vinyl] pyrimidine hydrogen tartrate. Janssen's R 8299 is 2,3,5,6-tetrahydro- 6 -phenylimidazo[2,l-fo]thiazole hydrochloride. It's the most promising broad-spectrum anthelmintic in a long series of derivatives of 6-arylimidazo[2,l-&]thiazole, Dr. Paul A. J. Janssen told the medicinal chemists. It's active at low, atoxic oral, and parenteral dose levels against adult and immature gastrointestinal and pulmonary nematodes in 14 different hosts [J. Med. Chem., 9, 545 (1966)]. The compound has been tested and early results confirmed in diverse areas of the world, including Africa, Argentina, Australia, a number of western European countries, and the U.S.
