Nov., 1961
2109
NOTES
Hz AXD C1-CT YIELDS FROM THE RADIOLYSIS OF %,%,4TRIMETHYLPENTANE BY J. A. KNIGHT,R. L. MCDANIEL,R. C. PALMER AND
FREDSICILIO
Radioisotopes Laboratory, Engineering Experiment Station, Georgia Institute of Technology, Atlanta, Georgia Keceizled June 16, 1861
Work current in this Laboratory is concerned with a detailed study of the radiation chemistry of branched hydrocarbons. Radiolysis products, hydrogen :md hydrocarbons through C,, from the X-irradiation (50 kvp.) of 2,2,4-trimethylpentaneJ have been identified and quantitative yields established by gas chromatography. Products with G 2 0.1, in order of decreasing yield, are hydrogen, isobutnne, isobutylene, methane, 4,4dimethyl-cis-pentene-2, propylene, neopentane, 2,2- and/or 2,4dimethylpentane, and propane. Experimental Materials.-The 2,2,4-trimethylpentane, Eastman, used in this work was percolated through activated alumina before use. The infrared spectra of the “treated” 2,2,4-trimethylpentane and of a sample of Phillips research grade niaterial were identical. Authentic hydrocarbon samples for identification of radiolysis products were obtained from Matheson, Phillips, or National Bureau of Standards. X-Ray Apparatus .--A Machlett OEG-60 end-window (beryllium) tube, housed in a lead-lined box, was used with a GE XRD-5 X-ray diffraction unit power supply with voltage stabilizer. All irradiations were performed a t 50 kvp. and 50 ma., at a dose rate of 8.2 X 10’7 e.v./g./min. for the volumes of samples studied. Irradiation Cell.-Irradiations were performed in an openwindow stainless-steel cell with an inside diameter of 67 mm. and inside depth of 37 mm. The cell, supported by a magnetic stirrer on a laboratory jack, is sealed to the end of the X-ray tube with an “0” ring above the liquid level. Inlet and outlet tubes allow inert gas to be bubbled through the cell. The outlet tube is connected to a condenser ao that condensable vapors are returned to the cell. The cell is jacketed, and coolant water was used during these irradiations to maintain the cell a t approximately 20’. Irradiation and Sampling Technique for Volatile Radiolysis Products.-During the irradiation of 2,2,4-trimethylpentane, the sample W:LP stirred and helium gas a t a known flow rate in the range of 5 to 10 ml. per min. was bubbled continuously through the hydrocarbon. This served to constantly remove volatile radiolysis products-hydrogen, rieopentane anti the C, through Cahydrocarbons. The exit helium gas, :i,loug with the volatile radiolysis products, passed through ’/4-incli o.d. copper tubing to a sampling loop of :L solciioid-oper:tted gas sampler’ connected to a gas chromatographic unit. Thus, it was possible periodically to subject the helium sweep gas to chromatographic aiialysis. In the analysis for hydrogm and methane, tlic sweep gas, after leaving thr: irradiation cell, was passed through a cold trap containinx 3 nil. of 2,2,+trimethylpentane, :it a temsteady state conditions were a t perature of -78”. t:iined, hydrogen and methane passed through the trap. Thc sweep g : ~ sthen w‘its passed through the g:is-sampliiig loop. In tlic an:tlysis for volatilc ratliolysis products other than hytirogvii aiicl Incthnnc, the cold trap w:tY omitted. Hydrocarbon Radiolysis Products in the C& Range.S:Lmplrs of tfi c: irratli:t t (:ti 2,2,4- t riinet hylprntarie mero fmctiorially distillot1 with :I 1.2 by %-an. colrimri packed with Podbicluiak IIeli-I’ak 3013 stainless steel packing. TIE distillations were performed very slowly in order to concentr:ite the radiolysis products of cnrbon content, less than Cs in the initial fractions. Fractions of 1 ml. or lcss were taken, and chromatograms obtained on each fraction. The chrol the radiolysis products of cnrmatograms showed thttt, : ~ l of bon content less than CS were coritaincd in the first three fractions. (1) R. C . Palmer, D. K. Davis and W. V . Willis, Anal. Chem., 32, 884 (1960).
Analytical Work.-Hj drogen and methane were identified and determined quantitatively with a 50-ft. column of Linde 13X Molecular Sieve, 20 to 40-mesh. Other hydrocarbon radiolysis products were identified with two columns, one a 50-ft. column of 20% (by weight) tri-m-cresyl phosphate on 30 to 60-mesh Chromosorb and the other a 50-ft. column of 207” dimethylsulfolane on 30 t o 60-meah Chromosorb. The gas chromatographic unit was calibrated for thermal response to hydrogen and to each of the hydrocarbon radiolysis products. Microliter syringes with fixed needles were utilized for injecting samples of known amounts for calibration, and sample sizes were used which corresponded to the ranges of the amounts of the radiolysis products. Peak areas were measured with a planimeter. Thermal response was linear in the range of sample volumes employed. Dosimetry.-The energy absorbed by 2.2,4-trimethylpentane was determined by polyethylene-ferrous ion dosimetry measurements,2 in which the dose received by a hydrocarbon is compared to the dose received by a corresponding quantity of polyethylene, in g./cm.2. The dose determined in this manner compares to within 5% of the dose received by an equivalent volume, as determined by electron density ratio, of Fricke dosimeter solution [G(Fef++) = 15.51.
Results and Discussion The radiolysis products, hydrogen and those hydrocarbons in the range Cl-C,, found in this investigation are listed in Table I. The data represent the results of some 30 individual irradiations. TABLE I RADIOLYSISPRODUCTS FROM 2,2,4TRINETHYLPENTANE (HYDROGEN AND C1-C, CARBON CONTENT) Producta
Gb
Hydrogen 2.2 Methane 1.1 Ethane 0.05 Ethylene .02 Propane .1 Propylenc .3 Isobutane 1.6 Isobutylene 1.4 Neopentane 0.2 Isopentane .05 Neohexane .02 .2 2,2- and/or 2,4dimethylpentane 4,&Diniethylpentene-l .04 4,4Dimethyl-trans-pentene-2 .06 4,4-Dime thyl-czs-pentene-2 .45 a Three additional products with low yields have been identified tentatively as: 2-methylpentane, G = 0.02; 4mcthylpentene-1, G = 0.003; 4-methyl-trans-pentenc-2, G = 0.005. Yield in molecules per 100 e.v., as determined by estrapolation to zm-0 dose.
Hydrogen and Methane.--Dewh~rst,~ in his work on branched chain alkanes, reported G-values for hydrogen and methane of 3.0 and 0.7, respectively, but did not report any other radiolysis products for 2,,2,4-trimethylpentane Our values of 2.2 arid 1.1 for G(H,) arid G(CI&), respectively, differ from the values reported by Dewhurst. However, he employed SO0 kvp. electrons and a sealed system whereas we used 50 kvp. X-rays with a dynamic system. Tolbert and Lemmon4calculated G(H2) and G(CH,) as being 1.9 and 0.82, ( 2 ) R C. Palmer, R. W. Carter and D. C. Bardwell, Intern. J . A p p l . Radzution and Iaotopes 9, 60 (1960) (3) I€. 4. Dewhurat, J . Am. Chem. Soc., 80, 5607 (1958). ( 4 ) B. &Tolbert I. and R. M. Lemmon, Radiafron Reusarch, 3 , 52 (1955).
NOTES
2110
respectively, from the data reported by Schoepfle and Fellows5 for the irradiation of 2,2,4-trimethylpentane with 0.17 mev. electrons. C1-C7 Radiolysis Products.-The radiolytic hydrocarbon products in the C1-C? range can be divided into two groups. Group I consists of those products that can be related to some structural portion of the carbon skeleton of the parent molecule, 2,2,4-trimethylpentane. There are ten of these products, with G-values ranging from 1.6 to O.O%, viz., in order of decreasing yield: isobutaiie, rsobu tylene , methane, 4,4-dimet hyl-cis-pentene-2, propylene, neopentane, 2,2- and/or 2,4dimethylpentane, propane, 4,i-dimethyl-transpentene-2, and 4,3-dimethylpentene-l. Group I1 consists of those products that cannot be related to the carbon skeleton of the parent molecule. There are seven of these products, all with low yields, with C-values ranging from 0.05 to 0.003, viz., in order of decreasing yield : ethane, isopentane, ethylene, neohexane, 2-methylpentane (probable) , 4-methyl-trans-pentene-2 (probable), and 4-methyl pentene-1 (probable). Inspection of the structure of 2,2.,4-trimethylpentane shows that there are 13 possible products that c:m be formed directly by cleavage of the carbon-c:wbon bonds, assuming t8heformation of alkanes or alkenes througheither uptake or elimination of hydrogen. Two of the possible products of cleavage wcire not found in this investigation, vix., 2,4dimetliylpentene-1 and -2. The hydrogen and methane yields are linear up to a total dose of 1021e.v./g. The yields of propane, propylene, isobutane, isobutylene, neopentai Le, 4,4-dimethyl-trans-pent ene-2 , 4,4-dimethyl-cis-pentene-2, and 2,2- and/or 2,4-dimethylpentane are linear up to a total dose of 3.5 x 1021 e.v. '8. The yield of the terminal olefin, 4,4-di1nethylpentene-l, is linear up to a total dose of 2 X loz1e.v.jg. and then decreases. Acknowledgment.-This work was supported in part bg the United States Atomic Energy Commission, Division of Research, Contract No. AT (40-1)--2490. ( 2 ) C S SchoepHe and C . H. Fellows, Ind. Eng. Chem., 23, 1396 (1931).
ON POLSROGRAPHIC ,4ND COULOMETRIC INVESTIGATIONS OF T H E REDUCTIOS RATE OF COBALT IOh'S IhTT H E PRESENCE OF SOME AMISO ACIDS AXD PROTEINS BY E. B. WERONHKI Department of Chemtstry, Unzverstty of Warszama, U'arszaua, Poland Recezred M a y 8 , 1961
In :irecently published paper by Shinagalv-a, et al., the authors reported results obtained by
Vol. 65
comparing the radioactivity-potential curves of cobalt Co60, in the presence of cysiine and histidine in Brdicka's electrolyte, with corresponding current-potential curves. In the case of histidine they came to the conclusion that the enhancement of current is due to an increase in the reduction rate of cobalt(I1) ions rather than to the catalytic reduction of hydrogen. This conclusion is identical with that reported in our previous paper2 concerning the reaction in the presence of cystine. However, in the case of cystine the results' lead to a different conclusion. I n our experiments we compared two decreases in currents of cobalt(I1) in Brdicka's electrolyte, measured polarographically in the presence of cystine. One decrease was measured at - 1.35 v. after coulometric electrolysis carried out a t the same potential (at which the curve of cobalt was assumed to be uiidistorted by the amino acid). The second decrease mas measured under similar conditions a t - 1.65 v. (ie., a t the top of the catalytic maximum of the current-potential curve). Although the half-wave potentials of the currentpotential curves of cobalt presented by Shinagawa and eo-workers were not in all cases identical with those of the radioactivity-potential curves (the reason has not been elucidated by the authors), the radioactivity-potential curves should be compatible with our results provided that the possible influence of ionizing action of cobalt-Co60 on the electrode reaction may be neglected. To explain the discrepancy we examined their and our results in different ways. It v a s found that if the coulometric electrolysis of Brdicka's electrolyte is carried out during the same period of time and under identical conditions in the absence or in the presence of cystine a t -1.35 v., the decrease in the current in the absence of cystine is several times greater than in the presence. These results suggest, despite the previously made assumption, that during the electrolysis a t - 1.35 v. the current-potential curve is increasingly affected by cystine. To avoid errors in explaining the results of similar measurements in the presence of other amino acids or proteins, the changes of current after coulometric electrolysis should be compared both in the absence and in the presence of amino acids or proteins using the previously described techniques.2 Then the results obtained with the radiometric and coulometric methods are and should be compatible also in other cases provided that the influence of the ionizing action of cobalt-Co60 on the electrode reaction may be neglected. (1) .\I. Sliinapaaa, H . Nezil, H. Aunaliwa, F. Nakaslii~iia,H. Okasliita and T. Yaniailn. Proc. 11. Intern. Congr. Caiiibridge 1959, in "Advances in Po1arogral)hy" (Ed. by I. S. Longuiuir), Perganion Press, London. 1960, Vol. 3 , g . 1142. ( 2 ) E. B. Weronski. J . r h u s . Chem., 66, 561 (1961).
