,Tunc, I961
RADIOLYSIS
OF
HYDROGEN CHLORTDE
SOLUTIONS I N CYCLOHEX-4NE
053
THE 7-RADIOLYSIS OF SOLUTIONS OF HYDROGEN CHLORIDE IN CYCLOHEXANE BY P. J. HORNER AND A. .J. SWALLOW T.I.Research Laboratories, Hinxton Hall, Cambridge, England
I\Tuck?aT Tech?ioloy~~ Laboratory, Department of Chemical Engineering and Cheniical Terhnology, Imperial College, London
S.W. 7, England
Receiied October 88, 1960
Solutions of hydrogen chloride in cyclohexane, when irradiated with y-rays, give hydrogen with G = 6.05 compared with G = 4.85 irom cyclohexane alone. When iodine is present (4 x lom4M )the yield for iodine removal is G = 7.0 atoms removed per 100 e.v., compared with G = 4.8 in the absence of hydrogen chloride. Solutions of hydrogen chloride also differ from pure cyclohexane in that they become yellow on irradiation, and give cyclohexyl chloride and a highly unsaturated material of high molecular weight. No cyclohexene is produced. Many of these results can also be obtained with ultraviolet light instead of y-rays. The facts cannot be explained by any existing theory, and it is concluded that excited molecules must be responsible for the effects.
The cyclohexane used for most of this work was a grade prepared specially pure for spectroscopy, obtained from British Drug Houses. The urity of this material was examined spectrophotometrical y using 10 cm. cells. Benzene and possibly some aliphatic unsaturation were present, but the concentration of benzene was below 5 X 10-4 M. For some experiments (including all irradiations with ultraviolet light) the cyclohexane was purified further by fractional distillation followed by passage through a silica gel column. This procedure removed all but slight traces of impurity, the hrnaene concentration being reduced by a factor of ten at least. The two grades of cyclohexane were dried by distillation from €',Ob in sztu. They gave identical yields of hydrogen on yirradiation, and both gave the same results in the iodine removal experiments. Cyclohexene was the purest avsi1:thle grade, and was purified further by distillation befort. me. Other chemicals, including iodine, ammonium chloride and sulfuric acid, were of reagent quality. Hydrogen chloride was prepared i n vacuo by tipping a weighed >mount of ammonium chloride into concentrated sulfuric acid. The hydrogen chloride was then distilled into the vessel containing the cyclohexane. Trans-
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fer into the vessel was quantitative as shown by titration. The concentration in the solution was estimated from the volumes of the cyclohexane and the vessel, using the solubility data of Wiegner.4 All solutions were thoroughly degassed to a pressure of about mm., and were sealed in all-glass vessels before irradiation. In the case of the solutions for ultraviolet irradiation, degassing was with an oil diffusion pump using silicone oil, t o avoid the possible presence of traces of mercury vapor in the system. The vessels used for the yirradiations were of three types. The type used for determinations of gas production consisted of a glass tube 14-15 mm. in external diameter, fitted with a break seal a t the lower end, and sealed off a t the top. The total capacity was about 12.5-13 ml. These vessels were filled with 12 ml. of cyclohexane. A second type of vessel was used for experiments avhere relatively large amounts of product were needed, as in the determination of cyclohexyl chloride and involatile residues. These vessels were also used for cyclohexene determinations. The vessels consisted of a glass tube approximately 125 ml. in capacity, but without a break seal. One hundred-ml. quantities were irradiated in these vessels. The third type of vessel, used for iodine scavenging experiments, was U-shaped. One limb consisted of a P ectrophotometer cuvette, the other of an irradiation 'cute. During yirradiation the cuvette was shielded with lead. The total capacity of the vessel was about 17 ml., the volume of cyclohexane used being 4 ml. Ultraviolet irradiation vessels consisted of a quartz cylinder 2.2 cm. external diameter and 2.2 cm. deep (light path 2.0 cm.), sealed on a tube (for filling) 15 mm. external diameter and 7.3 cm. long. A break seal was also included. Six-ml. quantities of cyclohexane were used in these vessels. ?-Irradiations were conducted with a 500 curie cobalt-60 irradiation unit constructcd by Nuclear Engineering Ltd. The Fricke dosimeter (in 0.1 N H2S04)was used to measure the dose, G being taken as 15.5.6 The dose in the system was obtained by multiplying the dose received in the Fricke dosimeter by the ratio of the electron densities of cyclohexane and water. Dose-rates were 400-4,000 rad. per minute. Ultraviolet irradiations were with a hydrogen discharge lamp made by Thermal Syndicate. The quartz irradiation cylinder was placed flat against the window of the lamp in such a way that the liquid was irradiated directlv, without vapor or air between it and the lamp. Radiolytic gas was measured volumetricallv, special care being taken to remove all gas from solution. After nieasurement of total volume the gas was transferred to a rising temperature gas chromatographic apparatus containing an alumina column with hydrogen as carrier and a cathetometer for detection. Gases other than hydrogen were detectable in this may, the remainder of the radiolytic gas being taken to be hydrogen. Cyclohevvl chloride was identified by comparison with authentic samples using gas phase chromatography with n-octane as marker. It was estimated in the same way after a preliminary concrntration by distillation of the irradiated solution. Some loss occurred a t this stage and was allowed for in the final calcuiation. 10-
(1) R. R. Williams and W. IT. Hamill, Radiation Res., 1, 168 (1954). (2) W. West, Trans. Faraday Soc., 28, 688 (1932): W. West and W. E. Miller, J . Chem. Phys., 8, 849 (1940). (3) H. A . Denhurst, .I Phvu Chem., 63, 813 (1959).
(4) F. Wiegner, Z . Blektrochem.. 47, 163 (1941). (A) J. L. Haybittle, R. D. Saiindrrs and A. J . Swallow, .I. Chem. Phys., 25, 1213 (195fi).
Very little is known about those stages which are intermediate between the passage of an ionizing particle through matter and the formation of free radicals and molecular products. An attempt to study these stages was made by Williams and Hamill' who irradiated solutions of various substances in hydrocarbons. Williams and Hamill found that the minor component, for example methyl iodide, became involved in reaction, and attributed this to the capture of electrons by the solute. However as Williams and Hamill pointed out, many of their results can also be explained in other ways, for example transfer of positive charge may explain some of the results, while transfer of excitation energy from solvent to solute, as in the photochemistry of similar systems,2 may be occurring in other cases. A simple system of the type studied by Williams and Hamill has now been selected for further study. Cyclohexane was chosen as solvent because its radiation chemistry is relatively simple3 and because its physical properties are well understood. Hydrogen chloride was chosen as solute because it played a special role in Williams and Hamill's experiments, and because its ionization potential and other properties are known, and stand in a favorable relationship to those of cyclohexane. Experimental Methods
P
954
150
1
P.J. HORKER - 4 A.~J. SWALLQW
0 0
1 2 3 Dose ( lo8 rad.). Fig. 1.-Production of hydrogen in ?-irradiated cyclohexane: X, with HCl; 0, no HCl.
4 1
5 10 Dose ( lo4 rad.). Fig. 2.-Rate of removal of iodine in ?-irradiated cyclohexane: X, with HC1; 0, no HC1. 0
cline concentrations were measured spectrophotometrically using a Unicam SP600 spectrophotometer. The molar extinction coefficient of iodine in cyclohexane a t 520 mp was taken to be 940.B Ultraviolet absorption curves were measured with a Unicam SP500 spectrophotometer. Infrared measurements were done in the University Chemical Laboratory, Cambridge, by kind permission of Dr. N. Sheppard. Organic microanalyses were performed in the microanalytical laboratory of the Chemistry Department, Imperial College.
Results The production of hydrogen in y-irradiated cyclohexane is shown in Fig. 1as a function of dose. KO gas other than hydrogen is produced, either in the absence or in the presence of hydrogen chloride. For pure cyclohexane the hydrogen yield calculated from Fig. 1 is G = 4.85. This is significantly lower than G = 5.85 as found by Burton, Chang, Lipsky and Reddy,? but does not differ appreciably from G = 5.2-5.4 as found by most other workers.8
VOl. A 3
The hydrogen yield for cyclohexane containing 0.20 M hydrogen chloride is calculated from Fig. 1 to be G = 6.05. Dissolved iodine (lo+ M ) reduced the hydrogen yield from pure cyclohexane by 10-15% (dose -2 X 1019 e.v./g.) in agreement with results by other w0rkers.7~~Iodine produced a similar decrease from cyclohexane containing 0.20 Af hydrogen chloride. Cyclohexene (3 X M ) produced less than 5% change in hydrogen yield, either in the absence or in the presence of hydrogen chloride. The loss of iodine from a 4 X lo-&iM solution of iodine in cyclohexane is shown in Fig. 2 as a function of dose. For pure cyclohexane the yield calculated from the average of three such experiment is G = 4.8 f 0.5 atoms removed per 100 e.\’., compared with G = 5.6 f 0.3 as found by Fessenden and Schuler.lo These results are in fair agreement within the experimental error of the two determinations. Hydrogen chloride (0.016 M ) produces a marked increase in iodine consumption, the yield calculated from curves like those shown in Fig. 2 being G = 7.0 atoms removed per 100 e.v. It was noted that whereas pure cyclohexane yolutions remained almost colorless on irradiation, solutions containing hydrogen chloride became yellow. ,4 typical absorption curve of an irradiated solution of hydrogen chloride in cyclohexane (0.20 iM, y-ray dose -3 x 1019 e.v./g.) is shown in Fig. 3 together with the curve for a similar solution which had been irradiated with ultraviolet light (30 hours) instead of y-rays. Seither hydrogen chloride nor cyclohexyl chloride show any absorption in this region at the concentrations present, but it seems probable that the shape of both curves can be partly accounted for by the presence of dienes. Examination of y-irradiated solutions of hydrogen chloride in cyclohexane by gas phase chromatography showed cyclohexyl chloride to be the only irradiation product present with a boiling point less than about 150”. However, an unidentified high boiling material was also detected (“Polymer”). Attempts were made to measure cyclohexyl chloride formed a t low dose levels by hydrolyzing the irradiated solution with alcoholic potassium hydroxide and precipitating chloride as silver chloride, but these were unsuccessful, and the yield was finally estimated by gas phase chromatography for solutions (0.12 M HC1) given a y-ray dose of -2.5 X loz1 e.v./g. The result was G (cyclohexyl chloride) = 2.5 i 0.5. The “polymer” was estimated by evaporating the irradiated solution (-2.5 X loz1e.v./g.) to constant weight in an oven a t 120’. I n the case of pure cyclohexane this technique gave a yield for cyclohexane converted to polymer of G = 4.2 in excellent agreement with independent determinations of the same
(6) E. N. Weber, P. F. Forsyth and R. H. Schuler, Radiation Res., 3 , 68 (1955) ; we have confirmed this figure. (7) M. Burton, J. Chang, S. Lipsky and M. P. Reddy, zbid., 8 , 203
(8) R. H. Schuler and A. 0. Allen, J . A m . Chem. Soc., 7 7 , 507 (1955); G. E. Adams. J. H. Baxendele and R. D. Sedgnick, J . Phys. Chem., 6 3 , 854 (1959); W. S. Guentner, T. J. Hardwick and R. P. Nejak, J . Chem. P h y s . , SO, 601 (1959); E. S. Waight and P. Walker, J . Chem. Soe., 2225 (1960); G. R. Freeman, J . Chem. Phus., 33, 71 (1960). (9) R. H. Bchuler, J . Phys. Chem., 61, 1472 (1957). (10) R. W. Fessenden and R. H. Schuler, J . ,4m. Chem. SOC.,79,
(1958).
273
(1957).
.June, l!)lil
Y-RAD~OLYSIS O F HYDROGEN CHLORIDE SOLITTIOW IS CYCLOHEXAXE
cluantity," and with G = 2 0 for dicyclohexyl (z.e., G = 4.0 for cyclohexane converted) in the electron irradiation of c y c l o h e ~ a n e . ~I n~ ~the ~ caqe of irradiated cyclohexane-hydrogen chloride wlutions (0.12 M ) the measured yield for loss of cyclohexane to give polymer was G = 2.8, but the product (dark in color) was later found to absorb oxygen rapidly from the air, so that the true yield must be somewhat less than this. Elementary analysis of a sample of this polymer evaporated to dryness in vacuo showed its empirical formula to 1)e CeH7.sClo.l. Irradiated solutions were examined by infrared bpectroscopy for the presence of cyclohexene, vhich absorbs a t 13.95 I.(. Irradiated pure cyclohexane absorbed a t 13.95 p consistent with publiihed results, but irradiated cyclohexane-HC1 (0.12 M, 2.5 X 1021e.v./g.) showed little or no aixorption at this wave length. From these experiments it is concluded that the yield of cyclohexene in the cyclohexane-HC1 system must be less than G = 0.4, and may be zero. Measurements were made of the yield of hydrogen from cyclohexane irradiated with ultraviolet light. The results for pure cyclohexane were much leis reproducible than for the 7-irradiations, posiihly because small traces of impurities play an important part by absorbing light. It was found that the presence of hydrogen chloride (0.04 M ) doubled the yield of hydrogen from cyclohexane. Complete spectroscopic data are available for hydrogen chloride,I3 but although it is known that cyclohexane absorbs little light above 1700-1750 A.,14 the actual extinction coefficients in this region do not appear to be known. Hence it is iniposiible to be certain whether the increased hydrogen yield is caused by the direct absorption of light by hydrogen chloride, which then splits into atoms which @.e molecular hydrogen, or is caused by iome other effect. I n other experiments with ultraviolet-irradiated qolutions it was noted that the solutions containing hydrogen chloride became yellow (Fig. 3), whereas the pure cyclohexane remained almost colorless. .Also cyclohexyl chloride was detected by gas phase chromatography in the irradiated solutions containing hydrogen chloride. Discussion Williams and Hamill found that hydrogen chloride (0.23 M ) inhibited the formation of methyl radicals from methyl iodide dissolved in cyclohexane. Our results confirm that hydrogen chloride can exert a powerful effect on a radiolytic reaction, and show that the yield and even the nature of the products from cyclohexane are quite different in the presence of HC1. The main products become cyclohexyl chloride (G = 2-3)) unsaturated "polymer" (G-cyclohexane = 2-3), and
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(11) J. Lamborn and A. J. Swallow, in preparation. (12) With y-rays a t low dose rates, as in the present experiments, the polymer would not be pure dicyclohexyl (H. A. Dewhurst and R. H. Schiller, J . A m . Chem. Soc., 8 1 , 3210 (1959) but the yield should still be close t o G = 4 as found here. (13) J. Romand, Ann. Phys.. 12th series, 4, 529 (1949). (14) G Scheibe and H. Gi-ieneisen, 2. phyaak. Chem., 86, 52 (1934), L W Pickett. hf Muntz and E M. Mopherson, J . A m . Chem. Soc., 7 3 , 4862 (1951)
l.O
955
t\ 250
300 350 400 Wave length (mp). Fig. 3.-Absorption spectra of yellow colors produced by irradiation of cyclohexane-HC1: a, ultraviolet; b, -prays.
hydrogen (G = 6.05). The rough material balance is reasonably good. I n seeking an explanation of our results it should be noted that several phenomena which are often invoked to explain results in radiation chemistry cannot be occurring in the present system. First direct action. If, in the absence of full information, we assume the importance of direct action to be proportional to the electron fraction of the solute, then in the most concentrated solutions studied here, direct action on hydrogen chloride, even if 100% efficient a t causing homolytic fission, could only give products a t the rate of 0.43 molecule per 100 e.v. absorbed in the whole system (assuming each atom produces one molecule of product). The actual amount of change due to direct action will be very much less than this. Secondly, the ionization potential of cyclohexane is 11.0 e.v. and of hydrogen chloride 12.8 e.v. so that transfer of positive charge cannot make a significant contribution. Thirdly it does not seenz possible to explain the results on the assumption that hydrogen chloride does not affect the primary radiation act on cyclohexane, but does enter into subsequent free radical reactions. I n particular, the activation energy for the reaction H
+ HC1+
Hz
+ C1
is low enough for this reaction to be favored over the corresponding abstraction of hydrogen from cyclohexane, but the chlorine atom so produced would abstract hydrogen from cyclohexane rather than enter into other reactions such as addition to the double bond of cyclohexene or combination with a cyclohexyl radical. Hence the over-all reaction should be unaffected by hydrogen chloride. Fourthly the results of Leprince and Limido'j show that hydrogen chloride does not react with cyclohexene in the absence of a catalyst such as stannic chloride, so that this reaction cannot be playing a part in the present system. Magee and Burton have concluded,16 and Williams and Hamill have implied,l that hydrogen chloride can capture electrons in irradiated systems presumably according to e
+ HC1+
H
+ C1-
From the hydrogen-chlorine bond strength and the electron affinity of the chlorine atom, this reaction (l,5) P. Leprince and J. Limido, Compt. rend., 244, 2044 (1957). (16) J. L. Magee and X. Burton, J . 4 m . Chem. Soc., '73, (1951).
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956
G. CZAPSKI, J. JORTNER AND G. STEIN
Vol. 65
is endothermic to the extent of 0.6 e.v., and so could phase. I n the liquid phase it is possible that eleconly occur before electrons have slowed down to tron capture by LLsolvated”hydrogen chloride thermal energies. From early experiments in the might be associated with the formation of molecular gas phase, the probability of electron capture in hydrogen in the reaction this reaction reaches a maximum a t 0.5 e.v.17 but e C6H12.HC1-+ C6Hll Hz c1more recent work indicates that the probability is a t a maximum a t a slightly higher value.l* The but this reaction is endothermic to the extent of 0.1 recent data are more extensive, and in better agree- e.v., and the above argument still applies. The ment with the thermochemistry of the system. To conclusion that hydrogen chloride does not capture calculate the yield for electron capture we may ac- electrons in the present system is reinforced by our cept provisionally the figure of 5 X for the results with ultraviolet light of a wave length which maximum probability of capture a t a c~llision.’~could not produce ionization. The effects obtained For simplicity of calculation we can further assume are very similar to those with y-rays, showing that that the capture probability is 3 X in the the y-ray result should be explicable without posturange 1.5-0.5 e.v., and zero outside this range. lating electron capture. In viev of the above process of elimination we Now if we assume with Mageels that an electron loses about 201, of its energy per collision (the exact are inclined to conclude that the radiolysis prodvalue depends on the medium, but no value is ucts must arise in some way from the reactions of available for cyclohexane), then the electron collides excited molecules. Unfortunately no reactions about fifty times in slowing down from 1.5 to 0.5 seem to have been reported in the literature which e.v. For the highest hydrogen chloride concen- could provide a precedent for the present case. tration employed in our experiments, about 1%, The simplest reaction scheme would be the number of collisions with HC1 would be 0.5. C6H12 C6H12* Hence the probability of a given electron being CsH,z* HC1 ---f CsHnC1 HI captured by hydrogen chloride is a t most 1.5 X H C1 Assuming a yield of G = 3 for the producCeHiz* CaH8 2H2 tion of electrons, the yield for electron capture by but this is highly speculative and further work is HC1 is G < 4.5 X which is negligible. The required. It may well be that similar reactions of principal uncertainties in this calculation are the excited molecules are occurring in other systems, for the cap- for example those studied by Williams and Hamill, assumption of the values of 3 X ture probability and of 2% for the percentage but that knowledge of their existence has so far energy loss per collision, but these values between been obscured by other possibilities. them would have to be low by a t least a factor of Acknowledgments.-The authors wish to thank 10-3 to affect the argument, and this seems unMr. W. H. T. Davison and Mr. D. G. Lloyd for likely. The argument so far applies to the gas their cooperation in this work, and many colleagues (17) N. E. Bradbury, J. Chem. Phys., 2 , 827 (1934); H. S. W. Masfor helpful discussions of this puzzling system; sey, “Negative Ions,” 2nd edition, Cambridge, 1950, p. 76. also the Chairman of Tube Investments Ltd. for (18) R. E. Fox, J . Chem. Phys., 26, 1281 (1957). permission to publish this paper. (19) J. L. Magee, Ann. Reu. Nuclear Sei., 3 , 171 (1953).
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+ +
+
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THE MECHANISM OF OXIDATION BY HYDROGEN ATOMS I N AQUEOUS SOLUTION. I. MASS TRANSFER AND VELOCITY CONSTANTS BY GIDEONCZAPSKI,JOSHUA JORTNER AND GABRIEL STEIW Department of Physical Chemistry, Hebrew University, Jerusalem, Israel Receked October 31, 19130
The conditions of the oxidation of various scavengers in aqueous solution by atomic hydrogen introduced from the gas phase were investigated experimentally and theoretically. Approximate equations are derived for the kinetics in the case of purely diffusion controlled mass transfer of H atoms and for the case of forced convection. First-order reaction with the scavenger in competition with second-order recombination, as well as consecutive scavenging mechanisms are considered. The results are compared with the treatment based on homogeneous kinetics.
I n the investigation’ of the reactions of H atoms with various scavengers atomic hydrogen was produced in the gas phase and introduced into the aqueous solution containing the scavenger. In this heterogeneous system we assumed, in view of the vigorous bubbling and stirring occurring under our experimental conditions, that homogeneous kinetics will provide a good approximation (1) (a) G. Ceapski and G. Stein, Nature, 182, 598 (1958): (b) G. Czapski and G. Stein, J. Phya. Chem., 63, 850 (1959); (c) G. Czapski, J. Jortner and G. Stein, ibid., 6 3 , 1769 (1959); (d) G. Czapski and G. Stein, ibid., 6 4 , 219 (1960).
t o the actual situation. I n the present paper this assumption is examined and the conditions under which our work is carried out is investigated both experimentally and theoretically. Mass transfer problems in heterogeneous systems are of great interest in several fields of chemical -~ kinetics. They have been i n v e ~ t i g a t e d ~mainly (2) L. L. Bircumshaw and A. C. Riddeford, Quart. Eev., 6 , 157 (1952). (3) T. K. Sherwood. “Mass Transfer between Phases.” Pennsylvania State University, 1959. (4) P. V. Danckwerts, Trans. Faraday Soc., 46, 300 (1950).
