THE JOURNAL OF
PHYSICAL CHEMISTRY (Rsgistsred in U. 8. Patent Office) (0 Copyright. 1963, b y the American Chemical Society)
MARCH15, 1963
VOLUME67, NUMBER3
THE EFFECT 01F BENZENE MODERATOR ON THE (n,y) ACTIVATED REACTIONS OF BROMINE ATOMS WITH ETHYL BROMIDE BY MIRIAMMILMAN Ecole Xormale Supe‘rieure, Laboratoire de I’Acce‘ldrateur Line’aire B.P. 5, Orsay (Seine et Oise), France Received January 11, 196‘8 The nature of the chemical reactions following neutron capture in mixtures of ethyl bromide and benzene were studied. After having ascertained the own reactivity of the benzene moderator, it was possible to determine the contribution of c‘hot” and “diffusive” reactions t o the yield of organic activity. A tentative description of the nature of reactions following radiative neutron capture in the solid system is proposed.
Introduction A nucleus absorbing a thermal neutron emits the excess energy in the form of one or more yrays. As a result of the radiative emission the atom acquires recoil energy, positive charge due to internal conversion or Auger electron emission, and also can be left in an ex-, cited state. The activated atom can enter stable chemical combinations after having lost some of its excess energy in successive collisions with the surrounding molecules; the experimental evidence seems to point out’ that if the medium is closely packed (liquid or solid phase) charge and excitation energy also are lost in this process. The atoms having ai kinetic energy higher than the thermal energy of the environment are called “hot atoms” and their chemical reactions are known as “hot reactions”; they have the following characteristic features which distinguish them from the reactions of the same atom a t thermal energies: (a) Their yield is temperature independent (b) Since they take place after only a limited number of collisions of the recoiling atom, their yield is not affected by small concentrations of scavengers added to remove thermalized free radicals. The use of scavengers in the study of the chemical effects of nuclear transformations helped to give a measure of the yield of reaictions taking place in the immediate vicinity of the capture event and of those taking place further away, after the active atom and free radicals start diffusing (“diffusive reactions”).2 (c) Hot reactions are sensitive to additives (moderators) that remove the kinetic, vibrational, or electronic energy of the excited species. (1) M. Milman, Radiochim. Acta, 1, 15 (1962). (2) (a) F. 9. Rowland and W. F. Libby, J . Chem. Phys., a i , 1495 (1953); (b) L. Friedmanand W. F. Libby, tb$d., 17, 647 (1949); ( 0 ) F. Goldhaber. R. 8. Chiang, a n d J. E. Willard, J . Am. Chem. floc., 7 3 , 2271 (1961); (d) G. Levey and J. E. Willard, ibzd., 74, 6161 (1952); (e) N.Milman and P. F. D. Shaw, J . Chem. SOC.,1303 (1!257); (f) N. Knight, G. E. >Idler, and P. F. D. Shaw, J . Inorg. Nucl. Chem., 2 3 , 15 (1961).
Recent studies3of the hot hydrogen reactions in gaseous hydrocarbons have led to the development of a model that was successfully applied to tritium and methane reactions moderated by inert gases, and also was used4 to estimate the contribution of reactions due to the kinetic energy of hot bromine and iodine in methane. Unless inert gases are used as moderators, the reactions of the hot atom with the additive further complicate the interpretation of the results. The work described in this paper was undertaken to investigate the influence of the addition of benzene moderator on the organic yield in liquid and solid ethyl bromide. Experimental Materials.-Ethyl bromide (chemically pure, Touzard and Matignon) was purified as described previously.6 The retention (percentage of the total activity in organic combinations) of 32%, now generally admitted for neutron irradiated ethyl bromide in the presence of about IO+ molar fraction of bromine, was used as a criterion for the desired purity of the material. The benzene (Merck p.p.a.) was stored in daylight with elementary bromine for about 30 min., to saturate olefinic impurities. To avoid extensive addition of bromine to benzene this treatment must not be prolonged, and further processing is necessary. After extraction of excess bromine with aqueous bisulfite and drying over magnesium sulfate, the benzene was shaken with successive portions of concentrated sulfuric acid (until the acid remained colorless) followed by extractions with sodium bicarbonate solutions and water. The material was dried over magnesium sulfate, filtered, and distilled in a column (50-cm. length, 1.5-cm. diameter) packed with glass helices. The middle 5001, was retained and passed dropwise (directly from the condenser attached to the distillation column) through a chromatographic column (50-cm. length, 1.5-cm. diameter) filled with silica gel (chromatographic grade). The benzene obtained in this way was recrystallized from an ice bath, discarding about 15%. (3) P. J. Estrup and R. Wolfgang, J . Am. Chem. Soc., Sa, 2661, 2665 (1960). :1(4) (a) E. P. Rack a n d A. A. Gordus, J . Chem. Phys., 34, 1855 (1961); (b) J. Phys. Chem., 65, 944 (1961).
537
MIRIAMMILMAS
538
I
1 oJ
1
'1 0 MOLAR
I
10" F R A C T I O N OF B R O M I N E .
Y 1
Fig. 1.-Scavenger effect of bromine on the reactions of BrBO activated by the (n,-,) process with benzene (semi-log plot).
MOLAR
~ R A C T I O NO F B R O M I N E
Fig. 2.-Scavenger effect of bromine on the reactions of Br80 with: 0, benzene (liquid); A, benzene (solid); 0 , hexane (liquid); 0 , cyclohexane (liquid) The hexane and cyclohexane were purified using the same procedure as for benzene. Neutron Irradiations and Determination of Organic Yields.The irradiations were performed with a 1.5-curie poloniumberyllium neutron source. The geometry and experimental procedure after irradiation have been described in an earlier work.5 The samples were irradiated for 40 min. and only Brso (half-life of 18 min.) was studied; for longer irradiations, or for delayed counting of the activity, second measurements were made in order to account for the presence of any Br80'" (halflife of 4.4 hr.). The results were corrected for density variations and, when necessary, for dilution.
Results and Discussion Reactivity of Moderators.-It was found previously6 that the presence of pentane, hexane, heptane, and decane in neutron-irradiated ethyl bromide and iodide leads to an increase of the organic retention (in the order mentioned above), and that this increase was practically independent of the presence of scavenger. This was interpreted to mean that the higher the molecular size of the solvent the more effective is the caging of fragments in the recoil volume, hence the higher the yield due to "hot reactions".' On the other hand, the organic yield of bromine from irradiated carbon tetrabromide dissolved in ethyl alcohol was found in an early experiment to fall to zero at high dilutions.* This apparent contradiction shows the complexity of the problem and suggests that the sensitivity of the solvent to hot halogen atom attack has to be ascertained before drawing conclusions about mechanisms of moderation and dilution. (5) M. RIilman, J . Am. Chem. Soc., 80, 5592 (1958). (6) S Aditya a n d J. E. %-illaid, z b z d , 79, 3367 (1957). (7) T h e term "hot reactions" a s used in ref. 6 applies t o direct displacement reactions a s uell a s t o the thermal recombination reactions of t h e tagged a t o m with radicals it has formed in the sloning d o u n process. For a detailed discussion of this interpretation see ref. 1. ( 8 ) W. F. Libby, J. Am. Chem. Sac., 62, 1930 (1940).
T'ol. 67
In order to determine the specific reactivity of the moderators, the scavenger curves (the organic yield as a function of bromine concentration) for benzene, hexane, and cyclohexane were established, and are shown in Fig. 1 and 2 . X comparison of the behavior of the three hydrocarbons clearly shows that in the case of benzene there is no sharp fall of the retention for small concentrations of bromine scavenger. The retention obtained from the extrapolation of the scavenger ciirve to zero bromine concentration was taken as the extent to which the moderator reacts with halogen in the absence of scavenger. Using the least mean squares method, the value obtained for benzene was R [liquid benzene] = 20.5 =!= 0.2%. Similar irradiations of quickly frozen mixtures of benzene and bromine gave an organic yield of 22.0 f 0.4% that remained constant (within experimental error) over the range of scavenger concentrations studied (Fig. 2 ) . In all the cases in which a radical scavenger is present during the irradiation of an organic bromide or iodide, small concentrations of additive cause a large drop of the organic retention, indicating its interference with the formation of radiohalides by diffusion controlled reactions. As the scavenger concentration is increased, the fall of retention is less abrupt and is attributed to the presence of the molecular halogen a t the very site of the capture I n the case of bromine and hydrocarbon mixtures, ai1 extremely small fraction of the halogen (about molar fraction) is activated by the capture process and the rest acts as an ordinary scavenger. I n consequence the scavenger curves of such systems should have the usual shape. This was found to be the case for bromine and hexane or cyclohexane mixtures and also for other hydrocarbon halogen ~ y s t e m s . ~As can be seen from Fig. 1 and 2 benzene seems to be an exception, as the retention is independent of bromine concentration over quite a big range (3 X lou3to 7 X 10-2), and falls linearly with higher concentrations of scavenger (10-1 to 1 molar fraction). The absence of a marked scavenger effect in benzene and bromine (or iodinegb) systems indicates that no organic radicals are available for diffusive reactions with the tagged atom. This can be due to the known stability of the benzene molecule; its low G-values are currentlylO attributed to a selfprotection effect characteristic of the highly resonant structure, so that any excitation energy is distributed immediately over the whole molecule without any decomposition. It is quite possible that benzene shows the same inertia to the collisions with hot halogen atoms. On the other hand it was found by Evans1' that 85yo of the organic retention of active bromine in benzene is due to the formation of bromobenzene, the rest being dibromo compounds. Hence, the only radical apparently involved is the phenyl radical (to a lesser extent the bromophenyl radical). This radical can be removed either by immediate recombination with the active bromine atom C6H5. Br* 4 C6H6Br* or by the reaction
+
Co&. (e)
+ C6& --+ C ~ H ~ . C B+HH~.
(a) C. E. RloCauley a n d R. Schuler, J . Phus. Chem. 62, 1364 (1958); (b) P. r. D. Shaw, private communication. (10) S.Gordon a n d 11. Burton, Dzscussfons Faraday Soe., 1 2 , 88 (1952). (11) J, B , Evans, Ph.D, Thesis, Wniverstty of D'ioconsin, 1957.
539
( n , ~ACTIVATED ) REACTIOXS OF BROMINE ATOMSWITH ETHYLBROMIDE
March, 1963
a type of reaction with is known to occur9b,12 only in aromatic systems. The result of the competition between the two reactions would be that the phenyl radicals would react to become diphenyl before having a chance to meet the thermalized active bromine atom. Effect of Moderator. (a) Liquid Phase.-The effect of benzene moderator on the retention of activity in liquid mixtures of ethyl bromide and benzene (in the presence of 3 x lop3 molar fraction of bromine) i s shown in the upper cuive of Fig. 3. To estimate the contribution of hot reactions to the total organic yield in benzene and ethyl bromide mixtures, it was necessary to establish the scavenger curves for a number of specific mixtures of the two components. The results are given in Table I. The part of the retention due to hot reactions is given1 by the intercept on the axis (at zero scavenger concentration) of the back-extrapolated scavenger curves a t high bromine concentration.2e These values as well as those obtained from the irradiation of ethyl bromide and benzene (at vanishingly small scavenger concentration) were corrected for the presmolar fraction of bromine during the ence of 3 x irradiation of the moderated systems studied and are plotted in Fig. 3, curve h.
--r-
I
~
0
0,2 M O L A R
94 0,6 F R A C T I O N OF
0,a
1
BENZENE.
Fig. 3.-Liquid phase: effect of benzene moderator on the reactions following the (n,y) process in ethyl bromide (3 X molar fraction bromine scavenger present): curve h, retention due t o “hot” reactions; curve d, retention due t o “diffusive” reactions. 80
t
-
A
0
1 0
“
o n
ox-----
TABLE I Molar fraction of benrene
Molar fraction of Rrz
x x x x ox 4 x 8x 5 x 0 x
Retention due Retention, to hot reactions,, % %”
0
6.0 2 4 3 7 5 0
10-4 10-1 10-1 10-1
30.0 14 0 11.4 9 3
18.4
1.8 x 10-1
6 2 3 4
10-4 10-1
19.2
10-1
29.0 14.7 12 0 10.6
10-4 10-1 10-1 10-1
28 0 15.0 12.2 10.8
19.7
2 4 x 10-1 4 . 0 X 10-l
14.9 12.5
20.0
3 7
7.8
x
x
10-1
10-1
6 2.4 X 3.9 x 4.5 x
10-1
tt
I0
2 . 5 X 10-1 15.2 20 56 4 0 x Io-’ 12 2 See a Determined using the back-extrapolation method.ae also Fig. 1 and 2. 1
For a system of t m components it is probable that the retention due to thermal diffusive reactions should he a sum of two independent terms
R (diffusive) -- mol. fr.l.R1
2o
+ mol. fr.*.Rz
where RI and RBare the diffusive retentions for the pure substance and mol. fr.Land mol. fr.2 their respective molar fractions. It was shown in the first paragraph of the Discussion that in the case of benzene there are hardly any thermal diffusive reactions. Consequently we expect the ther-. mal retention in a mixture of ethyl bromide-benzene to be a function of the concentration of ethyl bromide only. I n other words benzene acts as a diluent and the yield of such reactions will be proportional to (1 molar fraction of benzene). This was verified to be the (12) (a) For review see D. R. Augood and G. H. TV liiams, Chsm. Rev., 67, 123 (1957): (b) W. A. Waters, “The Chemistry of Free Radicals,” Second Ed., Oxford Clarendon Press, 1950, p. 149
I
I
I
44
0’2 MOLAR
I
FRACTION
I
q6
1
48
I
I
1
OF B E N Z E N E .
Fig. 4.-Solid phase: effect of benzene moderator on the reactions following the (n,?) process in ethyl bromide ( 3 X molar fraction bromine scavenger present). The irradiations were performed at -196’.
case by subtracting the experimeiitally determined retention due to hot reactions (see Table I) from the total retention. The result is the linear plot starting a t R {diffusive) = 9.6% for pure ethyl bromide and decreasing linearly to R (diflusive) = 0 for pure benzene. (b) Solid Phase.-Any conclusions drawn from the radiolytic or radiochemical studies of solid mixtures are subject to the condition of homogeneity. Owing to the scarcity of data available on the parameters of molecular crystals and even more so about mixtures of Substances forming molecular crystals, it is difficult to affirm with certainty that quickly frozen mixtures of hydrocarbons, organic halides, and halogen have kept the same homogeneity as in the liquid phase. I n the following study it is assumed that the extremely small size of the crystals obtained by abrupt freezing is a sufficient condition to establish a homogeneous distribution of the reagents. This is evidently an approximation confirmed only by the results of the Laue X-ray back reflection analysis6 and by the reproducibility of our results for a given mixture. Having this restriction in mind, the interpretation of the results from solid phase (n,y) reactions in CzH&3rand GHs mixtures is only tentative.
F. 0. SHUCKASD H. L. TOOR
540
It was found previously that diffusion controlled reactions hardly contribute to the total organic yield in solid ethyl bromide6and the retention of activity in solid benzene is independent of bromine concentration up to about 3 x 10-1 molar fraction of scavenger (Fig. 2). There are thus very good reasons to believe that the chemical reactions following neutron capture in solid ethyl bromide-benzene mixtures are confined to isolated regions in the immediate vicinity of the recoil. It is expected that the organic yield of the system should decrease from R = 78% for pure ethyl bromide to 2270 for pure benzene. As can be seen from Fig. 4 the retention seems to vary linearly to an extrapolated value of 71.5%, but the experimental points fall abruptly for benzene concentrations greater than 0.8 molar fraction to a value of presumably 22y0 characteristic for pure solid benzene. The abrupt change in retention for the addition of small amounts of ethyl bromide to benzene can be explained either by energy transfer from excited benzene molecules to ethyl bromide that can decompose further. or by competition between reactions of the type CF,H~* CeH6 -+ CeHb.CsH6 Ha (1)
+
+
which does not produce radicals susceptible to contribute to organic retention, and C6H5’
+ C2H5Br
-+
+ C2H6.
C6H6Br
(2)
which produces ethyl radicals. Similar reactions have been postulated to account for retentions in solutions of ethyl iodide and iodine in benzene.13
Vol. 67
The phenyl radicals involved in these reactions probably were created by the hot halogen atom while slowing down to energies where stable chemical compounds can be formed. It was proved and it is immediately obvious that because of the dilution and the high diffusion coefficients involved, the hot atom has no chance of recombining with any of these fragments in scavenged gaseous systems. It can be shown that the great majority of the available data from liquid halides can be rationalized without involving these fragments. I n solids, the radicals so formed are frozen, probably very near the site of hot reactions. In the experimental procedure it is necessary to melt the system before performing the reductive extraction; the inorganic species containing the radioactive atom are in this way given the opportunity of reacting thermally with the fragments found in their vicinity. Such reactions are only apparently “hot” in the sense that they are localized and only a high concentration of scavenger would affect them but are in fact ‘Lthermal,’las they are subject to normal kinetics.l4 Acknowledgments.-The author wishes to express her gratitude to Professors H. Balban and A. J. Blanc Lapierre for encouragement and to Professor R. Wolfgang for many helpful discussions. Financial support from the “Commissariat B 1’Energie Atomiyue” is gratefully acknowledged. (13) P. F. D. Shaw a n d J. Macrae private communication. (14) Such “post effects” also could be partly responsible for the increase of retention through annealing.13 (15) ?VI. M de Maine. A. G . &laddock, and K. Taugbol, Dzscussions Faradau Soc., 23, 211 (1957).
DIFFUSION I N THE THREE COMPOKENT LIQUID SYSTEM METHYL .ALCOHOL-n-PROPYL ALCOHOL-ISOBUTYL ALCOHOL BY F. 0. SHUCK’ AND H. L. TOOR Department of Chemical Engineering, Carnegie Institute of Technologg, Piltsbiirgh IS, Pennsylvania Received June 1$?,196% The diffusion behavior of the system methyl alcohoI-n-propyl alcohol-isobutyl alcohol ma8 studied at 30”. The diffusion coefficients for the binary systems which form the borders of the ternary system as well as those for the ternary system were measured using the diaphragm cell method. The binary as well as the four ternary diffusion coefficients were found to be linear functions of mass or volume fraction.
Introduction 4 study of the diffusion behavior in completely miscible ternary liquid systems was recently undertaken in these Laboratories. The beharior of a relatively ideal system of this type has already been reported.* As a continuation of this program the diffusion behavior of the system methyl alcohol+-propyl alcoholisobutyl alcohol has been examined over the full concentration range. Experimental The diffusion coefficients for the system were measured using a modification of the diaphram cell method which was described previously.2 The methyl and n-propyl alcohols used were “certified” reagent grade obtained from the Fisher Scientific Company and the isobutyl alcohol was obtained from the Matheson, Coleman and Bell Division of the Matheson Company. (1) Department of Chemical Engineering, Iowa State University, Ames, Iowa. ( 2 ) J. K. Burchard and H. L. Toor, J . P h ~ s Chem., . 66, 2015 (1962).
These alcohols were used without further purification. Concentrations were measured by gas-liquid chromatography, using a Eeckman GC-SA instrument with helium as a carrier gas. A six foot, 1/4 inch diameter column packed with 15 weight yo of polyethylene glycol (carbowax COO-Union Carbide Chemicals Company) on 40/60 mesh firebrick gave reasonable resolution and short elution times. The accuracy of analysis depended upon the sample composition. The amount of a component present in a sample could in general be determined to within 1.0 t o 0.2% of the amount present. The cell factors, p, for the four cells which were used ( p is defined by eq. 1) were determined by calibrating with 0.5 11‘ HCI diffusing into pure water at 25.0’. The results of several replicas and the 95yoconfidence ranges are
Po(cell 0) = 0.1685 i 0.0046 om.+’ pl(cell 1) = 0.2121 rt 0.0042 &(cell 2) p3(ce113)
0.0046
=
0.1990
=
0.2602 rt 0.0042 ern.-%
f
