August, l!f(il
1431
SOTI%
implies an initial “unsteady state” condition dur- vious studies of aliphatic hydrocarbons4 h a r t ing which the rate constant changes continuously indicated that for aromatic systems the over-all until a steady-state condition of the reactive inter- reactions are considerably more complex than the mediates is attained. Thereafter, a kinetically simple scavenging of radicals. Because of this a second-order exists for the reaction. (This con- more detailed examination of the radiation chemcept is in good agreement with the results for p- istry of benzene-iodine solutions has been undertoluenesulfinic acid. l ) taken in order to examine the suitability of using Because of the limitations imposed by the con- iodine as a radical detector in these systems. As is ductance method (resistance readings should be seen below the observed reactions are even more between 1.000-10,000 ohms), the p H could be complex than suspected in the earlier work. Exvaried over n limited range, 2.0-2.7. Also be- treme caution must be urged in the interpretation cause of the rapid change in pH during the early of the results of scavenging experiments in arostages of reaction at the higher initial pH values, matic materials. the pH soon approached a limit and its variation Experimental during much oi’ the reaction was smaller than would Reaction was followed by spectrophotometric observaappear. Thus, the initial high pH values do not tion of the iodine concentration during irradiation and by have much iignificance. Rut at low p H (2.0), the measurement of the uptake of radio-iodine to form organic iodides. Baker and Adamson reagent grade benzene was pH did not change much (0.05 unit) and hence used after three crystallizations with the rejection in each these lower pH values are much more representa- case of about half the material. Phillips research grade tive of p H conditions during most of the reaction. benzene also was used in preliminary experiments but was l:or example, although the initial pH was 2.7 for not regarded to be suitable since a small amount of thermal was observed to occur with dissolved radio-iodine. run 5, after several hours it dropped to 0.25 and a t reaction No observable reaction occurred with the triply crystallized the end of the run it was 2.3; hence, for most of the material. run it tvas 2.3-2.5. Because of the preceding, a Solutions of known concentrations of radio-iodine (1131) truer representation of the pH range for the runs were degassed by repetitive freezing and pumping, sealed and irradiated. Irradiation methods were very similar to listed would be much smaller than 2.0-2.7. There- those employed in previous work on the cyclohexane-iodine fore, it would be hazardous to attempt any quantita- system.6 Both 2 MeV. Van de Graaff electrons and cobnlttive correlation between rate constant and initial 60 -prays were used, the latter a t dose rates of 30,000 and 250.000 rad. per hour. Radiation yields were determined PH. the case of the ?-irradiations by comparison with the I t may be argued that if reaction rate ivas a func- in rate of oxidation of ferrous ions in the Fricke dosimeter tion of p H , theii the change in p H could account [G(Fe+++) 15.51 and in the case of the fast-electron irfor the curvatures obtained. However, curvatures radiations by the power input method. Optical absorbance measurements were made a t the existed for riins 2 and 4 even though the pH’s were absorption maximum of iodine in benzene (500 mp) with ~1 constant during the reaction times (the initial Beokman DU spectrophotometer. Auxiliary measurements pH’s were low, 2.0-2.1). This indicated that in- were made a t longer wave lengths. I n a number of cases duction periods undoubtedly existed during initial the complete absorption spectrum was recorded with a Cary reaction periods. This is in accord with results spectrophotometer. I n the fast-electron experiments absorbance measurements were made in an absorption cell obtained in the case of p-toluenesulfinic acid.l sealed to the irradiation vessel during the course of irradiaWhen iodide was not present, as a catalyst,’ the tion, initial reaction rate was small; after iodide had In the radio-iodine experiments determinations were been added, its effect on the reaction rate became made of the fraction of activity found in organic combinaafter thiosulfate extraction. I t was found that about noticeable. However, it did not affect the final tion one-third of this activity readily exchanged a t room temvalue of k . This aloo has been observed for p- perature with molecular iodine. This activity could be toluenesulfiiiic acid.’ Chloride ion has also been removed by a subsequent thiosulfate extraction after equireported as a catalyst,?but in n relatively large con- libration of the sample with a large excess of inactive iodine. measures of the radiation-induced reaction of iodine centration it produced only a small change in the Three with benzene are thus obtained: the total yield a8 measured rate constant (run 6). by the disappearance of iodine determined spectrophotoE
(7) T. P. Hilditcii, J . Chem. Soc.. 1091 (1910).
THE RADIATIOS-IKDUCED REACT103 BETWEEK BESZENE BSD IODINE BY
iiLBERT
metrically, the organic yield determined after thiosulfate extraction and the stable iodide yield measured after equilibration of the irradiated samples with inactive iodine. In the previous studies of cyclohexane-iodine solutions6 these three quantities were found to be equal,
Results and Discussion
T. FELLOWS% AND ROBERTH. S C H U L E R ~ ~ Figure 1 illustrates the changes which occur in
Chrmastry Department, Brookhaven National Laboratory, Cpton, Long Island, N E W York Receiaed February I S , 1961
Brief observations on the radiation-induced reaction between iodine and benzene3 during pre(1) Research performed under t h e auspicrs of the U. S. Atomic Energy Commission. (2) (a) Guest Chemist on leave from Socony hlobil Laboratories, Research and Development Department, Socony hlobil Oil Co., Inc., (b) Radiation Research Laboratories, Puiilsboro. S e w Jersey: Mellon Institute, Pittshiirph, Pennsylvania. (3) G. L. Clark and L. W.Pickett, J . Am. Cheni. Soc.. 62,465 (1930), first reported t h e X-ray induced uptake of iodine b y benzene 86 one of the early examples of t h e ohemical effekts uf ionizing radiation.
the absorption spectrum of a 4 X 10-3 111 solution of iodifie in benzene during and after a 5 microampere fast electron bombardment. Curve a represents the degassed solution before irradiation. Curves b, c and d represent the progressive decrease in absorption observed during irradiation. After completion of the irradiation (5 X 1021e.v./ g.) the sample was allowed to remain sealed, A darkening through shades of gray and dark green (4) E. N. Weber, P. F. Forsyth and R. H. Schiller, Radzatzon Research, 3, 68 (1955). (9) R. d’. Feawnden s n d R H. Sohuler, J 4m. Chem. Soc.. 76, 278 (1057).
NOTES
1452
WAVELENGTH A = M p
Vol. 65
WAVELENGTH-Mp
e
,
Fig. 1.-C!hanges in absorption spectrum of degassed 4 x 10-8 M solution of iodine in benzene during and after irradiation: a, initial absiorption spectrum (maximum depressed instrumentally); b, after irradiation of 5 X 1020e.v./cc.; c, after irradiation of 1 X 1021 e.v./cc.; d, after 5 X 10*1e.v./cc.; e, after standing for 4 hours after irradiation; f, after standing 10 hours; g, after standing 24 hours; h, after exposure of solution to air and sunlight.
was obsenred and is illustrated by curves e, f and g. Upon further standing the absorption represented by Q did inot increase and was unaffected by light in the absence of air. After exposure of the sample to air the coloring faded very quickly in bright sunlight. The resulting spectrum (h) was essentially identical with the absorption spectrum immediately after irradiation. The color of the aerated sample (g) was reasonably stable when stored in the dark. The nature of the colored substance formed here is unknown. The coloration is however extremely intense since, as indicated in Fig. 1, the integrated absorption in the visible range is considerably in excess of that of the original iodine. For more dilute solutions the iodine concentration decreases esponentially with dose. This behavior is in contrast to the linear decrease observed in the case of aliphatic hydrocarbons. Since the initial slope of the iodine disappearance curves has no pronounced dependence on concentration, the decrease in yield with dose is attributed to the build up of a reaction product which is capable of competing with the iodine in the scavenging reactions. The over-all results of fast electron experiments are given in Table I. It is seen that of the iodine reacting only 30-50% is firmly bound as organic iodide. Of the remainder 25-5070 is removed during the initial extraction and 15-20% rea,dily exchanges with iodine. The distribution of the iodine among the various components varies considerably from run to run but can be broadly described in terms of the following 0.7, G(organic) 0.4 and Gyields: G(tota1) (stable iodide) r- 0.3. In the previous X-ray experiments G(tota1) was reported to be 0.66.4 Gaumarine has shown that the addition of iodine to benzene after irradiation results in the conversion of phenyl cyclohexadiene to biphenyl. At least part of the difference between the total and organic yields can be ascribed, therefore, to the dehydrogenation of cyclohexadiene type com-
-
(6)
-
T.Gauniann, H t l v . Chim. Acta, to be published.
pounds and the resultant production of hydrogen iodide. The stable iodide fraction presumably represents the formation of organic iodide from radicals such as phenyl while the difference between the organic and stable fractions (- 0.1) represents the formation of more labile materials. TABLE
1
REACTION BETWEEN BENZENE AND IODIXE INDUCED BY 2 Mav. VANDE GRAAFELECTRONS Iodine
concn. moles! 1. x 10s
Dose. e.v.1 ml.
x
10-Js 0.166' 7.0 14.0 0.498 1.02 28.0 0.996 49.5 2.045 55.7 0.996 370
reaction--
-Radiation
7 %
Or-
Or-
yieldso-
ganic 8.0
Stable 4.6
Total 0.49
ganic 0.23
7.7
5.3
.G7
16.1
12.2
8.8
.71
.34 .63
29.9
17.7
12.4 7.7 44.3'
.74 ..
.44
.45
.35
.28
.32
.14
Total
17.3 15.5
..
86.6
10.1
G7.2'
Stable
0.13 .23
.39 .31
a Molecules/100 e.v.-as 1/2 I,. * Decreases to 55.070 on standing 16 hr. Remains constant on sttanding 16 hr.
A large number of cobalt-60 yirradiations were carried out at various iodine concentrations. The results are, however, complicated by the fact that irradiation periods are long with respect to the postirradiation effects noted above. While the results are not very systematic it is possible to make the following general comments. As the irradiation progresses the stable iodide continues to build up at the expense of the other fractions long after all of the iodine has been used up. The initial yields M iodine observed in the y-ra,y experiments at are approximately as indicated above for fast electrons. The organic yield a t 2.5 X 10-5 ill is essentially the same as at the higher conceritration while the yield of the stable fraction is initially somewhat less. Any quantitnt,ive interpretation of the yields from the y-ray experiments would seem to have very liniit'ed validity. The above observations amply illustrate the highly complex nature of the radiation-induced reaction between iodine and benzene. Observations on this system arc complicntetl by the pro-
NOTES
August, 1961
1453
gestively close to the value R In 2 = 1.377 e.u. predicted for a structure with molecules randomly placed in one or the other of two possible positions. Anorderedstructurewhichiscloselyrelated can be ob tained byremoving the center of symmetry and placing all atoms in positions (a) of space group T4-P2J: z,z,z; '/2 2,'/2 - z, - r; - Z/',. 4 1 / 2 - r; '/* z,--2,'/2 2. Because of the near equality of the scattering powers of nitrogen and oxygen, the X-ray diffraction experiment is extremely insensitive to any possible ordering of the N20 molecules. The postulated disordered structure thus rests entirely on the thermodynamic data, and there remains the possibility that the structure is ordered and that the excess entropy arises from COKFIRISIATIQX O F DISOItDEIt IN SOLID some other source. It seemed desirable to repeat the diffraction experiment with neutrons, where the NITROGS OXIDE BY SEUTIlON more favorable ratio of scattering factors (bo = DIFFRACTION' 0.58, b~ = 0.94) would allow a more sensitive test of any departures from the disordered model. 131- VALT TI^ C. HAMILTON AND MARTHA PXTRIE Commercial N20 was condensed to a depth of 8 cm. Chim,e'ry Departmeni, Brookharen A-ational Laboralory, Uplon, L. I . in a quartz tube 20 mm. in diameter. The tube was N e w York placed in a stainless steel cryostat maintained a t Receined January IB, 1961 liquid nitrogen temperature (77"K.), and several X-Ray diffraction studiesZ have indicFted that neutron diffraction patterns were obtained, using solid nitrous oxide is cubic, CQ = 5.72 A.,a space the apparatus described by Corliss, Hastings group Th6-l'a3, with four molecules per unit and Brockman.s The neutron wave length Ts;as cell. The structure is apparently isomorphous 1.064 A. A typical diffraction Dattern is showii to that of ( 2 0 2 , and de Smedt and Kcesom thus in- in Fig. 1. terpreted the X-ray powder data in terms of the following structure4
duction of reactive and unstable species as products which lead to pronounced post-irradiation and apparent intensity effects. While the results are, in general, difficult to interpret in terms of specific reaction intermediates, radiation yields of the order of several tenths molecule per hundred electron volts are observed for each of several components. These yields are considerably higher than that observed for hydrogen formation [G(H2) = 0.041. Reactions resulting in the disappearance of iodine therefore account for a preponderant fraction of the intermediates produced. Presumably a t least the stable activity indicated above results from the reactions of free radicals.
+
-
+
+
4 0 in positions (a): 0,0,0; O,l/p,l/s; 1/2,0,1/2; l/z,'/*,O x,l/z - x, - x; 8 Tu' in posit,ions ( c ) : 4 (z,x,x;l / z x,'/? - .'; '/s - x, - x,I/2 x ) with x = 0.1167 -2,
+ +
+
This describes 11 symnietrical niolecule N-0-N with equal bond lengths of 1.1.6 A. The infrared spectrum of nitrous oxide can be interpreted, however, only in terms of the unsymmetrical molecule N-N-0. Extremely careful measurenieiit of the rotational constants in gaseous NzO allows interatomic distances to be derived5 rr =T Y - N
1.1257 It 0.002 A. 7.2 E ~ x - 0= 1.1863 f 0.002 A. 1') $- T? = 2.3120 rt 0.001 A.' =
With t,liesc unsymmetrical moleculcs, the symmetry requircnients of space group Pa3 can be satisfied in a statistical way only. The N2O molecules iuust be presumed to pack randomly in the ttwo possible orieiit,ations available to each molecule; the molecule a t the unit cell origin, for example, may have the oxygen pointed in either the positive or negative [111] direction. This stJructurewas first proposed by Blue and Giauque' who discovered a residual entropy in the solid of 1.14 cal. degree-' mole-' (e.u.). This is sug( I ) Work performed under the aiispices of the U. S. Atomic Energy
Commission.
(21 J . de Smedt and W.11. Keesom, Proc. Acad. Sci. Amsterdam, 27, 839 (1924). (3) A value of 5.656 A. at -190' has been reported by L. Vegard. Z . Phypilo, 71, 465 (1931). (4) See "International Tables for X-Ray Crystallography," Vol. I, the Kynoeh Press, Birmingham, 1952. ( 5 ) A. E. Douglas and C. K. Moller, J . Chrm. Phys., 22, 275 (1854). (6) An electron diffraction investigation (also in the vapor phase) remlted in a value of (n 4-ra) = 2.32 f 0.02 A., and the ratio r d r s was determined to lie beteen the liniits 0.925 and 1.08. See V. Sohomaker and R. Spurr, J . A m . Chem. SOC.,66, 1182 (1942). (7) W. Blue and W. F. Giauque, ibid., 67, 991 (1935).
I
40'3
!
1-
-
Fig. l.--Observed neutron diffraction pattern for N10. The shaded areas are due to the cryostat and sample tube and have been subtracted from the tabulated intensities. The solid curve in the inset shows the net observed intensity in the region of the (110) reflection. The dashed curve shows a reflection (not observed) of intensity 1.0 on the scale used in Table I. The bar in the inset has a length twice the standard deviation of the individual points.
Intensities were calculated for several sets of structure parameters for the disordered model (space group Pa3). The scattering factor for the central atom (a) was taken as b N and that for the bo)/2. A series of least outer atom (c) as ( b N squares refinements led to these best values for the parameters
+
x = 0.1196 f 0.0013 B(c) = 1.87 f 0.38 k2 B ( a ) = 1.29 i 0.39 A.Z.9 (8) L. M. Corlise, J. M. Hastings and F. G. Brockman, Phyis. Rsa., 90, 1013 (1953).
(9)B is the Debye-WaUer factor which enters the atructure factor expression a8 em [- B sinW/B/X*].
