Y.
586
required for double hydrogen bonding. The heat of adsorption for this step can be approximated by the activation energy of desorption of species I1 of about 20 kcal/ mol. This value is the same as that reported by Ustinov water species adsorbed on tungand Ionovls for their sten. It is also similar to the 19 kcal/mol reported by Holmes, et al.,I7 for the heat of adsorption encountered in the hydroxylation of thorium dioxide. Step III. Physisorption. The final step in the adsorption of water vapor is the actual physisorption of water molecules on a water covered plutonium dioxide surface. Interactions in this step are primarily between water molecules. The heat of adsorption for this step was not determined, however, it should approximate the heat of liquefaction of water.
Hatano, K. Takeuchi, and S. Takao
Summary and Conclusions Gravimetric and thermal desorption studies have illustrated the complexity of the water adsorption process. Thermal desorption spectra showed water to be chemisorbed as two different species with different binding energies. Hydroxylation of the oxide surface took place with a heat of adsorption of 68 kctil/mol while chemisorption on the hydroxylated surface took place with a heat of adsorption of 20 kcal/mol. A single physisorption process accompanied the two chemisorption processes. Acknowledgment. The author gratefully acknowledges the assistance of T. L. McFetters and D. I. Hunter in making some of the experimental measurements and processing the data.
xygen-Containing Products in the Radiolysis Solutions of Nitrous Oxide Yoshihiko Hatarto,” Ken-ichi Takeuchi, and Satoshi Takao laboratory of Physical Chemistry, Tokyo lnstitute of Technology, Meguro-ku, Tokyo, Japan (Received July 13, 1972)
0xygen-containing products in the radiolysis of cyclohexane solutions of NzO have been quantitatively measured and found to be HzO and c-C~HIIOH,denoted by ROH. The yield of ROW, most of which is HzO, is in good agreedent with that of Nz. This finding shows that the decomposition of NzO in liquid cyclohexane finally leads to Nz and ROH with the ratio of one to one. A further study of the correlation of the nitrogen yield with the decrement in the hydrogen yield has been made in order to estimate the nitrogen yield per electron captured by NzO. Possible processes on the decomposition mechanism of NzO in the radiolysis of cyclohexane solutions are discussed.
Introduction Nitrous oxide has received more attention than any other solute among electron scavengers, particularly because of the comparative ease with which the nitrogen formed can be measured. The formation of Nz from cyclohexane solutions of NzO has been studied by many gr0ups.l The results, however, cannot always be interpreted solely in terms of electron capture. The difficulty in interpreting the results arises from the fact that the observed yield of Nz is much greater than the expected yield of electron in the radiolysis of liquid cyclohexane and also from the fact Chat the overall study on the formation of oxygen-containing products after the decomposition of NzO has not been established yet. As the oxygen-containing products, thus far, the formations of C - C , $ H ~ ~ Qand H ~ Hz05 -~ were reported in the cyclohexane-NzO system.6-8 The yield of c - C ~ H ~ ~ Owhich H, occupied only a few per cent of the expected yield of oxygen-containing products, was measured q ~ a n t i t a t i v e l y . ~ The yield of HzO, however, has scarcely been known. In the present study, the measurements of H20 and other oxygen-containing products in the radiolysis of cyclohexane solutions of NzO have been accurately carried The Journalof Phyticrrl Chemistry, Vol, 77, No. 5, 1973
Further, an attempt has been made on the correlation of the nitrogen yield with the decrement of the hydrogen yield upon the addition of NzO in the radiolysis of liquid cyclohexane. Assuming that electron scavengers depress the hydrogen yield from liquid cyclohexane by an amount approximately equivalent to the yield of scavenged electrons,I1 one can estimate the nitrogen yield per electron captured by NzO. (1) J. M. Warman, K . 4 . Asmus, and R. H. Schuler, Advan. Chem. Ser., No. 82, 52 (1968), and the references cited therein. (2) R. Blackburn and A. Charlesby, Nature (London), 210, 1036 (1966). (3) N. H. Sagert and A. S.Blair, Can. J. Chem., 45, 1351 (1967). (4) R. A. Holroyd, Advan. Chem. Ser., No. 82,488 (1968). (5) S. Sato, R. Yugeta, K. Shinsaka, and T. Terao, Bull. Chtrm. SOC. Jap., 39, 156 (1966). (6) In hydrocarbon-N20 systems other than cyclohexane7q8the formation of H20 and alcohols was also detected. (7) A. Menger and T. Gaumann, Helv. Chim. Acta, 52, 2477 (1969). (8) R. C. Koch, J. P. Houtman, and W. A. Cramer, J. Amer. Chem. SOC.,90, 3326 (1968). (9) Recently in our laboratory the quantitative analysis of H20 in the gas-phase radiolysis of NZO-hydrocarbon mixtures has also been carried out gas chromatographically.10 (10) S. Takao, Y. Hatano, and S. Shida. Bull. Chem. SOC.Jan. 44, 873 (1971). (11) K.-D. Asmus, J. M. Warman, and R. H. Schuler, J. Phys Chem., 74, 246 (1970).
-
Radiolysis of Cyclohexane Solutions The results will be useful for the understanding of the decomposition mechanism of N2O in the radiolysis of cyclohexane solutions. xperimental Section Phillips Research grade cyclohexane was used as supplied. Since the 65' value of hydrogen did not change within experimental errors by the chromatographed treatment hexant> OP silica gel, further purification was mpusities of less than 0.01% in the cyclohexhalf of which was 2,4-dimethylpentane, were detected by gas chromatography with a flame ionization detector. Cyclohexene and other olefins were not detected. Nitrous oxide supplied by Takachiho-Shoji Co. was thoroughly degassed and stored under vacuum through a -120" cold tIap. The cyclobex ane w13s completely dehydrated before irradiation by using liquid Na-K alloy under vacuum. In the cyclohexane, cyclohexene and other olefins were again absent. The gas chromatograni showed the same impurities as in the case of nondehydrated cyclohexane. The dehyarat ed cyclohexane solution of N20 was sealed in uacuo into a small sampling glass tube (about 2.5 ml) with a breakable seal. The concentration of N20 a t room temperature was calculated by using the Ostwald absorption coefficient of 2.62.5 Special caution was used in order to avoid the contamination of the samples with HzO. The samples, 2 ml of the cyclohexane with various concentrations of VzO, arere irradiated by ' W o y rays to a total dose of 8.3-9 2' x eV/g at room temperature. In calculating the dose, G(Fe3+) = 15.6 was used in Fricke dosimetry. The products not condensable at the temperature of liquid nitrogen were measured by ai gas buret attached to a Toepler pump and a copper oxide furnace kept at 250". The composition of the noncondensable gases after the removal of H2 through the furnace was mass spectrometrically confirmed to be nitrogen and a trace of methane. Nitrogen could nct be detected in the radiolysis of cyclohexane samples not containing N20. After the measurements of Nz and HZ yields and the removal of remaining N2O using a -120" cold trap, all the mixture condexisable a t - 120" was collected and sealed into the prepared glass tube with a clean liquid Na-K alloy. The total yields of HzO and alcohols, denoted by ROH, were calculated stoichiometrically on the basis of the hydrogen yield produced by the reaction with Na-K alloy. Great caution was again used for the analysis of ROW containing the main product, H20, by the method described els ewhew .I2 Other products in the solution were analyzed by a gas chromaltogaph with a flame ionization detector using a 6.0-m d i m e t l ~ y ~ s u ~ ~column o l a ~ e a t 30" and a 0.75-m polyethylene glycol 600 column a t 100".
Results and Disearssim~ Figure 1 shows Ike yield of oxygen-containing products together with the yields of Hz and Nz in the radiolysis of cyclohexane soflutiom of NzO. The yield of ROH, the total yield of W 2 0 and alcohol, i s estimated to be twice the yield of H2 produced by the reaction with Na--K alloy. The reaction has been justified stoichiometrically by the prepared cyclohexane solutions of c - C ~ H I ~ O or H HzO. The yield H20 calculated from the difference between the yield of H and that of c-CsH11OH which is
02
0.L
08
0.6
Concentration of
587
1.0
N20 ( ~ o I / l )
Figure 1. Yields of H 2 ( 0 ) , N2(.), ROH( and H20(- - - -) from the cyclohexane solul of H 2 0 is estimated by the difference between and that of c-C6Hl10H.
the
yield of ROH
measured by gas chromatography. HzO was not detected before and after irradiation in the dehydrated pure cyclohexane. The gas chromatogram did not reveal other oxygen-containing products, such as aldehydes, ketones, etc. The hydrogen yield for pure cyclohexane i s 5.57 i 0.05 by the average of 5.52, 5.55, and 5-63 a t a dose of 8.8 x eV/g. In a recent paper,ll the observed hydrogen yield of 5.60 was reported at a dose of about 1 x 1019eVjg and the effect of C02 as an impurity in cyclohexane on the hydrogen yield was pointed out. Since cyclohexane in the present experiment has been purified by using Na-K alloy, impurities such as C02, 0 2 , and H28 initially dissolved in the cyclohexane may be eliminated.13 The h i tial yield of hydrogen should be taken 5.77 f 0.05 by the correctionll for the dose dependence of hydrogen yield. This value is a little larger than that reported earlier, 5.67 rt 0.05.11 The yield of ROH is surprisingly in good agreement with nitrogen yield a t all concentrations from 0.6 to 0.01 mol/l. The yield of c-CeW1lOH a t 0.4 mol/l. of N2O is about 0.3, which is only a few per cent of the total yield of oxygen-containing products. The yield of C-C&II~OHapproximately agrees with that previously r e p ~ r t e d .The ~ yield of H2O was measured as an oxygen-containing product in the radiolysis of n-hexane solution of N20,7 in which the method using LiAlH4 was applied. The result, however, involves large experimental errors and the method itself remains a problem of stoichiometrical reactivity. As described above, the yield of ROW, most of which is HaQ, is in good agreement with that of N2. This finding shows that the decomposition of N28 in liquid cyclohexane finally leads to N2 and ROH with the ratio of one to one. Possible schemes of ROW formation through ionic processes may be deduced as follows. c-C&I,, e-
N,O-
+
+
c-C,H,,+
c1-C6H,,'
N,O
-r-
e-
(11
m
* N,ON7 4- OW 9 C-C~HII (3a) N, 4- R,O 4- C"66M1" (3b)
_ 1 1 1 _
NZO- f c-C6Hl2 ----t NP $. OH- f c-C&I,, OH 4- cC6H12 -----ic H,O ..6. c-C&I,, OH- f c-C6Hlp+ --+ WZO 4- CvC
(4) (5)
(12) K. Takeuchi, K. Shinsaka, S. Takao, Y. Hatarso, and S . Shida, Bull. Chem. SOC.Jap., 44,2004 (1971 ). (13) K. Ueno, K. Hayashi, and S. Okamura, J. Poiym. Sci., Part B, 3, 363 (1965); K. Tsuji, H. Yoshida, and K. Hayashi, J. Chem. Phys., 46, 810 (1967); K. Funabashi, C. Hebert, and J. L. Magee, J. Phys. Chem., 75,3221 (1971).
The Journal of Physical Chemistry, Vol. 77, No. 5, 1973
Y. Hatano, K. Takeuchi, and S. Takao
58
OH 4- c-C,K1l
c-CsH11OH C-CsHlo -4-
HZO
The ion-moleculc reaction (4) is not established conclusively, since the electron affinity of NzO is not yet known accurately. At present, however, this reaction seems to be po~sible.1~ If both the neutralization (3a) or (3b) and the ion-molecule reaction (4) are possible, it may be rather difficult to decide which process is more important. The above scheme may explain the yields of N:! and ROH, at least arising from the electron scavenging, but not their yields in excess of the expected yield of electron. The excess yields of Nz and ROH will be interpreted effectively as N,O X Nz ROH (8)
+
+
where X might be intermediates other than N2O- or an electron which is captured by NzO as shown in reaction 2. The intermediate X is not determined here, but it must finally lead to the formation of N2 and ROH with the ratio of one to one. A following attempt has been made in order to realize the relative importance of reactions 2 and 8. The decrease in the hydrogen yield by the addition of NzO to cyclohexane has been corasidered,lJl at least at lower concentrations, as a result of electron capture by N2O with subsequent interference in the normal ion-electron recombination. e- .C- c-C&I,~+ f’Hz eN 2 0 + NzON,O‘ 3- C - C ~ R -~-+~ ~fNHZ where f’ axid f “ are the efficiencies of Hz formation. If hydrogen is formed with unit efficiency (f’ - f” = 1),11y = G(N~)/(G(Hz) ~ G(H2)) gives approximately the ratio of nitrogen and electron contributing to the electron scavenging of NzO. G(N2) is obtained by the correction for the direct radiolysis of N z 0 using G(N2) = 12.9,15 although its adequacy has not yet been certified. G(H& is estimated to be 5.73 $: 0.05 by the correctioni1 for the ionic part of close effect on the hydrogen yield from pure cyclohexane at a dose of 8.8 x 1019 eV/g, and G(H2) i s the observed hydrogen yield at this dose from the cyclohexane containing NzO. The ratio y in Figure 3 can be estimated by using the experimental results in Figure 2 where the yields of N2 and Hz are shown as a function of Ns(N20), the mole fraction of N2Q. The ratio y is larger than unity, which indicates, as is now generally recognized at least at the higher N20 concimtrations, more than one N2 is formed per electron captured. The ratio y seems to be approximately con&tant, 4.6, at all NzO concentrations, although it is not so char at Ns(N20) < 10-3. Even in the lower concentration region it appears to be evident that y is larger than unity, which seems to be different from the previous suggestiun.lJ1 Because of the following reason, however, this conclusion in the lower concentration region might be unwarr~nted.The value of G(H& may be reduced by the unknown impurities in the cyclohexane if they had high reactivities for electrons or other precursors for hydrogen formation via neutralization. Assuming the ratio -y at the 1ower N2O concentrations to be unity, one can estimate the value of G(M& to be more than about 5.9. Various species will be considered as possible sources of
+
1
(7a) (7bi
4
(3
2I :
10-4
Mole fraction of N20
Figure 2. Yields of H 2 ( 0 ) , N 2 ( 0 ) . and R O H ( 0 ) as a function of N20 mole fraction. The corrected yield of N z ( X ) for the direct radiolysis of N 2 0 is also shown, taking G(N2)o = 12.915for pure N20.
ri.
-
1o - ~ 16~ Mole fraction of N2O
8
162
--+
-
T h e Journal of Physical Chemistry, Vol. 77, No. 5, 1973
Figure 3. The y values, y = G(N*)/(G(H*)o
function of NzO mole fraction.
-
G(Hp)),
as a
the excess nitrogen in the radiolysis of cyclohexane solutions of N20, that is, the intermediates X in reaction 8. Nitrous oxide has been generally recognized to be inert with respect to reactions with thermal hydrogen or alkyl radicals.1 The reaction with OH radical to produce N2 may also be excluded, because this reaction seems to be very slow compared with reaction 5.1fi Excited cyclohexane may not be directly responsible for the formation of the excess nitrogen at least at lower concentrations of NzO . Secondary reactions with N20 of negative ion formed on electron capture by N2O have frequently been proposed to explain the excess nitrogen at higher NaO c0ncentrations.l The secondary reaction of negative ion, however, could not easily explain the experimental results in the present study on the oxygen-containing products. The reaction of NzO- with NzO would be expected to produce NzOz-,l which is the precursor of the excess nitrogen at higher concentrations of N20. To explain the experimental result of oxygen-containing products the process
NyOZ-
c-C,H,,+or c-C,H,,
+.
N,
i-ROH
must be provided, and N2Oz- must exclusively give ROH as the oxygen-containing products. It seems rather difficult to expect such a process. In the case of the secondary reaction of 0-, it also seems difficult to expect that the reaction of 0 2 - with c-C~H12+or c - C S H ~ Zexclusively (14) S. Takao. Y. Hatano, and S. Shida, J. Pbys. Chem., 75, 3178 (1971). (15) M. G. Robinson and G. R. Freeman, J. Phys. Chem., 72, 1394 (1968). (16) N. R. Greiner, J. Chem. Phys., 53, 1070 (1970); FI. Simonaitis, J. Heicklen, M. M. Maguire, and R. A. Bernheim, J. Pbys. Chem., 75, 3205 (1971).
Hydroxylation of Nitrobenzene, Chlorobenzene, and Toluene gives ROH as oxygen-containing products. Ketones or other oxygen-containing products might be produced in such a process. Especially a t lower concentrations it also seems difficult to assume that N20- or 0- reacts predominantly with N20 itself. The rate of the reaction between 0- and ~ L - C ~ HinL the , ) gas-phase is 1.2 x 10-9 cm3 molecule-1 s%ec-1.17If we take this value for the rate of reaction 4 and assume the excess nitrogen a t lower concentrations of MzO to be due to the secondary reaction of NzO- with N-20, the rate of this secondary reaction should be a t leaist about lo4 times as large as that of reaction 4. Thus the possibility of the excess nitrogen forma-
589
tion via secondary reactions of negative ions with N2O may be excluded at least at lower concentration of N2O. Other possibilities for the formation of the excess nitrogen, such as the role of positive ions, electrons which cannot be captured by N20, etc., should also be examined in detail.
Acknowledgment. The authors wish to thank Professor S. Shida and Dr. S.Sato for valuable suggestions. (17) D. K. Bohme and F. C. Fehsenfeld, Can. J. Chem., 47, 2717 (1969).
Radiation-Induced Homolytic Aromatic Substitution. 1. Hydroxylation of Nitrobenzene, Chlorobenzene, and Toluenela Manfred K. Eberhardt" and Massayoshi Yoshida Puerto \?iCo Nuclear Center, 'b Caparra Heights Station, San Juan, Puerto Rico 00924 Publication costs assisted by the University of Puerto Rico
(Receiyed May 75, 7972)
The radiolysis of aqueous solutions of nltrobenzene, chlorobenzene, and toluene was investigated. The effect of 02 and NzO on the isomer distribution was studied. In presence of 0 2 a substantial increase in the yield of phenols was observed as expected for a homolytic substitution. Our results indicate a selectivity in the disproportionation step. Evidence is presented for the formation of p-nitrophenol by a mechanism involving the nitrobenzene anion radical. A change in isomer distribution of nitrophenols in presence of' NzO as compared to deaerated or oxygenated solutions indicates the involvement of a more nucleophilic species like 0- or NzO- in the hydroxylation reaction. In the case of chlorobenzene phenol was formed in addition to the chlorophenols. In deaerated toluene solutions NzO proddced a three- to fourfold increase in the total yield of substituted phenols, but no change in isomer distribution was observed. On the other hand, 0 2 produced a significant change in isomer distribution particularly in the case of toluene. The ratio of G(bibenzyl):G(total cresols) was found to increase in going from an argonsaturated to NzO-saturated solution of toluene. This indicates the involvement of a nucleophilic species like 0 - or NzO-. Evidence for a direct displacement of chlorine by OH radical is presented. The experimental results are interpreted on the basis of SCF-MO (CNDO-2 and INDO) calculations.
A large amount wf work on radiation-induced hydroxylation of aromatic compounds has appeared in the literature.2 Several workers have recently investigated the hydroxylation of nitrobenzene,3 but no quantitative work on the hydroxylation of chlorobenzene and toluene has been carried out. We have started a program on radiation-induced hydroxylation of a series of aromatic compounds, and we are trying to interpret our results on the basis of SCF-MO theory.
Experimental Section Materials. All solutions were prepared using triply distdled water as solvent. Nitrobenzene, chlorobenzene, and toluene analytical reagent grade were redistilled prior to use. Argon, nitrogen, oxygen, and nitrous oxide saturated solutions were prepared by bubbling the gas through 1 1. of triply distilled water for about 1 hr. The gas was introduced by means of i l hypodermic needle inserted through
a silicone stopper. The saturation was enhanced by frequent shaking. After saturation the aromatic solute was introduced with a Hamilton syringe and dissolved by vigorous shaking. Irradiations. Irradiations were carried out with a 6OCo room source. The dose rate was determined by Fricke dosimetry using a value of 15.6 for G(Fe3+). The dose rate was 0.875 x 103 rads/min unless otherwise indicated in the tables. Samples (1 1.) were usually irradiated for 30 min or a maximum of 1 hr. All irradiations were carried out in unbuffered solutions and a solute concentration of (1) (a) Presented at the Metrochem Meeting, April 30-May 3, 1971. San Juan, Puerto Rico. This work was part of a Ph.D. Thesis presented by M. Yoshida, at the University of Sao Paulo. Brazil, 1971. (b) The Puerto Rico Nuclear Center is operated by the University of Puerto Rico for the U. S. Atomic Energy Commission under Contract No. AT-[40-1)-1833. (2) E. J. Fendler and J. H. Fendler. Progr. Phys. Ofg. Chem., 8, (1970). (3) (a) R. W. Matthews and D. F. Sangster, J. Phys. Chem., 71, 4056 (1967); (b) J. H. Fendler and G. L. Gasowski, J. Org. Chem., 33, 1965 (1968) The Journal of Physical Chemistry, Vol. 77, No. 5. 1973
