1538
stant for the long-lived excited state was found to be 1.0 X lo7l./mole. The lifetime is computed to be 1.5 X sec. assuming that every diffusional encounter (6.6 X 10Y/sec./l. for a 1 ,1/1 solution) between quencher and excited molecule is effective. It is significant that the concentrations of retarders for this reaction are close to those for the retardation of the photoreduction of the fluorescein-type dyes. It appears that the triplet species of these dyes have a strong affinity for the solid substrates. Thioniiie dyes in the long-lived state undergo certain photochemical reactions when bound to specific high polymers.1° In the present case, however, the long-lived species wanders through the solution until it encounters a solid particle; failing such an encounter during 1 msec. it drops to the ground state. This may account for the low quantum vield of photobiiiding for rose bengal. This is approximately l/aoo that of the maximum (extrapolated to infinite reducing agent concentration) for photoreduction. Increasing the amount of zinc oxide should increase the chance of such an encounter and hence, increase the rate of the reaction. However, the system is unaffected by concentrations of the powder beyond 1.5 mg. per ml. due to the light scattering by the suspended substrate which decreases the amount of light allowed into the solution. For zinc oxide powder of this mesh size, the surface area is estimated from extrapolation of published data on ziiic oxide powderll t o be about 100 cm.Z/g. For a 10-ml. solution of rose bengal at 1 0 F M in the presence of 15 mg. of zinc oxide, complete binding takes place with prolonged illumination. That is, 4 X 1OI6 molecules are bound per cm.2 of surface. Since the dye molecules occupy an area of the order of 100 it is obvious that niultilayer adsorption occurs. Apparently the light-excited dye has an af€iiiity for solids coated with adsorbed dye as well as for the original solid substralx i. (10) K Wotherspoon and G Ostet, J . Am Chem (1957). (11) C W Siller, zbid., 66, 431 (1943).
Sol
, 79, 3992
IlhDIOLk-SIS OF TOLUENE : JIECHANISXL OF FORMATIOK OF BESZYL RADICALS BY 1,. H. GALE,B. E. GORDON,G. STEINBERG, AND C. D. WAGSER Shell
'Vol. 66
NOTES
022
Compantj, Ma.il%nez Caldornm und Shell DeLdopment Company, Ernes y u d i e , Cahfool nzn
radica1s.l When isolated molecules of toluene in the gas phase are ionized by energetic elertrons in a maws spectrometer, the principal fragment ion is the cyclic tropylium ion, C7H7+, as shown by %!eyerson and Rylander.4 (They found by isotopic means that the probability of loss of the single hydrogen to form the ion is the same for all types of hydrogen on toluene, and that all carbon atoms in the C7H,+ ion are equivalent.) Although a t 25' some of the benzyl radicals arr undoubtedly formed by hydrogen abstracatioii by tolyl radicals,l they also must be formed by a dirwt process. They may be formed directly by detachment of an alpha hydrogen froni an excited toluene molecule, as in (a), or by rearratigenieiit of Ihc tropylium ion on neutralization by an electron, as
in (b). 111 the present study, t,oluene-a-C14 was irradiated with 3 Mev. electrons in order to differentiate between these pat'hs of reaction. Experimental A sample of two microcuries of t01uene-cu-C~~ (Research Specialties Go., Richmond, California) was diluted with 71 g. of research grade toluene, and placed in a circulation system containing an irradiation cell.6 The cell was cooled to -30" and degassed by successive addition and removal of dry, oxygen-free nitrogen while the liquid wa,s circulated through the cell. The irradiation was conducted with a 45-painp. 3-1Iev. electron beam, collimated by a 1 /2 in. 0.d. pipe before striking the thin window of the cell. A i irradiat,ion timc of 4800 see. gave an est>imateddose of 8 X lo8 rad. Toluene was separated from the heavier material by distillation, and the dimer fraction was recovered from t'hc residue by several gas chromatographic separations o r il. 6-mm. diameter X 1.5 n:. silicone column at 170". The recovered fraction (1 51 mg.) was diluted with 299 nig. of bibenzyl (Eastman Nodak White Label) and oxidized by chromic acid. The resulting benzoic acid was purified by sublimation, yielding 307 mg. of material ( 5 1 % ) . llost, ol t.his (274 mg.) was degraded by the Schmidt reaction t>oaniline (8656) and carbon dioxide (8275), which in turn wcrc convert,ed to acetanilide and barium carbonatc.
TABLE I SPECIFIC ACTIV~TIEB OF SCHMIDT D E C ~ ~ A D A TPIE OO S U~CW Starting material
Products
Bibenzyl-benzyltoluene-bitolyl
Benzoic acid Acetanilide Barium carbonate Benzoic acid Acetanilide Barium carbonate
Toluene
IZecezbed January 11, 1962
Toluene in the liquid phase reacts under ionizing radiation to give hydrogen and polymeric products. The Clinierip materials have been shown by HoigiiB , i i d Gaumann to consist of bihenxpl, tht. h i u y l toluenes, and the six bitolyls. Bridge2 a i d Chiltoir and Porter3 has-e shown that ionizing radiation acting on toluene in a solid matrix a t low temperature generates benzyl radicals, and the bibenzyl appears to be formed by rombinatiou of the benzyl ( I ) J Jloigiid and 1' G&*ninrirn,Jiclv C h 7 n i l d n , 44, 2141 (1Uhl) (21 N. I'i Ui,dge, Iature, 186,30 (1960) ( 3 ) H. T. Chiltoil and 6 Porter, J. Pitus Chern , 63, 904 ilSSD)
Nc./mole
0.42 i.0.03 o.ooo=koo.oo15 0.37 =k0 . 0 1 2.47 0.000f0.0051
2.21
Results and Discussion lmizoic :wid wits derir-cd solely froin LIIC id)eiizyl, siirtae thr, tliiiieric acids From the other 7'11tk
dimers were discarded. All of the activity was found in the carboxyl carbon. The reduced specific: activity in the carboil dioxide may be due to dilution of the sample by atmospheric carbon dioxide (4) S. Meyersoiu and P. X. Rylander, , I . Am. C h r m . S n c . , 79, 812 (3957);d . Chenz. Phys., 9 7 , !)01 (i!457). (j) Tlic agparatiis was dcscribod prcvioualy: C. 11. WUIICU../. I'iiys. CRem., 64, 231 (1960).
August, 1962
NOTES
or self-abrsorption in the barium carbonate suspension counting technique. The absence of activity in .the ring excludes a symmetrical intermediate such as the tropylium ion in the formation of benzyl radicals in the liquid phase. Therefore, it is concluded that the direct mechanism for formation of benzyl radicals is by direct C--1H scission and does not involve tropylium ioii. A check was made on the possibility that rearranged toluene could be found in the radiolysis. Oxidation of a sample of the toluene following radiolysis, and measurement of the activity in the ring and carboxyl by the same method, gave an upper limit of G = 0.025 for such a rearrangement reaction.
period. One day was roughly the time taken for an experiment. The t,emperature was measured and controlled with a platinum-rhodium thermocou le placed outside the reaction chamber, which had been stown to agree with another couple inside the chamber. Both couples were accurate within 2” at thc melting point of lu. The sample consisted of Mo, MOO*,and “Mo~C,”whose X-ray diffraction patterns gave no indication of any impurities. Complete dissolution of “Mo&” in nitric acid indicated the absence of carbon. These were intimately mixed together, ressed into a pellet, and put on a piece of molybdenum foifin an alumina boat. This was placed in the reaction chamber, which was evacuated and closed. The sample was heated t o about 850’. The gas evolved was pumped off and the sample left a t this temperature for 2 days. A further small amount of gas was evolved and this also was pumped off. This procedure removed water vapor and other extraneous gases from the system. The sample was heated to a fixed temperature and readings of the mercury manometer were taken until a constant pressure was reached. The temperature was raised slightly in order to increase the gas pressure, dropped back to its original value, and a new constant pressure obtained. The equilibrium lies between these two pressures. The system then was completely evacdated and the process repeated, the second equilibrium being always in excellent agreement with the first. This procedure removed any further gaseous impurities and indicated the independence of the equilibrium pressure upon composition of the sample. An X-ray diffraction pattern of the quenched sample a t the end of the experiment showed only Mo, MoOz, .and “Mo2C” present. Complete solution of the sample in nitric acid without residde indicated the absence of carbon. A series of equilibrium pressures was obtained in this fashion at different temperatures from 926 to 1068’. During the first experiments an alumina rod was placed inside and along the porcelain reaction chamber. This wae removed at the end of the experiment and examined visually for traces of carbon which could have formed by disproportionation of carbon monoxide, resulting in an alteration of the equilibrium pressure. Only the most minute traces of carbon were ever formed. To obtain gas samples for analysis, the ground-glass stopper was replaced by a rubber stopper through which passed a thin silica sampling tube, one end of which was centered over the sample, the other end being connected to a sampling bulb. When the total pressure in the furnace beca,me equal to the equilibrium pressure measured previously, a small gas sample was quickly withdrawn througb the silica tube so that the equilibrium was not disturbed perceptibly. Thc sample was collected over a mercury reservoir and analyzed in an Orsat apparatus, using a standard procedure5 in a-hich the COZwas absorbed in a potassium hydrovide solution and the CO in an acid cuprous chloride solution. Tlie samples were found to consist of CO and COz and a small uiianalyzed residue comprising always between 2 and 3y0of the sample. The recorded partial pressures in Table I have been corrected accordingly, and the ratios p ~ o ~ / have p ~ obeen used to obtain values of the free energy6 shown in Fig. 1.
E’REE ENERGY OF FOB1LIATIOK OF M,IOLYIBDENCM OXIDE AND CSRBIDE B YMOLLYGLEISERASD JOHN CHIPMAK A !, par tsient
of iWetalluigy, Massachusetts Instztute of Cambrzdge, ildass.
Technology,
Recezved January 18, 1962
Despite their growing importance as useful materials, little has been learned concerning the thermodynamic properties of the metallic carbides. A knowledge of their free energies a t elevated temperature need not await the results of calorimetric investigations since in many cases it is possible to obtain dependable values through simple equilibrium measurements. This paper presents the results of such a study. When a metal and t m other solid phases comprising its oxide and carbide are brought into equilibrium a i t h a gas phase, the free energies of formation of both the oxide and the carbide are determined by the partial pressures of CO and COZ and their known thermodynamic properties. If the free energy of formation of the oxide is known, a himple measurement of the total pressure fixes that ot the carbide. The oxide arid carbide must be in g r i ~ ~ the a l most metal-rich stable compositions of thc system and the solubility of each in the metal iiiust be sinall. In the case of molybdenum, this last requirement is fil1ed.l The oxide phase is l;\1002,while the carbide is the metal-rich extreme of the solid solution the composition of which varies from &loz 2‘ICto I \ I o ~ . ~ ~This C , nonstoichiometric compositioi~~-~ will be designated “MozC” for present purposes, and its molar thermodynamic properties will be taken as those associated with 1 g.atom of C. Method.-The apparatus consisted of a gas-tight porcelain tube, closed a t one end, and resting inside a platinummound furnace. Thr rest of the apparatus was made entirely of Pyrex glass, fused to the porcelain tube. It comprised a mercury manometer, a vacuum tap, and a groundglads stopper ma mhich specimens would be inserted. I t was found that with the system evacuated n o change in pressure could be detected by the mercury manometer over a 24-hr. (1) I\X. Hansen, “Constltution of Binary Alloys,” MoGraw-HI11 Book Co., New York, N. P , 1958, p. 371. (2) T. T a k a , Scz. Repts. Toholcu I m p . Unzu., 17,939 (1928). (31 A Westgren and G PhragmBn, Z anorg allgem. Shenz , 166, 27 (1926) (4) \V P. SgLcs, 1C li T a n Horn, and C. M. TuLkei, 2’7ans 4 m I n A Wzn J f p t Ungrs , 117. 173 (1935).
1539
Discussion Complete thermodynamic data on R!to02 were summarized in the recent work of King, Weller, and Christense~i,~ who based their free energy values entirely on calorimetric data. Earlier equilibrium studies of the reaction hI00z
+ 2Hz = 310 + 2HzO
W ‘ P ~ Prwiewed
by GokceiP along with his O M 11 dat :I on thik Gquilibrium. Of the four sets of data con-
( 5 ) Burrell, “Manual for Gas Analysts.” Catalogue No. 80, Burrell Technical Supply Co., Pittsburgh, Penna. (6) “Selected Values of Chemical Thermodynamic Properties,” Series 111,Natl. Bur. of Standards, 1949. (7) E. G. King,W. W Welier, and A. C . Christensen. Bureau >lines R e p t Inr eetigations 5664,1960 (8)N. A. Goheon, T r a n s Am. Inul. X i 1 1 J l c t . L n g ~ b, 197, 1019 (1953).
