1802
AI. A.
TABLE IV G-VALUESFOR GASEOUSPRODUCTS FROW
GBEENBBUM,
R. E. Y A T E S ,
AND
111. FARBER
Vol. 67
There are small differences in the hydrogen G-values
7-IRRADIATION OF obtained from the three compounds which can be BENZOYLCYCLOPROPANE rationalized by the opportunities for molecular loss of
Products
coz co
----Electron 62 5"pb
THE
G-Values volts per gram X lO-ig-310" 545' 816'
.
625O
0.0010 0.0057 ,.. . . 0.0052 .033 ,030 0.028 0.021 .032 Hz .027 .029 ,023 .023 .030 CK .0026 .0022 .002 ,002 .0017 CzHz ,0038 .0055 .011 .006 .0029 C& .018 .012 .013 .028 ,014 CJhd .0039 .0041 .004 ,002 .0040 C& (propylene) ,0071 ,0046 .004 .003 .0047 CgH6 (cyclopropane)e .004 .002 CsHs .o .0080 .O .O .0050 C6H6 .0039 .0026 .O .O ,0071 Total ,100 0.104 0.089 0.087 0.107 a Gas analyses by mass spectroscopy. Average values from duplicate samples. Gas analyses by gas chromatography. Suspected cyclopropene, calculated as allene by mass spectroscopist. e Mass spectrometer cannot distinguish cyclopropane from propylene in these mixtures. Presumably the G-values for propylene listed in columns 1, 2, 5 (left to right) are due approximately 40% to cyclopropane from consideration of the values determined by gas chromatography.
the mechanisms of photochemistry, mentioned in the introductory paragraphs, and the radiolysis of the aralkyl ketones.12 Thus, n-butyrophenone forms acetophenone and ethylene in relatively high yield, an observation consistent with the fact that the NorrishBamford Type I1 dissociation is not as markedly inhibited in condensed phases as is the a-cleavage reaction. The formation of benzaldehyde, carbon monoxide, propane, and propylene from isobutyrophenone implicates a-cleavage as the predominant mode of decomposition. The resistance of benzoylcyclopropane to radiolysis also supports the analogy being constructed. Thus, the crude analogy with the photochemistry of dialkyl ketones seems safe insofar as the experiments reported herein are concerned. It is clearly recognized, however, that the data are incomplete, and that the establishment of the mechanisms requires much more experimentation. (12) It has been found in a series of preliminary experiments t h a t the photolysis with 2537-A. radiation of the neat, aralkyl ketones zn vacuo yields products anticipated on the basis of the photochemistry of dialkyl ketones.
hydrogen, but lack of data prevents substantiation of such a rationali~ation.1"'~ It is probably indicative of H-atom scavenging by t,he phenyl moiety that the hydrogen G-value is, in every case, less than that of dialkyl ketones.lK The decrease in G-values with increasing dose which is exhibited by the olefinic products from n-butyrophenone and isobutyrophenone may be due, in part, to scavenging action by the olefins. There is a concomitant decrease in G(H2) from both ketones, but the magnitude is not as great as the decrease exhibited by the olefins. I n the absence of more detailed knowledge, the results of these experiments can most reasonably be interpreted in terms of intermediates of the types recognized in the photochemistry of simple aliphatic ketones. The formation of hydrogen is an exception, but nothing about its mechanisms of formation can be deduced from these experiments. The mass spectra of the aralkyl ketones fail to shed much light upon the possibility of ionic intermediates contributing to the products in an extensive fashion. Acknowledgments.-This iiivestigat'ion was supported under a pre-doctoral fellowship, granted by the Xational Institutes of Health, Public Healt'h Service, to D. J. Coyle during the 1959-1960 academic year. The author wishes to express appreciation to Professor Weldon G. Brown for guidance during this research. (13) Gamma radiation produces hydrogen from alkanes by both atomic and molecular paths. See: (a) M. C. Sauer and L. 31.Dorfman, "Molecular Detachment Processes in the Vacuum Ultraviolet," I. Ethylene: 11. Butane. Abstracts of Papers, 136th National Meeting of the American Chemical Sooiety, New York, N. Y., September, 1960, p. 45; and (b) L. U. Dorfman and M . C. Sauer, Jr., "Molecular Detachment Processes in the Radiolysis and Vacuum Ultraviolet Photolysis of Gaseous Hydrocarbons," Abstracts of Papers, 139th National Meeting of the American Chemical Society, St. Louis, Mo., March, 1961, p. 23R. (14) The C-H bond strength of cyclopropane and its derivatives has not been determined, b u t from a variety of sources it. appears t h a t secondary or tertiary hydrogen atoms on a cyclopropane ring are more tightly held than the corresponding open-cbain compounds. See: (a) J. R. McNesby and A . 8 . Gordon, J . Am. Chem. Soc., 79, 625(1957); (b) R . C. Brown and M. Borkowski, ibid., 74, 1694 (1952); (0) G. S. Hammond and R . W. Todd, ibid., 76, 4081(1954): (d) H. E. Gunning and E. W. R. Steacie, J. Chem. Phys., 17, 351 (1949). (15) P. Ausloosand J. F. Paulson, J . Am. Chem. Soc., 80, 5117 (1958).
THE THERMODYNAMIC AND PHYSICAL PROPERTIES OF BERYLLIUM COMPOUNDS. IV. HEAT AND ENTROPY OF SUBLIMATION OF BERYLLIUM CHLORIDE' BY MICHAEL A. GREENBAUM, ROBERT E, YATES,AND MILTOXFARBER Rocket Power, Inc., Research Laboratories, Pasadena, Cal. The vapor pressure curve of BeClZ(s) has been determined over the temperature range 440-6OO0Ii. using Knudsen gravimetric and torsion effusion procedures. From analysis of the vapor pressure data, second-law values for the heat and entropy of sublimation of BeClz are 32.9 f 0.4 kcal./mole and 42.7 =t1.4 cal./deg. mole. By means of estimated thermal functions AHsui,zg8 was found to be 33.1 f 0.5 kcal./mole and AssUbzg8was 43.2 xk 1.5 cal./ deg. mole. The corresponding third-law value for AH,,b is 32.1 kcal./mole.
I. Introduction The first vapor pressure data for BeClzwere reported b y Rahlfs and Fischer in 1933.2 These ill\wstigators (1) This research was supported b y the Air Research and Development Command of the United States Air Force.
measured the vapor pressure of solid BeClzfrom 613'K. ~~~~ to its m.P. (677'K.) using a ~ X U M X I E Procedure. The liquid vapor pressure curve was determined from the melting point to 733'K. using the same procedure. (2) 0. Rahlfs and W. Fischer, 2. U R O T B . allgem. Chern., 211,349 (1933).
Sept., 1963
HEATAND EXTROPY OF SUBLIMATION OF BERYLLIUM CHLORIDE
The authors used quartz vessels for their studies and reported substantial reaction between these vessels and their BeC12. From their data, the heat of sublimation of 30.0 kcal./mole (for an equilibrium mixture of monomer and polymer) was reported. Brem~er,~ in 1946, made some calleulations which indicated that the major species existing in the gas phase up to the boiling point of BeCl2 would be (BeC12)~a t 1 atm. In 1958 Buchler and Klemperer4 confirmed the presence of (BeC12)2up to temperatures of 1000° and 1atm. Above this temperature the dimer was shown to break down into monomer. In 1952, Rossini, et u Z . , ~ using the data available a t that time, obtained values of 29.2 kcal./mole for the heat of sublimation of BeClz and 43.1 cal./deg. mole for the entropy of sublimation. A recent mass spectrometric determination of the heat of sublimation has been reported by Ryabchikov and TikhinskK6 From mass spectral data over the temperature range 496-578OK. the authors obtained a value of 34 f 1 kcal./mole for A H s u b . KO value for A S s u b was presented, however. The authors further observed mass spectrometrically that over their experimental temperature range the concentration of Bed314 was 0.51.5%. These are the only values currently available for the heat and entropy of sublimation of BeC12. Due to the extensive reactions between the BeCl2 and the containers used to obtain the vapor pressure data as reported by Rahlfs and FischerZ and the relatively narrow ranges over which the vapor pressure measurements were imade by Ryabchikov and Tikhinskii,6 it was considered advisable to obtain new vapor pressure data for Be(% over a wider temperature range. This would allow second-law plots and a subsequent determination of the entropy of sublimation. The present work employed both gravimetric and torsion effusion methods. Materials employed for the effusion cells showed no reaction with the BeC12. The torsion effusion measurements confirmed the previous observations6 that the vapor due to sublimation consisted primarily of the monomer of BeClz(g),
11. Experimental The vapor pressure measurements of BeClz(s)were carried out over the temperature range 440-600'K. Measurements were made by means of the 'Knudsen gravimetric effusion procedure previously described in the first of this series of papers on the thermodynamic properties of beryllium compounds .7 I n addition, the torsion effusion method was used to provide confirmatory vapor pressure data in the temperature range 470-510'K. Effusion cells constructed of graphite were used for all runs after it was established that no reaction occurred between BeClz and graphite. Two different effusion cells were used in the gravimetric measurements: cell 1 had a single 0.5-mm. diameter orifice and cell 2 had four 1-0-mm. diameter orifices. The Clausing factors for these cells were calculated from the hole dimenaions and were 0.514 and 0.672 for cells 1 and 2 , respectively. The cells were otherwise identical, with cylindrical sample cavities 18 mm. in diameter and 18 mm. deep. The torsion effusion cell consisted of a graphite block containing 4 sample cavitiee 7 mm. in diameter and 15 mm. deep. The effusion orifices were 1 mm. in diameter with axes a t a (3) L. Brewer, Metallurgical Lab. Report CC-3455 (1946). (4) A. Buchlerand W. Klemperer, J . Chem. Phye., 29, 121 (1968). (6) F. D. Rossini, et al., "Selected Values of Chemical Thermodynamic Properties," National Bureau of Standards Circular 500, U. S. Govt. Printing Office, Washington, D. C., 1962. ( 6 ) L. N. Ryabohikov and G. F. Tikhinskii, Fie. Metal. i Metalloved Akad. Navk SSSR,10,635 (1960); Chem. Abstr., 56,11988d (1961). (7) M. A . Greenbaum, et a i . , J . Phys. Chem., 67, 36 (1963).
1803
distance of 5.5 mm. from the center of rotation of the cell. The Searcy factor for the orifices was 0.734. A chromel-alumel thermocouple, used to regulate the furnace temperature, was secured to the outside of the Pyrex furnace tube. Another chromel-alumel thermocouple was inserted into a graphite block and the entire assembly positioned a t the center of the tube and approximately 1 in. below the center of the hot zone of the furnace. Measurement of the temperature profile inside the tube indicated a variation of less than 2" over the central 3 in. of the heated zone. Although the surface temperature of the tube and the temperature of the graphite block were not the same, thermal equilibrium was established after about 15 min. a t a constant surface temperature and the temperature of the graphite block remained constant. The BeClz employed in the present investigation was obtained from the Brush Beryllium Co. The material is extremely hygroscopic and is obtained in the form of a fluffy, microcrystalline mass. All handling of the material was done in a drybox in an atmosphere of dry, purified nitrogen. Purification of this material was found to be necessary, however, and was accomplished by vacuum sublimation a t 300". The resulting material is more dense and nearly pure white in appearance. Use of the sublimed BeClz in all work reported here resulted in highly reproducible vapor pressure data. Because of the extreme hygroscopicity and large surface area of the sublimed BeC12, it was never stored for more than a week before use. Wherever feasible, the sublimed material was loaded into an effusion cell immediately after preparation and the cell placed in the vacuum apparatus. Total sample weights were 350 to 700 mg. At the conclusion of each experiment where purified material was used, the condensation of small amounts of BeClz on the inner surfaces of the sample cavity and lid was noted. In each experiment the sample was heated (in the effusion cell) for a period of 1-16 hr. a t a temperature of 100-200" under vacuum to ensure that all moisture was removed. This resulted in the conversion of traces of water in the sample to HC1 by the reaction
If the HC1 is not driven off prior to measuring the vapor pressures, erroneously high vapor pressures would be obtained. It is felt that this could, a t least in part, account for the higher vapor pressures reported by Rahlfs and Fischer,2 compared with the values obtained in this study. Approximately 30 vapor pressure points were obtained by the gravimetric effusion method. Each of these points is based on the average rate of weight loss as determined from 5-10 individual rates of contraction of the quartz helix. The contraction of the helix was determined using a Gaertner cathetometer, read to 0.05 mm. The temperature used was the average of the 5-10 individual temperature readings made concurrently with the reading of the helix position. The e.m.f. of the chromelalumel thermocouple was read to 0.01 mv. Eight vapor pressure points were obtained by the torsion effusion method. Each of these points was determined from the angular displacement of the torsion effusion cell, measured optically by mean6 of a mirror, mounted on the axis of rotation of the cell. The displacement angles were measured to 10-3 radian. The temperature was measured concurrently with the measurement of the displacement angle. System pressures during both gravimetric and torsion measurements were in the range 5 X to 10-6 mm. The reaction of BeClZ(g) and Si02 previously reported by Rahlfs and Fischer2 a t temperatures in the range 613-733'K. has been observed to occur in the present investigation and at lower temperatures. Apparently the BeClz vapor effusing from the cell reacted with the hot surface of the Pyrex furnace tube resulting in the mild etching of the tube in the vicinity of the cell.
111. Discussion of Results The vapor pressure data obtained in the temperature range 440-600'K. are presented in Table I. The data, as plotted in Fig. 1, demonstrate that the measured vapor pressures are independent of the orifice areas, which differed by a factor of 16 in the two gravimetric cells.
31. :IGIWEXBAUM, . It. E. YATES,A N D AI. FARBER
t
indicates that up to 1000" and 1 atm. the predominant vapor sprcies is (UeC12)2. From the data of B r c ~ w r ~ based 011 the work reported by Rahlfs and Fischcr2 the AF for dissociation of the dimer
I\
-3.0
L a ,
-3.5
Vol. 67
BezCla(g) +2UcCls(g)
I
(2)
is AF
=
31,000
+ 111'10g 1' - 661'
(3) In tlie tcrnpcraturc and pressure range of this invcstigation, based on eq. 3, the vapor species would consist of a mixture of dimcr and monomer. However, a recent experimental determination of the vapor species above UeC12(s) in the tcmpcrature and pressure range employed in the present work, using a mass sprctrometer,6 has shown that no more than 1.5y0of BezC14 is present. l'licreforc, a value of 80 was employed for M in eq. 1. The vapor pressure of BeClz at several temperatures was calculated using the torsion cff usion technique by employing the equation I
I
&I
-7.0
1.6
1.7
18
19 10a/T, O K .
I
I
20
2.1
2.2
I'ig. l.--\'apor pressure of BeC12(s) as it function of tempcraturc: 0, grnvimetric effusion cell 1; 8 , gravinietric effusion ~ 1 2;1 0, torsion effusion.
VAPOR PRESSURE
--Cell T ,OK.
TABLC I DATAFOR fk?Cl,(S). M mw o u 1 p x ins,
-
468 477 478 483
485 401
4% 499 503 507 514 518
Cell 2
T,'IC.
II1lll.
0 0929 0 765 1 63 1 65 2 40 2 77 4.76 4 54 6.75 6 i8 10 2 15.1 16 5
441
G R A l IJIETHIC ErFUSION
p X 108, inrn.
7 20 70 52 123
497 31 0 534 537 544 551 559 565 570 574 580
164
281 292 466 5i7
-
a r iGd
863 1,0.56 1,430 1 ,770
583 586 59 1 600
log patnl = -(7200 z t 90)/T
+ ((3.39
f
0.20)
'1'1~vapor prrssurr of BeCl, at each cxprrinirntal tcnipcrat ure \t as calculated from the modified effusion rquation
I'",,
=
17.14
G T.I 0 al
dT/,tl'
uhrrr G is thc grams of material effusing during 1 seconds, 'CYois the Clausing factor, a is the orifice area, and :If is thr molecular weight. A discussion of the validity o f assuming that the Z' calculated from the above equation is thc rquilibrium pressure has been presented in a
prrvious paprr.* In order to calculate the vapor pressure from thc Knudseii equation it is necessary to know or to assume t hr average molrcular \\eight of the species ef'fusirig I'rc:m the tell. 'l'kir u ork of Ruchlcr a i d Iilemprrcr4 ( 8 ) A I l i a t l ~ r n r r d\
nlierr P is the vapor pressurc in dyiirs/cm.2, D is the torsion constant of the susprnsion wire, 2-mil molybdrnum, in dyne-cm./radian, B is thc angle of deflection in radians, a1 is the cross-sectional area of the hole i in cm.2,fi is the Scarcy correction factor, and q1 is the moment arm. The vapor pressures measured by this procedure are included in Table 11.
I D s i ~ ! i l l I Ciiem I ' h ~ a 25,526(19i0).
TABLE I1 EFFUSION VAPORPHESSURE DATAFOR BeCll(s). TORSION RIETFIOD
T,OK. 47 1 482 4s2 49 1 494 5 30 1 508.5 510 5
log pi,$,
p X
IO*, min.
0.76 1.58 1 58 2.9Y 3.97 5.97 10 2 11 6
-(7260 f.9O)/T
+ (9.39 zk 0.18)
Llvan't Hoff calculation was made from a lrast squares plot of the data in Table I in the form of log p 2's. l/T. The resulting equation for the vapor prrssure has been given in Table I. The van't Hoff calculation leads to a value for AZ13,,b of 32.9 =t0.4 kcal./mole in the cxperimciitsl temperature range. Extrapolation from 500 to 2!18"K., using estimatrd thermal fun(.yicldcd AH3,1b29e= 33.1 f 0.5 kcal./mole. The corresponding third-law value of AIIs,,bzg8is 32.1 kcal./ molr. Similarly, a least squares plot of AF us. T gave A s s u b = 42.7 f 1.4 cal./drg. mole in the experimental temprraturr range, in good agrecinent with thc value of 43.1 cntropy units at G78"1
