NOTES
Aug., 1956 dure outline in the Parr Manual.4 The only modifications of this procedure in the experimental runs were that the sample was contained in glass bulbs as described by Tannenbaum, Kaye and Lewenss and that the usual Parr sample container was placed in a tight fitting 98% nickel sleeve to increase the depth by about inch and thus prevent spattering of the unburned sample on the cold bomb walls. A pressure of 30 atmosphere of oxygen was used in all cases. Analysis.-Carbon dioxide was determined by bleeding the bomb gases through a standard carbon dioxide absorption train containing caroxite (Fisher Scientific G O . )as the absorbent, and then flushing the bomb three or four times with oxygen under 10 atmospheres pressure. Boron analyses were made by determining. the boric acid present in the reaction products. The determinations were made by titrating samples of the washings with sodium hydroxide, using a pH meter to determine the end-point. Treatment of Data.-The total raw heat value obtained from the combustion measurement was corrected for the contributions of the fuse wire and masking tape (which was used to hold the fuse to the sample bulb) and for the sample weight to give A E l (cal./g.). To this value were added the correction factors shown in Table I which are as follows: the conversion to 1 atmosphere pressure and to a constant pressure process giving AHl = AE1 - 29 cal./g.; corrections for the conversion of boric acid t o BzOa, AHa = 13 cal./g. sample; and correction for the oxidation of unburned carbon (assumed to be amorphous) to Cor, AHs. Since it was not possible to obtain accurate boron analyses in the concentration range used in this work, it was assumed on the basis of the rough values (which ranged from 95 to 100% of the theoretical value) that the combustion of boron was 97.5% complete in all cases. In view of the low weight per cent. of boron in the sample, this approximation seems justifiable. The correction for unburned boron (assumed to be elemental) is thus constant a t A H , = -21 cal./g. sample. The heat of combustion ( A H , ) is then given by AH1 AHP A H 3 AHd.
+
+
+
Results and Discussion
1137
felt that by using the corrections mentioned above, increased confidence can be placed in the resultsin fact, it does not seem likely that our values should be in error by more than 0.3%. The presence in the residue of unburned boron and carbon in the elemental state has been found by other workers who have studied compounds of this type. On the basis of the foregoing statements, the corrections applied, and the consistency of the values in Table I it may be concluded that the heat of combustion of tri-sec-butylborane is 2130 f 6 kcal./mole. From this value and the following heats of formation: B2O3(s), -306 kcal./mole6; C02(s), -94.1 kcal./mole7; HzO(c), -68.3 kcal./ m0le,7 the heat of formation of this compound may be calculated to be -75 kcal./mole. (6) E. J. Prosen, W. H. Johnson and F. Y. Pergiel, Natl. Bur. Standards Report No. 1552, March 26, 1952. (7) F. D. Rossini, D. D. Wagman, W. H. Evans, 8. Levine and I. Jaffee, “Selected Values of Chemical Thermodynamic Properties,” Circular No. 500, Natl. Bur. Standards. Feb. 1, 1952.
STUDIES OF THE RECOIL TRITIUM LABELING REACTION. 11. METHANE AND ETHANE’ BY RICHARD WOLFGANG, JOSEPH EIGNER~ AND F. S. RowLAND
Received February 4, 1968 Brookhaven National Laboratoy. Upton, Lone Island, New York Princeton Universzty, Princeton, New Jersey
A tritium atom produced in the Li6(n,a)Tnuclear reaction recoils with an initial kinetic energy of 2.7 Mev. This energy makes it potentially more reactive chemically than hydrogen atoms produced by TABLE I more conventional means. About 12% of these Sample -AH‘ - AHa, AHw kcd./;. kcal./g. kcal./g. wt., coz, recoil tritium atoms enter non-labile positions in g. % theor. sample sample sample the parent molecule when stopped in crystalline 0.5085 11.590 0,078 98.8 11.68 s ~ g a r s . It ~ is ~ not ~ practical, however, to determine .3442 96.1 11.453 .248 11.71 the chemical fate of all of the tritium reacting in .5318 11.665 ,039 99.4 11.71 sugars because of the great number of possible .121 98.1 11.518 .5151 11.65 degradation and polymeric products which could 11,646 .030 .a202 11.68 99.5 have been formed. With the simple alkanes of 11.559 ,131 .5209 98.0 11.70 the present study, these products are such that a determination of the location of almost all of the Av. 11.69 tritium is quite feasible. Since the precision of the values of AH, is quite The irradiation mixtures consisted of low tempergood despite variations in the degree of combustion ature slurries of approximately 40 mg. of Li salt and in the sample weight used, it appears that the and 3 mi;. of alkane, sealed in quartz capillaries. corrections employed are satisfactorily self-con- These tubes were irradiated for 30-60 seconds in the sistent. Brookhaven reactor a t - 160’. After irradiation, The value 11.69 kcal./g. for tri-sec-butylborane they were warmed to room temperature in vacuo corresponds to 2130 kcal./mole in comparison with and then broken mechanically. A protective Tannenbaum and Schaeffer’s value of 2110 f 10 brass cup prevented the resultant explosion from kcal./mole for tri-n-butylborane, This latter was shattering the vacuum line. calculated without corrections for unburned carbon Measured volumes of carrier ethane, propane and and boron which averaged about 1% of the theo- n-butane were introduced into the line, mixed with retical. On consideration of these corrections and (1) Research carried out under the auspices of the U. S. Atomio of the inherent differences in the alkyl groups, Energy Commission. (2) Chemistry Department, Harvard University, Cambridge, these two values seem quite compatible. It is also The results of six consecutive determinations are given in Table I.
(4) “Oxygen Bomb Calorimetry and Oxygen Bomb Combustion Methods,” Parr Manual No. 120, Parr Instrument Co., Moline, Ill. (5) S. Tannenbaunl. S. Kaye and G. F. Lewens, J . A m . Chem. Sac., 76, 3753 (1953).
Massachusetts. (3) R. Wolfgang, F.,S. Rowland and C. N. Turton, Science, 121,715 (1955). (4) F. S. Rowland, C. N. Turton and R. Wolfgang, J. Am. C h e w SOC.,78, 2354 (1956).
i 138
NOTES
the irradiated alkane, and condensed in a trap cooled with solid nitrogen, reducing the vapor pressure of methane to about 50 p . Hydrogen gas was then admitted, and an aliquot removed and counted as described below. After pumping off the remaining hydrogen, repetition of this procedure with a second batch of carrier hydrogen always yielded a negligible activity. This, together with the reproducibility of the results, indicates a valid assay of the amount of H T produced. Following removal of the hydrogen carrier, the alkanes were separated from each other by distillation successively from liquid nitrogen, frozen isopentane and liquid isopentane. As gas was evolved, it was removed batchwise for counting. Traces of higher alkanes were permitted to distil into the final fraction by warming the trap to room temperature. The ethane run was performed similarly, with the inclusion of methane carrier gas. All counting was done using silver-walled glass, gas proportional counters.6 The sample to be counted was expanded into the counter and then filled to a pressure of 70 em. with "P-10" (90% A, 10% CH4) counter gas. Figure 1 shows the results of a typical run on a slurry of methane with LizS04.Hz0 particles. The specific activity of the successive fractions is plotted against the cumulative volume of carrier
VOl. 601
signed to particular compounds. The rise in specific activity at the end of the distillation is attributed t o still higher hydrocarbons. Table I summarizes the data on the percentages of the total activity found in each carrier for five methane and one ethane runs. Three different lithium salts were used as the sources of the energetic tritium atoms. The Li&03 and LiF particles were 1 /I or less in diameter, while the LizS04.Hz0 had been sieved to select particles of diameter 53-61 p . The reproducibility appears to be quite satisfactory and presumably indicates that the surfaces of the lithium compounds do not influence the reactions and final combination of the ('hot'' tritium. Thermal hydrogen atoms do not react with gaseous methanee and it is reasonable to assume they would not attack the liquid a t - 160". Hence the finding that recoil tritium reacts with methane, giving labeled products, confirms that recoil tritium labeling reactions do indeed proceed' while the tritium still has excess kinetic energy, i.e., they are "hot-atom" reactions. However, the result (Table I) that most of the tritium goes into H T and the tagged form of the alkane irradiated, is also consistent with the earlier hypothesis6 that this excess energy has become quite small when the tritium finally reacts to enter stable combination. Probably most tritium atoms, having lost nearly all their recoil energy, are finally stopped by a collision in which they break a single bond and then combine with one of the radicals formed to yield HT, CH,T or CaH6T. Walden inversion and hydrogen abstraction reactions by these "epithermal'' atoms would also yield these products. (These possible alternate mechanisms are presently being studied using other systems.) TABLE I CHEMICAL COMBINATION OF TRITIUM FROM IRRADIATED ALKANE-LITHIUM SYSTE~IIS (PERCENT.OF TOTAL ACTIV-
50L 0
AMOUNT OF CARRIER REMOVED (rnlrcrns Hg).
Fig. 1,-Activity spectrum from carrier distillation of tritium irradiated methane.
removed. In this plot, the areas corresponding t o each carrier fraction are proportional to the per cent, of the total activity present in that carrier fraction. The hydrogen carrier was removed in one fraction, and the successive methane samples showed substantially constant specific activity. However, the distillation of the ethane carrier showed a small but steady decrease in specific activity, consistent with the preferential distillation of carrier-free radioactive ethylene in the earlier fractions. The amount of C2H3Tis estimated from the area of the histogram above the ethane carrier asymptote. A number of other C3 and C4 compounds would also be removed in the propane and n-butane fractions, and the activity cannot be as(5) W. Bernstein and R. Ballentine, R e v . Sci. Instr., '21, 158 (1950).
ITY)
-Irradiation
mixtureCH4 CXH6 Lias 4 Best Lip LiaCOa COa LiF LiF ,Ha0 values LiF H2 40.3 39.1 40.6 38.2 39.3 39.1 41.7 CH4 44.7 47.6 47.0 49.4 48.9 48.6 5.4 .CaHe -10 6.8 5.90 4.80 5.5 37.5 'C2HiaJ ,. -0.36 -0.31 -0.3 -4.0 ,. 4.14 4.27 4.2 5.3 Ca fraction -4.9 6.5 1.86 2.05 2.0 5.9 C4fraction . C > d fraction" ,. -0.30 -0.38 -0.3 >0.3 Active Droduct
1
a
"
1
..
No corresponding carrier present.
The occurrence of many-carbon and unsaturated molecules among the labeled products requires a more complex reaction path. In these cases, the tritium must have sufficient energy left on arriving at the end of its track to rupture several bonds, producing radicals such as CH2, or charged fragments, etc. The decreasing probability of finding labeled higher hydrocarbons is a measure of the decreasing probability of producing larger numbers of fragments in the immediate vicinity of the end of the range of the tritium atom. (6) E. W. R. Steacie, "Atomic and Free Radical Reactions," 2nd ed., Reinhold Publ. Corp., New York, N. Y . , 1954.
