RALPHKLEINAND MILTOND. SCHEER
324
Vol. 65
HYDROGEN ATOM REACTIONS WITH PROPENE AT 77’1L DISPROPORTIONATION AKD REC0,MBINATION BYRALPH KLEIN’AND MILTOND. SCHEER National Bureau of Standards, Washington, D.C. Received August 16,1960
The investigation of the reaction products of deuterium atoms with ordinary propene helps to establish the relative importance of the atom addition to the propyl radical with respect to disproportionation of two propyl radicals. The distribution of the isotope species among the propenes and propanes is caculated. It is concluded that the D atom addition to the propyl radical accounts for little, if any, of the propane formed, and the radical disproportionation reaction is the predominant one.
The reaction of hydrogen atoms with solid olefins such as propene a t 77°K. gives as products the corresponding alkanes and the alkyl radical dimer. Deuterium atoms, generated from molecular deuterium on a hot tungsten ribbon in the gas phase, impinge on a layer of condensed propene a t 77” to diffuse into and react with it. So-called “cracking” products, resulting from C-C bond ruptures of excited species, are never observed. This contrasts with the corresponding gas phase reactions. Another striking difference is that the ratio of the alkane to the radical recombination products is found t o be an order of magnitude higher in the solid a t 77°K. than in the gas a t 300°K. It has been observed that even with a hundred-fold dilution of propene with an inert diluent such as butane or dichlorodifluoromethane, the alkane/dimer ratio remains unaffected. This would indicate that isopropyl radicals diffuse rather readily in the solid. The reaction products of the hydrogen atom addition t o solid propene a t 77°K are propane and 2,3dimethylbutane. The possible reactions that may occur include
+
C3H0 H +C3H7 (isopropyl) 2C3H7 --+ CaHs CaHB 2C3H7 +CeHla C3H7 H --+ CJ% C3H7 H +C ~ H B Hz
+
+
+
+
(1)
(2) (3)
(4) (5)
Mass spectrometric analysis showed that the propene was deuterated, Table I, so that a t least (4) is accompanied by ( 2 ) , the only mechanism by which propene deuteration could be effected. (Exchange reactions have too high an activation energy.) The calculation of the isotope distribution among the products requires some care. It is possible in principle, by using the steady-state hypothesis for isopropyl radical concentration directly, to arrive a t a distribution, but the complexities inherent in the variety of the isotopically labelled species makes the method ineffectual. It is only necessary to consider the stable species. Since it has been shown thnt propane formation is first order with respect to p r ~ p e n e the , ~ individual isotope species must also be represented by first-order processes. In detail, a propene species al gives the radical ?,+I upon D atom addition. The index refers to the number of deuterium atoms in the molecule. The radical Tit1 disproportionates with any other radical r,. In the process, r i + l may lose an H or a D to become a,+l or a,, or it may gain an H or a D to become propane or ,4i+2. This can be expressed as c . - S - i dt
ka,-l
(z = 0, 1,
-2 - 5k 6
. . ., 5 and a-l
~ -, k’ar =
(A)
0)
The first term on the right refers to the D atom The ratio propane/2,3-dimethylbutane was found addition to ai-1 to form r,, the r, then disproporto be 9 over a wide range of conditions such as tionating with loss of H to give ai. The numerical propene dilution and hydrogen atom concentration. coefficient (6 - i ) / 6 accounts for the 6 - i abstractThe ratio of 9 contrasts with the gas phase value of able H’s in The second term describes D atom 0.5.3 It may be speculated that (4) occurs more addition to al to give rl+l followed by H atom abreadily in the solid than in the gas (especially in straction to give ai+1. The third term accounts for view of the easy paths for energy transfer out of loss of a i by way of propane formation, and if (4) the molecule of the exothermic heat of the C-H did not occur, IC’ mould equal k. bond formation). The occurrence of (4),however, There is derived from (A) would not be in accord with the constancy of the a. = aDOe-(Ed6 k 4-k’)t propane/2,3-dimethylbutaneratio since (4)would be expected to increase with dilution relative to (3) al = 5a00 [e-(4/6 k + k?l - e-(6/6 k + k 3 f ] and hence increase the ratio. The occurrence of az = 10aoO [ e - ( 6 / 8 k + k 7 f - 2e-(4/6 k + k31 + e-(3/6 k + k 3 f ] (B) ( 5 ) would be shown by the formation cf HD, but this was absent. Insofar as the isotope distribution etc. among the products is concerned, it must lie within The analogy to a sequence of radioactive decay the limits imposed by (a) the formation of propane processes is obvious. by (4) exclusively, and (b) the absence of reaction 6 (4). Case (a) would be evidenced by the complete a, = aooe-k‘t (C) 2 = 0 absence of deuteration of the residual propene. (1) Now a t bfelriar, Inc., Falls Church, Va. ( 2 ) R. Klein and RI. D. Scheer, THISJOURNAL, 62, 1011 (1958). (3) P. J. Boddy and J. C . Kobb, Pmc. Rou. SOC.(London), 249, 518
(IDBD).
(4) R. Klein, M. D. Scheer and J. Waller, THISJOURNAL, 64, 1247 (1960). (5) The hydrogen on the carbon containing the free spin is not
considered abstractable.
Feb. , 1961
SOLVEKT EXTRACTIOX STUDIESOF INTERHALOGEN COMPOUNDS OF ASTATIXE
The isotope distribution among the propanes Ai can be derived from the set dt
= (1
- ~ ) k / ‘ a i -+~ Bk”ai-l
+ k”’ai--2
(D)
B is the fraction of the potentially abstractable hydrogen in the radical which is of the H variety. The first term on the right of (D) arises from the D atom addition to ai-2 with its subsequent participation in a disproportionation reaction in which it abstracts a deuterium from an isopropyl radical. The second term refers to the D atom addition to ai- 1 followed by its abstracting hydrogen from another radical. The third term is associated with reaction (4). The derivation of R from set (B) is simply
5
fj =
( 5 - i)ai
czO--
2 6ai
= .5/fie-k/6
8
Propene with butane as a diluent reacted at 77” The propene mas diluted so that a condition of constant atom concentration through the film could be maintained.4 After reaction, the propene and propane mere separated by gas phase chromatography and each fraction analyzed by mass spectrometry. IC’ was determined from equation (C) with the analytical data for diminution of total propene with time. With the assumption that k = k‘ = k”, the isotopic propene fractions were calculated from the set (B). Although the fractions could not be determined unambiguously from the data of Table I because of lack of reference spectra, the results are in moderately good agreement with the calculated values.6 This is confirmatory evidence that (4)occurs, if a t all, only to a small fraction of (2) under the experimental conditions used in this work. E(. with deuterium atoms.
(E)
TABLE I MASSSPECTRA OF PROPENES FROM THE D REACTION=
2 = 0
Calculation of the A’s gives
d e
etc. 7
A, 2 = 1
This gives k” which is small,
=
aoo[l - e-(k”+ k”311
+ k”’ 5
a, i=o
k’ since, neglecting (3)
=
+
5
Q
A,
325
0
Reaction time, minutes
10
36 1.0 1.0 37 4.8 4.3 38 6.5 6.0 39 28.0 24.8 40 10.8 11.8 41 41.7 36.3 42 25.9 26.1 43 1.0 4.5 0.9 44 45 0.1 46 Relative to the 36 peak.
+ PROPENE
30
100
1.0 3.8 6.0 23.6 12.4 33.0 27.2 8.4 3.3 0.3 0.1
1.0 4.0 6.0 19.3 13.3 27.3 25.3 13.3 7.7 1.7 0.3
= ao0
i = l
If (4)does not occur, then 1;”’ = 0 and k = k‘ = k“.
(6) Data for the propane fractions were also obtained but again lack of reference spectra for the more highly deuterated species precluded a rigidly critical comparison between calculated and observed values.
SOLVENT EXTRACTION STUDIES OF INTERHALOGEN COMPOUNDS OF ASTATINE’ BY EVANH. APPELMAK~ The Department of Chemistrg and the Lawrence Radiation Laboraforg of the UniversitzJ of California, Berkeley, Colifornia Recewed Aueuet 88, 1960
The distribution of astatine between aqueous solutions and CC4 has been used to study the reactions of this synthetic element with 12, I-, IBr, Br- and C1-. The species AtI, AtRr, AtI2,- AtIBr-, AtICI-, AtBrr- and AtCL- have been characterized, and the equilibrium constants interrelating them have been evaluated. These constants have been shown to correlate well with the analogous constants involving the lighter halogens. I n the course of this investigation the equilibrium constant for the distribution of IBr between water and CCl,, and the formation constant of IBrz- have been redetermined, and the results are in substantial agreement with previous work.
Introduction The investigations of the solvent extraction behavior of astatine that have been carried out heretofore have been qualitative in n a t ~ r e . ~ -The ~ (1) Based on work performed under the auspices of the U. S.Atomic Energy Commission. (2) (a) Argonne National Laboratory, Argonne, Illinois. (b) Abstracted from the Ph.D. thesis of the author, University of California (Berkeley), June, -1960 (UCRL-9025).
principal obstacle t o quantitative studies has been the necessity of working with this highly radioactive element only at exceedingly low concentrations. At such concentrations it is difficult to preventthe astatine from reacting with impurities in the experimental system’ (3) G. Johnson, R. Leininger and E. SegrB, J. Chem. Phzls., 17, 1
(1949). (4) H.M. Neumann, J. Inorg. Nucl. Chem., 4,349 (1957). (5) E. H.Appelman, J. Am. Chem. Soc., 82, 000 (1960).
