3478
R. T. K. BAKERAND R. L. WOLFGANG
On the Occurrence of Simultaneous Double Displacement in Hot-Hydrogen Reactions by R. T. K. Baker’&and R. L. WolfgangIb Department of Chemistry, Universitu of Colorado, Boulder, Colorado
(Received A p r i l 4,1969)
Two general mechanisms for reaction by hot hydrogen to replace two atoms or groups are considered. In this connection the pressure dependence of the reaction of recoil tritium with ethane was measured. As expected, the methyl radical, CHzT., was found to derive largely from the decomposition of excited ethane, CzHsT*, formed by the familiar single-displacement process. However, a significant yield of ethyl radical, C2H4T.,is also formed. The appearance and relative lack of pressure dependence of this product is consistent with, but does not prove, a direct or simultaneous double-displacement mechanism. A reexaminationof earlier data does not support any conclusion that the direct double-displacement mechanism is negligible. The question of whether such a process occurs is still open, although it remains clear that this is not a major mode of hot-hydrogen reaction.
+
+
When hot hydrogen atoms interact with hydroT* R’-CXY-R+ R’-CTY-R* X (5) carbons the two main classes of reaction are abstraction, R’--CTY-R* ----t R’-qT-R Y (6) to form molecular hydrogen, and the replacement of various atoms or groups by the incoming fast a t ~ m . ~ J Alternatively, the process could go (B) by a direct Single replacement is quantitatively the most impordouble displacement, with two particles being ejected tant type of replacement reaction. Using recoil triessentially simultaneously. The entire process would tium as the source of hot hydrogen results in labeled occur within about sec and no longer-lived intermediate would be involved. hydrocarbons Mechanism A would be pressure dependent since T* R X ---f R T X (1) reaction 6 which yields the final product must comT* R’R” ----f R’T R” pete with collisional deactivation of the intermediate
+ +
--f
+ + R”T + R’
(2)
There is now general agreement that the mechanism of these processes involves a single ~ t e p . ~ *The , ~ reactions are direct and no intermediate species having a lifetime longer than about the period of molecular motions sec) is involved. Double displacement is also found
T”
+ R’-CXY-R
+ (X,Y) +R’QTX + (R,Y) etc.
--.)
R’CTR
(3)
(The radical products thus formed may be observed as the corresponding halide if halogen scavenger is present; e.g., see reaction 4.) R’CTR
+ Br2-+
R’CTBrR
+ Br
+ M +R’-CTY-R
+ M*
(7)
Mechanism B would, of course, be pressure independent. Since excess energy is available in hot-atom reactions, it is plausible that primary reaction products may be sufficiently excited to decompose.2 Hence, it is not surprising that pressure-dependence studies show that double replacement by mechanism A is significant, particularly in systems where the primary single-displacement products have a low barrier toward decomp~sition.~-~ The important question remains whether hot-atom reaction to simultaneously replace two bound entities can occur at all, ie., whether any double-replacement
(4)
This type of process generally occurs in much lower yield, a finding which has been explained in terms of limitations in energy transfer in a fast, localized colli~ i o n . ~Beyond ,~ this, however, the mechanism of double replacement is still not fully understood. On the basis of early work, two possible mechanisms were suggested :4 (A) single-displacement which yields a product sufficiently excited to then undergo unimolecular decomposition; e.g. T h e Journal of Physical Chemistry
R’-CTY-R*
+
(1) (a) Atomic Energy Research Establishment, Harwell, Didcot, Berkshire, U.K. ; (b) Department of Chemistry, Yale University, New Haven, Conn. ( 2 ) M. A. El Bayed, P. J. Estrup, and R. Wolfgang, J . Phys. Chem., 62, 1356 (1958). (3) (a) R. Wolfgang, Progr. Reaction Kinetics, 3, 99 (1965); (b) R. Wolfgang, Ann. Rev. Phys. Chem., 16,15 (1965). (4) D. Urch and R. Wolfgang, J . Amer. Chem. Soc., 83,2982 (1961). (5) E. K.C. Lee and F. S. Rowland, ibid., 85, 897 (1963). (6) Y.Tang, E. K. C. Lee, and F. S. Rowland, ibid., 86, 1280 (1964). (7) A. J. Johnston, D. Malcolme-Lawes, D. 8. Urch, and M. J. Welch, Chem. Commun., 187 (1966).
SIMULTANEOUS DOUBLEDISPLACEMENT IN HOT-HYDROGEN REACTIONS products are formed by mechanism B. An earlier review of relevant data suggests, but by no means proves, that simultaneous displacement might indeed occur. This view received some support from the finding that formation of CH2T by recoil tritium attack on methane was essentially pressure independent.* Tang and R,o~vland~ have recently attempted to determine whether mechanism B was completely negligible. Double-displacement reactions of tritium with propyl chlorides would yield radicals such as
-
T
+ CH3CH2CH2C1+CHTCH2CH2C1+ 2H or H +CH&TCH2C1 + 2H or H2 (8)
Products from further decomposition of such radicals were sought CHTCHzCH2Cl+CHT=CHCH2C1 CH&TCHZCl+
CH&T=CH*
+H
+ C1
(sa) (9b)
It was convincingly shown that the propylenes corresponding t o reactions 9 were produced in less than 0.1% yield. A conclusion that an upper limit of 0.1% can be placed upon single-step double displacement, reaction 8, is, however, more doubtful. It rests on the implicit, but rather questionable, assumption that the radical products of reaction 8 will inevitably decompose according to the modes shown in (9). I n the case of CHTCH2CH2C1,for instance, eliminating of CH2Cl takes less energy than elimination of H , and therefore should be much mbre likely than (sa). However, the corresponding product CHzCHT was apparently not sought. What is even more questionable is the extent to which these radicals will decompose a t all. Experiments by Rabinovitch’O on decay of propyl radicals indicate that an excitation of greater than 3 eV will be required for such species to decompose efficiently a t pressures of about 0.1 atm. There is no evidence that any singlestep double displacement by hot hydrogen will normally leave such a considerable excitation in the radical products. Thus the assumption that limits placed on reaction 8 are also limits on reaction 7 lacks adequate justification. I n a further consideration of the importance of single-step double displacement (B), Tang and Rowlandlo reexamined evidence on yields in the reaction of recoil tritium with methane.8 They showed that the very small pressure dependence of CH2T. formation, as well as its relative yield, could be explained by assuming that the primary single-displacement process
T*
+ CH,
CH3T
+H
(10) left the CH3T with a uniform spectrum of excitation energies from 0-5.7 eV/molecule. Decomposition of the more highly excited CH3T would yield the right amount of CHzT-, and there would be little effect of pressure in the range measured. Thus, direct production of CHzT. by mechanism B would not be ----)
3479
required to account for the results. However, in all fairness, the consequences of other, equally arbitrary, assumptions should also be examined. Suppose, for instance, that the CH3T is formed with any distribution of excitation energies below 4.4 eV/molecule. I n that case none of it could decompose to CH2T., and this product would then have to be formed entirely by mechanism B. Obviously, the only conclusion that seems defensible is that by making the appropriate assumptions as to excitation energies deposited in primary single displacement, one can “justify” almost any prejudice as to the importance of single-step double displacement (B). I n summary, we may say that the statemente “ . . .the occurrence of a “double substitution’’ process in a single step. . . (amounts) at most to a yield of 0.1%. . . ”, while it may well be true, lacks foundation (even when restricted to propyl chloride systems). I n an attempt to more definitively estimate the extent, or the lack of it, of direct double displacement, we have carefully examined certain reactions of recoil In this system, methyl radical tritium with CH2T. is produced in fair yield. Previous studies of the pressure dependence of this product’ indicate that it is in large part formed by the two-step mechanism (A)
+ C2Hs +C2HjT* + H CzH5T* +CH2T. * + CHI. T
(11)
A E = 3.82 eV/molecule (12) Our work focused particularly on possible production of the radical CZHdT
T
+ C2H6+C2H4T.+ 2H or H2
(13)
Reaction 13 is exactly analogous to the system of reaction 8, but in contrast to the latter case, it is not necessary that the product decays further in order to be detected. Formation of C2H4T. by the two-step mechanism involving CzH5T* CzHST* +C2H4T.
+H A E = 4.25 eV/molecule (14)
should be relatively small. This is because unimolecular decay strongly favors the less endoergic’l route (12) Furleading to CHzT., over (14) leading to CZH4T thermore, wThat C2HIT. is produced by mechanism A might be expected to show a pressure dependence similar to that of CHzT., at least to the extent that the precursor of both species is C2HsT” excited to the same energy. a .
(8) D. Seewald and R. Wolfgang, J . Chem. Phys., 47, 143 (1967). (9) Y.Tang and F. S. Rowland, J . Phys. Chem., 72, 707 (1968). (10) W. E. Falconer, B. S. Rabinovitch, and R. J. Cvetanovic, J. Chem. Phys., 39,40 (1963). (11) 8. W. Benson, J. Chem. Educ., 42,502 (1965).
Volume 73,Number IO
October 1969
R. T. K. BAKERAND R. L. WOLFGANG
3480 Table I: Pressure Dependence of Recoil Trhium Reaction with Ethane
Ethane Helium-3 Bromine
2710 5.5 36.9
1960 5.9 38.5
944 5.6 36.2
677.6 5.3 30.8
328.4 5.4 29.0
184.3 5.6 21 .o
119.7 5.2 6.9
Absolute" (a) and Relativeb (b) Yields Products
(a)
(b)
(a)
HT CHaT Ci"T CzHsT' CHzTBr CzH,TBr TOTAL
40.8 2.9 25.6
159 11 100.0
39.1 2.7 24.1
