21 86
W. S.Smith and Y . -N. Tang
portant and would mean that the amount of CHzCF2 formed by ionic pathways would be much larger than the amount of CHFCNF actually seen. The small yields of CHzFz and CH3F as well as the small yield of methrnne in the presence of NO seem to be the only indications of any molecular elimination of carbenes during the radiolysis of l,1-difluoroethane. No products resulting from the reaction of these carbenes were identified. What is clear is that this is EL much less important path in the radiolysis of I,l-tlifluoroethane than it is in the radiolysis of ethane itself.2a Summary The radiolysis of 1,l-difluoroethane produces CHF3 and CW3CF3 by fluoride ion transfer reactions involving fragment ions. Methyl radicals formed in the fragmentation of the parent ion eventually form methane and ethane. Of Iesser importance are the formation of CzH4, C ~ H S Fand , CH2CF2 via processes that are not completely identified, but lead to large yields of both hydrogen atoms and molecular hydrogen. Of minor importance are processes which produce CH4, C&F, and CHzF2 by direct elimination. The production of‘ CHFCHF hints at the possibility of scrambling among the fluorines and hydrogens of ethyl ions.
Acknowledgments. The authors thank the Nuclear Engineering Department of North Carolina State University for the use of the 6oCoirradiation facility. They also thank Dr. J . R. Hass and Mr. M. Hanafy for assistance in identification of some of the products. 5hppLementary Material Auailable. Table I and Figures 3,4,6, and 7 will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of
the supplementary material from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. C . 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number JPC-74-2183.
References and Notes (1) P. Ausloos, Annu. Rev. Phys. Chem., 17,205 (1966). (2) (a) H. H. Carmichael, R. Gordon, Jr., and P. Ausloos, J. Chem. Phys., 42, 343 (1965); (b) R. E. Rebbert and P. Ausloos, J. Res. Nat. Bur. Stand., Sect. A, 78, 329 (1972). (3) J. G. Calvert and J. N. Pitts. “Photochemistry.” Wiley, New York, N.Y., 1966. (4) R. F. Claridge and J. E. Willard, J. Amer. Chem. Soc., 87, 4992 (1965); D. W. Skeily, R. G. Hayes, and W. H. Hamill, J. Chem. Phys., 43, 2795 (1965); T. I. Balkas, J. H. Fendler, and R. H. Schuler, J. Phys. Chem., 75, 455 (1971). (5) A. Maccoll, Chem. Rev., 69, 33 (1969); G. E. Millward and E. Tschuikow-Roux, J. Phys. Chem., 78,292 (1972). (6) S. C. Chau, Y. Inel, and E. Tschuikow-Roux, Can. J. Chem., 50, 1443 (1972). (7) A. W. Kirk and E. Tschuikow-Roux, J. Chem. Phys., 53, 1924 (1970). (8) N. H. Sagert and A. S. Blair, Can. J. Chem., 48, 3284 (1968); M. B. Fallgatter and R. J. Hanrahan, J. Phjs. Chem., 74, 2606 (1970); E. Heckel and R. J. Hanrahan, Int. J. Radrat. Phys. Chem., 5 , 287 (1973); G. A. Kennedy and R. J. Hanrahan, J. Phys. Chem., 78,366 (1974). (9) P. M. Scott and K. R. Jennings, J. Phys. Chem., 73, 1513 (1969). (10) E. Tschuikow-Roux. W. J. Quiring, and J. M. Simmio, J. Phys. Chem., 74, 2449 (1970); B. Noble, H. H. Carmichael, and C. L. Bumgardner, ibM., 78, 1680 (1972). (11) (a) F. T. Jones and T. J. Sworski, J. Phys. Chem., 70, 1546 (1966). (b) See paragraph at end of text regarding supplementary material. (12) J. T. Bryant and G. 0. Pritchard, J. Phys. Chem., 71, 3439 (1967). (13) J. H. J. Dawson, W. G. Henderson, R. M. Q’Malley, and K. R. Jennings, lnt. J. Mass Spectrom. Ion Phys., 11, 61 (1973). (14) R. W. Fessendenand K. M. Bansal, J. Chem. Phys., 59, 1760 (1973). (15) M. S. 8. Munson and F. H. Field, J. Amer. Chem. Soc., 87, 4242 (1965). (16) P. Ausloos, S. G. Lias, and I. B. Sandoval, Discuss. Faraday SOC., 67 (1963).
Consecutive Unimolecular Decomposition Following Recoil Tritium Activation of 1,l-Diflruorocethanel
. !S.
Smith and Y. -N.Tang”
Depmtment of Chemistry, Texas A & M University, College Station, Texas 77843 (Received February 14, 1974; Revised Manuscript Received June 2 1, 7974)
Excited CzH3TF2 formed by T*-for-H substitution in C H ~ C H F gives Z CzH2TF and CzHT as the secondary and tertiary decomposition products, respectively. The observed trends demonstrate that the CzH3TFz yield increases, the C2HT yield decreases, while the CzHzTF yield exhibits a maximum with increasing pressure. These trends are consistent with a mechanism of consecutive unimolecular decomposition in which all the excitation energy is introduced during the initial substitution.
htroductioa Although consecutive reactions in thermal systems are discussed in every kinetics textbook,2 the consecutive decomposition initiated by hot atom activation has scarcely The Journal of Physical Chemistry, Vol. 78, No. 22. 1974
been i n ~ e s t i g a t e d . ~Previously, -~ Krohn and coworkers have presented some good evidence in a series of recoil 18F studies for the consecutive decomposition of CF318F to give CFlsF and CHzl8FCF3 to give CH18F.3 However, in order
Consewtive Llnimolecuiar Decomposition of 13 1-Difluoroethane to provide a better quantitative basis for future theoretical treatment, a system which yields nonradical species as decomposition products should be more appropriate. From the pressure ritudies of recoil tritium reactions with cyclobutane, Lee and Rowland have obtained a broad energy distribution with a “median” energy of 5 eV following the T*-for-H substitution.6 Subsequent studies with CH3NC demonstrated that the substitution products are always left in a high state of internal e ~ c i t a t i o n .This ~ quantity of excitation energy should be enough to initiate consecutive unimo~e~ular processes in some suitable system. In the present work, we wish to report an experimental study of a Consecutive decomposition following the T*-forH substitution in CH&HF2. In hot atom systems, if the excitation energy arising from the primary substitution is high enough, it is possible that the secondary decomposition product inay undergo a tertiary reaction. In the CH3CHF2 case, the secondary product, vinyl fluoride, may further decompose by HF elimination to give
+
T*
---
C2H4F2
I:C2H3TF,)*
+H
(C2H3TF2)*
(C2H2TF)* + H F
(Ce2H2TF)*
CzHT
(1) (2)
+ HF
(3 1
Experimental Section The standard techniques for recoil tritium studies were adopted. Samples containing CH3CHF2 with a small amourit of 3He and 0 2 were sealed by high vacuum techniques in Pyrex 1720 bulbs at pressures ranging from 100 to 900 Torr. Energetic tritium atoms were produced by the nuclear reaction 3He~:n,p)3H.All irradiations were carried out a t the Texas A & M University Nuclear Science Center Reactor, where samples were subjected to a thermal neutron flux of I X I 013 neutrons/cm2 sec for 5 min. The usual procedure for radio-gas chromatography was followed.lO A 50-ft dimethylsulfolane column (35%) a t 25O was used for the :separation of the major products, C ~ H ~ T F CzHzTF, Z, and C2HT. H T and CH3T were resolved with a 50-ft column of 10% propylene carbonate coated on alumina which was operated at Oo. Quantitative yields of each tritium labeled product were obtained by means of a gas proportional counter. Helium-3 with a tritium content of less than 2 X % was obtained&from the Monsanto Research Corp. 1,l-Difluoroethane was obtained from Matheson Co. in >98% purity. Oxygen with a minimum purity of 99% was obtained from Airco.
Results and Discussion Pressure Dependence of Reaction Products. Besides the T*-for-H substitution as shown in (l),other primary processes horn recoil tritium reactions with CHaCHFz include H abstraction, F abstraction, T*-for-F substitution, and T*-for- R displacements
rr*
t G~H,F,
T‘
4
CESCHF,
T*
4
CH,CMF,
-
-
HT
+
C~H,F,
(4)
TF
+
CH&HF
(5)
-+
T‘k iCHSCHF, -T+
CH,CHTF CH3T
+
L CHTF,
+
F
CHF,
+
CH3
(61 (7)
(81
2187
76-
72 -
..
g
1 I
w 64P
::
;20=:
-0” 16-
0
B
IZ-
81
0
100
200
300
400
500
600
700
800
900
IO00
Pressure ( T o r r )
Figure 1. Percentage yields of
stabilization and decomposition products from recoil tritium reactions with CH3CHF2 as a function of pressure. All of the expected primary products with the exception of T F have been experimentally observed. At a pressure of 1 atm, the yields of these products (with reference to the yield of H T as 1000) are CH&HTF (3), CH3T ( 5 7 ) ,CHTF2 (12), and CzH3TFz (520). Products expected from the consecutive HF-elimination processes, (2) and (3), are also observed. A t 1 atm, their yields relative to that of H T as 1000 are C2H2TF (72) and CzHT (30). The pressure dependence of the yields for these two products and that for the stabilization product, C2HsTF2, is shown in Figure 1. They are expressed as percentages of the initial amount of excited CzHaTFz. It is obvious from the figure that when the pressure increases from 100 to 900 Torr, the yield of CzH3TFz increases, the yield of CzHT decreases, and the yield of CzHzTF goes through a maximum. In reactions 2 and 3, T F may be eliminated instead of H F to give nonlabeled products. In order to account for the undetected TF, the observed yield of vinyl fluoride must be multiplied by 41’3 and the observed yield of acetylene must be multiplied by 2. A tritium isotope effect of unity is assumed here. All of the data shown in Figure 1 have already been corrected for these factors. Other decomposition paths for excited CzH3TFz such as H2 and F2 elimination are also possible. (C2H3TFz)* (C2H3TFz)*
-
-
C2H3T
CHT=CF2
+ F2
+
(91
H2
(10)
However, at 1 atm, the observed yields of CzH3T and CHT=CF2 are only 4 and 6, respectively, relative to H T as 1000. These small yields indicate that both (9) and (10) are relatively unimportant decomposition routes when in competition with (2). In order to show the complementary nature of the stabilization and decomposition products for the T*-for-H substitution route, t,he corrected yields of CzH3TF2, CzHzTF, and C2HT (expressed relative to H T as 1000) are summed up. An assumption that the H T yield is pressure independent has been made here. The sum of the three yields at various pressures is roughly a constant with a value of 670 f 10. This also confirms that the product contributions from other decomposition routes such as (9) and (10) are very minor. The Journal of Physical Chemistry, Vol. 78, No. 22, 1974
W. S. Smith
2188
Consecutive Decomposition Following Hot Atom Activation. There are some significant differences between the consecutive decomposition initiated by hot atom activation and that by conventional thermal activation. In the thermal activation case, the excitation energy is introduced into the activated molecule in two separate steps since the molecules recejve the internal energy as a result of bimolecular collisions. Whenevei* the parent molecule has accumulated enough energy, it will undergo the first decomposition process. The resulting product will only undergo further decomposition when enough excitation energy is again accumulated. Thih means that another collisional excitation is in general a requirement between the two decomposition step,o. On the other hand, in the case of hot atom activation, all the required excitation energy is introduced into the parent molecule during the primary substitution process. Bimolecular collisions with other species will have a negative effect. Instead of supiplying energy and causing the second decomposition, a collision will normally deactivate the excited species. As a wsult, an increased collisional frequency will enhance the stabilization and not the consecutive decomposition. Theoretically, consecutive decomposition similar to that o f the hot atom activation system can also be initiated by other nonthermal processes such as photolysis and chemical activation. However, there is essentially no report in the literature which is solely devoted to the study of consecutive reactions mitiated by these methodsa3 Scheme of Stabilization and Consecutive Decomposition in Recoil Tritium Activation of CH3CHF2. The thermal decomposition of CH3CHF2 has been studied by Tschuikow-Roux, Quiring, and Simmie with shock tube techrliques.8 The HF elimination from CH3CHFz has an activation energy of 61.9 f 1.8 kcal/mol while the decomposition of vinyl fluatride has an activation energy of 70.8 f 3.6 kcal/m01.~The heat of reaction for the former process is 20 kcal/moi while that of the latter is 17 k c a l / m ~ l . This ~,~ means that during the first consecutive decomposition step the total excitation energy of the two products, vinyl fluoride and HI”, will be decreased by an additional 24 kcal/mol due to the ther modyinamic requirement. The full scheme of stabilization and consecutive decomposition of recoil tritium activation of CH3CHF2 is shown in Figure 2. Axording to the amount of excitation energy, we have divided the primary substitution product, CzH3TF2, into three categories. The first type includes those with energy below 62 kcal/mol, the activation energy for reaction 2. These molecules, which are represented by one asterisk, have only one reaction path available to them, i e , collisional stabilization to yield the labeled parent. The second group of excited molecules, which are marked with two asterisks, possess energy between 62 and 91 kcal/mol. The latter quantity, which represents the minimum energy requirement to initiate the consecutive decomposition, is evaluated by adding the heat of reaction o f (2) to the activation energy for HF elimination from vinyl fluoride neglecting the amount of energy carried away by the H F in resction 2. Excited molecules in this category may either decompose to vinyl fluoride or be stabilized to give the tritiated parent. The stabilization process may be carried out by either strong or weak collisions. In other words, the excited molecule may either lose all its excitation in a single encounter, or it may cascade down in energy by multiple collisions,. The Journal of Phlsical Chemistry, Vol. 78, No. 22, 1974
( H-
C z C-T) * & F C -
-_
+M
Figure 2.
and Y . -N. Tang
Possible stabilization and decomposition modes of excited hot atom excitation.
C2HsTF2 subsequent to
E < E,
El< E
AH1 €2.
Acknowledgment. This research was supported by AEC Contract No. AT-(40-1)-3898.
The pressure dependence is such that A increases with pressure, C decreases with pressure, while B exhibits a
References and Notes
+
+
)3@-O-@H3,over the temperature range 184.5-230O in static reactors. The reactions in each case are first order with activation energies of 36.1, 29.6, and 36.2 kcal/mol, respectively. A correlation was found between the order of stability and electronegativity of the group attached to the tris(difluoroamin0) functionality.
Intraductiain
Recently several report^^-^ have appeared concerning the synthesis and chemistry of difloroamino (NF2) compounds. In a study of the decolmposition behavior of a series of poly(difluoroamino)fluoromethanes4 attention was drawn to the similarity in rate parameters to those of polynitro com-
pounds. As a result the rate-determining step was postulated as homolytic cleavage of the C-N bond as had been previously proposed for the polynitro compounds.6 Herein thermal decomposition results are reported for three compounds having the general structure (NF2)8C-Z with Z = C1, NH2, OCH3, not previously reported. A similar study of related compounds with Z = NFz or F has already The Journal of Physical Chemistry. Vol. 78, No. 22. 1974
