COMMUNICATIONS TO THE EDITOR
4455 distribution in Table I is consistent with this conclusion. I n addition to the possibility that allyl radicals are produced in a primary process such as reaction 1, one must also consider the possibility that they result from the decomposition of excited species formed in the primary photolytic process such as 2 and 3, and finally radical-olefin bimolecular reactions such as 4 and 5 must be considered as possible sources.
Table I
+
m
QW
Y
a, 0.001 Mimrlllo
0.1 0.4 1.0 6.0
0.99819 0.99260 0.98065 0.8496
1.015 1.062 1.177 2.845
0.99835 0,99325 0.98254 0.8761
Using values of a, and y a$eady determined,4 the 0.001 Mlrny/Z0 are calculated following values of a, in Table I. There is, therefore, no inconsistency between the semiideal behavior of sucrose solutions and the experimental data.
+
Table I : Relative Yields of Six Carbon Products from the Photolysis of Propane at 123.6 nm'
(4) R.A.Robinson and R. H. Stokes, J . Phgs. Chem., 65,1954 (1961)-
* To whom correspondence should be addressed at State University
of New York at Binghamton, Binghamton, N. Y.
UNIVERSITY OF NEWENGLAND PRMIDALE, N. S. W. AUSTRALIA STATEUNIVERSITY OF NEWYORK AT BINQHAMTON BINGHAMTON, NEWYORK
R. H. STOKES 5
R. A. ROBINSON*
Pressure of propane = 50 Torr.
CaHs
RECEIVED JULY24, 1970
+ h~
=
CaHb
+ Hz + H
+ h~ = Hz + CaHs* = CaH6 + H CaHs + h~ = H + C3H7" = C3H5 + H2
CaH8
C3He 3. H = CaH6
Allyl Radicals in the Vacuum Ultraviolet Photolysis of Propane
Sir: The previously unreported products, 4-methyl-lpentene, l-hexene, and 1,5-hexadiene have been identified in the product mixture resulting from the photolysis of pr0pane.l The photolysis was carried to less than 0.1% conversion in the gas phase using a microwave powered krypton resonance larhp. The only other sixcarbon products are 2,3-dimethylbutane, 2-methylpentane, and n-hexane. It is noteworthy that only unsaturated products containing terminal double bonds are observed. This strongly suggests that the precursor of the unsaturated product is an unsaturated radical. If a hexane formed with excess energy via the recombination of propyl radicals were to decompose by hydrogen elimination, instead of the more favorable carbon-carbon bond scission, it would certainly decompose to give some of the thermodynamically favored products w%h interior double bonds. Moreover, the production Of Olefins from saturated six-carbon precursors would easily be pressure quenched at one atmosphere of total sample pressure.2 The most important six-carbon olefin, $-methyl-l-pentene, is still present in significant quantities at pressures greater than 5 atm. It seems most reasonable to conclude that the observed unsaturated products result from the combination of allyl radicals with isopropyl, npropylI and allyl radicals, respectively. The product
1000 106 260 93 2 15 9
Methane 4-Methyl-l-pentene 2,3-Dimethylbutane 2-Methylpentane 1,5-Hexadiene l-Hexene %-Hexane
CH3
'
+ H2
+ CzHz = C3Hs
(1) (2) (3)
(4)
(5)
Previously determined rate constants for reactions 4 and 5 suggest that they, in fact, will not be important relative to other reactions of H and CHa. At 0.1% conversion, abstraction from propane and addition to propylene by hydrogen atoms will effectively use 97% of the available H relative to reaction 4.3 The remaining small fraction of H can account for less than 1% of the observed allyl radical yield. To be certain that some unusual reaction involving propylene was not occurring, e.g., with energetic H atoms, excess propylene was added to the reactant propane. Up to 1% propylene by volume did not appreciably affect the yield of .hexenes relative to hexanes. Similar considerations for the methyl radical show that it is a thousand times as likely to recombine with other radicals in the photolytic system as to add to acetylene (1) (a) A. L. Lane, Ph.D. Thesis, University of Illinois, 1968;Nucl. Sci. Abstr., 23 (14),28652 (1969); (b) R.E.Rebbert and P. Ausloos, J . Chem. Phvs., 46, 4333 (1967); (01 D. W. L. Grifith and R. A. Back, &id., 46, 3913 (1967); (d) A. H.Laufer and 3. R. McNesby, dbid., 70, 4094 (1966); (e) P. Ausioos and S. G. Lias, ibid., 44, 521 (1966); (f) J. R. McNesby and H. Okabe, Aduan. Photochem., 3, 157 (1964); (g) P. Ausloos, S. G. Lias, and I. B. Sandoval, ~ i Faraday ~ sot., ~ 36, 66 ~ (1963);~(h) H.~Okabe .and J. R. McNesby, J . Chem. Phys., 37, 1340 (1962). (2) B. 8. Rabinovitch and D. W. Setser, Advan. Photochem., 3, 1 (1964). (3) K.R. Jennings and J. R. Cvetanovic, J . Chem. Phys., 35, 1233 (1961). The Journal of Physical Chemistry, Vol. 74,No. 26,1970
COMMUNICATIONS TO THE EDITOR
4456 PRESSURE DEPENDENCE OF ALLYL R A D I C A L S
P (Atm)
Figure 1.
at the concentrations of acetylene available at low conversion (0.1%) . 4 Any other bimolecular mechanism that one can conceive involving the fragments available from the photolysis of propane can be discounted in a,manner similar to that used for the examples of reactions 4 and 5 . The essential difference between reaction 1 and reactions 2 and 3 is whether all three bonds are broken simultaneously or in a stepwise fashion. The contributions of decompositions such as 2 and 3 cannot be easily determined. Increasing pressure will tend to stabilize such excited species. However, as these species may decompose by more than one path, a direct relationship is difficult to establish. Allyl radicals are apparently produced via unimolecular decompositions. However, whether it is the unimolecular decomposition of CaHa,* C3H,*, or CaHa* or some combination has not been established. The
The Journal of Phgsical Chemistry, Vol. 74, No. $6, 1070
shape of the curve in Figure 1 suggests that more than one precursor (or one precursor decomposing via different modes) are important. Using the simplest case of multiple precursors, two, we have determined two lifetimes (reciprocal of the unimolecular rate constant). The lifetime of the short-lived precursor was determined from the linear high-pressure region of the curve. This contribution was subtracted from the low-pressure region and a lifetime for the longlived precursor was calculated. These calculations give lifetimes of 2 X lo-" sec and 9 X sec for the short- and long-lived precursors, respectively. Since it is not apparently known how energy partitions among fragments in the photolysis of propane, the lifetimes determined cannot be associated with specific fragments or states as precursors on the basis of presently available data. Since most other fragments arising from the photolysis of propane show a much less striking dependence on pressure than allyl radicals, it is reasonable to conclude that primary photodecomposition in propane occurs in 5 2 X lo-" sec.
Acknowledgment. The authors wish to thank the National Science Foundation for financial support of this work. (4) L. Mandelcorn and E. W. R. Steacie, Can. J. Chem., 3 2 , 474 (1954).
* To whom correspondence should be addressed. DEPARTMENT OF CHEMISTRY NORTH DAKOTA STATEUNIVERSITY FAROO, NORTH DAKOTA58102 RECEIVED AUGUST10, 1970
J. H. VORACHEK R. D. KOOB*
