11. AVRAHAMI AND P. KEBARLE
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Vol. 67
PRIMARY REACTIOXS I S THE MERCURY-PHOTOSESSITIZED DECORfPOSITION OF PROPYLENE BY M. AVRSHAMI* AND P.KEBARLE Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada Received August 10,1963 It is shown that propylene decomposes by three primary reactions (1) (2) (3) There is no molecular elimination
+
C8Hs*-+- allyl H CaH6* vinyl CH3 C3Ha* +cyclopropane of hydrogen in the primary step.
+
Introduction The mercury-photosensitized decomposition of olefins is known to proceed by an excited molecule mechanism. Excited molecules may decompose by one or more reaction paths. For a considerable time it was believed that the olefins decompose only by cleavage of a C-H bond until, comparatively recently, Lossing2 showed that molecules like butene-I decompose by breaking the allylic C-C bond in addit,ion to the allylic C-H bond. Such multiple readion paths are of interest since they throw some light on the properties and unimolecular decomposition kinetics of the excited molecule. Propylene has been s h ~ w to n ~decompose ~ ~ by (1) allyl
+ H + Hg
(1)
The present work gives evidence for two additional primary reactions. Experimental The propylene was decomposed in a flow system attached t o a mass spectrometer. The apparatus, conditions, and methods of analysis have been described in detail elsen~here.2~4+The pressure of the helium carrier gas was 10 mni. and that of the propylene 2 p. The temperature of the reaction zone was 52" and the gas stream was saturated with mercury at this temperature. The contact time mas 1.4 msec. The reaction products were measured directly with the mass spectrometer or collected by freezing in liquid nitrogen and analyzed by gas chromatography.
Results and Discussion The products from a typical run are given in Table I. Losing2 has found that the allyl radical and hydrogen atom formed by (1) give rise to allene, hydrogen, and 1,5-hexadiene. It will be shown that the other compounds result from two additional primary steps C3H6* -+vinyl
+ methyl
cyclopropane (3) The secondary reactions consist, a t the specific conditions of the present apparatus, almost exclusively of radical-radical and radical-excited mercury interactions. All but one product in Table I can be explained if the presence of H, CH3, vinyl, allyl, and isopropyl radicals is assumed. The methyl and vinyl are thought to originate by ( 2 ) and the small amount of isopropyl by the addition of hydrogen atoms to pro-+
(1) University of Slberta poRtdoctora1 fellow. (2) F. P. Lossing, D. G. H. Xarsden, a n d ,I. B. Farmrr, Can. J. C h e w . . 34, 701 (1956). 13) H.E.Gunning a n d E. W. R. Steacie, J. Chem. Phw., 10, 926 (1948). (4)
P. Kebarle a n d MdAvrahami, Can. J . C h e d , , t o be published,
89 % 11%
0.2%
pylene. The recombinations of these radicals lead to hydrogen, ethane, 1,3-butadiene1 1,5-hexadiene1 2,3dimethylbutane, methane, ethylene, propane, (propylene), butene-1, isobutane, 1,4-pentadiene, 3-methylbutene-1, and 4-methylpentene-1. All these products, with the exception of 3-methylbutene-1, were found and identified. The retention volume of the 3-methylbutene-I, on the dimethylsulfolane column used, is such that the compound should appear in a region where the very large allene peak tails. N o special effort was made to find this very minor product. TABLE I PRODUCTS FROM DECOMPOSITION OF PROPYLENE^ H2 15.3 Isobutane 0.09 1.2 Methane 0.75 Butene-1 Ethane 0.26 1,3-Butadiene 0.07 Ethylene 0.23 Pentene-1 .025 Acetylene 2 12 2,3-Dimethylbutane .01 Propane 0.48 1,4-Pentadiene .18 Cyclopropane 0 052 4-Methylpentene-1 .17 Allene 12.6 1,j-Hexadiene 1.8 a Products expressed per 100 molecules initial propylene. Final propylene -75.
The allene and acetylene probably result partly from disproportionation of H and allyl and vinyl, respectively, and partly from the decomposition of these radicals by excited mercury atoms. The cyclopropane must originate from the primary reaction (3). I n order to confirm ( 2 ) and (3), experiments were conducted in which the propylene (1 p ) was decomposed in the presence of an excess of fully deuterated mercury dimethyl. The decomposition of mercury dimethyl gives almost exclusively methyl radical^,^ so that the experiment aims to "scavenge" the allyl and vinyl radical with CDB producing C3H&D3 and CtH3CD3. Figure I gives plots of C3H5CD3and C2H3CD3formed per molecule of propylene decomposed as a function of the initial pressure of mercury dimethyl. The data were obtained by mass spectrometric measurement of the ion peaks a t mass 42 (AC3Hf,), 45 (C2H3CD8), and 59 (C3H6CD3). More detailed information could be obtained by trapping out one mercury dimethyl), separating by gas run6 (10 chromatography, and analyzing the fractions with the mass spectrometer. The results are shown in Table 11. A study of the decomposition of mercury dimethyl alone has shown5 that aside from the ethane (450)' ( 5 ) P. Kebarle, J . Phvs. Chem., 67, 351 (1963). (6) I t IS shown in the Appendix t h a t downstream reactions occurring be-
fore trapping are unimportant. (7) The figures i n braokets refer to quantities obtained in a run with 10 r mercury dimethyl, alone, a n d adjusted to ethane 460,
-
Feb., 1963
MERCURY-PHOTOSENSITIZED DECOMPOSITIOK O F PROPYLENE
355
+
H allyl = C3H6 accounts for about 26y0 of the total decomposition. Setting (1) ( 2 ) = 20.5 equal to 74YGwe can calcuhte the relative contributions of the primary steps: (1) 90.2%, ( 2 ) 9.7%, ( 3 ) 0.19%. There is relatively good agreement between the two sets of data.
+
10
-.HD H2
Fig. 1.-Methyl addition products in decomposition of propylene (1 p) and deuterated mercury dimethyl (10 p): 0. CzH3CDd;0 , C3HbCD3in molecules formed per 100 propylene molecules decomposed.
small amounts of ethylene (1.3), acetylene (5.13), propylene ( 2 . 7 ) , and propane (3.1) are formed. Accordingly, the highly deuterated products in Table I1 must be due t o the mercury dimethyl alone. These are: ethylene (CzDI 1.2) acetylene (CzD2 and CJIR 6.6), propylene (C3HBand C3D6H2.4), and the total propane TABLEI1 CODECOMPOSITION OF 1 p PROPYLENE ASD 10 p DEUTERATED MERCURY DIVETHYL~ Ethane 450 450 C2D6 Ethylene 1 . 4 1.2 CzD4, 0 2 CzH3D Acetylene 8.2 3.0 CzDz, 3.3 CzDHs, 1.9 CzHz Cyclopropane 0.05 Propane 3 3 3.3 C3DB Propylene 81.5 2 1 CaDe, 0.3 CID,H, 1.1 CaD3H3, '7.0 C,H,D, 71 C3H6 Allene 1.0 1.0 CgH4 Butene-1 16 0 16 0 C3H6CD3 a Products given per 100 molecules initial propylene. The methane formed was not determined.
(C3Ds 3 . 3 ) . The remaining species can be used to establish the relative importaiice of the primary reactions. The butene-1, allene, and C3H,D8 counted together give 24 as contribution of reaction 1. Tlhe C,H,CD,, C2r12,and C2H3Dadded together give 3.0 for reaction 2. Finally, the cyclopropane gives 0.05 for reaction 3. Expressed in per cent the figures are: (1) 89% ( 2 ) 11%, and (3) 0.2%. A similar eEkimate is obtained from the decomposition of propylene alone (Table I). A methyl radical mass twice ethane isobutane balance (methane butene-1) gives 2.6, while the corresponding vinyl acetylene twice butaradical balance (ethylene diene 1,4-pentadiene) adds up t o 2.7. Thus the methyl split corresponds to -2.7. The allyl vinyl butene-1 pentene-1 t radical balance (allene 1,4-pentadiene 4-methylpentene-1 twice I , 5hexadiene) gives 17.8. However one important reaction has been omitted from the last balance. As is shown in the Appendix the normally not visible recombination
+
+
+ +
+
+
+ + +
+
+
(8) T h e presence of CsHsD is surprising. Some D atoms are released from t h e mercury-sensitized decomposition of t h e &De. Since t h e recombination of D (and H) atoms on t h e walls is slow they might be able to compete with +he methyl radicals.
Fig. 2.-Formation
of hydrogen in mixtures of C3H8and C31)~.
A primary step occurring in the sensitized decomposition of ethylene is the loss of molecular hydrogen. I n order to check whether this reaction occurs in the propylene decomposition a set of runs was made w ~ t h C3H6 and C3D6 mixtures. The results are shown in Fig. 2. Keeping the total propylene pressure constant (2 p ) the ratio C3H6/C,D6was changed and the corresponding Hz and HD formation determined. I n a plot of the Hz/HD ratio vs. the propylenes ratio the intercept with the ordinate should give the ratio of molecular hydrogen to atomic hydrogen formed in the primary step. Since the plot gives a straight line through the origin, we conclude that the elimination of molecular hydrogen does not occur. It is interesting that although the bond dissociation energy difference D(C2H3-CH3) - D(C3H,-H) is 16 k~al./mole,~ reaction 2 still occurs. The isomerization to cyclopropane is similar to the rearrangement of butene-1 to methylcyclopropane,found by Cvetanovi(,,lo but occurs t o a much lesser extent. An isotope effect was observed in the experiments with the deuterated propylene. It was found that the net decomposition of the two isomers, done under as nearly identical conditions as possible, was 28y0for the C3H6 and only 20% of the C3D6. It is most probable that this effect is not due t o differences in secondary reactions but reflects different rates of the primary decomposition. Appendix Recombination of Hydrogen Atoms and Allyl Radlica1s.-The extent of the recombination reaction in (9) A. G. Harrisonand F. P. Lossing, J . A m . Chem. Soc., 88, 519 (leeo). (10) R. J. Cvetanovie a n d L. C. Doyle, J , Chem. P h y a , t o be pubhshecl.
M. AVRAHAMI A N I P. KEBARLE
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Vol. 67
composition. The amount of recombination is given by the difference of the decomposition at mole fraction unity and zero and is found equal to 6.5% of the initial CaDs. Plot B gives the ratio: C3D6H/C3DB decomposed, in function of the mole fraction. The value of the ratio a t mole fraction unity equals 26% and corresponds to the recombination in per cent of the total decomposition. The results of plot A expressed in the same terms give an identical result: 6.5 X 10/25 = 26%. Downstream Reactions.-Since the data discussed were obtained partly by direct mass spectrometric analysis of the reaction stream emerging from the illuminated zone and partly by collection in a trap situated about 10 msec. downstream, a few remarks on possible differences in product yields due to down18 stream reaction night be of interest. No important 28 radical-molecule reactions could be occurring downstream. Otherwise, in the methyl saturation experi0 2 ment, one would have detected isopentane formed by methyl addition to propylene followed by recombixao20 D nation. Using equations and rate data obtained in the m u preceding paper,6 one can shorn that this is to be exa pected and that it is due to the rapid downstream decay 20 12 of the methyl radicals. 0 m V Since hydrogen atoms can recombine only on the a surface and their decay (at higher concentrations) is relatively much slower, downstream hydrogen-mole4 cule reactions are more probable. Thus in the decomposition of propylene alone the isopropyl radicals should be due to hydrogen atom addition to propylene. C3H5 Such reactions may become really important, downC3H6+C3D6 stream, if the reactor walls become accidentally poisoned Fig. 3.-Recombination reaction: allyl + H or D, respectively, for H-atom recombination. studied by the decomposition of mixtures of C3Hs and CoDe. Since downstream radical-molecule reactions generally are unimportant, it is of interest to establish whether the decomposition of propylene alone was determined by radical recombination and disproportionation adds using mixtures of C3H6and C3D6a t constant total pressignificantly to the collected product. An illustration sure ( 2 p ) but variable ratio. The results of such that the product increases little is obtained from the experiments are given in Fig. 3. Plot 4 shows the ratio: data of the preceding paper.5 Emerging from the observed decomposition of C3Da,over the initial C3Da (lamp off), in function of the mole fraction of C~HB. illuminated zone were 2 p of ethane and 0.25 p of methyl. Thus after complete downstream recombination, the As the mole fraction approaches unity the visible deethane should increase only to 2.13 p. composition becomes equal to the total primary de26
A
,
