M. BELLAS,J. WAN,W. ALLEN,0. STRAUSZ, AND H. GUNNING
2170
The Reaction of Hg 6(3P,) Atoms with Vinyl Chloride
by M. G. Bellas, J. K. S. Wan, W. F. Allen, 0. P. Strausz, and H. E. Gunning Department of chemistry, University of Alberta, Edmonton, Alberta, Canada
(Received February 7, 1964)
The reaction has been studied both by 202Hg-monoisotopicand natural mercury sensitization techniques. Principal primary products are 1,3-butadiene, acetylene, hydrogen chloride, calomel, and polymer. Above 200’ the three isomeric chlorobutadienes appear. Three primary steps have been established
+ CzHaCl+ Hg* + CzH,Cl+ Hg* + CzHaCl+ Hg*
+ C1 + Hg CzH3* + HgCl (CZH&21)* + Hg
CzH8.
(0.49)
(1)
(0.06)
(2)
(0.45)
(3)
(0.17)
(4%)
(0.28)
(4b)
followed by (CZHaCl)* +C2H2
+ HC1
(CzH&l)* +CzHaCl (+ h ~ ) (CZH3Cl)*
+ M +CzHlCl + M
(4c) where the numbers in parentheses give the quantum yield values a t zero pressure. It is shown that quenching a t the chlorine atom leads largely to (1) and (2) and quenching a t the a-bond to (3). The heavy chlorine atom promotes intersystem crossing or more probably phosphorescence in the excited triplet molecule, CzH3C1*, thereby shortening its lifetime as compared to the corresponding CzH4species. Butadiene and chlorobutadienes arise from displacement-type reactions of CzHa. and CzH2C1. radicals, respectively, with CzH3Cl. Radical-radical reactions are unimportant even a t 10 mm. substrate pressure. Above 200’ chlorine atoms abstract hydrogen atoms from C2H3C1initiating a chain process. An approximate value of 16.0 kcal./mole was found for the activation energy of this abstraction reaction.
Introduction The mercury-photosensitized reactions of the simple olefins have been studied extensively, including ethylene1,2 various deuterated ethylenes, propylene,’ the butenes, 1-pentene, ~yclopentene,~ cyclohexene,6 allene, butadienes,’ biallyl,8andcyclooctatetraene.’ In each case the primary step appears to yield vibrationally-excited triplet biradicals which undergo collectively a variety of reactions, including unimolecular isomerization, molecular and free-radical elimination, and collisional deactivation to the ground electronic state. Tetrafluoroethylene’ excepted, only one study has been reported on the reactions of halogen-substituted olefins. Koizumi and ly’akatsuka,s from an in-
‘,,
The Journal of Physical Chemistry
vestigation of the photopolymerization of vinyl chloride, concluded that the reaction was photosensitiz$d by mercury atoms, the effective light being 1849 A. ~~
(1) R. J. Cvetanovid in “Recent Advances in Reaction Kinetics,” G. Porter, Ed., in press. (2) . . J. P. Chesick, J . A m . C‘hem. SOC.,8 5 , 3718 (1963) (3) D. W. Setser, B. S. Rabinovitch, and D. W. Placzek, ibid., 85, 862 (1963). (4) W. A. Gibbons, W. F. Allen, and H. E. Gunning, Can. J . Chem., 40, 568 (1962). (5) G. DeMar6 and H. E. Gunning, 46th Chemical Institute of Canada Conference, Toronto, June, 1963. (6) R. Srinivasan, J . Phys. Chem., 67, 1367 (1963). (7) B. Atkinson, J . Chem. SOC.,2684 (1952). (8) M. Koisumi and K. Nakatsuka, J . Chem. SOC.Japan, 72, 431 (1961); 75, 205 (1954).
2171
REACTION OF Hg 6(3131)ATOMSWITH VINYLCHLORIDE
Now halogen atom substitution should alter the im-. portant features of the olefinic reaction in a number of ways, and the investigation of such systems could provide valuable information on the chemistry of triplet states and t h e mechanism of energy transfer processes. The details of the vinyl chloride study follow.
Experimental zozHg-Monoisotopic Sensitization. The technique of monoisotopic photosensitization wherein a specific isotopic species (‘Hg) is uniquely excited in natural mercury vapor (”Hg) to the 3P1 state for reaction with the substrate has recently been detailed in a review article from this laboratory. The experiments were performed in a single-pass system, using a helical quartz reaction cell, wound from tubing 15 inm. in i d . and 60 cm. in length. The light source was a quartz electrodeless discharge tube containing mercury, 98.3 atom % in 202Hg,axially mounted in the spiral cell with the discharge maintained by a 2450-Mc. oscillator (Baird Associates). The lamp temperature was kept constant at 18”. The experimental orientations of the cell-lamp-microwave antenna assembly and that of the rotating sector used here (vide infra) are shown in Fig. 5 and 6 of ref. 9. Further experimental details have been given elsewhere.1° Lamp intensities mere measured by propane actinometry. 11,12 Natural X e r c u r y Sensitization. ‘The reaction was investigated in a circulating system (300 cc. in volume) comprised of a cylindrical quartz reaction cell, 5 cni. in diameter and 6 cm. in length, a gas circulating pump, a stripper, and a mercury saturator. The gas circulating pump was specially constructed to have essentially no “dead” volume. The rotor blades were of stainless steel with the rotor mounted on Rulon bearings. Coupling for the external magnetic drive was provided by a glass-encased iron bar affixed to the top of the rotor. The stripper was a simple straighttube condenser through which water, thermostated at 20.5 f 0.lo, was circulated. For the saturator a U-tube was used containing ca. 1 cc. of mercury maintained at 25.1 0.1’ by a constant temperature bath. I n each cycle the gases were driven sequentially to the stripper, the saturator, the reaction cell, and back to the pump. Thereby a constant mercury vapor pressure in the cell mas maintained regardless of the cell temperature. In the high temperature runs the stripper served to cool the hot gases emerging from the reaction cell and thus prevented a rise in the surface temperature of the mercury in the saturator. Tests showed that the efficient circulation provided by the
pump effected a complete replenishment of mercury vapor in the reaction zone in spite of its consumption in the reaction through calomel formation. The cell was housed in a large aluminum block furnace, the temperature of which did not fluctuate at 300’ by more than f1’. For the light source a Hanovia SC-2537 mercury resonance lamp, thermostated a t 25.5O, was used. A Vycor 7910 filter was employed to remove radiation below 2000 A. The absorbed light intensity a t 2537 8. was determined with propane as actinometer l~~~ (taking +E2 for this reaction as 0.50 at room temperature) as well as with n-butane-nitrous oxide mixture^,'^ assuming unit efficiency for nitrogen formation. The two determinations showed excellent agreement. The gases used were obtained from Matheson of Canada. The vinyl chloride (99.9% min.) was further purified by low temperature distillation. Gas chromatography (g.c.) on two different columns, a 6.5-m. dimethyl sulfolane column (I) and a 8.3-m. dibutyl maleate column, both operated a t 0’ revealed no detectable impurities. Repeated low temperature distillations of the sulfur hexafluoride (98.070 niin.) resulted in 99.5% purity as indicated by mass spectrometric analysis. Kitric oxide (99.0Y0 min.) was triply distilled through liquid oxygen prior to use. Mass spectrometric analysis showed it to be a t least 99.9% pure. The gases were circulated for 5 niin. prior to irradiation to allow for proper equilibration. At the end of each run the fraction, noncondensable at -159.5’, was collected and measured in a gas buret and subsequently analyzed by g.c. on column I. Ethylene and acetylene were easily separated and directly measured. What remained, i.e., the difference between the total fraction and the amount of C2H4 C2H2as determined from peak areas, was shown to be HC1 in separate experiments by reaction with AgN03. The material condensable at - 159.5’ was divided into two parts, one of which was analyzed on column I, and consisted of a trace of acetylene, the 1,3-butadiene
+
(9) H. E. Gunning and 0. P. Strausz in “Advances in Photochemistry,” Val. I, W. A. Noyes, Jr., G. S. Hammond. and J. N. Pltts, Jr., Ed., Interscience Publishera, New York, N. Y . , 1963. (IO) J . K. S. Wan, 0 P. Strausz, UT. F. Allen, and H. E. Gunnmg, to be puhlished. (11) S. Bywater and E. R. R. Steaoie, J . Chem. Phys., 19, 319 (1951). (12) Y. Rousseau and H. E. Gunning, unpublished results. (13) R. J. Cvetanovib, J . Chem. Phys., 23, 1208 (1955); R. J. Cvetanovib, W. E . Falconer, and K. R. Jennings, ibid., 35, 1225 (1961); Y. Rousseau and H. E. Gunning, Can. J . Chem., 41, 465 (1963).
Volume 68, Number 8
August, 1964
2172
M. BELLAS,J. WAX,W. ALLEN,0. STRAUSZ, AND H. GUXNING
product, and unreacted vinyl chloride. The remainder was also analyzed on column I in the low temperature runs (
