T H E HXLOGEX-SENSITIZED OXIDATIOX OF CARBON hIONOXIDE* BY ROBERT LIVINGSTON
The results of an exhaustive investigation of the reaction kinetics of the chlor-sensitized oxidation of carbon monoxide and the related formation of phosgene have been presented by Bodenstein and his co-workers' in a series of very interesting papers. I n light of their results it seemed worth while to study the analogous system involving bromine. Brom-phosgene is known to be unstable, and its decomposition is accelerated by visible light in the presence of bromine*; it seemed possible, therefore, that bromine might sensitize the photochemical synthesis of carbon dioxide, without introducing any complicating side reaction analogous to phosgene formation. However, a series of experiments with this system indicates that bromine is not capable of sensitizing the photochemical oxidation of carbon monoxide. The carbon monoxide, used in these experiments, was generated from formic acid and concentrated sulphuric acid, was bubbled through concentrated sulphuric acid, and passed through a liquid-air trap. The oxygen was made by heating a mixture of potassium permanganate and barium peroxide, was passed through a plug of g¶ass wool, and then through a liquid-air trap. The bromine used in several of the experiments was distilled a t low temperature and pressure from a commercial sample of bromine. I n two of the experiments the bromine used was prepared from a mixture of potassium bromide and bromate and sulphuric acid, was distilled a t atmospheric pressure off of potassium bromide, and was then fractionated at low temperature and pressure. The reaction system was made of pyrex, and was isolated from the generating system by a three-way stopcock, lubricated with a mixture of ortho and meta phosphoric acids.3 The reaction cell was a water-jacketed cylinder 17 cms long, with fused-on pyrex ends. The pressure was measured by means of a Bodenstein quartz-spiral m a n ~ r n e t e rsealed ,~ to the apparatus by a Pyrex-to-quartz graded seal. h liquid-air trap was placed between the pump and the system to prevent the admission of mercury vapor to the reaction system. The source of light was a zoo watt tungsten lamp at 40 cm distance. Experiments were performed at room temperature, and approximately at atmospheric pressure. The bromine pressure was varied from 13 to 16 -
* Contribution from the School of
Chemistry, University of Minnesota. 'Bodenstein: Re.. Trav. chim., 41, 585 (1922);( b j Bodenstein: Sitz. preuss. rlkad. 13 104 (1926)'( c ) Schumacher: Z.phvsik. Chem. 129, 241 (1927);( d ) Bodenstein: 130 2 ; (1927); (e') Bodenstein and Onodk: 131, 153 (1927); cf', Bodenstein. Lehner and, k a g n e r : (B) 3,4,59(1929). 2 Bonhoeffer: Z. Physik, 13, 94 (1923). Stephens: J. Am. Chem. SOC., 52 (1930). 4 Bodenstein and Dux: Z. physik. Chem., 85, 300 (1913).
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ROBERT LIVIS(;STOS
cm of Hg. The oxygen and carbon monoxide were approximately in stoichiometric proportions. The maximum time of illumination varied from two and a half to ten hours. I n no case was there any indication of the occurrence of a reaction. The maximum change in pressure observed was 2 . 0 nim for the ten hour experiment, and was less than 0.5 mm in the orher experiments. This small change in pressure was probably a temperature effect. Since there was some possibility that the observed negative results might be due to impurities in the gases or faults in the apparatus, chlorine was substituted for bromine in one experiment. The chlorine was prepared by heating auric chloride, and was fractionated at low temperature and pressure. In this experiment, using the same apparatus and the same oxygen and carbon monoxide generators, the reaction wits half completed in 2 hours and 2 4 minutes, and was practicall)' complete in 6 hours. These results indicate with considerable probability that, at room temperature and atmospheric pressure, bromine is not capable of photo-sensitizing the oxidation of carbon monoxide. It follows from this, that bromine atoms, which are present at relatively high concentration in the illuminated gas m i ~ t u r e ,are ~ not capable of inducing the homogeneous reaction between carbon monoxide and oxygen. I n terms of the Bodenstein mechanism for the analogous chlorine system ( I f), this may be due to the extreme instability of the intermediate substance, COBr. It does not seem very probable that the difference between the action of bromine and chlorine is due to the slightly lower heat of dissociation of br0mine.j Since the completion of this work the publications of Lenher and Rotlenstein6 have come to our attention. They find (6a) that a mixture of pure bromine and carbon monoxide reacts in visible light at a measurable rate t o form bromphosgene. In their experiments the steady state was reached ivhen about 10% of the mixture had reacted This is in interesting contrast with our result: that in the presence of oxygen exposure to light did not produce any diminution in pressure (as would have resulted from the formation of either bromFhospene or carbon dioxide). It seems probable that this difference was due entirely to the presence of the oxygen; however, it is possible that other factors, such as the intensity of light, may be in part responsible for the observed difference.
summary Evidence is presented to show that bromine is not capable of photosensitizing the oxidation of carbon monoxide a t room temperature and atmospheric pressure. .Ill li lle~17~011 s, M~i~iieaotu. 5 6
Compare Kistiakowsky: "Photochrmiral Reactions," pp, 9;, 98 (1928). (a)Lenher and Bodenstein: 2. physik. Chem., 135, 85 (1928); (b) Ber. 61B,1671 (1928).
