Micromethod for Determination of Gaseous Olefins

Azoisopropane has been photolyzed by 3660 A radiation over the temperature range 30-120" C. The effect of pressure indicates an excited molecule rr~echar~ism. Excited molecules which decompose give nitrogen and isopropyl radicals; the latter either corl~bine, disproportionate, or react with azoisopropar~e. The activation energy difference between the two reactions C3111. + C3H?.N:N.C3H7 -+ C~H B + CBHB.N:N.CJHI 2C3Hi.-3 CsH14 has been found to be 6.5 f 0.5 lical. per mole.The difference in activation energy between the disproportionatio~~ a~~d combination reactions is rendered ambiguous by the possibility of CJH~.N:N existing a t the lower temperatures; but this is certainly s~nall. 'l'he ratio of the rates of the two reactions is 0.5 a t room temperature. INTRODUCTIONThe photolysis of azoisopropane has not been previously reported in the literature although tlie thermal decomposition has been studied by Ranisperger (10). ?'his author did not ailalyze the gaseous reaction products and did not propose any free radical mechanism for the decomposition. 1 t seeins reasonable to suppose that the initial reaction in deconlposition is to form nitrogen and isopropyl radicals, these radicals then would react with themselves aiid the remaining azoisopropane.Weininger and Rice (12) have shown t h a t a molecule of azoethane on absorbing a q u a n t~~m of radiation is raised to an excited state having a lifetime reasonably long conipared to the time between collisions with a normal molecule and hence a proportion of the radiation absorbed by the azoethane is dissipated without reaction. The quantum yield of nitrogen formation is therefore pressure dependent, and although recent work on the photolysis of azomethane (3) has shown that this effect does not occur with the lower homologue it should appear in the pliotolysis of azoisopropane which has a greater number of internal degrees of freedom.The object of this research has been therefore to investigate tlie disproportionation and combination reactions of isopropyl radicals, the rate of reaction of isopropyl with azoisopropane, and t o determine whether an excited molecule is produced in the photolysis oE azoisopropane. EXPERIMENTALThe azoisopropane was prepared after the method of Lochte, Noyes, and Bailey (6). I t was purified by vacuum fractio~latiori and stored in a darliened trap attached to the apparatus by means-of a mercury cutoff. About 0.2 per cent of C6HI4 was always present in the stored azoisopropane which, if allowed t o remain, would cause an appreciable error in the CSH14 analysis of runs of low conversion. The procedure used in filling the reaction cell therefore entailed con-

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

The Photolysis of Azoisopropane

Durham¹,

Steacie²

1953

“…The methane in the non-condensable fraction was separated from hydrogen by dilfusion of the latter through palladium (8). The olefin content of the Cz, Cj, and C4 fractions was determined by the method of Pyke, Kahn, and Le Roy (13). Infrared spectra of the C5 and CG fractions were taken.…”

Section: Methodsmentioning

confidence: 99%

THE MERCURY PHOTOSENSITIZED HYDROGENATION OF PROPYLENE AND THE ACTIVATION ENERGY OF THE REACTION C₃H₇+H₂ = C₃H₈+H

Hoey¹,

Roy²

1955

The reactions initiated in hydrogen-propylene mixtures by Hg(3PI) atoms were studied over the temperature range from room temperature t o 320°C. A t 260" and above, the rate of formation of propane and the rate of pressure decrease are linear functions of the hydrogen pressure. This effect is attributed t o the reaction C3H,+H2 = CaHs+H and its activation energy is estimated to be equal t o or slightly greater than 12.5 kcal. per mole. This is 1.2 Bcal. per mole greater than the corrected value for the activation energy of the analogous reaction &Ha+ Hz = CzHs+ H. The ratio kco,blnntlon/kdlspropOrtlonatlon is estimated t o be approximately 2.0 a t room temperature in the case of isopropyl radicals. INTRODUCTIONKinetic data are now available for the reaction of methyl (10,17,14) and ethyl (18, 9) radicals with hydrogen as well as for the reverse reactions of atomic hydrogen with methane (2) and ethane (1). The present investigation was undertaken for the purpose of obtaining data on the reaction of propyI radicals with hydrogen, C~H T + H ? = CaHE+H.The method chosen was similar to that used by Le Roy and Kahn (9) in their study of the analogous reaction, Their method can be described briefly. At temperatures below approximately 200°C. the mercury (3P1) photosensitized hydrogenation of ethylene was interpreted in terms of the following reactions :In agreement with this mechanism, the rate of pressure decrease was independent of hydrogen pressure. At high temperatures, however, the rate of pressure decrease and the rate of formation of ethane increased linearly with the hydrogen concentration. This effect was attributed to reaction [a]. When this reaction is incorporated into the mechanism the predicted slope of the rate vs. (Hz) curve is proportional to k,/(k,+k,)~~. From the effect of temperalManuscript

“…of the impurity showed some similarity t o t h a t of 1,3-cyclopentadiene, and the material was shown to react with the olefin reagent (22). More than 40 maxima in the spectrum were observed, but will not be reported here.…”

Section: B Reagentsmentioning

confidence: 78%

“…I t was possible t o separate the Cr products into an ethylene-rich and a n acetylene-rich fraction. T h e latter was shown by infrared analysis t o contain no ethane or ethylene; the former was analyzed for acetylene (2) and ethylene (22) by che~nical means, and the residue was taken to be ethane. T h e n-butane was readily identified by its vapor pressure, infrared spectrum, and its nonreactivity to the olefin reagent (22).…”

Section: Productsmentioning

confidence: 99%

Free Radical Mechanisms in the Mercury Photosensitized Reaction of Hydrogen With Acetylene

Cashion¹,

Roy²

1954

The Hg(JPI) photosensitized reaction of hydrogen with acetylene has been investigated with particular emphasis being placed on the identification of products and their dependence on hydrogen pressure. The results have been interpreted in tertns of a number of elementary reactions in which vinyl and ethyl radicals play an important part. INTRODUCTIONAlthough the reaction of H atoms with acetylene has been the subject of a number of investigations, no colnpletely adequate mechanism has been proposed to explain the results obtained under different conditions. With t h e high H atom concentrations used in the discharge tube method acetylene appeared to act simply as a catalyst for atom recombination (3, 27,8).However, Geib and Steacie (9) found that when D atoms were used the acetylene was rapidly exchanged. They suggested that the catalytic effect could be explained in terms of one of the follo~ving mechanisms:[4]The catalytic effect was confirmed by Tollefson and Le Roy (24) and by Dingle and Le Roy (6). These workers used the hot filament method for producing H atoms and total pressures of the order of 5 mm. The latter, using an improved technique, found the steric factor and activation energy of the rate controlling step, [I] or [3], to be 4X10a4 and 1.5 ltcal. per mole, respective1 y.Le Roy and Steacie (18) used the method of mercury pl~otosensitization t o produce H atoms in the presence of acetylene and found that appreciable quantities of ethane, ethylene, and butane were formed, as well as the polymer which had been observed by others.The wide difference in the nature of the results obtained by the discharge tube and hot filament methods on the one hand and photosellsitization on the other undoubtedly arises because of the very great differences in the relative concentrations of H atoms, H Z molecules, and perhaps certain radicals. Any reasonable mechanism for the fornlation of the gaseous products of the photosensitization experiments would seem to require the existence of vinyl radicals, regardless of whether the primary reaction is [I] or [3].In comparison with the saturated alkyl radicals, very little is known about the reactions of free vinyl radicals; the present reinvestigation of the mercury 'Manarscript received M a y 17, 1954.