R. F. E’OTTIE,
UT.H. HAMILL
for cyclobutene than cyclobutane, but the difference in the energies of activation i’or the reactions of cyclobutene and cyclobutane is much larger than the resonance energy of butadie.ie. In a consideration of the isomerization of cyclobutene, the simplicity of the o ver-all process, the first-order character, and the lack of effect of nitric oxide or propylene would be in agreement with a uniniolecular mechani:;m. Likewise the frequency factor is of the magnitude expected for a unirnolecular reaction.” By a c:omparison of the expression tie(kT/h)e’.s‘,’R wit L1 the observed frequency factor,22athe entropy 2f activation A S a t 150’ can be estimated when a value of ti, the traiismission coefficient, is assumed. For K = 1 the calculated entropy of activation is - 1.4 cal.,’ deg. mole and for ti = 0.5 the entr2py of activation is essentially zero. X negative .iralue of the entropy of activation does not appear likely for the reaction. In view of the possibi1:ty of an uncertainty in K or an inaccuracy in the :‘requency factor due to a small error in the activai,ion energy, the
numerical values estimated as above may be somewhat incorrect. However, the results seem to indicate t h a t the entropy of activation is relatively small in contrast to the value of +S-9 cal./deg. mole found for cyclobutane, methylcyclobutane and ethylcyclobutane. I n this connection it is of interest to note that from the data reported in the literature the increase in the entropy of 1,Ybutadienez3 over t h a t for c y c l o b ~ t e n ea~t ~127’ is 4.5 cal./deg. mole. On this basis the apparent smallness of the entropy of activation for the isomerization of cyclobutene does not appear unreasonable. Acknowledgment.-The authors wish to express their appreciation to the National Science Foundation and the Celanese Corporation of America for financial support during various phases of this study. They also wish to thank Dr. R. C. Lord for a sample of cyclobutene, Mr. R. C. IYilkerson for mass spectrometric analyses, and N r . Carl 11-hiteman for the infrared measurements.
+
123) J G Aston, G bzasz, H W. Woolley and F G Brickwedde. ( 2 2 ) (a) S. G l a s t o n e , K. J. Laidler and H . E l r i n g , “ T h e Theory of J . Chcm. Phys , 14, 67 (1946) Rate Processes,” AIcCraw-Hill Book Co., I n c . , New York, N. Y., (24) A. Danti, z b r d , 27, 1227 (1957) 1911, pp. 26, 295-29(i; ib) N. B. Slater. Trans. R o y . SOL.( L o n d o n ) , 246A, 57 (1033) ROCHESTER, S. Y
[CONTRIBUTION FROM
THE
DEPARTMENT
OF
CHEMISTRY, UNIVERSITY OF
SOTRE
DAME]
Diffusion and Hot Radical Kinetics in the Photolysis of Methyl Iodide in Cyclohexane’ BY ROSWELL F. POTTIE, WILLIAXH. HAMILL AND RUSSELL K. WILLIAMS,JR. RECEIVED FEBRUARY 24, 1958 Solutions of methyl iodide in cyclohexane have been photolyzed a t 2537 A. Products and their quantum yields were: methane 0,190, cyclohexyl iodide 0.085, cyclohexene 0.057, iodine 0.048, hydrogen iodide 0.009. Methane is attributed to reaction of hot methyl radicals with solvent and to reaction of thermal methyl radicals ivith hydrogen iodide. Cyclohexene is attributed to a diffusion controlled reaction between cyclohexyl radicals and iodine atoms which remain in proximity after the hot reaction. Hydrogen iodide is twice as effective as iodine in reacting with methyl radicals. Recombination of methyl radical-iodine atom pairs following dissociation varies exponentially with the square root of hydrogen iodide concentration, indicating a diffusion-controlled process.
Introduction The present study is based upon, and extends, recently reported work.*m3 I n particular i t attempts to illustrate further the kinetic peculiarities of reactions of atoms and free radicals in the liquid state which depend largely upon diffusion-controlled processes. Such effects can be recognized and isolated since their kinetic behavior differs notably from reactions occurring in the steady state, to which the more familiar methods of gas kinetics apply. Insofar as steady-state processes occur in liquids, the same type of approach may be expected to apply. The system methyl iodide-cyclohexane was chosen for study because Schuler’s* preliminary (1) This article is baied on a dissertation submitted by R. F. P. i n partial fulfillment of the requirements for t h e Ph.D. degree. T h e work was carried o u t under the auspices of the Radiation Project, Department of Chemistry, University of Notre Dame, supported in part b y the U. S. Atomic Energy Commission under contract AT(11-1)X8 and Navy Department loan contract Nonr-06900. Presented at the 132nd meeting of the American Chemical Society, New York, N Y . , 1967. ( 2 ) R . 11. Schuler ;and W. H. Hamill, THISJOURNAL, 73, 3460 (1‘331).
(3) D. L. Bunbury, R . R. Williams, Jr., and W. H. Hamill, ibid., 78, 6’28 (1!)56).
observations identified or indicated the products of photolysis as iodine, hydrogen iodide, methane, cyclohexene and cyclohexyl iodide. Of these, hydrogen iodide and cyclohexene could not then be accounted for. Subsequently Bunbury3 showed that the photolysis of ethyl iodide in the liquid phase produced ethylene and hydrogen iodide by reaction of iodine atoms with ethyl radicals. I t would be expected that cyclohexyl radicals and iodine atoms might behave analogously. Two such reactants must be originally juxtaposed, and this condition is met by a prior reaction of hot methyl radical with cyclohexane. Furthermore, thermal encounters between CH3-1 or CGHI1-I, yielding CHJ, CGHIIIor C ~ H I O and H I , are expected to be diff usion-controlled. Such encounters in the steady state are impossible for the light flux used in the presence of even minute concentrations of molecular iodine because of its very efficient scavenging action. Experimental Materials.-Methyl iodide was purified by passage through a 25 cm. colmin of activated silica gel, followed by distillation in a 4 ft. column packed with l/* in. glass helices a t a reflux ratio of 10: 1. The middle one-third was re-
Aug. 20, 195s
PHOTOLYSIS OF METHYL IODIDE IN CYCLOHEXANE
tained and stored in a brown bottle over copper wire. The refractive index, n % 1.5281 ~ was constant and the samples remained water-white for periods as long as one year. Cyclohexane was purified by various methods, depending on the initial purity. Technical grade cyclohexane was washed with fuming sulfuric acid, dilute base and water, dried and fractionally distilled. In some cases the distillate was passed through a column of activated silica gel. Spectral grade cyclohexane (Eastman Organic Chemicals) was either used as received after proving that peroxides were absent and checking the ultraviolet transmission, or further purified by refluxing over lithium aluminum hydride, distilling and passing through a column of silica gel. Phillips research grade or a C.P. grade of cyclohexane was treated with fuming sulfuric acid, etc., and then passed through several successive columns of silica gel until the ultraviolet spectrum approximated that of the best spectral grade sample. In all cases the purity was tested by the optical transmission and by refractive index. Several samples were further tested by determination of the mass spectrum of both total liquid and a vapor fraction in order t o check for other hydrocarbon impurities. Cyclohexene (Eastman Organic Chemicals) was washed with successive portions of dilute acidified ferrous sulfate. The product was washed with distilled water, dried over calcium chloride and fractionally distilled. Samples were withdrawn from a constant boiling middle one-third fraction and introduced into small Pyrex ampoules. These samples were degassed by repeating freezing and pumping and were sealed under vacuum. The ampoules were opened as required t o recalibrate the method devised t o analyze for cyclohexene. Iodine (Rascher and Betzold C.P. resublimed) was used without further purification. Hydrogen iodide was prepared from phosphoric acid and potassium iodide. Neopentane (Phillips research grade) was transferred on the vacuum line directly into a cell for photolysis. Paraffin oil (Nujol) was used as received. Sodium-biphenyl reagent was prepared' for analysis of organic iodides. Apparatus.-The source of illumination was a Hanovia SC-2537.4 low pressure mercury discharge tube in the form of a helical grid. The lamp was housed in a ventilated box and the light intensity was controlled by manual adjustment of the input Variac. The lamp was maintained at a temperature of 45 + 2" by a stream of air circulating through the housing. Samples could be photolyzed either inside the helix or a t a window in the lamp housing. Cylindrical Vycor cells joined to Pyrex through graded seals were used. The vapor space above the liquid sample was shielded with black tape. The light absorbed by methyl iodide was thus confined t o 2537 A . since Vycor (Corning 7910) cuts off above 1849 -4. The remainder of the light emitted by the lamp is in the visible spectrum. Preparation of Samples.-The cells for photolysis were provided with two side arms equipped with break-offs. The central stem was joined to the vacuuni.line through a capillary constriction. A prepared solution of methyl iodide and cyclohexane was dried over phosphorus pentoxide and partially degassed by several cycles of freezing, pumping and thawing. The sample was then distilled twice trap-to-trap while pumping, and then by distillation into the cell. Contamination by air was reduced t o the point where its contribution to the mass spectrum of product gases was indistinguishable from the normal background. In some experiments iodine was added to the solution before degassing and was distilled simultaneously into the reaction cell. If hydrogen iodide were to be added the central stem of the reaction cell was fitted with a stopcock and the mcasured volume of gas immediately condensed into the cell. The solution for photolysis then was introduced. These precautions were necessary in order to avoid loss of hydrogen iodide by absorption in stopcock grease or by reaction with mercury. The results of several tests demonstrated that both iodine and hydrogen iodide were introduced quantitatively into the reaction cell. Solutions of methyl iodide in neopentane were introduced into the reaction cell by the same technique. In experiments using Nujol it was added directly t o the cell before sealing to the vacuum line and degassed a t 100". Previously degassed methyl iodide was then distilled into the cell.
The light flux was determined for both positions using ethyl iodide as a n actinometer and taking the value 0.13 as the quantum yield for production of 12.5 All runs of methyl iodide in cyclohexane were monitored by simultaneous photolysis of ethyl iodide. Analysis.-No adequate method for analysis of cyclohexene could be found in the literature so the following technique was devised. Cyclohexene and iodine were known6 t o undergo a reversible atom-catalyzed addition. Reaction is ca. 90% complete under practical experimental conditions and a bright tungsten light must be used to accelerate attainment of equilibrium. By calibrating under carefully standardized conditions the method becomes quantitative. Our procedure involved (a) adding a considerable excess of iodine t o the exposed sample, (b) equilibrating a t On, (c) removing excess iodine by washing with a solution of sodium thiosulfate, then with water, (d) destroying the diiodide a t some higher temperature and then determination of the iodine released. h 1 M solution of iodine in methyl iodide was prepared for estimation of relatively large amounts (ca. 50 micromoles) of cyclohexene, while 0.15 M iodine in chloroform was used for smaller concentrations. The method was calibrated with solutions of known concentration of cyclohexene and factors were determined relating iodine released t o cyclohexene added. The sample for analysis was made up of 5 ml. of the solution of cyclohexene in cyclohexane, 7 ml. of carbon tetrachloride, 5 ml. of the iodine solution. Equilibration was performed with the testtube containing the sample immersed in an ice-water-bath under illumination of a tungsten lamp. One hour sufficed for equilibration. Following elimination of excess iodine, the diiodide was decomposed by placing the sample a few centimeters above a lamp and titrating iodine until no further release occurred. Under these conditions the factor for converting iodine uptake to cyclohexene is 1.173 for the methyl iodide solution and 1.168 for the chloroform solution. It is estimated that cyclohexene can be determined within 14.; for A0 micromoles, and within 596 for 10 micromoles. Following illumination the cell was sealed to the vacuum line zlia capillary constriction and break-off, cooled t o - 196' and gaseous product collected, measured and reserved for mass analysis. Degassed water was distilled into the cell, thecontents frozen and the cell sealed and removed from the line. After shaking the warmed cell to extract hydrogen iodide into the aqueous phase the contents were again frozen, sealed t o the vacuum line via the second break-off and residual gas collected as before. If more than a fem micromoles of gas mas collected, the operation was repeated. The mixture in the reaction cell was poured into a separatory funnel along with additional distilled water and carbon tetrachloride used to rinse the cell. The phases were separated; the aqueous phase was washed with carbon tetrachloride t o remove iodine. If cyclohexene were to be determined, the washings were added to the organic phase t o a total volume of 7.0 nil. of carbon tetrachloride. Any additional carbon tetrachloride washings were added to a separate flask. Otherwise all washings, regardless of amount, were added t o the organic phase. The aqueous phase was titrated t o determine hydrogen iodide. To obtain clear end-points with biom cresol purple, it was necessary to bubble carbon dioxide-free air through the solution during titration. Concentrated aqueous potassium iodide was added to the organic phase and the iodine was titrated with standard thiosulfate. The phases mere separated once more and the organic layer was analyzed for cyclohexene as described above. In several runs, after iodine had been estimated, the organic layer was run into a semi-micro still. A few ml. of normal hexane was added and methyl iodide distilled over. After approximately half of the n-hexane had been collected, the residue in the still pot was analyzed for organic iodide by means of sodium-biphenyl reagent, and then potentiometric titration with silver nitrate.4 Complete removal of methyl iodide was checked by analysis of the last distillate. The method was further checked with a known solution of n-amyl iodide in cyclohexane and n-hexane. After a few ml. of n-hexane was removed by distillation, analysis of the residue showed quantitative retention of the iodide. Since ( 5 ) E. Cochran, W. H . Hamill and R. R . Williams, Jr., THIS JOUR7 6 , 2145 (1954). (6) R . J. Birkenhauer, Ph.D. dissertation, U n i v . of S o t r e Dame 1941.
NAL,
( 4 ) F. L. Benton and W. H . Hamill, A t d Chent., 20, 269 (1948): L. M. Liggett, ibid.,26, 748 (1954).
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