May 20, 1957
RADIOLYSIS OF ALKYLIODIDES
primary spectra. I n fact, it is tempting to decide that the C4Ha+ intermediate is more similar to the 1,3-butadiene or 1-butyne ions than to the other two primaries shown.
[CONTRIBUTION FROM
THE
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Acknowledgment.-We wish to express our appreciation to Mr. Burl L. Clark for his invaluable help in making the measurements and calculations. BAYTOWN, TEXAS
DEPARTMENT O F CHEMISTRY OF
THE
UNIVERSITY
OF
WISCONSIN]
Effects of Structure, Product Concentration, Oxygen, Temperature and Phase on the Radiolysis of Alkyl Iodides BY EVALYN0. HORNIGAND JOHNE.WILLARD RECEIVED DECEMBER 20, 1956 The yields of elemental iodine as a function of radiation dosage have been determined for the radiolysis with Cow 7-rays of seven purified degassed alkyl iodides. Doses >16 X lom e.v./ml. were used in some cases. Although six of the iodides show linear dependence of GI, on radiation dosage a t low doses, all depart from linearity a t higher doses, and the GI, for isobutyl iodide decreases with increasing dose even a t the start of irradiation. The linear dependence is attributed to a balanced competition between H I and 11for thermal radicals and, consistent with this concept, the rate of exchange with radioiodine during radiolysis is found to be dependent on the concentration of added 12, or 0 2 . The dependence of GI, on the p-hydrogen content of the alkyl iodides is confumed and ascribed to increased probabllity of H I formation by decomposition of excited molecules as the number of P-hydrogens per molecule increases. Added 0 2 increases the initial GI, for C2HJ by an amount concentration increases with increased dosage GI, returns to independent of the 0 2 pressure from 2 t o 188 mm., but as the 1% its degassed level a t a rate inversely dependent on the oxygen pressure. With added iodine present a t a concentration elevenfold greater than the dissolved oxygen, the initial GI, was the same as that in the absence of additives. GI, for four iodides tested is essentially independent of temperature in the liquid phase from 20 to -78" and for C2Hd from 108 to -78", but the GI, values for both CHII and Cd&I have a positive temperature coefficient in the crystalline solid phase. . G values for two iodides studied in both the glassy and crystalline states a t - 190" were higher in the glassy state, a result which is tentatively ascribed to molecular orientation favoring a stereospecificity for hot radical reactions. tem, and then were vacuum-distilled through P20s into the irradiation vessels and sealed off. During the irradiation, the samples (5 ml.) were contained in an annular vessel,' which surrounded a 40-curie Cow y-ray s o ~ r c e . The ~ radiation intensity on the samples was 2 X IOs roentgens/hr., the absorption of energy in the alkyl iodides being a t the rate of about 2 X lO1O e.v. per ml. per hr. Iodine analyses were made with a Beckman DU spectrophotometer. A spectrophotometer cell made from square Pyrex tubing was attached to the annular vessel so that iodine analyses could be made between successive irradiations of the same sample without exposing it to the air. Concentrations were read a t the absorption maximum a t Experimental 478 mM and for high concentrations a t 550 and 625 mM. The alkyl iodides, Eastman or Matheson best grade, were The high concentration readings were checked a t 478 m r purified by passage through activated alumina, followed by in methyl and ethyl iodide irradiations by using 0.98 and distillation through a two-foot Vigreux column, the middle 0.95 cm. silica inserts in the analysis cells. Determinations of molar extinction coefficients were made a t the three wave 50y0 being retained. Identical iodine yields were obtained from the irradiation of samples of ethyl iodide purified as lengths for methyl, ethyl and n-propyl iodides, the values obtained being 1280, 356 and 72.8 l./mole cm., respectively. above, and from those purified by shaking with concentrated H2S0, and washing with Na2SOs solution prior t o Since the extinction coefficients were the same for the three distillation. The purified iodides were degassed by several iodides, it was assumed that the same values would also be cycles of freezing, pumping and thawing on a vacuum sys- applicable to the other iodides. To irradiate samples a t -78, 123 and - IQO", they were (1) R. J. Hanrahan and J. E. Willard, THISJOURNAL, 79, 2434 surrounded during the irradiation by baths of solid CO2 and (1957). acetone, butyl chloride slush, and liquid air, respectively. (2) Previous investigators have determined the relative yields of Ia A thermostatically controlled bath of mineral oil was used from X-ray and a-particle irradiation of air-saturated ethyl iodide.& for the irradiations a t 108". from y-irradiation of eight air-saturated alkyl iodide,sb from fast Oxygen of known pressure was admitted to degassed electron irradiation30of degassed CHzI, CHYII,CtHsI and n-CrH71, and samples of ethyl iodide on a vacuum line, either from a from X-ray irradiation of degassed CHd,3a,dC ~ H S I , ~ O p* r~p *~ y l ' ~ . ~cylinder or by heating KClOr containing RInO~. The and butyl iodidesad The yields of various products from the X-ray number of moles oxygen introduced to the sample vessel was irradiation3"lh and very low intensity CoB0 y-ray irradiationad of determined from the pressure change in a known volume of degassed CHII have also been determined. Similarities and contrasts the vacuum apparatus, measured with a phosphoric acid between the radiolysis and photolysis of the alkyl iodides have been manometer. The solubility of oxygen in ethyl iodide was c ~ n s i d e r e d . ~ " *Distillation ~~~~' with added carriers following irradifound to be 8 X 10" mole/l. per mm. pressure a t 23',5 so ation in the presence of radioiodine has been used to identify interthat the concentration of dissolved oxygen in the samples mediate free could be determined from the gas pressure. (3) (a) M. Lefort, P. Bonet-Maury and M. Frilley, C o m p f . rend., Iodine labeled with ILalwas prepared from carrier-free 2%6, 1904 (1948); (b) P. Sue and E. Saeland, Bull. SOC. chim. France, iodine solutions by adding the desired amount of K I and 437 (1949); (c) R.H.Schuler and W. H. Hamill, THISJOURNAL, 74, oxidizing with acidified KI03. The iodine was transferred 6171 (1952); (d) E. L. Cochran, W. H. Hamill and R. R. Williams, into the irradiation vessels by vacuum distillation through Jr., ibid., 76, 2145 (1954): (e) C. R. Petry and R. H. Schuler, ibid., P206. A solution-type Geiger counter was used to measure
Introduction The purpose of the work of this and the following paper' mas to elucidate further2 the mechanisms of radiolysis of the alkyl iodides. The investigations have included studies of: (1) the competitive reactions of 0 2 , IZ and HI with thermal radicals produced in the radiolyses and (2) the effects of temperature, molecular structure and irradiation in the solid crystalline and glassy states on the yields of elemental iodine.
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76, 3796 (1953); (f) W. H. Hamill and R. H. Schuler, ibid.. 73, 3466 (1951); (6) L.H. Gevantman and R . R. Williams, Jr., J . P h y s . Chcm., 6 6 , 569 (1952); (h) R.H. Schuler and R. C. Petry, TEISJOURNAL, 78,
3957 (1956).
(4) R.P. Firestone and J. E. Willard, Rcr. Sci. Inst., 24,904 (1953). (5) E. 0.Hornig, Ph.D. thesis, University of Wisconsin, Feb. 1956, available from University MicroElms, Ann Arbor, Michigan.
EVALYN 0. HORNIG AND
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JOHN
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Energy absorbed per ml. (e.v. X 10-19). Fig. ].---Iodine production in the radiolysis of degassed alkyl iodides as a function of energy absorbed: 0, methyl; 0 , ethyl; 8 , n-propyl; 6 , isopropyl; @,n-butyl; Q, isohutql; 0 , s-butyl. the distribution of activity between organic and itiorganic combination after radiolqiis.
of 15.6 Fe++ oxidized per 100 e.;. absorbed was a ~ s u m e d . ~The absorption in the aqueous FeS04 solution (1.33 X e.v./ml. hr.) was multiplied by factors5 to correct for the difference between its density and average atomic number and those of each of the iodides. The rates of energy absorpe.v./ml. hr. for tion so obtained were 2.64 X CHJ, 2.26 X 1019 for CzHjI, 2.05 X 10lg for nC3H71, 2.02 X for i-CaHiI, 1.94 X IO19 for Y Z - C ~ H1.92 ~ I , X lOI9 for i-CeHsI and 1.91 X 10'''
Results Variation of Iodine Yields with Radiation Dosage.-Figure 1 shows the change of iodine concentration with time of irradiation for seven alkyl iodides. Six of these showed no change in slope e.v./mL6 up to radiation doses as high as 4.5 X The seventh, isobutyl iodide, gave yields which dropped from 3.36 I atoms (as '/z 1 2 ) produced per 100 e.v. absorbed initially to 1.89 after 0.64 X TABLE I e.v./ml. was absorbed (see insert of Fig. 1). G VALUES" FOR IODINE PRODUCTION IS THE RADIOLYSIS OF Identical results were obtairied with each of three LIQUID -4LKYL IODIDES separately purified i-CdHgI samples. For the much Present w o r k - - - -------Temperatures, 'e,Other work, higher radiation doses plotted in the main portion Room 108 -78 -120 -190 room temp. of Fig. 1, all of the iodides except methyl showed a CHrI 2.52 0.94' 0.64' 3 . 3 , b2 . 7 , c 2 . 4 d region of decreasing rate of 1 2 production. For CiHaI 4.25 4.05 4.34 2.05 0.95' 4.l,*4.1,' 4 06' 3.03* those compounds which were subjected to the 3 . 5 . c 2.88C largest dosage (isobutyl, isopropyl, n-propyl) the rr-CsH11 3 . 2 0 5.36 5.26 0 05' 5.3e i-CsHII values of G i , 2 ~ 2for increments of irradiation after n-CdHpI 3 . 3 8 3.3Sg the samples had been exposed to 10 X lozoe.v./ml., i-C,H,I 1. 89'L 1.86 3 . 1 9 1 2.60' 4.80 4.430 all came to the same value of 0.95 f 0.03. The s-GHeI 5 . 0 4 curvature is of interest because it is indicative of The radiation yields (G values) are expressed as equivacompeting reaction steps which change in impor- lents of iodine ( I / * 12)produced per 100 e.v. absorbed. They tance with changing concentration of the radiolysis are for irradiations a t iodine concentrations of
