Xov., 1960
THERADIOLYSIS OF LIQCIDTL-HEXASE
1623
THE RADIOLYSIS OF LIQUID TZ-HEXANE BY THOMAS J. HARDWICK GulfResearch & Development Company, Pittsburgh, Pennsylvania Received April 6 , 1960
The continuous decrease in hydrogen gas yield during the radiolysis of n-hexane is attributable to the addition of H atoms to the hexenes concurrently being produced. Unsaturation in the CrClz products arises from the addition reaction of alkyl plus alkenyl radicals; the alkenyl radicals are derived from hydrogen abstraction from hexenes by H atoms. The overall kinetics of radiolysis are satisfactorily explained by invoking free radical mechanisms; ion-molecule reactions appear to play little or no part. Both "hot" and thermal H atoms are produced, the former invariably forming hydrogen gas. An explanation for the mechanisms of formation of the so-called "molecular" products is given. The yield of products can be explained quantitatively by a relatively simple reaction scheme.
Introduction Section I The radiolysis of liquid n-hexane has been studied Irradiation of Pure n-Hexane by several investigators. Dewhurst' found the Irradiation Facility.-A 4-liter volume of hexane was hydrogen yield from n-hexane to be constant up to irradiated by electrons from a Van de Graaff accelerator doses of 100 j./g. ( G H ~= 5.0 mo1./100 e.v.). In (Model KS, High Voltage Engineering Corp.). The liquid the presence of free radical scavengers, the hydrogen was placed in a circulating loop, the details of which are elsewhere.5 Briefly, the liquid was circulated yield dropped to a limiting value of 3.0. Davison2 published through a loop consisting of an irradiation cell, a degassireported a lower hydrogen yield ( G H ~= 4.0 mol./ fier, a heat exchanger and a pump. Gases produced by 100 e.v.). Recently, Allen and Schulera showed radiolysis were stripped from the liquid in the degassifier that there was no effect of linear energy transfer and were removed from the system through a condenser maintained at 0". A dry test meter placed in series with (1.e.t.) on the hydrogen yield. this condenser measured the total volume of gas evolved. We have previously reported that a decrease in Provision was made for sampling the gas without interrupthydrogen yield occurs on irradiating cy~lohexane.~ing the flow. The rate of energy absorption for the particular electron When a similar experiment with n-hexane resulted and beam current used was measured using the in the same behavior, it became desirable to investi- energy sodium formate dosimeter .a Values were considered acgate further into the n-hexane system to explain curate to the reproducibility of machine operation (+2%). this phenomenon and to reconcile it with published Using the gas collection arrangements described above, data. Accordingly, a varied program was under- the rate of gas evolution could be measured a t a constant input. The ratio of these two quantities, d(gas)/dt/ taken to determine the primary processes and sub- power (power input), expressed in appropriate units, is the yield sequent reactions which occur on the radiolysis of (G-value) of the gas evolved: furthermore, this yield can liquid n-hexane. In view of the discrepancies in be measured continuously throughout the experiment. In a typical run the hexane was put into the loop and previous results, it was felt that a quantitative into 25". Nitrogen was passed through the degasvestigation was necessary in which accurate yields brought sifier to purge oxygen from the system. For a total of of primary processes could be determined and the 1500 j./g. energy absorption about 40 1. of gas was evolved rates of subsequent reactions measured. In a during radiolysis. The power absorption in the liquid was practical sense, such data would enable one to alter usually in the range 500-700 watts. Samples of gas were taken about every 80 j./g. of energy reaction sequences and thus predictably modify absorbed and were analyzed mass spectrometrically . By the product spectrum. interpolating the analytical results, a continuous measurement of the individual gas yields was obtained as a function Experimental Materials.-Phillips Pure Grade %-Hexane was used throughout. The concentration of unsaturated material, presumably some Ce isomer, was 0.25 millimolar or 0.003%. It will be shown later that the small amount of this impurity does not affect the various yields. Analytical. Determination of Unsaturation.-The hydrocarbon sample (5-50 ml.) was added to 20 ml. of glacial acetic acid. About 25y0 excess bromine was added rn a 1.5 M solution in acetic acid. After two minutes, 25 ml. of water containing excess K I was added, converting the excess bromine to iodine. The whole mixture was titrated with 0.05 M sodium thiosulfate in the usual iodometric manner, Titrations with pure olefins gave theoretical values of the unsaturation. This method is considered satisfactory for light, (C,). discrepancy between these results and those previously A 4liter volume of hexane containing 4~0/vol.hexene-1 published may arise from difficulty in calibrating absolutely was irradiated a t 25" in the circulating loop. The method gas chromatographic units or masa spectrometers for the of radiolysis and the analytical procedures were identical highly branched materials resulting from radiolysis. with those described in Part I. The ylelds of lower hydroOne noticeable result is the extent of unsaturation in carbons are shown in the last column of Table I. The values all product groups. A high degree of unsaturation has are essentially the same as those found for pure n-hexane. been noted before in n-hexane radiolysis by Dewhurst,' was found t o The product distribution in the range G-CIZ and by Keenan, et, al.,8 on liquid butane radiolysis. Fur- be about the same with and without the presence of hexene-1, ther experiments were made to determine the role of un- although the yield of unsaturation was increased by about saturation in the radiolytic reactions. 20% throughout. r
(7) A.S.T.M. Method D-1319-58P. (8) V. J. Keenan. R. M. Lineoh, R. L. Rodger8 and H. Burwaaser.
J . Am. Clem. Soc., 79, 5125 (1957).
(9) H. A. Dewhurst, THIBJOURNAL, 61, 1400 (1957). (10) A. M. Brodsky, Yu A. Kolbanovaky, E. D. Filatova and A. 5. Tchernyskeva, Inter. J . Applied Rad. and Isotopes, I , 57 (1959).
THERADIOLYSIS OF LIQUIDHEXANE
Nov., 1960
1625
The great difference in reaulte from the radiolysis of hexane and h e x a n d % hexene-l was found in the hydrogen yield. This yield was constant for the first 600 j./g. of energy absorbed (OH, = 3.4) and gradually decreased with increasing dose. The results are plotted as the dotted line in Fig. 1. The presence of hexene in n-hexane during radiolysis has two effects: (1) the hexene acts as an intermediate in the formation of higher unsaturated hydrocarbons and (2) reduces the hydrogen yield to a constant value over a wide range of energy absorption. From these and other unreported results it becomes obvious that one key to understanding the mechanism of hexane radiolysis was to determine quantitatively the effects of addition of small amounts of hydrogen atom scavengers; e.g., hexene, on the hydrogen eld. The development of this investigation is reported in &tion 111.
Section III Hydrogen Production in the System: Hexane Scavenger
+
Experimental-Irradiations were carried out in an irradiation cell of the design which permitted removal of dissolved gases prior to irradiation and of hydrogen after exposure. This cell (-250 ml.) was filled with 100 ml. of liquid, a connecting tube containing a stopcock was added through which the cell was attached to a vacuum system. Gases were removed by conventional chilling and pumping techniques. The liquid was irradiated at room temperature (23') bv X-ravs from a Van de Graaff accelerator. The irradiat
