BY AUSTIN11. YOUNG AEU JOIIN
E. WILT,AI?D
Depurtmenl of Chemistry of the Uninersity of lYiscoiasin, Mudison, Wise. Received.August 8,1981
This paper reports studies of the radiolysis of liquid and gaseous CCI3Ur, of t,hr phot,olysis of gaseous CChBr and of the rndiation induced cxchange of hrotnine between Urz and CCIaBr in the liqi~idand thc gaseous phases. Cob0 w a y s and 2537 A. light were the activating radiations. Typical product yields in units of moleculw/100 e.v. absorbed are Dose,
CC1,
e.v./g.
CCleUrz
CClBrr
CoCls
Bro
CoClaBr
Radiolysis (lis., 20’) 1 x 1021 3.5 2.9 0.1 0.5 1 0.15 1.2 0 7.5 6.1 .a2 1.2 1 x 10” Rad. (gas, 108”) 0 0 5.1 5.7 .3 0 Rad. (gas, 108’, 1 mole % Brd 1 x 1021 0.7 0 0.5 0.5 .01 0.9 Photolysis (gas, 108’) 1 x 1022 The apparent activation energies for the formation of the products are about 3 Iccal./mole, which is lowcr than required for thermal radical and atom abstraction reactions in this system. This rnny result from a small contribution from thermal processes superimposed on a rntijor contribution from ternpcrature insensitive reactions of vibrationally excited species or ions. The reaction cross sections for thcse hot processes must be significantly lower than the collision cross sections since the yields are reduced by 1 mole 70 added Brz. The G-values for the exchange caused by -prays in the gas and in the liquid a t 108” are both of the order of 600 atlo~ns/lOOC.V. absorbed while that in the liquid a t 20’ is about 150. These valucs indicate chain reactions with an over-sll activation energy which appears to bc too low to be explained on the basis of the known thermal chcmistry of the system, arid so may include :I contribution from ion molecule react,ions. G(cxchange) is sensit,ive to some unknown variable but is not affected significantly by 2 mole yo0 2 in the presence of 1 mole yoBrs.
Introduction This work \vas initiated to compare the radiolysis of CCLBr in the gas phase with that in condensed phases.2 It was designed to determine to what extent the yields, the temperature effects and the scavenger effccts in the condensed phases are dependent upon diffusion controlled and caging processes. Characteristics of the gas phase radiolysis, in which ions are formed, have been compared with the photolysis, in which no ions are formed. Previous work 011 the radiolysis of the liquid has been extended by gas chromatographic analysis of the organic product,s. The exchange of gaseous CC13Br with Br2 induced by y-rays has been investigated and found to occur by a chain reaction. Experimentalib Thc purifieiition and sample preparation procdrires uwd were, with some variations, similar to those described earlicr.2 Irradiation of samples to be analgzcd spectrophotometrically was carried out in an annular vcssel3 into the conter of which a 400 curie CoGo source about 1.0 cm. diam. and 2 em. long could be inserted t,o give a dosage rate of about 1 X lozoe.v. hr.-l (6. of CCl3Br)-1. A Rcckman-type cell of square Pyrex tubing attached to the annular vessel by sevcral inches of glass tubing was used for spectrophotometric :tnalgsis of the liquid or gas aftor each of successivc periods of irradiation. For analysis of g:isc~oussamples steam heated “thcLrmospaccrs” werc usod on the Bockman DTi spoctrophotometer cell compart,mont,, which T V : ~provided with an electrically heated covw box to accommodate the protruding mnular vessel. Caro was taken to mix t,horoughly the irradiated and unirradiatcd portions of the sarnplc before analysjs. I’he at)sorbancy index of I3rf in gaseous CCI3I3r at 4160 A . was detmmined to be 166 1. mole-1 crn.--I. (;:M 7-irradiations wcre carried out, in a thcrmostuted mineral oil-t)ath in which was mouutcd a l o d shicAl(l to reduce the exposurc of the spectrophotometer cell to irradiac
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_
(1) (a) Presented ut t h e International Congress of Iladiittion
Re-
tion while the gas in t,hc annular vc:ssel was being exposed. l3P for exchange studies was prepared by irradiation of small ampoules of liquid Brz a t a flux of 10l2neutrons scc.-1 cm.-2 in the CP-5 react)or of the Argonne Xational Laborator.. Mixtures of CC13Rr and radiobrominc which had undcrgone cxchange were purtitioncd between CClS and aqueous sulfite solkit ion to determine by counting methods the extent, of exchangr. The 0.3-nil. liquid smiplcs to be analyzrd by gas chromatography wen: sctaled in 6 em. long 4 mm. i.d. Pyrex tubes following degassing, and during irradiation were positioned so that they rcxoivcd the same dose rate as samples irradiated in the annular vesscl. Detection of the components in the effluent from thr: BiIiconc-oil-on-firc‘r,rickchromatographic columns was doric with a Gow Mac thcrmal eonductivit,v detcctor. When radiobroniinc had t)con used in the sample a scintillation countor also w i s used.‘ The photochcmicsl expcrimeiits were carried out in a 10 em. long, 2.5 cm. diam. quartz cell with flat end windows, mountcd in an ttluminiim block furnacc. The radiation sonrce was a IIanovia SC 2537 low pressur(: mercury arc, filtered by s i Vgcor plata t,o exclude wave lengths bclov 2100 1. All of thch incidont 2837 A. radiation was adsorbed by thc CCl& at! the prcssaros usod. The intensity was Lwsumed to be the same as that previously foundKfor the sanie type of lamp a t the same current* and used at the same geometry with redpcct t,o the rcsction cell.
Results G(Br2) of Liquid Phase Radiolysis.-Itl has been rcported2 that, G(Br2) for the radiolysis of liquid CC13r at 20’ dccrcases with increasing txominc concentration up to about 0.02 M and then remains constant, t,he value at Ill Br2 being 1.5 and ihe “terminal” value being 0.94. These observations werc made a t a dose rate of 1. X 10l9 e.v. g.-I hr.--l and total doscs up to 2 X IO2” e.v. g.-’. Thc prcscnt work has shown that thc same trend occurs with a 10-fold highcr intcnsity (1 x 1020 C . V . g.-I hr.-l) and for 10-fold highcr doses (2 X 1O2I e.v. g.--I). The absolute G-valucs at the lower bromine concentrat ions show evidence of being slightly higher a t thc higher int,ensit>yb u t the terminal G-values appcar to he indistinguish-
seurch, Burlington, Vt., A w . 19.58; (h) Additional det:iils of work a r e given in the Ph.1). tlresis of A. 11. Young. filar1 with tilt! University of Wisconsin Library in 19.58 a n d available froin University Microfilms, Ann Arbor, Mieh. (2) 1%. F. Firestone and J. E. Willard, J . A m . Chem. S o c . , 89, 3851 (1061). (3) R. F. I’irestonn ant1 J. 12. Willard, Rev. Sei. Insir., 94, 904
(4) J. B. Evuns. ,J. E. Quirilun und J . E. Willard, Ind. l h g . Chem., 60, 192 (1958). ( 5 ) ( i , h l . Ilarriu and .J. 1:. IViIlard, .I. Am. Chcm. Soc., 7 6 , 4678
(1 953).
(19.54).
able at the two intensitica and diffcrcrit total doses. Organic Products of Liquid Phase Radiolysis. . CC1, uiid CC12Br, are thc pidominunt pruducts of the radiolysis of liquid CCI313r a t 20' They are formed with constant G-values of 3.5 arid 2.0, respectively, up to doses of a t least 2 X loz1e.v.
G-vnlnrs of thc organic products shown for 108" a11 increased by a factor of 1.4 l o 1.5 except for OCIIJr3which iricrcuscd from ci - 0.8%to C = 1.8. Br,-CCl,Br Exchange during Gas Phase Radiolysk--When gaseous CCl,,Ur L: t 108' w.ns r d i olyzcd in the grcsence of 1-11 mole % Brz labeled g.-'. Other organic products, produced with with Brg2well over 90% of the E3P2in organic commuch lower G-values, are C2C16 (0.5), C2C15Br h i a t i o n appeared as CClsUr (Table I). In 15 (0.15) and CC1Br3 (0.1). The accuracy of the low cxperimcnts in which G(exchange) us deterG-value measurements is not adequate to indicate mined the average value for atoms exchanged pcr whether the rate of formation of these is independ- 100 e.v. was 497 f 225 in the absence of oxygen cnt of bromine concentration. and 266 f 93 in the presence of 0.04 to 2 mole Br2-CCljBr Exchange during Liquid Phase Radi- % oxygen (Table 11). From thc data it may be olysis.-Some thirty determinations of the ex- said that the exchange must be a chain reaction, and that it is not highly sensitive to Brz concentrachange reaction CCljBr Br2* --t CCluBr* Brz were made using Br2 labeled with BrS2. G- tion, radiation dose or oxygen concentration within (exch.) is about 650 a t 108' and 150 a t 28', thc ranges tested but is sensitive to somc unknown apparently independcnt of radiation intensity variable, as in the case of the liquid phase cxchangc over a 50-fold range from 0.03 X 10l8 to 1.3 X reported above. lo'* e.v. g.-' min.-', and independent of Br2 conTABLE I centration over the twenty-fold range from 5 X I)ISTRlBCTION O F Br*2 h\IC)SG ORGANIC ~'liODI2Cl'S 01' 'PIJE to 1 x 10-1 AI. Rathcr largc unexplained OF GASEOUS CCI3Br-Br2 ~fIXTURXSCOATAINIXC: fluctuations in individual G-values were observed RADIOLY~IS Br2(Br-82) between certain supposedly duplicate runs, the (log0, 1.64 X IO2' e v. absorbed pcr 9.) extreme values at 28' being 30 and 350. Control ----Prnction of organic inIOZl? [Brzl, runs in which the samples were prepared, heated CCkh CCIlBrz CClBra mole % mole& ,' and analyzed in the standard manner but not 0.032 Trscc 0.97 1 0 subjected to radiation showed negligible exchange. .077 Tr:icc .92 5.3 1.2 .048 0 01 Gas Phase Radiolysis.-TyricaI G-values for .04 10 0 .027 Nonc the prcducts of radiolysis of CC13Br in the gas .07 10 1.2 phase a t 108' both with and without added Brz TABLE I1 are tabulated in the Abstract. Thc values withG( ExciiaNoE) DURING RADIOLYSIS OF G a s ~ o v s CC1,13r-k out added Brz are slightly higher than those in the ~IIXTUR CONTAIXING ES Br,(Br-S2) liquid at 20°, except for CzCl5I3r which is not e.v. g.-l hr.-*) (108", 7.7 X observed in the gaseous products. The presence of 1 mole % added Br2 reduces all of the yields slightly and seems to completely eliminate CzC16 and Brzproduction. 1 ,* 1.96 0.90 680 A comparison of the solid lines of Fig. 1 illus1 .. 0.75 .54 590 trates the increase in G(Br2) with temperature 1 .. .13 .26 1400 in the radiolysis of gaseous CC13Br. The G-values 1.'I .. . i8 .43 620 calculated from the slopes a t 0.2 mole % Br2 1.3 .. . i8 .42 510 concentration on the 108 and 177' radiolysis curvcs 1.2 .. .78 .13 1'10 are 1.2 and 2.2. A value of 1.3 was obtained for 0.5 .. .i 8 .74 500 a similar determination a t 141'. Other experi0.5 .. .i8 .58 330 ments in which the bromine concentration was 0.2 .. .30 .42 170 followed as a function of dose a t various tempera1 .. .i 6 .27 250 tures up to 177' and a1 doses up to 18 X lo2' e.v. 1 .. .iG .43 460 g.-l showed that G(13r2) approaches zero for pro1 .. .i6 .31 280 longed irradiations a t 108 arid 141'. At 177" the 1 2 .i G .21 190 data also shorn this trend, but a t a dose as high as I 0.4 .i6 .2s 200 17 X lo2*e.v. g.-' G(Hrz) is still 0.9 for an avcragc 1 0.04 .i G ..I1 '110 bromine concentration of 3.2 mole %. The Br2 F/F is the fr:iction of cyuilibrium cxchange ucliirvcti. concentrations at which G(Br2) becomes zcro Gas Phase Photolysis.-Gaseous CCl& ilincrease with increasing tempcraturc, being about 1.5 mole yoat 108', 2.5 mole yo a t 141' and higher luminated with 2537 A. radiation produccs 13rz a t than 3.5 mole yo a t 177'. When a sample of a rate which dccreascs with increasing bromine I3rz had been concentration and increases with increasing tcmCCl8Br vapor in \vhich 1.5 mole produced by irradiation was allowed to stand for perature, as shown by the broken lines of Fig. 1. 18 hr. a t 177' without irradiation no incrrnse in 'I'hc initial yield of Br2 per unit of energy absorbrd the Br2 concentration occurrrd, showing thermal is somewhat higher for the photolysis than the radidecomposition to be negligible. Likewise an un- olysis. The initial G of photolysis decreases more irradiated sample showed negligible bromine pro- rapidly with iiicreasing Br2 concentration than duction when held for 37 hours at 180'. that of radiolysis but, in contrast to the radiolysis, When thc radiolysis was carricd out a t 182' is not reduced to zero at 108' at, the bromine conwith an absorbed dose of 1.5 X 1021e.v. g.-I the centrations achieved in Fig. 1. F'ollon-ing Ihe
+
+
-
5
dctermiiiation of bromiiic production as a functioii of time of photolysis shown in Vig. 1 the two Aamplcs uscd to dcfintt the curvc at, 108' cach w m aiitlyzed by gab chrorristtography. 'I'he yields of organic products for these samples which had each been exposed to C.V. g.-1 of 2537 A. radiation arc given in Table 111 both as G-values m d as yuaritum yields. 'rAHLE
3.0
-
l80$ I
111
O F PRODUCTS PnODUCED BY TIIE PIiOTOLYSlS O,F G A S h O U S CC13Br A T 108' WITH 1020 e.v. g.-l OF 2537 A.
\'IELuS
RADIATION -Suinple
-1
Quantuni Compound
G-value
CCI4 CC13Br CC12Brz CCIBrS C2CI6 Hrz (spcrtrophotomctrically)
0.33 -2.4s 0.56
-Sample
yield
G-value
0.016 .121 .027
.80
.039
0.47 -2.77 0.52 .O1 .90
.69
.034
.iO
..
-
...
2Quantum yield
0.023 .13G ,026 .0005
-
.041 .034
From the curvcs of Fig. 1 it is apparent that the average quantum yiclds of Br2 for the complctc experiment arc much smaller than the initial yields. This presumably is true also for C2C16, one molecule of which is produced for cach molccule of Br2formed. Discussion Mechanism of Br, Production in Radiolysis.G(Br,) as estimated from the slopc of the 108" curvc of Fig. 1 a t 0.2 mole yo Br2 is 1.2, which is similar to the value of 1.7 obtained2a t this bromine conccntration in the liquid a t $18'. The absence of a sharp discontiniiity in yield a t the liquid nvapor transition probably is fortuitous. Caging effccts in the liquid undoubtcdly cause some primary rccombination with no net product yicld, while reactions of adjacent radicals in the spurs must lead to Brq production which could not occur in the gas phase, wherc the radicals must react with Br2in the bulk of the gas becausc thc high localized radical conccntration of thc spurs does not exist. It has been noted earlicr2 that thcrc is no discontinuity in G(Br,) a t the solid phase transitions a t -33.5 and -13.5Oor a t the mcltingpoint a t -5.6'. These transitions arc not, however, accompaiiicd by a density change of the magnitude involved in the change from liquid to vapor. An immcdiatcly apparent diff crcncc bctwccn the liquid and gas phase radiolysis is that in the latter G(Br2) decreases v ith incrcasing Rr2 concentration, approaching zero a t n conccntratiori which is dcpcndcnt on temperature. This was observed in the range of 0.1 to 3 molc yo Br2, while in thc overlapping rang(%of 0.01 to 0.3 mole Yo in the liquid constant G-valucs werc obscrved above 0.1 mole % a t all tcnipcrat urcs. Three types of available evidence are hclpful in considcring the mechanism of bromine production in the gas phase radiolysis, ix., the tcmpcrature dcpendencc, the dcpcndcricc on hromine concentration, and comparison with thc photolysis. A comparison of the rates of the gas phaw radiolysis a t 0.2 mole % 13r2 a t 108, 140 arid 177" shows an
2 4 G S 10 Energy absorbed, e.v. g.-1 X 10-21. Fig. 1.-Production of bromine by the radiolysis (solid lines) and photolysis (dashed lines) of gaseous CCLBr. 0
apparent activation energy of only 3 kcal./mole. The similar apparent activation energy in thc liquid phase radiolysis sccmcd to bc ascribed best to the increasing probability of CC13and Br atoms escaping primary or secondary Such recombination does not occur in thc gas so the tempcraturc cocfficiciit in this phase must be associated with a chemical process. Threc kcal./molc is, howcvcr, much lomcr than thc activation energy (8-10 kcal./mole)6 of rcaction 6 listed below, or tho activation encrgy of rcaction 7, uhich appcars to bc about 24 k c a l . / m ~ l e . ~Reactions 5 and 7, uti-
+ + + + + + + +
CCLBr --+CC13 Br CClaBr "-+ CCl,Br+ eCClaBr m-f CC13+ Br ccc13 M +CCI, &.I CCI3 CC1, +C2Cle CC4Br Br -+ CCI3 Br; CCla CClaBr --f C2Cls 13r CC13 f CClaBr --f CZC16 13r CCI3 Brt +CC1,Br Ur
+ + +
+ +
+
(1)
(2) (3) (4)
(5) (6) (7) (7') (8)
lizing cc13 radicals formed by (1) and (4) or ((i) appear to be the only plausiblc thermal rcuc%oiis of neutral spccies which can lead to 1ic.t bromiiio production in the systcm. Ion-molrculc reactions cannot be excluded, but the photochcniical production of Brz with similar low activation cncrgy (Fig. 1) indicates that ncutral specks can g i w yields of the magiiitiidc obscrvcd. 'l'hc fact thut Br2 and C&ls production can be climiiiatccl by thc prescncc of a few mole yo of Br2 aiid that thc 13r2 concentration requircd to accomplish this iiicrca with increasing temperature indicates that the rcactivc spccics must undergo on the avcragc many (0) (a) A. A. Miller a n d J. E. ~ \ i l l a r d ,.I. Chem P h i i s , 17, It18 (1949); (ti) N. 1)svidwm arid J. 11. Sullivuri, %hid.,17, 170 (1919,. (7) E. N. neoker, 1'ii.D. ttieuis, Univrrsit, of Tiiscoiisin, 195.3.
(Ytudics at dmcs L c h ~&out 2 x 1 ~ f’ 1v. g - 1 r q u i r c d the comhiiintioii of srp~1rt~Ltely irradiated samples in ordcr to oblaiii enough product for analysis.) If the stcady-stalc ooiicciritration of CCI3radicals is coiltrolled b y reaction 8, as appears to bc the case a t most of the Urz coiwcntrations uscd in this work, their concentration will be inversely proportional to the Br2 concentration. I n t,his caw -0.5 I I I I I I I I the rate of production of Brz would be proportiorial -1.0 -0.8 -0.0 -0.4 -0.2 0 0 to 1/[Br2] if reaction 7 is responsible and to I / log jljr?]. [BrzJ2if reaction 5 is responsible. Reaction 7’ Fig. 2. -I.:valuatioii of depcndcncc of G(Br?) on broniine also would give a 1; [firz]dcpendeiice if it involves concentration. G( Url) is in units of molccwleu produced/ 100 C.V. absorbed during thc particular interval of irrrrdia- long lived vibrationally excited radicals which tion; bromine conrentration is in units of mole ‘& of original may be deactivated, and so prevented from reacting, CClYBr and is thr average coriccntrat ion during the interval by collision with Brp. Tho bromine dependence for which G is calculated. for a run at 108’ with a total dose of 4.9 X IO2’ collisions with CCl& before consummatiiig the C . V . g.-l and a final Brz concciitration of 0.8 molc bromiiie producing reaction, arid that thc prob- 7’ hiis been determiiied by evaluating b in the rrability of reaction on collision is increased by in- lation G(Br2) = a [Brtlbfrom the slope of thc plot creasing the tcmpcrature. The most plausiblc of log G ( h ) us. log [Br,] shown in Yig. 2. The explanation of the observed results srems to be slopr of the line which has been drawn is -0.75. that the major portion of bromine production is Within the experimental accuracy (diameters of the result of the temperature independent reaction vircles indicate error of 0.001 absorbance unit) of vibrationally excited CC13 radicals by (7’), the slope of the correvt line could be -1 but combined with a small yield of the temperature de- riot - 2 , pendent reaction 7. Vibrationally excited CC13 Mechanism of BrS Production in tke Photolyradicals which undergo many collisions before sis.-Absorption of a photon of 2537 A. radiation reacting and which can be scavenged by Br2 before gives a CC13Br mt:lecule sufficient energy to break reacting likewise have seemed to offer the best the C-Br bond (49 kcal./mole) explanation of the bromine dependence of G(Br2) CC1& --+ CCll + 13r ($1) in the liquid phase radiolysis. Clear evidence for the role of vibrationally excited C1-f~radicals in the plus 64 kcnl. which must appcar as \ribrational vas phase radiolysis of CHJ has been observed.b energy of the CCls radical or kirictic encrgy of the ?he hot CC13 radicals from the gas phase radiolysis CCls and of the Br atom. If the energy in excess of CC13Br must be vibrationally rather than ki- of thc bond encrgy all appears as kirictic energy netically hot because kinetic cwrgy would be lost conservation of momentum requires that it bc in a very few collisions with CC13Br molecules divided, 26 kcal. to the CCIJ and 38 k d . l o thc and so the reaction of the radicals would not be Br. Comparison of the curves for bromirie protiucsubject to the scavenger action of bromiiie a t the tiori as a function of dose for the radiolysis arid photolysis (Fig. 1) indicatcs that G(13rp) decreases concentrations used. The question of whether C&16 production (and with increasing [Br,] concentration at low [13r2]colihence Brp production) occurs predominantly by centrations for both methods of activation. Above reaction 5 or reactions 7 and 7’ also can be cxamincd about 1 mole % Br2thc G of brominc production by by estimating the relative probability of thew the photolysis a t 108’ is constant at 0.5 to at least 2.3 reactions from estimated steady-state concentra- mole Yo (lower dashed line) ; whercas G(Rr2) for the tions and rate constants. Such estimates are highly radiolysis is zero a t 1.5 mole 96,as seen from thc uncertain, with the greatest uncertaintics arising horizontal portion of the upper solid line plot of from lack of information on the relative frequency Fig. 1. The lincar portion of the 108’ photolysis factors of the radical reactions. Assuming equal curve indicates thatj 13r2 is being formed by a hot frequency factors, a Br2 concentration of 0.1 mole process which is not susreptible to scavenging by yo, and activation energies for reactions 7 and 8 Br2 molecules in this concciitration rangy. The which are 24 and 4 kcal./mole, rcspeetivcly, higher vibrationally or k i n c t i d y exited CCL radicals than that of (5) the calculated rates of (7) and (5) involved in this process miist have a high probare 6 X 10-9 and 3 X of that of (8). Since ability of reacting with CCI3I3r or being deactivated the rate of ( 5 ) is second order with respect to [CCl,], by it before reacting with or bring deactivated it increases rapidly with decreasing concentration by Br2 present a t 2 mole %. If CCl, radicals arc of Brz. Fragmentary evidence that, such increase formed with 26 kc*al./mole of kinetic cnergy thcy is observable experimentally a t concentrations in might meet this criterion, some of them producing the range below 0.2 mole yo Br2 was given hy the 1/2 Brp by reaction 7’ and others being reducctl fact that G(C2Cle)for an exposure of gaseous CC13- to energies hclow the iiecessary activation ciicrgy Br to 1.5 X 1021 e.v. g.-I (maximum [Rrg] = 0.2 on the first few collisions. The slope of thc mole yo)was 1.2 while it was 5.2 for an exposure straight, h i e portion of the 108’ curve (lower dashcd of0.13 X 1021e.v.g.-l ( m u . [Brz] = 0.02 mole yc). line) indicates a quantum yield of about 0.023. If the rate of the hot photolysis reaction shown ( 8 ) (a) G. A I . Harris and J. E, Willard, J . A m Chem Soc., 7 6 , by the straight portion of the 108’ ciirve is sub4878 (19.54): (b) F. P. IIiidson, I?. 11. \\ illiarnx. .Jr , itnd W. I 1 ITamlll. tracted from the rates of the reaction l~clow1 mole J . Chcm. I ’ h p . , 21, 1801 (1953).
*
yo Br2 at both 108 nnd L80° the rates of the bromlne semitive reaction arc givcn. Comparison of thcsc yiclds gives ail apparerlt aclivatiorl encrgy of aboul d ltcal./molc. This is lower than the activation energy of eitlier reaction 6 or 7 and sllggcsts that, ns in the radiolysis discussed above, there may be both a thermal and a hot reaction contributing to the bromine sensitive yield, processes 7 and 7' beiiig possible reactions. Organic Products of the Radiolysis and Photolysis.--The presence of CC14, CClzBrzand CC1Br3as products of both the radiolysis and photolysis of gaseous CClaBr and of C2CI5Bras a product of the radiolysis in the liquid phase indicates that some or all of the following reactions must be occurring
+ CCl& --+ CCI, + CCl2Br + CCll13r --+ CCl, + CClzBr + CC12Br2-+ CCl, + CC'lIh + CCILT3rL--+ CCl, + C C l l h CCIJ3r -+ CC1213r + C1 C:C1,13r3_, + Urz +CXI,Br4-. + n r (Xla CCl? CCh CC13
CCls
+ CC1J3r +C2C16Br
(10) (10') (11) (11') (12)
(13) (14)
(omitting consideration of possible ion-molecule reactions). One mole % of initially added Br2 reduces the yields of CCL, CCI2Br2arid CC1Br3 in the gas phase radiolysis slightly (see table in hbstract for comparison of typical runs), but does notJ eliminate them. That portion of the products not eliminated by the added Brz must be produced as a result of the hot processes 10' and 1 I' or 12. If reaction 12 does not occur stoichiometry requires that G(CCL) = G(CC12Br2) 2G(CClBr3). Within the precision of the data the results are in agreement with this requirement indicating that C-C1 bond rupture in the radiolysis occurs as a very minor process if at all. The material balance observed in the photolysis suggests that 1% or so of the photons absorbed may cause primary rupture of a C-C1 bond rather than a C-Br bond. The yiclds of CC14 and CClZBr2from the radiolysis exceed the yields of CzC16and Brz, but the reverse is true for the photolysis under the conditions of the table given in the Abstract. In view of the great differences in both kinetic energy and vibrational energy which may exist between the CC13 radicals formed by the absorption of 2537 A. radiation and those formed by excitation by electrons or by ncutralixation of ions in the radiolysis system it is plausible that the relative probability of reactions 7' and 10' would be distinctly different for the photolysis and radiolysis. The yields of organic products from the radiolysis a t 180" are somewhat higher than a t log", indicsting some contribution from the thermal reactions 10 and 11. This might be expccted from the finding of Becker7 (using CCh radicals labeled with C13*) that the analogous chlorine abstraction reaction
+
CC13*
thc liquid phasc radiolysis but riot tfiosc of thc gas phase prcsumubly is thc result of ieaction 14 occurring III the spurs, Similar reactions of CUI3 aiid CClzRr radicals to form CzClr;13r arc indicated by earlier studies of solutions of l3rg in CCl, activated by the Br*'(n1y)lW2process.$ Br2-CC1,Br Exchange during Radiolysis.-Significant characteristics cf the exchange reaction induced by Co6" radiation include: (1) the Gvalues in the range of several hundred indicate that it is a chain reaction; ( 2 ) comparison of the Gvalues a t 108 and 28' in the liquid phase indicates that the activation energy probably lies between 4 and 6 kcal./mole; (3) the G-values in the gas and liquid phases a t the same temperature are indistinguishable; (4) the yield is not markedly affected by the presence of O2 at a concentration equal to that of the Brz present. The only possible mechanism for a chain reaction for the exchange seems to be SL repeating sequence of reactions 6 and 8 or an ion-molecule sequence. lo If the observed temperature dependence, suggesting an apparent activation energy of 4-6, is correct the sequence 6 and 8 cannot be responsible for the total exchange since the activation energy of ( 6 ) is 8 kcal./mole or more.6 The evidence at present available suggests that the chain is the result of the (6)-(8) sequence occurring simultaneously with an ion-molecule reaction of lower activation energy. Schultell has found G(C2C16) = 0.86 for the CoG" irradiation of liquid CC1, a t 21', a value similar to those we h a w observed in both liquid CC13Br a t 20' and in gaseous CC13Br at 108". He finds a value of G for the exchange of Clz with CCli of 3.5, much lower than our observation for the Br2CCI3Br exchange. The comparisons suggest that the C2C16 may be formed in the two systems by analogous hot processes of the type of (7'). The fact that the activation energy for C1 CCl, .-t CCls C12 is at least 14 kcal./mole12 may account in part for the lower yield of the exchange reaction. Acknowledgment.-This work was supported in part by the United States Atomic Energy Commission (Contract AT(l1-1)-32) and in part by'thc University Research Committee with funds made available by the Wisconsin Alumni Research Foundation. During part of the work one of the authors (A.H.Y.) held a fellowship given by the Standard Oil (Indiana) Foundation. (9) J. F. IIornig a n d J. C. Willard, J . A m . Chem. S O C ,71, 461 (1053).
(IO) BY analoay with the rneolianisin postulated b y Thompson arid Shaeffer (abad , 80, 553 (19%)) for the Hz-Dz cxchango tlic ion rnolecule sequence might be
+
-
+ + +
CC1J3rf 131-2~ CCIJBr2*+ l3r CC13Br2* CC1313r +CCI?Br* CCLI3rZ+ CC13Br2+ Dr,* --f CClaBr2*+ 131-2 +
+ CClr +CCll* + CCll
has an activation energy of about 10 kcal./mole. The presence of CzCl5I3ramong the products of
+
+
+
+
(11) J 1%' Siliultr, J .
(12)
(1961).
,477~
Chem S o c , 79, 46l.3 (1957)
F. ,J. Jobnston and J E. Willard, J P h y s Chcm ,
06, J17
