RADIOLYSIS OF DEUTERATED BIPHENYLS
Oct., 1960
We also note that this decrease in “total ionization” is owing to the decreased probability of the processes in which loss of Eix, seven and eight, carbon atoms are lost. The probability of the other carbon loss processes appears to be closely similar in all of these molecules.
1367
Acknowledgment.-We wish to acknowledge the invaluable assistance of R. A. Meyer who obtained all the spectra reported in this paper; we also wish to acknowledge many helpful conversations with D. E. McKenzie, J. P. Howe and H. Eyring in particular.
THE RADIOLYSIS OF DEUTERATED BIPHENYLS : MECHANISM OF HYDROGEN FORMATION1 BYJ. G. BURRAND J. 31. SCARBOROUGH Research Department of Atomics International, A Division of North American Aviation, Inc., Canoga Park, Calif. Received Januaru 26, 1960
Certain general characteristics of the radiation chemistry of aromatic hydrocarbons are summarized. The values of G(hydrogen), G(H), G(D) and the H*/HD/Dz ratios observed in the radiolysis with cobalt-60 ?-rays of biphenyl, biphenyl4,4’-d2, biphenyl-2,2’,6,6’-d4, biphenyl-3,3’,5,5’-d4, biphenyl-2,2’,4,4’,6,6’-da, biphenyl-2,2’,3,3’,5,5’,6,6’-ds and biphenyl-dl, are reported. The formation of radiolytic hydrogen is insensitive to any difference between the ortho, meta and para positions of biphenyl (although it is sensitive to the 1.3 kcal./mole difference between C-H and C-D bonds) since the specific rates of H and D formation are insensitive to the location of deuterium on the hydrocarbon and the total hydrogen yield is a linear function of the deuterium content of the biphenyl. The isotope effect in the formation of radiolytic hydrogen and deuterium (3.00) is observed to be different from that observed in the loss of one or two hydrogen atoms (or deuterium atoms) in the dissociation of‘the corresponding molecule-ion in the mass spectrometer (1.8). It is shown that this same isotope effect of 3.00 and simple statistical considerations can be used successfully to account for the Hz, H D and D2 ratios in the hydrogen from individual hydrocarbons. However, the Hz, H D and D2 content of the hydrogen from a mixture of biphenyl and biphenyl& cannot be so described, and is also observed to be unequilibrated. It is suggested from this that at least two processes are involved in the formation of radiolytic hydrogen from biphenyl.
I. Introduction I n this article we are reporting the results of using partial selective deuteration to obtain information about the mechanisms of hydrogen formation in the radiolysis of biphenyl. An accompanying paper2 contains the results obtained by applying this same technique to study the mechanism of the molecule-ion dissociation processes of biphenyl in the mass spectromet,er; we are report,ing elsewhere on the radical scavenging ability of several types of aromatic substance^.^ We have chosen biphenyl as the subject for this research because biphenyl is the simplest aromatic molecule which :has sets of distinguishable carbonhydrogen and ca,rbon-carbon bonds and which still retains much of the structural and electronic simplicity of benzene. We did desire an aromatic molecule which possessed distinguishable sets of carbon-hydrogen bonds to enable the use of selective deuteration as an aid in investigating the mechanisms of hydrogen formation. This proPedure has been found useful in examining the radiation chemistry of ethanol,* acetic acid,6 choline chloride6 and methanol.; With this aim in mind (1) Work performed under AEC Contract AT-(lll)-Gen-S. This material was presenti:d in part before the Boston Meeting of the American Chemical Society, April 5-10, 1959; and before the Congress of Kiiclear Energy, Rome, Italy, June 15-19, 1959. (2) J . G. B w r , J. M. Searborough and R. H. Shudde, TEXIS JOURNAL, 6 4 , 1359 (1960). (31 J. G. B-irr and J. D. Strong, “The Radiolysis of Organic Solutions: 111. Mixtures of Biphenyl with Propanol-2 and of Biphenyl with Benzophenone,” Abstracts of the 137th Meeting of the American Chemical Society, p . 43-K, J. A m . Chem. Soc., 8 1 , 775 (1959); J. G. Biirr and J. D. Strong, THISJOURNAL, 63, 873 (1959). (4) J. G. Burr, ibid., 61, 1477 (1957); J . A m . Chem. Soc., 79, 751 (1957). ( 5 ) .J. G. Burr, T H I :JOURNAL, ~ 61, 1481 (1957).
IJoE
we have prepared a number of selectively deuterated biphenyls.* H()H H
D
IiVD
H
IV
1 H 1 0
I
I
v
VI
Degassed samples of these substances have been exposed t o the ionizing y-radiation from a 1500curie cobalt-60 source. The yield of radiolytic hydrogen and its isotopic composition were de(6) R. 0. Lindblom and R. M. Lemmon, B E C Report UCRL-8204. p. 5 , 1958. (7) J. G. Burr, Abstracts of t h e 137th MeetinE o f the American Chemical Society, p. 43-K. (8) The preparations of I, 11, I V and V I are reported in a n article by R. I. Akawie, J. hl. Scarborough and J. G. Burr, J . O r g . Chem., a4, 946 (1959). The preparations of I11 and T’ ~ ~ be1 rc,ported 1 in a subsequent similar article together with some other aspects of deiiterating aromatic hydrocarbons, J . Org. Chem., i n press.
,J. G. BURRAND J. hl. SCARBOROUGH
1.365
\
/
‘\\/
CAPSULE BREAKER C060
:---7/
Vol. 64
mined by mass spectrometry using a modified CCC Model 21-820 Mass Spectromctrr. (c) Irradiation.-All samplcs were irradiatrd with robalt60 g:tmmas from a source of about 1500 curies in thc clhape of a hollow rylindrr.0 The source could be lowered around a heated capsule and holder, approximately 1 inch diameter by 4 inch height, as shown in Fig. 1. The dosage rate w1t9 determined by conventional ferrous sulfate dosimetry and measurement of hydrogen yield from benzene. Appropriate corrections for electron density were m d e for biphenyl samples. All samples were irradiated at 100 +L 5’ The dose rate waa 1.4-1.5 X 1018e.v./ml watei-min. Experiments with biphenyl-& showed that G(hydrogen) was independent of dosage within the limits of error of these evperiments sinre all values lie within the region of 0.00768 & 0.0002 for dosages ranging from 1 X loz1e v./g. to 4 X 1021 e.v./g. Since dosages for most samples reported here were very similar, little error would be introduced unless there were a very large, dose dependency. Although the scanty existing data10 for gas yields from biphenyl suggest that these are not very sensitive t o temperature up t o about 300°, the mechanisms tor hydrogen formation ivhich the work presented here has led us to propose (see below) require a sensitivity to temperatrlre. It mav very well be that some of the scatter of our data is caused by insufficiently precise control of the biphenyl temperature during the radiolysis.
111. Results The 100 e.v. Yields of Hydrogen-G(Hz, HD, D2).-The observed yields of hydrogen (as H2 HD D2) are shown in Table I. These values mere obtained by combining the experimental values of G(gas) with the experimental composition of the gas reported by mass spectrometry. Our value for G(hydrogen) for biphenyl, 0.00768 A 0.00019 (at a dose of 4.3 X loz1e.v./g.) can be compared with the value of 0.0098 reported by Colichmari and GerckeIO a t the considerably higher dose, 22.6 x lo2’ e.v./g. ; and the value of 0.0067 reported” for a dose of 1.9 X lo?’ e.v./g. at 82”. The variation of G(hydrogen)I2 with the deuterium content of the various biphenyls is shown in Fig. 2 . 2. G(H),I? G(D)I? and the Specific Rate Factors, y and n.-The isotope effect in the formation of hydrogen and deuterium during these radiolyses has been determined by comparing the fractional hydrogen and deuterium contents of the radiolytic hydrogen with the fractional hydrogen and deuterium contents of the substrate hydrocarbon. We have found it convenient to make this comparison in terms of specific hydrogen and specific deuterium yields which have been normalized to the same process in biphenyl. Our definitions are 1.
CAPSULE LOADER
ELECTRICAL/’ ,I’ LEADS THERMOCOUPLE
CAPSULE IN HEATED HOLDER IN CO-60 SOURCE
Fig. 1.
termined. The total dose of radiation was kept very low in order to minimize any possible dose dependency effects; under such conditions the quantities of hydrocarbon gases (which comprise only 2 4 % of the gaseous prodLtcts from biphenyl radiolysis) and the quantities of polymer were too low for measurement. 11. Experimental (a) Preparation.-The synthesis of the deuterated biphenyls used in this investigation has bren reported elsewhere.8 Gas chromatographv a t two temperaturrs, infrared sp~~ctroplriotometry and mass spectrometry of these materials indicated chemical purities greater than 99.5yo. Small amounts of less deuterated materials were present but the isotopic purities ranged from 95.1 t o 98.0 mole % of the species sbown .e ( b ) Encapsulation and Capsule Opening.-Capsules were prepared from 22 mni. Pyrex tubing, in a form shown in Fig. 1. These were filled in inert a,tmosphere with liquid biphenyl by means of the capsule loader shovn in Fig. 1. The body lengths of t,he loaded capsules were about 75 nim. The filled capsiiles were attached to a vacuum system and degassed try conventional freezemelt technique (the “freeze” portion of the cycle was to room temperatiire only since lower temperatures prevented effective degassing). The eflectiventm of of sample was obtained from the weights of the empty capsule and the two parts of the filled, degassed and sealed ampule. The sample weights of the biphenyls were about t,en grams in nearly all cases. The irradiated ampules were opened on a high varuum line, using an ampulf. breakw of the design shown in Fig. 1. The radiolysis gases wwe removed from the ampule with a Toepler pump through an ice-cooled trap and a liquid nitrogen-cooled trap, and were collected in a calibrated gas buret. During this piimping proreps t>hebiDhenyl was degassed by subliming it into the ice-cold trap. The volume of gas in the gas buret, was determined by pressure and temperature measuremi:nts, and the composition of the gas was deter-
+
+
Specific hvdrogen yield = G(H) per C-H bond in hydrocarbon molecule ____._ G(H) for biphenyl/lO--= Y
Specific deuterium yield = G(D) - per C-n bond in hyArrocarbon molecule G(H) for biphenvl/lO (9) E. L. Colichman, P. J. Mallon and .i. . 4..Jarrett, S u c i e o n i c s . 16, No. 4, 115 (1957). (10) E.L. Colichman a n d R. H. J. Gercke, . \ ~ I L ~ I Y O I I ~ C 14, S . YO. 7, 50
(1956). (11) K. L. Hall and F. A . Elder, J . Chem. Phua., 31, 4120 (1959). (12) We define G(hydrogen) as G(H2 €ID + 11:): G(1.11 as G ( H z ‘/&ID) and G ( D ) as G(Dn ‘/rHD).
+
+
+
Oct., 1'360
RADIOLYSIS OF
1369
DEUTERATED BIPHENYLS
TABLE I I'RODUCTS Substance
Biphenyl I IV
v
I1 111
VI
+
Biphenyl mixture
G(hydroxen)" No. of X 103 dctns.
7.68 :& 0.19 6 . 0 3 :& .28 5 ~ 7 92~ .69 6.03 4.30 f .05 3.30 2.49 4 .31 V I 50/50 5.01 f .12
8 3 2 1
3 1 3
FROM
% H,
100 83.8 f 0.9 66.8 69.8 f 2.6 40.2 f 3 . 0 1i.5 3.6
2
60.3
RADIOLYSIS OF DEUTERATED BIPHENYLS 70 DI
% HD
........
... ....
G(H)r" C(D)a" 74 X 10' X 108 (gamma)
..
.d
(pi)
-r/ra
..
7.68 15.5 f 1.0 0.77 i 0 . 2 5.53 3 0 . 7 f 2 . 2 2.56 k 0 . 3 4.75 29.3 0.8 5.10 45.7 f 1 . 7 14.2 f 1 . 3 2.72 40.7 42.0 1.39 5.9 90.5 0.16
1.000 ... 0.50 0.903 0.325 1.04 1.03 .338 0.88 1.10 ,296 .343 1.58 0.80 .342 1.99 0.91 2.34 ( - - - ) * .31
2.78 3.05 3.72 2.60 2.80
27.6
1.30
2.86
I
12.2
3.71
0.97
.34
Av 2.07 f 0 . 2 8 G(hydrogen) = G(Hz H D Dz); G(II) = G(H2 l/zHD); G(D) = G(D2 '/ZHD). * iZ value for this of 1.6 can be estimakd ver.y roughly since the 7% of H in the gas must have come from the approximate 1% of hydrogen in the It biphenyl-& (although it is possible that some of the Hz may have originated in water adsorbed on the ampule walls). should be noted that the ratio G(H,)/G(.D*) from biphenyl and biphenyl-&, is 7.68 X 103/2.49 X l o 3 = 3.08. L)efiriitiori of these quantities will be found in section 111-2 of the text.
+
a
+
These definitions are similar to the definition^'^ of y and T factors which have been used elsewhere2 to analyze the data obtained from the mass spectra of these biphen:yls. As an example, we show computatioii of the specific hydrogen and specific deuterium yields for hydrogen obtained in the radiolysis of biphenyl-& (11). Y =
+
+
2.70 x 10-3 x io = 0.88 1.68 X X 4
-.--
7 rr)'
2 6 X h
5
The specific. hydrogen yield for biphenyl is 1.00 by definition. It will be noted that the Y/T ratio is a measure of the apparent isotope effect for hydrogen formation relative to deuterium formation in the radiolyds of a particular hydrocarbon. These dttinitions provide a simple means for comparing the radiolysis and molecule-ion dissociations of thwe deuterated biphenyls. Discussion of t,hiij comparison will be found in a later section of this paper. Compilation of the y and T factors will be found in Table I, and the variation of these factors as a function of the deuterium content of the hydrocarbons is shown in Fig. 3. H D and D2.-The relative 3. Formation of Hz, amounts of Hz, IID and D, are of interest because the H, is a hydrogen species which can be derived from the CJ-H konds in the molecules; similarly the HD represents the result of interaction (by one or another mechanism, such as H and D atom abstraction) of the C-H and C-D bonds in the molecule OT mixture of molecules; and finally the DSmust be derived only from the C-D bonds in the molecules. Measurements of this sort provide information about the degree of equilibration of these species. This may be evnluated by calculating the ratio (HD)2/(Hz)(D2).If the species arise from equilibration of ET2 D, or HD, then this ratio has the value at 25" of 3.2514 (it will be shown later that when the hydrogen species arise randomly via the intermediacy of an organic molecule, the value
+
(13) F. >I. Field and J L. Franklin, "Electron I m p a c t Phenomena," Academic Press. Ne\% York, N. Y , 1957. pp. 204-217. (14) S. 0. Thompson and I?. C Schaeffer, J . Chem. P h g a . , 23, 759 (1955); J. also the original calculations by A. J Gould W Bleakney. and H. S. Taylor, d a d , 2 , 362 (1934).
5
c3
0 0 4
>.
I Y
0
3 2
~
I
Yo DEUTERIUM IN THE HYDROCARBON, Fig. 2.
of this constant seems to be 4.0). The values of this ratio for the gases evolved from the deuterated biphenyls are shown in Table 11. Ordinary probability theorems can be used to calculate the relative amounts of Hzr H D and D, to be expected in the hydrogen from each of the deuterated biphenyls if this formation is a purely random process. I t is necessary to insert a weighting factor into these calculations for the comparison of calculation with experiment to be realistic. Thus the relative probabilities of H2, HD and D2 formation correspond to the coefficients in the (normalized) binomial expansion of (xNH Y M D ) ~ ,where N and M are the fractional abundances of C-H and C-D bonds in the molecule, and x/y is a weighting factor which can be considered to represent the isotope effect in the formation of H and D (as the molecular speries Hs, H D
+
,T. G. BURR.4ND ,IM. .
1370 1.6 r
i
0’
% a
&
I
I
/
I
I
I
1
I
I
1
I
1.4
1.2
w
3 1.0
-I
’
a
0.8
-J
f 0.6 a
& 0.4 3
0
IO
20
I
/
50 60 70 80 90 100
30 40
PERCENT DEUTERIUM IN HYDROCARBON. Fig. 3.
TABLE I1 D? IN THE RADIOLYSIS OF DEUTERATED BIPHENYLS
FORIIAT~OX OF Hz, HD
AKD
(1.83
Substance
Biphenyl-&(I)
Species
t
NH MD)
HZ
77.2 H D 21.1 D, 1.4 Biphenyl-& (IT-) Hf 5 3 . 7 (Yl 53.7 (IV) H D 39.2 (V;l 39.2 (Iv) D, 7.2 7.2 Bipheny!-d8 (11) H2 30.0 HD 49.3 D, 20.2 Biphenyk& (II’I) H? 9.9 H1> 43.1 Dz 47.0 BiphenylH? 41.6 biphenyl-& HD 4 5 . 5 50/50 Mixture D, 12.4 K = (HD)‘!/(Hz)(Dz).
or:
(3.0
+
,VH MD)2
Obsd.
85.2 14.2 0.59 67 67 29.8 29.8 3.3 3.3 44.5 44.5 11.1 18.4 49 32.7 56.2 37.5 6.25
83.8 15.5 0.8 66.8 69.8 30.7 29.3 2.56 0.8 40.2 45.7 14.2 17.5 40.7 42.0 60.3 27.6 12.2
K”
3.72 5.52 ( IV) 15.4 (T:)
3.67
2.25
1.04
and Dz). We use two values of z/y: 3.00 and 1.83. The results obtained by use of each of t,hese values are shown in Table 11. TABLEI11 THE Loss OF H AND I) DEUTERATED BIPHENYLS
THEISO~YJPE EFFECTIN PHENYL’ A S D
Type of measurement
1. G(H21/G(D2)from radiolysis of biphenyl anti biphtnyl-dlo 2. -y/r from radiolysis of deuteratrd biphtlngls from m:tss sprctra of tieutrrntrd 1)i3. phmyls a Average val.ie.
~ ~ 0 BIx 1
.. .
i
q
o
. ~
~
3.08
’.
‘j7
* O . 28
( 1 .83)“
IV. Discussion The radiatioll chemistry of aromatic hydrocarbons is distinguished by low product yields15 (15) The relatim? product sield8 observed i n the irradiation of organic conicounds a.re well reported in numerous reports and review
articles.
P.mong rhe most useful of these are E. Collinson and A. J.
Vol. 64
SCARBOROUGH
(such as gases and “polymer”), and by the fact that the yield of polymer is about ten times the yield of hydrogen, whereas in the radiolysis of most saturated organic molecules the yield of hydrogen is two or three times the yield of polymer. It is also noteworthy that thcsc materials are remarkably stable to ultravioletl light,16 and that there is great predominance of the molecule-ion peak in the mass s p e ~ t r a . ~The ~ ‘ ~yield of radicals from aromatic hydrocarbons is about equal to the yield of “polymer.”17 In the radiolysis of aromatic hydrocarbons some processes must be operative which, in a relative way, enhance the polymer yield and decrease the hydrogen yield. The formation of hydrogen in the radiolyses of saturated and aromatic substances may be discussed in terms of this set of elementary reactions. RH
--+
RH*
+H + H? RH* + R H +R ” + H: H + RH +H? + R H + RH --+ RH2, RH* --+ R . RH* --+ R’
(la) (1b) (2)
(3) (4)
(3)
Reactions 1, 2 and 4 are sufficient in general to describe the formation of hydrogen in the radiolysis of saturated substances,18 so that practically every molecule and atom of hydrogen which is formed in the dissociation process must necessarily appear in the product hydrogen. The effect which structural changes or isotopic substitution might have upon the dissociation processes must be reflected in the yield or composition of the product hydrogen. On the other hand, aromatic hydrocarbons, and most of their derivatives, can be regarded as unsaturated substances to attack by hydrogen atoms and organic free radicals. It has been shown repeatedly that methyl radicals add readily to aromatic rings,lg that hydrogen atoms can add readily to liquidz1 and gaseous aromatic hydroquite readily t o solid olefins a t liquid nitrogen temperature,22 and that even phenyl radicals add readily to benzene at least at high temperatures in the gas phase.21 The effect of dissolved aromatic hydrocarbon^,^^ aromatic ketones,24and quinones18 upon the radiolytic hy9ivallow, Chem. ifess., 56, 171 (1956); B. AX. Tolhert a n d R. 11. Lemmon, 4.E.C. Report UCRL-2704 (1954); J , 0. Burr, Proceedings of the Second International Conference on the Peac.efirl Cyes of Atomic ~ Energy, ~ ~ &Val. 20, p. 187, Geneva. 1958; and t h e chapters on Radiation Chemistry i n the sevcral volumes of “The Annual Reviems of Physical Chemistry,” Annual Reviem, Inc., Palo Alto, California. (16) G . K. Rollefson and 1f.Burton, “Photochemistry,” PrenticeHall Book Co.. New York, Ti. Y., 1939, Chap. V I l I . (17) A. Chapiro, THISJOURNAL, 63,801 (1959), gives a summary of radical yields for benzene, toluene, xylene, ethylbenzene and sryrenr as measrired b y several different methods. (18) G. E. .%dams, J. H. Baxendale and R. 1). Sedgn-ick. i b i d . , 63, 854 (19%). (19) J . Smid and AI. Szivarc, J . A m . Chem. SOC., 78, 3322 (1956),and earlier pauers. (20) P. E. RI. Sllen, €1. T. Melville and J. C. Rohb, Proc. KO!,.Soc. (],ondon), 218, 311 (1933); H. W. Jlelrille a n d J. C. Robb, ibid.. 202,
18:2::gE)iv. Taylor, Can.
J , Chem,, 3F, 739 ( , 9 B ~ ) , (22) R. Klein and AT. D. Scheer, THISJOURXAL 62, 1011 (1958). (23) J. H. Baxendale. personal communication. (24) J. G. Burr and J. D. Strong, THISJ O U R N A63, L , 87:3 (1959).
oct., 1960
RADIOLYSIS OF DEUTERATED BIPHENYLS
1371
drogen yield from several saturated substances has been shown to follow a simple kinetic scheme based upon a competition between hydrogen atom abstraction from the saturated substance and hydrogen atom addition to the aromatic substance (we do not wish to imply that we think that it is certain the effecl, of these additives is owing to addition sca~enging,but that this is the most likely explanation at present). It is thus apparent that the radiolysis of aromatic hydrocarbons differs from that of saturated Gubstances at least to the extent that any hydrogen atoms arising in the dissociation step (1) might be removed not only by reaction 4 but also by reaction 5 and that more complicated radicals, such as phenyl, which also arise in the dissociation step might also be reinoved via a reaction similar to ( 5 ) . The high value of the radical yield measured for benzene, ranging from 0.8 to 3.1,17325suggests that the dissociative split of aromatic hydrocarbons into radical pairs is an important process. The fate of the hydrogen atoms produced by the radical pal r dissociation of the aromatic molecules iq indicated clearly by several other studies. The yield of hydrogen in neutral, degassed aqueous benzene solutions has been observed to be 0.42i26 equal to G',(H2),27sensitive to pH or the presence of 0xygei1.~~I'hung and Burton cite this as evidence that the hydrogen atoms produced from radiolysis of water do not abstract (reaction 4) from benzene mdecules under these conditions and this conclusion is reinforced by the observation of these same that only a trace of HD is formed by the radiolysis of benzene-ds in water or the radiolysis of benzene in D20. This eridencc. for the fate of water hydrogen atoms in the presence of benzene taken together with the low yield of hydrogen in the radiolysis of pure benzene suggests that hydrogen atoms produced in the bmzene radiolysis disappear almost entirely b,y add ition to surrounding benzene molecules. This coiicept is supported by the studies of Gordon, VanD ykeii and Dournaniz8 who found that the yields of both hydrogen and biphenyl were low (0.044 and 0090, respectively). The polymer, which was formed with a yield of about 0.90 (molecules of benzene appearing as polymer per 100 e.v. ab:.orbed), consisted largely of hydrogenated biphenyls and terphenyls. These are the products which would be expected to be a consequence of the addition of hydrogen atoms to benzene, as suggested by the earlier observation of unsaturatioii in the polymer from irradiated benzene.2s Thus the evidence quoted here suggests that d i ~ ) c i a t i o i iinto ~ ~ radicals is the predominant primarr chemical process in the radiolysis of
aromatic hydrocarbons ; t,hat the hydrogen atoms produced in this primary process are scavenged pract'ically quantitative1y3l by addition to the surrounding aromatic hydrocarbon, and that other radicals produced in this primary process would also be extensively scavenged by a similar addition process. However, hydrogen is produced in the radiolysis of benzene and in t'he radiolysis of biphenyl in yields n-hich are about 10% of the radical yields or polymer yields: if the secondary reactions of hydrogen atoms are not' important contributors to the formation of this hydrogen, then non-radical processes such as react'ioii 2 or reaction 3 must be the important contributors. The hydrogen and deuterium obtained in the radiolysis of biphenyl and the deuterated biphenyls, I-VI, therefore must reflect characteristics of non-radical processes such as reactions 2 and 3. Products which would enable charact'erization of the radical reactions 1 and 5 would only be found among the monomer, dimer and ot,her high boiling fract,ions of the radiolysis products. Other consequences of the non-radical processes 2 and 3 may possibly be found among the light hydrocarbon fractions of the radiolysis products, such as the acetylenes, which were obtained in too low a yield in t'his investigation to warrant examination. 1. The Yield of Hydrogen a s a Function of Hydrocarbon Deuterium Content.-Selectivity in the format'ioii of hydrogen during radiolysis of deuterated ethanols4 was revealed in part by a non-linearity in the plot of G(hydrogen) versus deuterium cont'ent of the carbinol. i Z similar nonlinearity has been observed in the radiolysis of deuterated methanols7; for the deuterated cholinesj6the G(-M) value was a non-linear function of the deuterium content of the substance. It is apparent from inspection of Fig. 2 (and column 2, Table I) that t.he yield of hydrogen from the deuterated biphenyls is best considered as a linear function of the deuterium content of the biphenyl. The yield of hydrogen from the 50/50 mixture of biphenyl and biphenyl-dlo also falls accurately on this straight line. Thus not only is the yield of hydrogen quite insensitive to the posit.ion of deut'eration, but it is also insensitive to the manner of deuteration. The equation of t,he line in Fig. 2 can be presented as G(hydrogen) = 7.68 X - (0.528 X 10-3)N, where N is the average number of deuterium atoms in the biphenyl, without regard to the position where the deuterium atoms are located on the biphenyl skeleton and, for a given N , without regard to the molecular species actually present. It is thus apparent that the very real differences in reactivity among the ortho, meta and para posi-
125) AI. Biirtnn a n d S. Lipsky, zbzd., 61, 1461 (1957). (26)' i T' Plinng a n d 31 Burton, Radzatzon Research, 7, 199 11957). 127) T J ~ n o i ~ hJi , A m Chem SOC.,76, 4687 (1954) (181 P Gordon, 4 R. VanDSken and T. F. Doumani, THISJ o w n v a ~ , 6 2 , 20 (1858) (29) R Y Patiirk and 32. Burton, J. Am. Chem. Soc., 76, 26LG ( 195%) ( 3 0 ) It is not meant b y thm statement t o infer anything specific about the intimate mwhanism of t h e radlcal formation for example, It could occur ~ z asimple dissociation of exclted or ionized molecules or ma some i n i i c l i mnre complex process.
(31) For example. the G(scavengeab1e hydrogen atoms) i n the Co-GO radiolysis of water is 3.70. All of these n w e scavenged b y the concentration of benzene-di used (ref. 25) since t h e observed G(H3) m-as 0.42. T h e G(HD' was obserred t o be 0.013. T h u s (0.013/3.70) X 100. or 0,35y0of the liydrngen atom reactions with t h i s benzene were abstractive. If G(hydrooen atoms) from benzene is taken t o be 0.4 (i.e.>one-half the radical yield), and 0.35% of these react with benzene via abstraction, then t h e G(hydrogen) produced b y abstraction would be 0.0014 or about 2.8% of t h e observed total G(H9) from benzene, 0.040. We consider mechanistic contributions of this order of magnitude to be negligible.
.
11. SCARBOROUGH
0.6
0.4 i
l
c
I
I
U
I
I
0
20
40
60
i
I
80
100
PERCENT DEUTERIUM IN CARBINOL, Fig. 4.
tions of biphenyl, which show up in ionic substitution reactions and in free radical substitution reactions of this molecule are not able to influence the over-all yield of hydrogen in this radiolysis. 2. The Deuterium Content of the Hydrogen.It was oberoed in the earlier studies on saturated aliphatic substmces that selectivity in the formation of hydrogen was ex-idenced not only in the xwiation of hydrogm yield with hydrocarbon deuteriiirn con:ent but was revealed also by coiisideration of the deuterium content of the radiolytic h y d r ~ g e i i . ~The simple form of analysis usable for this problem when only the abstraction reactions of hydrogen atoms need be considered is obviously inapplicable to the radiolysis of biphenyl, since it appears that abstraction reactions are not important in the radiolysis of biphenyl. We hare found it convenient to represent the relative 11 and D content of the hydrogen from these biphenyls as G(H) and G(D), and the values of these qumtities are found in Table I, columns 7 a n d 8. The values of G(H) and G(D) were made specific aiid normalized according to the definitions of 7' and A factors given in the Results sections. The valuw of these functions are shown in Table I,
T'ol. 64
columns 9 and 10. It will be noted on Fig. 3 that these fuiictions seem, within experimental variation, quite insensitive to the deuterium content of the hydrocarbon, and again no evidence of position specificity appears in these normalized specific rates of hydrogen and deuterium formation. Both hydrogen and deuterium are formed from these partially deuterated bipheiiyls a t specific rates which do not depend upon the arnouiit of deuterium present in the molecu1ei.e , the presence of deuterium in the biphenyl does not increase the specific rate of hydrogen formation or otherwise affect the breaking of carbon-hydrogen bonds. If the data for the H and D content of the hydrogen produced in the radiolysis of the deuterated ethaiiols4are similarly reduced arid plotted, as in Fig. 4-specificities in the dissociatioii of the C-H bonds in the ethanol molecule and in abstraction processes of the hydrogen arid deuterium atoms are reflected very clearly in the variation of these y and ir factors with deuterium content of the carbinol. Thus in radiolyses where the formation of hydrogen may be reasonably considered to occur via simple dissociation into radicals followed by simple abstraction processes any position specificity is clearly revealed by this method for analyzing the data. The insensitivity in biphenyl of the radiolytic specific hydrogen and deuterium yield factors (radiolytic y and ir factors) may be an artifact of the special nature of the liquid phase. The sensitivity of these yield factors in the dissociations of the molecule-ion to the deuterium content of the molecule-ion was assigned in the earlier paper? to the increased non-fixed energy content of the moleculeion which is a demonstrable consequence of increasing the deuterium content. This effect can only influence the dissociation rates if the lifetimes of the molecule-ions are long enough for the excess energy to be distributed among all the possible degrees of freedom. This situation exists in the highly dilute gas phase of the mass spectrometer where the lifetime of the molecule-ion can be lo7 normal vibrational periods or greater. However, in the liquid phase this complete equilibration of absorbed ionizing energy may not be possible, since the collision freqiieiicy in the liquid is sufficiently high that all of the excess energy of the molecule-ion probably is removed within a few molecular vibratioii periods. Thus dissociation of a C-H bond in the energized molecule would not result from equilibration of energy but simply from excitation to a dissociative level of the potential energy curve for that bond in the energized molecule. The effect of increasing nonfixed energy with increasing deuterium content mould never have a chance to influrnre the dirsociation processes, and the liqiiid phase y and A factors could be illvariant with the tlrutc.riuni content of the molecule.32 By either the measure of over-all liydrogen yield or by the measure of the specific hydrogen and (32) However, since t h e excitation and ionization occiir vertically, t h e Franck-Condon principle can be applied t o explain why C-H bonds should still be broken faster t h a n C-D bonds even i n t h e liquid state-cf.. Field and Franklin, ref. 13, pp. 205 a n d 216.
deuterium yields, any difference among the bond strengths of the ortho, meta or para C-H bonds in biphenyl33 apparently is not reflected in the rates of hydrogen formation from these deuterated biphenyls. Either there is no effective difference in the strerigths of these bonds, or the difference is too slight to affect the radiolysis process (however, the radiolysis process is sensitive to the 1.3 kcal. differeiice hetween the C-€1 and the C-D bonds), or the mechanism is such that any differences in the bond slrengths ( L e , , the structure of the molecule) would no; be reflected in the radiolysis process-a:; an tastreme example, the mechanism might be that of a completely random disengagement of hydrogen molecules from ai1 essentially structureless transition state. 3. Consideration of the Isotope Effect.-In the first paper of this series2 it was shown that the isotope effect observed in H and D loss from the deuterated biphenyl molecule-ions was a smooth function of the dieuterium content of the nioleculeion with a n average d u e of 1.83. It was suggested that t h k was a characteristic value for dissociation of wigle C-H and C-D bonds in the molecule-ions of unsaturated substances. It was also reported in that paper that the isotope effect, for the loss of two hydrogens from these molecule‘/&D),‘?zH/(2D l/&ID)/(lO ioiis, (2H n)D, was approximately the same as that for the loss of one hydrogen. For the biphenyl radiolyses, we have two apparent measures of the isotope effect: (1) the ratio of the hydrogeii yield from biphenyl to the deuterium yield from biphenyl-&; and (2) the calculated ratios of ? / T reported in Tables I and IV. The numeric:tl near-equality of these two isotope effect s seems consistent with the conclusion reached above 3 bout the relative unimportance of the secoidary reactions of hydrogen atoms as contributors to the formation of product hydrogen. I t does not seem likely that the isotope effects for the seconclary reactions of H and D atoms would all be equal to each other or be the same as the isotope effects in the dissociations of (3-13 and C-D bonds; the complex of possible reactions would bc different for the substrates containing both C-II aiid C-D bonds than for the pure biphenyl ai d p u i ~biphenyl-&. If the secondary rcnctioiis ncre important, it does not then seem likely that the ratios 7,’. would be the same as the ratio G(I-T,),’G(D2)for biphenyl and biphenyl-dlo, nor would the ratio T/T be so nearly the same for :dl the pnitially deuterated substances. 4. Information to be Gained by Consideration of the H?, E D and D, Content of the Radiolytic Hydrogen.--If the distribution of 112, H D and to be expvttd f i om each of the deuterated biphenyls is computed statistically, it is then observed that the computed abundances do not agree at all with the observed abundances; this lack of agreement 1s not iiiiexpected owing to the known
+-
+
(3’3) I n calcnlation of the potential function an$ aqsipnment of the peak- in t h e infrarrrl spectra of hiphensl and t h m e deutrrared blp h i nJ1s. the riioilcl udolpted for t h e asrionnients a n d c a l c ~ l a t i o n arzi n ti t h e i i e of ditfercnt force comtants for t i t i -e ortho, meta and p . v i C-11 hondz h l i n o r L >+ill be plibll>hcd shortly b> R H. hhiidde, L \’v Ltiiuian and J. hl Scarborough of this Laboratory. 51,
existence of an isotope preference for the dissociation of C-H bonds. If the statistical computations are corrected by iiicluding a single weighting factor, then the entire set of computed abundances for the individual partially deuterated biphenyls can be brought into agreement with the observed abundances. The results are shown i l l Table 11, columns 4 aiid 5 ; it is noteworthy thnt a single value of the factor, 3.00, suffices for the eiitire series. Iiispect’ioii of the data in Table I1 leads to t v o general conclusions. One of t,hese is that the experimental data are in quite good agreement with statistical prediction if the radiolysis value of the isotope effect (3.00) is used-the value .* 1.83 from the mass spectra does not prodixe a satisfactory fit. This agreement is so good for t>he entire set of compounds (except for the biphenyl-d,, where the data are from a single experiment) that it hardly can be fortuitous. The formatioii of hydrogen in these radiolyses is thuis agaiii shown t,o be a structurally random process, at least for HS, HD, D2; this conclusion is identical with the. previous discussed lack of position selectivity in these processes. It also can be noted from this evidence for a structurally random process t>hat the deviatioiis from equilibrium suggested in t he last column of Table I1 probably do not represe:it non-equilibrium situations of niechaiiistic iinportance. The abundances of species in i-he hydrogen from the equimolar mixture of biphenyl and biphenyl-dln are not, in good agreement with the statist,ical expectation, nor are these species present, i i i equilibrium amounts.34 There is t’oo much €1, and D2 and too little HD. A similar lnck of equilihratioii of the hydrogen species has been ohserved in the hydrogen from an equimolar mixture of other unsaturated organics and their perdeuteroanalogs,35 such as benzene-benzene-&, toluenetoluene-&, and acet,one-acetone-ds, Since any single random process which can bc written for the production of hydrogen from biphenyl would produce from a mixture of biphenyl and biphenyl-dlo either an equilibrat.ed mixture of hydrogen species or a mixture of pure H2 and Dz, the mixture of hydrogen species actually obtained must result from t,he opcration of at, least two such reactions simultaneo~isly.~~ If the arguments cikd above are sufficieiit too allow esclusion of the reactions (1) (4)as producers of
+-
( 3 4 ) T h e valiie of K shon-n in Table I1 for t h e Iiz, FIT) a n d Dt froin t h e equimolar hipirenvl-hiphenyl-rIln mixtiire. 1.01, is reduced even f i i r t,her if r h e data i n ‘Iahlc I are riirrected for t h e Ii. and Hn w!iicli originate froin tile biphcnyl-d:c romiwnc’nt of the nii.;tiire. The val,ii* of K t h e n becomes 0.8i. T h i s is a r,rononnced (IF.. iation from t h e expected e ~ i i i l i b r i ~ trmd i i e ranre, 3.76-4.00, for a pas with sltch laroe amounts of each of the components ( t h e K v s l ~ i efor biyhenyl-d?, biphenyl-d, and biphenyl-dio are mr;ch less reliable because t h e very small contents of Dt-in the first t w o cares-and II? in t h e last case make t h e valrir of K extremely senPitive to sniall errors in the analysis of the gas). (35) J. G . Burr, “Prorerdings of t h e Second Unit
