ADDITION OF TRITIUM TO OLEATE
1297
Mechanism of Addition of Tritium to Oleate by Exposure to Tritium Gas
by C. T. Peng Radioactivity Research Center and Department of Pharmaceutical Chemistry, University of C.zlifornia, S a n Francisco, California 94222 (Received October 22, 1966)
Formation of tritiostearate by exposure of oleate to tritium gas proceeds by a mixed radical and ion-molecule mechanism with 40% of the total addition caused by tritium atoms and 60% by ion molecules containing tritium. Argon sensitizes the addition by the formation of tritium argonium ion, ArT+, and oxygen inhibits the reaction by scavenging both radicals and ions. Gross tritium incorporation in the reactant is temperature dependent, which is interpreted as the result of an equilibrium between tritium in the adsorbed phase and that in the gas phase. Labeling by tritium a t cryogenic temperature is mainly decay induced. The activation energy for tritiation by addition is higher than by substitution and is of the same order of magnitude as those observed for addition of hydrogen atoms to unsaturates by others.
Labeling of organic molecules with tritium is frequently effected by the Wilzbach gas exposure method.' Under such conditions, addition of tritium atoms across the carbon-carbon double bond also occurs, giving rise predominantly to a saturated product of extremely high specific activity.2-4 Mechanisms of labeling by tritium substitution have been investigated in homogeneous systems of methanetritium15 ethylene-tritium,s propanetritium,' etc., and were found to involve decay-induced labeling and p labeling; the former is initiated by HeTf and the latter by electrons. In contrast, the mechanism of labeling by tritium addition remains relatively unknown. Although a free-radical mechanism has been postulated for the additionj3s8 the presence of free radicals in the reaction mixture was not at a level readily confirmable by electron spin resonance measurement .8 We report here the effect of temperature and the activation energies of tritium labeling by substitution and by addition, and give evidence for an ion-molecule mechanism for the addition of tritium to an isolated -C=Cbond, based on the results obtained from a study of the methyl palmitate-methyl oleate-tritium system. The inclusion of methyl palmitate in the system as a reference compound is necessary to allow a comparion of the rates of addition and substitution.
Experimental Section Methyl elaida te, methyl oleate, and methyl palmitate
of greater than 99% purity, purchased from the Hormel Institute, University of Minnesota, and tritium gas from the Oak Ridge National Laboratory, all without further purification, were used for tritiation. The mixture of fatty acid esters to be tritiated, consisting of either methyl palmitate and methyl oleate (28.66:71.34 wt %) (A) or methyl palmitate and methyl elaidate (27.03:72.97 wt %) (B), was deposited evenly onto 0.2 g of fine glass wool to provide a large surface area for contact with tritium gas in a 20-cc tritiation ~ h a m b e r . ~According to the need, three to four such chambers were connected to one inlet. After evacuation to mm or less, tritium gas was introduced by Toepler pumping. The individual chambers from one run, when sealed at the capillary and detached, contained approximately equal quantities of tritium and were separately maintained at different experimental temperatures for one (1) K. E. Wilzbach, J . Am. Chem. SOC., 79, 1013 (1957). (2) H. J. Dutton and R. F. Nystrom, Advan. Tracer Methodol., 1, 18 (1963). (3) R. F. Nystrom, ihid., 1, 46 (1963). (4) E. P. Jones, L. H. Mason, H. J. Dutton, and R. F. Nystrom, J. Org. Chem., 25, 1413 (1960). (5) T. H . Pratt and R. J. Wolfgang, J . Am. Chem. SOC.,83, 10 (1961). (6) K. Yang and P. L. Gant, J. Chem. Phys., 31, 1589 (1959). (7) K. Yang and P. L. Gant, J . Phys. Chem., 66, 1619 (1962). (8) University of Illinois Report TID 14787, Feb 2, 1962. (9) C. T. Peng, J. PhaTm. Sci., 52, 861 (1963).
Volume 70,Number 4 A p r i l 1966
C. T. PENG
1298
predetermined length of time. I n the first series of experiments, 0.5-ml portions of A were exposed to 4 curies of Tz at 77, 193, 273, and 353°K for 2, 5, 12, and 28 days. In the second series, 0.3-ml portions of A and of B were exposed to 2.5 curies of Tz at 77, 193, and 298°K for 6 days. I n the third series, 0.3-ml portions of A were exposed to 3.3 curies of Tzin the presence of argon or of oxygen at the above temperatures for 6 days. Exposure in the presence of nitric oxide was carried out with 3.0 curies of Tzfor 7 days. Argon and oxygen were passed through a molecular sieve column before being admitted to the tritiation chamber; nitric oxide (from the Matheson Co., Inc.) was introduced directly. Pressure of the added gas in the chamber was approximately 300 mm in all cases. I n the fourth series of experiment, photosensitization with 2537-A Hg-resonance radiation was carried out in a quartz flask with a Hg lamp (250 w, Hanau) on 0.3 ml of A distributed evenly over 0.1 g of fine glass wool and 10 curies of Tz for 30 min in the presence of finely dispersed niercury droplets. The isotropic light source was positioned approximately 10 cm away from the sample in order to avoid heat effect. After exposure, the unreact ed tritium gas was removed under vacuum. A given amount of methyl stearate in n-heptane, approximating the amount of oleate present in the sample, was added prior to purification as a carrier for the tritiostearate formed by the addition reaction. Analysis by gas radiochromatography was performed on samples before and after purification. Purification of the tritiated esters was carried out according to the procedure of Jones, et ( ~ 1 .Radioactivity ~ of the samples was determined by liquid scintillation counting. The individual fatty acid esters were analyzed by gas chromatography with a hydrogen flame-ionization detector using a 0.25 in. x 6-ft column filled with 100-120 mesh Gas-Chrom P coated with 14% EGSS-X (a siliconated ethylene glycol succinate polyester from Applied Science Laboratories, Inc.) or with 9Cb-100 mesh Celite coated with 15% diethylene glycol adipate polyester (Resoflex). The radioactivity of the effluent gas from the column was measured at a split ratio of 10 : 1 with a 275-cc high-temperature flow ion chamber connected to a critically damped vibrating reed electrometer; the sensitivity of this instrument was determined with a known sample of tritiated stearate. The area of the mass and radioactivity peaks, recorded with two synchronized potentiometers, was measured by planimetry for each individual ester.
Results When the product of gross incorporated tritium activity and temperature of exposure of the system The Journal of Physical Chemistry
C16:0-C18:1-T10 was logarithmically plotted us. either the exposure in curie-days at 77, 193, 273, and 353'K or the reciprocal of temperature during exposure at 2-, 5-, and 12-day intervals, linear or approximately linear relationships with practically identical slopes were obtained. Based on these observations, an empirical equation relating the gross tritium incorporation, A , in millicuries, exposure, E , in curie-days, and temperature of exposure, T, in degrees Kelvin can be derived; thus ET'//' A=L where L is a constant, characteristic of both the system and the compound under study. Figure 1 shows the plot of log A vs. log (ET"/') of all the systems investigated. Owing to the phase heterogeneity inherent in the reaction mixture and the uncertainty in the surface area available for tritium exposure, eq 1 is only approximate, and the scattering of the points is anticipated. However, over the extended range of temperature and duration of irradiation, points from similar systems fall along a straight line with an intercept equal to L on the horizontal axis.l' For the CmoC18:1-T systems, an average value of 2.31 X lo3 (from 1.72 X lo3 to 2.90 X lo3) for L was obtained. The presence of argon in the system caused a shift of the plot to the left from its normal position with L = 0.80 X 103, while that of oxygen was shifted to the right with L = 8.0 x lo3. Systems containing nitric oxide yielded widely scattered points, presumably because nitric oxide is a very reactive gas and can induce profound changes in the system as indicated by the presence of a pronounced unidentified peak in the gas radiochromatogram. Samples which were unadulterated and have been exposed for 28 days at 77, 193, 273, and 353°K did not show a proportionate increase in the gross incorporated tritium activity and gave points that were positioned to the right of the normal plot with L = 4.6 X lo3. This shift indicates that on long exposure the rate of labeling is adversely affected by a change in the tritium concentration from dilution by hydrogen from exchange and other gaseous products from radiation decomposition. The composition at the (10) The shorthand designation is from V. P. Dole, etal. [ J .Clin.Invest., 38, 1544 (1959)l. The numeral before the colon indicates the number of carbon atoms in the fatty acid ester and that after is the number of double bonds present, namely, CIE:O is for palmitate and C16:l is for oleate. (11) The plots for different systems have unit slope and will pass through the origin provided that the scale of the horizontal or the vertical axis is adjusted with the corresponding log L term for each system according to eq 1, namely, 1 = log A/[log (ETa//')- log L ] or 1 = (log A log L)/log ( E T * / z ) ,respectively.
+
1299
ADDITION OF TRITIUM TO OLEATE
L
I
'
1
I
I
1 I
IIII
I
I
1 1 1 ' 1 1 '
I
0
I
1 1 1 1 1
/ J /
A
Table I : Activation Energy for Addition and Substitution of Tritium I
E, System
+ Cl6:o + ClBt1 T + C18:1
T T
A
Tritiated product
Temp,
Substitution Substitution
Cls:0
Addition
C18:D
77-273 77-193 193-353" 77-193" 193-353
Reaction
c18:1
OK
kcal/ mole
0.4 0.2 1.3 0.3 2.8
A/
See text for the reaction mechanism associated with this temperature range.
Figure 1. See text. 0, m, 12,and 0 represent tritiated systems containing Tz, (Tz Ar), (Tz NO), and (T2 Od, respectively. A represents samples of CM:O-CLS:~-T system exposed for 28 days and A, the Hg-photosensitized sample. Solid line is for the 0 points only.
+
+
+
gas-liquid or gas-solid interface may also have been altered, resulting in subsequent impediment of tritium and radical diffusion. From our limited data, the L value appears to be invariant toward slight fluctuations in tritium pressure and in the quantity of material being exposed. Activation energies for tritium labeling by substitution and by addition for the C1a:o-C1s:l-Tsystem are listed in Table I. These values were calculated from Arrhenius plots of log (specific activity) us. 1/T. The Arrhenius plots for tritiooleate and for tritiostearate have different slopes above and below 193"K, suggesting two different mechanisms of formation of these products. The activation energies of labeling of these two products in the temperature range of 77 to 193°K are of comparable magnitude as that of palmitate. Since labeling of palmitate occurs exclusively by substitution through replacement of a hydrogen atom by a tritium atom in the molecule, therefore it is reasonable to suggest that from the magnitude of the activation energy observed, the labeling of oleate in this particular temperature range may also follow a similar mechanism. In the absence of preferred addition of tritium to oleate, tritiostearate is also formed from oleate and tritium by the direct action of the latter, with a maximum probability of formation of 0.12, based on a random attachment of tritium atoms to 2 out of 17 carbon atoms in the alkenyl chain of oleate. The fact that this ratio was actually observed
in the presence of oxygen at 77, 193, and 298°K lends credence to this proposed mechanism. As the lowtemperature substitution, to be discussed later, is mediated by decay-induced labeling, it should have a low, if not zero, activation energy. Our results, shown in Table I, are in line with this argument. At high temperatures, tritium addition to the -C=Cbond becomes predominant. But a finite probability also exists for a hydrogen atom elimination from the site of addition, thereby leading to the formation of a tritiooleate molecule. Formation of tritiooleate may occur in the condensed phase by disproportionation of the radicals formed from the addition of one tritium atom to an oleate molecule, but when the radicals are trapped at the surface, disproportionation with tritium atoms or other reactive species in the gas phase may become the only important mode of reaction. The plausibility of this explanation lies in its adequacy to account for the increased rate of formation of tritiooleate parallel to that of tritiostearate in this hightemperature range. The activation energy of addition of tritium to oleate given in Table I is in accord with the values reported for the addition of hydrogen atoms to unsaturates by other^.'^-'^ The ratios of tritiostearate to tritiooleate formed under various experimental conditions (henceforth called the addition ratio) are given in Table 11. The addition ratio increases with the temperature of exposure and reaches a limiting value of 14.5 at 298°K. Addition of tritium atoms to elaidate (trans-9-octadecenoate) is hindered at low temperatures, but a t 298°K the difference in the rate of tritium addition to cis and trans -C=C- bond disappears. It may be noted that this observation is in accord with the findings (12) M. D. Soheer and R. Klein, J . Phys. Chem., 6 5 , 375 (1961). (13) A. B. Callear and J. C. Robb, Trans. Faraday Soc., 51, 638 (1955). (14) K. Yang, J. Am. Chem. Soc., 84, 3795 (1962).
Volume 70, Number 4
April 1966
C. T. PENG
1300
Table I1 : Addition Ratio (Ratio of Tritiostearate to Tritiooleate) under Various Experimental Conditions Tritiating systema
Tz Tz Tz (€3: TZ Ar Tz NO Tz Ozc Tz (Hg-photosensitized)
+ + +
77
193
2.04 f 0.033' 1.81 1.70 3.34 0 0.12
2.94 f 0.29 2.61 1.74 8.14 0 0.11
...
Temp of exposure, OK 273
12.94 f 0.43
... ...
...
... ... ...
...
298
353
...
14.0 f 0.61
14.2 14.5 12.4 8.3 0.12 5.4
... ... I
.
.
... ...
...
'
Mixture -4was used in all systems unless indicated otherwise. Average value plus or minus standard error from samples exposed to 4 curies of TZat 77, 193,273, and 353°K for 2, 5, 12, and 28 days. ' Samples were assayed on new gas chromatographic columns to avoid desorption of tritiostearate from the used column.
on the addition of hydrogen atoms to cis- and trans2-butene at 296.5"K1* and on the disproportionation of sec-butyl-sec-butyl radicals at 77°K into 2-butene, which was formed predominantly in the trans configuration.l5 The presence of argon enhanced the addition ratio only at cryogenic temperatures, but a t 298°K the ratio approached the limiting value observed for the same system without argon. Oxygen inhibited the formation of tritiostearate by tritium addition to oleate at all temperatures and did not affect adversely the tritiation of palmitate and oleate. I n contrast, the presence of nitric oxide in the Cle:oCls:l-T system completely eliminated the formation of both tri1,iostearate and tritiooleate at 77 and 193"K, while it sensitized the formation of both these labeled compounds with an addition ratio of 8.3 at 298°K. Mercury-p hotosensitized addition of tritium at 298°K yielded an addition ratio of 5.4. Table I11 gives the G(T) value, namely, the number of tritium atoms incorporated per 100 ev absorbed, for some of the systems studied. The ratio of the G(T) values observed a t 298 and 77°K for the T2 system, at 298 and 193°K for the (Tz Ar) system, and at 193 and 77°K for the (T2 02)system agrees with the ratio calculated from the expression (TI/ T2)a'2according to eq 1. The lowest G(T) value is 0.018 calculated for the decay-induced labeling based on the assumption that energy released per tritium decay is 5.7 x lo3 ev. In the presence of argon the G(T) value is generally increased three- to fivefold for the reason that argon has a large ionization cross section and that the metastable argon atoms can cause more ionization of the reactant molecule to occur, thereby facilitating tritium labeling. At 298"K, the G(T) value for both the NO system and the Hg-photosensitized system is slightly increased but the presence
+
The Journal
of
Phydeal Chemistry
+
of nitric oxide did not cause a similar increase at cryogenic temperatures. As can be seen in Figure 1, the tritiating systems with larger or smaller G(T) values than normal are invariably distributed to the left or the right of the normal plot, respectively, and with different L values.
Table 111: G(T), Number of Tritium Atoms Incorporated per 100 ev Absorbed under Various Experimental Conditions Tritiating system
77
Tz" T2b T2 Ar TI NO Tz 0 2 Tz (Hg-photosensitized)
+ + +
0.046 0.051 0.18 0.015 0.013
...
Temp of exposure, OK 193 273 298
0.11 0.13 0.51
0.019 0.051
...
0.35
...
---. 353
0.35
.. . ...
0.38 1.05
... ...
... .. .
...
... ...
...
...
o.55c
...
Average values from 2- and 5-day exposure samples of mixture Values from 6-day exposure samples of This value is considerably mixture A to 2.5 curies of tritium. lower than that obtained with other systems [F. Cacace, A. Guarino, and G. Montefinale, Nature, 189, 54 (196l)l. The energy input for photosensitization was estimated to be approximately 1 w or below from the light source whose rating was 250 w owing to geometry and shading by glass wool. The dispersed Hg droplets appeared "oily" and were presumably covered with the fatty acid esters which further diminish the light intensity in the 2537-A region. a
A to 4 curies of tritium.
The stability of fatty acids toward p radiation during exposure to tritium has been noted.16 Under pro~~
(15) R. Klein, M. D. Scheer, and R. Kelley, J . Phys. Chem., 68, 598 (1964). (16) See ref 2, p 20.
ADDITIONOF TRITIUM TO OLEATE
1301
longed exposure, radiation decomposition takes place yielding both short- and long-chain fatty acids of high specific activity with no apparent detectable mass peaks. Among all the systems studied, it is generally observed that the extent of decomposition is usually greater at 353°K than at 77°K. Composition by gas radiochromatographic analysis of the 28-day exposure samples at these two temperatures is given in Table IV. At the high temperature, long-chain saturated and unsaturated fatty acids are the predominant radiation decomposition products presumably formed by radical disproportionation and combination followed by hydrogen abstraction or hydride transfer. At the cryogenic temperature, reaction cage effect and restricted diffusion of free radicals diminish the extent of radiation decomposition, and unsaturation by hydrogen abstraction is also less favored. The decomposition products are mostly short-chain fatty acids formed by chain scission of the parent compound. Table IV : Per Cent of Labeled Fatty Acid Esters Formed under Prolonged Irradiation with Tritium’ -Temp Esterb
77
1.43 1.47 3.28 1.97
c 1 2 : o c 1 3 : o
C14:O C1s:o
...
&:I
Cleo Cl6:l
+ Cl?:O(iao) C17:l + cm:o CU:I + Clg:O(iro) C19:o CM:Z+ CzO:O(isor c 1 7 : O
c18:O(iso)
Cm:a CZI:o(iso; c22:o(neo)
+
C Z I : ~ C22:0(i”a)
12.10 1.78 0.73
... 52.26 24.94
... ... ... ... ... ...
of exposure, OK353 ,..
... 0.01
...
0.48 2.06 0.79
... 0.39 82.76 5.91 3.05 1.86 0.81 0.32 0.29 1.03
See ref 10 Mixture A was used. Exposure was 28 days. for abbreviations. The is0 and neo homologs are so designated.
The identity of the decomposition products listed in Table IV was determined by comparison of the retention times on two different columns with available known compounds and from plots of retention time vs. the number of carbon aroms.17 The unsaturated products were identified by their absence in the gas radiochromatogram after bromination or hydrogenation; an example is given in Figure 2. The gaseous products and the short-chain fatty acids below laurate were neither determined nor identified.
Figure 2. Gas radiochromatograms of a sample of tritiated fatty acid esters before and after bromination.
Discussion Although the mechanism of tritiation in heterogeneous system is not well known,’* by analogy to the model developed by Klein and S ~ h e e r ~the ~ Jprob~ ability is a surface reaction between the reactive tritium species generated either in the gas phase or a t the gas-solid interface and the condensed reactant followed by diffusion into the interior of the latter. This model, however, assumes a priori the existence of an adsorption-desorption equilibrium of tritium between the surface of the condensed reactant, the wall of the container, and the gas phase which is temperature dependent. Diffusion of tritium to the interface where the primary reactions occur is also temperature dependent as evidenced by the variation of the coefficient of self-diffusion of hydrogen and deuterium with T”p.20 Since diffusion is usually a rapid process, it is therefore unlikely that the depletion rate of tritium at the interface of the Cla:o-Cls:l-T system is diffusion controlled. The primary reactions based on the findings for homogeneous systems6-’ may involve decay-induced labeling and p labeling. I n decay-induced labeling, the reactive species is HeTf formed from the r e a c t i ~ n ~ , ~
Tz +HeT+
+ e-
with a recoil energy of 1.65 ev and a G(T) value of 0.018. Because of the immutability of radioactive (17) J. W. Farquhar, W. Insull, P. Rosen, W. Stoffel, and E. H. Ahrens, Nutr. Rev., Suppl., 17, N o . 8 , 1 (1959). (18) K. E. Wilzbach in “Tritium in the Physical and Biological Sciences,” International Atomic Energy Agency, Vienna, 1962,p 3. (19) R. Klein and M. D. Scheer, J . Phys. Chem., 6 6 , 2677 (1962). (20) J. 0.Hirschfelder, R. B. Bird, and E. L. Spatz, Chem. Rev., 44, 205 (1949).
Volume 70,Number
April 1966
C. T. PENG
1302
decay and the small magnitude of the G(T) value, the decay-induced labeling is temperature independent and unimportant as a mechanism for tritiation under ordinary conditions.21 At 77"K, when tritium addition in the C16:0-C18:l-T system is suppressed by the presence of oxygen, a G(T) value of 0.013 is found, indicating that the decay-induced labeling is probably the exclusive mode for tritium incorporation. I n the absence of oxygen, tritium addition occurs, but the G(T) value for tritium incorporation by substitution in palmitate and oleate can be calculated to be 0.01922 from the addition ratio and their relative specific activity, which is in approximate proportion to their weight fractions, showing that the labeling is again by the decay-induced mechanism. Thus, at the cryogenic temperature, when the molecules are in a state of lowest vibrational energy23and the molecular excitation energy from irradiation can be degraded thermally, decay-induced labeling becomes the dominant mode of tritiation. I n @ labeling, the initiating step is the ionization of reactant m0lecules6~~ and tritium.24 The former on combination with electrons or negative ion molecules yield free radicals either directly or indirectly by dissociation; the reactions are depicted as
+
T2-*+ Tz+ eT2+ Tz +T3+ T
+
I
R--C-H
-+
I
I R-C:
l
R-C-H+
J I
+
+ eL
\M-
1
radicals
+H+
I The temperature dependence of tritiation can be reasonably explained by a mechanism which involves a relative predominance of either a gas phase or an interface initiation of the reactive tritium species as a consequence of the adsorption-equilibrium established throughout the experiment in the tritiation chamber; the mechanisms are given in Scheme I, Scheme I
Gas-phase initiation T2(g) --t HeT+
+ Ts(g) +Tz++ 2e-, etc. e- + T2(a) +T2++ 2e-, etc. + M (or wall) +M* + e- or M + + 2e(HeTf, T2+,etc.) + & + I tritiation e-
e-
+ e-
The Journal of Physical Chemistry
Interface initiation T2(a) +HeT+ e-
+ e-
+ M +M* + e- or LM++ 2eHeT+ + h/r +tritiation
where Tz(g) = tritium in gas phase, T2(a) = tritium in adsorbed phase, M = reactant, and M* = reactant in the excited state. At 77"K, it is likely that the concentration of tritium in the adsorbed phase greatly exceeds that in the gas phase; hence, decay-induced labeling should predominate. With increasing temperature, the relative concentration of tritium in the gas phase increases and thus favors the p labeling. It should be noted that from a mechanistic point of view, tritiation by substitution can be initiated by reactive species formed either from reactant molecules or from tritium, whereas tritiation by addition is only effected by the reactive species of tritium. Radical meachanisms have been postulated for tritium incorporation by addition3r8and by substitut i ~ n ~in- gas ~ exposure, for the addition of hydrogen atoms to alkenes and other unsaturate^'^ and for radiolysis of olefins.25 The importance of ion-molecule mechanism has also been stressed for the latter.26-28 Our results on tritium addition in t,he C16:0-C18:1-T system can best be explained on the basis of both mechanisms. Tritium atoms generated by quenching of 6(3P1)Hg*atomslZ9when acted upon the C16:&18:1-T system, gave an addition ratio of 5.4 via disproportionation.30 Under these conditions, the reactive tritium species are entirely radicals. It is likely that molecular radicals formed from oleate and a tritium atom, when diffused away from the surface, can undergo disproportionation among themselves in the con(21) P. E. Riess and K. E. Wilzbach, J . Phys. Chem., 62, 6 (1958). (22) This value is higher than 0.013 given previously, probably owing to additional contribution from tritiooleate formed by disproportionation. (23) L. P. Hammett, "Physical Organic Chemistry," McGraw-Hill Book Co., Inc., New York, N. Y., 1940, p 72. (24) 0. A. Schaeffer and S. 0. Thompson, Radiation Res., 10, 671 (1959). (25) R. A. Holroyd and G. W. Klein, J . Phys. Chem., 69, 194 (1965). (26) C. D. Wagner, Tetrahedron, 14, 164 (1961). (27) P. C. Kaufman, J. Phys. Chem., 67, 1671 (1963). (28) F. Collinson, F. S. Dainton, and D. C. Walker, Trans. Faraday Soe., 57, 1732 (1961). (29) H. Niki, Y. Rousseau, and G. J. Mains, J. Phye. Chem., 6 9 , 45 (1965). (30) By analogy t o the disproportionation of sec-butyl-sec-butyl radicals,'E the 9-octadecenoate formed may have existed predominantly in the trans configuration. Analysis was not performed because the cis and trans forms could not be separated by gas chromatography on the packed columns used, although the individual forms when chromatographed alone showed slightly different retention times.
ADDITION OF TRITIUM TO OLEATE
1303
densed phase, but when trapped a t the surface, they disproportionate with tritium atoms in the gas phase. The reactions are depicted below. Hg*(3Pi) H H R-C=C-R’
+ Tz
---j
Hg(lSo) H
+ 2T H
+ T +R-c-C-R’
T a. At interface H H R--C-C-R’ T
H
H
+ T -+ R-C-C-R’
or
T T H T
+ HT
R-C=C-R’ b. I n condensed phase H H 2R-C-C-R’
H
H
+R-C-C-R’
T
H
T
+ R-C=C-R’
H T H H or R-C-C-R’
H
H
+ R-C=C-R’
T T R = (CHJTCH3 R’ = (CH2)&OOH
perturbed tritium incorporation by substitution in palmitate and oleate in the presence of oxygen. Nitric oxide promotes the combination of hydrogen atoms34but because of its own tendency to form nitrosonium ion, NO+,35by charge transfer upon collision with other ion molecules, it cannot charge-neutralize reactive tritium ions. By contrast to mercury photosensitization, formation of tritiostearate by tritium addition a t 298°K in the system containing nitric oxide is probably initiated solely by tritium ion molecules with an addition ratio of 8.3. The complete absence of tritiooleate and tritiostearate in the gas radiochromatogram in samples exposed at 77 and 193°K is anomalous, which is not easily explained and shall be further investigated. The Cle:o-C18:1-T system, if unperturbed, gives rise to tritiostearate by the mechanism of addition of both tritium atoms and tritium ion molecules to the double bond of oleate. The numerical value of the addition ratio of the unperturbed system at 298°K is almost identically the sum of the ratios of XO-scavenged and Hg-photosensitized systems, indicating about 60% participation by tritium ion molecules and 40% by tritium radicals in the addition reaction. This mechanism of mixed addition for tritium incorporation by unsaturates is not in contradiction with the Scheme I1
Because of the large ionization cross section (by 75-v electron impact3’), argon sensitizes the tritiating system by increasing the proportion of energy supplied to the tritium by the reaction32 Ar+
H -C
+ Tz +ArT+ + T
This formation of the tritium argonium ion, ArT+, and the tritium atom elevates the G(T) value of the Cle:o-C18:1-T system at all temperatures of exposure. At 298”1
