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J. Phys. Chem. 1982,86,2062-2065
lO-'O esu for this series of compounds) which is in reasonable agreement with the charge found by Benson and required to give agreement with experimental heats of formation of alkanes. The importance of any model proposed lies in its extension to halogenated molecules containing more than one carbon. In order to investigate the potential of the model here for extension to the ethanes, we have calculated the AHF values of CzF6 and CZCl6. In the general case, extension of the model to the ethanes would require the introduction of two new parameters, B(C-C) and acc. However, with a symmetrical ethane, there is no net field at the midpoint of the C-C bond so acc is not required and B(C-C) may be calculated from ethane and transferred to
C2F6and C2C16. Table I1 shows that there is very poor agreement between the experimental and predicted heat in model I, while model I1 gives excellent agreement in the case of C2F6 and C2C16. The electrostatic model with polarization work included (model 11) presented here fits the experimental heats of formation and dipole moments of the chlorine- and fluorine-substituted methanes well and shows potential for application to the substituted ethanes. Ideally, extension of the model to any chlorine- or fluorine-substituted saturated hydrocarbon should introduce but two new parameters, B(C-C) and acc. The extension of the model to unsymmetrically substituted ethanes is currently being pursued.
Gas-Phase Reaction of Daughter Ions from the Decay of Multitrltiated Propane with Benzene and Toluene. Solution of a Longstanding Anomaly Fulvlo Cacace, Romano Clpolllnl, and Plerlulgl Glacomello University of Rome, 00 100 Rome, Itaiy, and University of Camerino, 62032 Camerino, Macerate, Itaiy (Received: Juiy 30, 198 1; In Final Form: December 29, 198 1)
The population of the daughter ions from the decay of multitritiated propane has been sampled by using as a probe their gas-phase reactions with benzene and toluene at pressures up to 400 torr. Tritiated n- and isopropylated arenes account together for 70-75% of the total activity of the decay ions. This observation-and the failure to detect aromatic allylation-removes an early radiochemical anomaly, by showing that decay of tritiated propane yields propyl ions as the most abundant daughter species, following a trend established for all other tritiated hydrocarbons. The abnormally high abundance of allyl ions measured by mass spectrometry is traced to the decomposition C3H7+ C3H6+ + H2allowed by its low activation energy, the lack of collisional stabilization,and the long residence time which characterizes operation of a "charge" mass spectrometer. The excitation energy required for the decomposition is likely to arise from the deformation energy of the propyl cations, born from the sudden nuclear transition in a shape reminiscent of the parent hydrocarbon molecule.
-
Introduction The chemical consequences of the 0 decay of tritium have been the subject of extensive theoretical, massspectrometric, and radiochemical investigation.'Y2 In particular, the decay of suitably tritiated molecules has been largely exploited as a technique for the mechanistic and kinetic studies of ion-molecule reactions in gaseous and condensed The mass-spectrometric data show that the process B GHmT
p
-
+ [C,Hm3He]+
3He + C,H,+
(1)
is the overwhelmingly predominant fragmentation pathway of tritiated hydrocarbons, giving an abundance of daughter C,H,+ ions that exceeds 70% in all cases. Furthermore, in the few comparable cases so far investigated, the position of the radioactive atom within the parent molecule appears, if at all, to affect the decay-induced fragmentation pattern to a minor extent, as shown by the abundance of (1) Wexler, S. 'Action Chimiques et Biochimiques des Radiations"; Haissinsky, Ed.; Maason: Paris, 1965; Vol. VIII. (2) Cacace, F. 'Hot Atom Chemistry Status Report";IAEA Vienna, 1975. (3) Cacace, F.Adu. Phys. Org. Chem. 1970, 8, 79. (4) Cacace, F.'Interactions between Ions and Molecules";Aualm, P., Ed.; Plenum Press: New York, 1975. (5) Cacace, F.'Kinetics of Ion-Molecule Reactions";Ausloos, P., Ed.; Plenum Press: New York, 1979. (6) Cacace, F. Adu. Chem. Ser., in the press. (7) Akulov, G.P. Usp. Khim. 1976,45, 1970.
the C7H7+daughter ions which is the same, i.e., 78 f 1.5%, from the four isomeric monotritiated toluenes.8 A conspicuous exception does exist however, namely, the decay of tritiated propanes, which give remarkably low and different abundances of C3H7+daughter ions: 56% from [l-3H]propaneand 41% from [2-3H]propane. The anomaly, tentatively traced to the abnormally low energetic requirements for the unimolecular fragmentation of excited C3H7+ions into allyl ions,8has not been removed by more recent theoretical"" and mas~-spectrometric~'-'~ studies. This represents a disturbing discrepancy in the current interpretation of the chemical phenomena triggered by the 0 decay. We report an experimental attempt to clarify the situation by sampling the population of the labeled daughter ions from multitritiated propane. As a probe we use a suitable nucleophile (benzene or toluene), whose encounter takes place within a short time subsequent to the formation of the decay ion, in a reaction environment characterized by highly efficient collisionalMabilization processes. (8) Wexler, S.; Anderson, G. R.; Singer, L. A. J . Chem. Phys. 1960,32,
417. (9) Ikuta, S.; Iwata, S.; Imamura, M. J . Chem. Phys. 1977, 66, 4671.
(10) Ikuta, S.;Yoshihara, K.; Shiokawa, T. Radiochem. Radioanal. Lett. 1977, 28, 435. (11) Ikuta, S.;Hashimoto, S. Chem. Phys. 1979, 42, 262. (12) Nishizawa, K.; Narisada, K.; Teramatau, H.; Iwami, H.; Shinagawa, M. Mass Spectrosc. (Tokyo) 1973, 21, 199. (13) Omori, T.; Kikuki. T.: Shio Kawa, T. Radiochem. Radioanal. Lett. 1979, 37, 233.
0022-3654/82/2086-2062$01.25/0@ 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 11, 1982 2063
Daughter Ions from Decay of Multitritiated Propane
TABLE 1: Composition of the Gaseous Systems system no.
partial press. of components,a torr decay time, C,H, C,H, 0, NH, month
~
1 2 12 13 3 4 14
10
401 400 400
201 30 200
400 200 360 200
10 10 10 10 10 10
18.7 22.5 4 4
18.0 22.8
4
18.7 23.0 25.0
a All systems were stored at 90 "C and contained a tracer concentration of multitritiated propane.
Experimental Section Materials. The multitritiated propane used in the present work, prepared, purified, and stored as previously de~cribed,'~ was analyzed by radio GLC, and its radiochemical purity was found to exceed 99.7%. The activity distribution, as determined by mass spectrometry, corresponded to 5.6% C3H7T,21.7% C3H6T2,37.6% C3H5T3, 20.9% C3H4T4, 7.3% C3H3T5,5.6% C,HzT6,0.3% C3HT7, and 1.0% C3TP The tritium content of the methyl groups and that of the methylene group were in the ratio of 2.40 f 0.3, as measured by tritium NMR spectrometry. The aromatic substrates, solvents, and products, employed as reference standards or carriers, were research-grade chemicals and used without further purification. Preparation of the Decay Systems. Tritiated propane (0.4 mCi), diluted with C3H8to a specific activity of 0.14 mCi mmol-', was introduced into carefully outgassed and evacuated Pyrex vessels that contained measured amounts of the aromatic substrate($ and the appropriate additives (02, NH3). The vessels were then sealed off and stored for 18-25 months in the dark at 90 "C. The composition of the decay systems is shown in Table I. Analysis of the Tritiated Products. The analysis was carried out by radio GLC, employing a Fractovap ATC/F 410 gas chromatograph from C. Erba Co., equipped with a hot wire detector. The outlet of the instrument was connected to a 10-mL internal flow proportional counter from Berthold-Frieseke which was heated to 150 "C. The total flow rate, through the counter, was adjusted to 90 mL min-' by diluting the effluents from the gas chromatograph with a 1:l mixture of He and CH4. Two different sampling techniques were adopted: (i) Small portions of the homogeneous gaseous content of the heated vessels were withdrawn with a gas syringe and injected directly in order to analyze the gaseous tritiated products. (ii) The vessels were cooled and thoroughly washed with n-pentane containing small amounts of appropriate inactive carriers. The tritiated propane was removed by repeated freezingthawing cycles, and measured aliquots of the organic solutions were injected into the radio gas chromatograph. The tritiated products were identified by comparing their retention volumes with those of authentic samples on at least two different columns chosen among the following ones: (A) 4 m X in. Porapak Q at 40 "C; (B) 2 m X '/8 in. Porasil C/phenyl isocyanate at 0 "C; (C) 14 m x '/8 in. Apiezon "L" grease on Chromosorb W-AW-DMCS operated at 120 "C. Results Nature and Yields of the Aromatic Products. The alkylation products, n-propylbenzene and cumene, and npropyltoluenes and cymenes from C6H6 and C&, re(14)Cipollini, R.;SchWer, M. J.Labelled Compd. Radiopharm. 1978, 15, 703.
spectively, represent the major tritiated species formed in all systems, together with smaller amounts of C6H5Tand C7H7T. The initial activity of multitritiated propane, its isotopic composition and storage time, the decay rate of tritium, and activity of the products were necessary in order to determine the absolute yields being defined as the ratio of the activity contained in the products to the total activity of the decay fragments formed within the system during the storage period. Owing to the propagation of errors from multiple sources in the experiment, the absolute values calculated must be regarded as crude estimates. Nevertheless, tritiated aromatics undoubtedly represented the major reaction products. Their combined absolute yields reached 7?75% in systems containing no ammonia. The balance was provided by gaseous tritiated products, including ethene, acetylene, and propene, accompanied by traces of tritiated ethane, cyclopropane, and other minor unidentified products. The relative yields, expressed by the normalized activity distribution, can be measured with a far better accuracy than absolute yields; these are reported in Table I1 for aromatic products. The isomeric composition of the alkylated products from toluene is remarkably constant in all systems investigated, corresponding to 15.0 f 1.6% ortho, 59.5 f 2.1% meta, and 25.6 f 0.8% para for isopropylation, and 29.7 f 0.7% ortho, 46.2 f 1.7% meta, and 24.8 f 1.3% para for npropylation. Gaseous Products. In the absence of NH,, the activity distribution of the gaseous products corresponded to 15 f 1%in acetylene, 22 f 1% in ethene, 53 f 2 % in propene, with traces of ethane (ca. 1%)and cyclopropane (ca. 0.5%). In the presence of NH3, the percent activity of propene increased to 60 f 2%, at the expense of ethene (17 f l%), while the percent activity of acetylene remained constant (14 f 170).
Discussion Alkylation Process. Aromatic alkylation represents a probe to sample the population of the decay ions. The failure to detect any aromatic allylation is in contrast to the large extent of propylation. In fact, allyl cations are known from mass-spectrometric studies15J6to react efficiently with arenes. They produce excited allylarenium ions, e.g.
which are stabilized effectively by few deactivating collisions.16 Any C3X5+(X = H, T) decay fragment can therefore be expected to react with benzene, giving a CgXll+adduct, that is collisionally stabilized at relatively high pressures prevailing in the system. Its deprotonation by a gaseous base, e.g., NH,, should lead to appreciable yields of tritiated allylbenzene or allyltoluenes. Consequently, failure to detect such products provides direct evidence against the formation of C3X5+ions following the dissociation of the daughter C3X7+ions before a reactive encounter with an aromatic molecule occurs. Under the prevailing pressure, in the systems investigated, this requires lifetimes of the C3X7+ions to exceed s. Propylation of arenes, which is the predominant reaction channel, must be traced to the C3X7+decay ions. The ionic (15)Munaon, M.S.B.; Field, F. H. J. Am. Chem. SOC.1967,89,1047. See also: Field, F. H. J. Am. Chem. SOC.1967,89,5328. (16)Houriet, R.;Elwood, T. A.; Futrell, J. H. J.Am. Chem. SOC.1978, 100,2320.
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The Journal of Physical Chemistty, Vol. 86, No. 11, 1982
Cacace et al.
TABLE 11: Tritiated Products from t h e Gas-Phase Reaction of Benzene and Toluene with the Decay Ions from Multitritiated Propane activity distribution,b %
1 2 12 13 3
4 14
benzene
cumene
n-propylbenzene
21.4
65.8
11.0
23.9
61.5
10.6
13.7 1.1 8.3
19.2 3.5 20.4
4.3 0.9 4.8
allylbenzene
toluene 2.0 28.8 2.1 27.6 20.2 23.0 17.4
a For the composition of the gaseous systems, cf. Table I. ca. 10%.
_
n-propylcymenes toluenes _
_
~
52.8
10.5
56.9 35.3 59.4 39.8
12.0 6.3 10.0 6.7
-
allyltoluenes _
~
others 7.9 1.9 3.5 1.0 2.0 2.5
Detection limit, ca. 0.5%. Standard deviation of the data,
Scheme I Unimolecular Decomposition
c3x7+
+
C Y R 6' 5
+
Fragments
* Intramolecular + C3X7 S h i f t s
- BH
K
+C6H5R
I n t c rmo l e c u l a r alkyl transfer
character of the alkylation is witnessed, inter alia, by the effect of NH3 It increases the yields of tritiated propene at the expense of the labeled propylated arenes, owing to the competition of the fast," exothermic18deprotonation process C3X7++ NH3
-
C3X6+ NH3X+
(3)
with aromatic alkylation. The present results make but a small contribution to the currently accepted picture of aromatic alkylation by gaseous propyl ions, drawn from detailed mass-spectrometric and radiolytic s t ~ d i e s , ' ~ J and * ~ whose prevalent features are outlined in Scheme I. However, the relative yield and isomeric composition of products provides new information on the C3X7+decay fragments. Isopropylation predominates over n-propylation by a factor greater than 5. Since statistical considerations and the isotopic composition of the multitritiated propane dictate that most C3X7+decay fragments are initially formed as primary (17) Lias, S. G.;Rebbert, R. E.; Ausloos, P. J.Am. Chem. SOC.1970, 92, 6430.
(18) The reaction is exothermic by more than 20 kcal mol-'. Cf.: Walder, R.; Franklin, J. L. Int. J.Mass Spectrom. Ion Phys. 1980, 36, 85.
(19) Cacace, F.; Possagno, E. J.Am. Chem. SOC.1973, 95, 3397. (20) Attinl, M.;Cacace, F.; Ciranni, G.; Giacomello, P. J.Am. Chem. SOC.1977, 99, 2611. (21) Attinl, M.; Cacace, F.; Giacomello, P. J. Am. Chem. SOC. 1980, 102.4768. -, -(22) Yamamoto, Y.;Takamuku, S.;Sakurai, H. J. Am. Chem. S O ~ . 1978, 100, 2474. (23) Takamuku, S.;Iseda, K.; Sakurai, H. J.Am. Chem. SOC.1971,93, 2420 and references therein. ~
~
- BH
ions, extensive isomerization to the more stable" secondary structure must occur. This has been noted in previous radiolytic s t ~ d i e s . ' ~ , In ~ ~addition, ~~' the experimental features, typical of the decay technique, allow one to specify the nature of the isomerization process. In fact, at the high ratios [ArH]/[C,X,] and pressures (400 torr), typical of the decay experiments, isomerization can only occur by an intramolecular mechanism. Probably this occurs via rapid 1,2 hydride-ion shifts, and its rate must be sufficiently high so to permit substantial conversions within the relatively short time frame available for isomerization of a n-C3X7+ion, prior to reacting with an aromatic molecule 8). n-Propylation can be traced to the fraction of n-C3X7+ions escaping isomerization and/or to some C3X7+moiety which shows a slower isomerization rate to the isopropyl cation. Protonated cyclopropane is a likely candidate whose role, suggested earlier,17has been recently demonstrated in a gas-phase aromatic alkylation.2l Indirect support of this view comes from the actual isolation of tritiated cyclopropane among the gaseous prodUCtS.25
Selectivity of the C3X7+ Decay Ions. The substrate selectivity, deduced from the competition experiments, is characterized by an apparent k T / k B ratio of 1.5 f 0.1 for n-propylation and 1.8 f 0.1 for isopropylation in systems containing equimolecular amounts of toluene and benzene, (24) The stability of i-CsH7+exceeds by 8 kcal mol-' that of c-C3H7+ and by 16 kcal mol-' that of n-C3H7+.Cf.: Chong, Shuang-Ling; Franklin, J. L. J. Am. Chem. SOC.1972,94, 6347. (25) Traces of cyclopropane are also formed in the radiolysis of C3H8 carried out in the presence of ammonia; cf. ref 17.
Daughter Ions from Decay of MultitrRiated Propane
The Journal of Physical Chemistry, Vol. 86,No. 11, 1982 2065
and 5d are almost thermoneutral when R = H and that or an excess of the latter. When the system contains an reaction 5c can be ruled out since no tritiated C3X4isomers excess of toluene, the selectivity is somewhat lower, e.g., have been detected among the gaseous products. b / k g is 0.9 (for n-propylation) and 1.4 (for isopropylation) at a [C7H8]/[C6H6]ratio of 12.01. It should be noted that Conclusion isopropylation by thermal s-C3H7+ions, highly diluted in The present results remove the anomaly represented by C3H8 at 720 torr, is characterized by a k T / k B ratio of the "abnormal" decay-induced fragmentation pattern of 0.8-0.9, as shown by earlier radiolytic experiment^.'^^^^ tritiated propane. In fact, they show that the abundance The positional selectivity of the C3X7+decay ions is of the propyl ions from reaction 1 is indeed high (>70characterized by the predominant meta orientation, which 75%), and fully comparable with those of the correcontrasts with the ortho/para substitution prevailing in sponding daughter ions from other tritiated hydrocarbons, radiolytic experiments involving thermal s-C3H7+ions, when measured by forcing them to react within a suffiunder conditions of kinetic control of products.20 The ciently short time after the nuclear transition. positional selectivity of the decay ions is reminiscent of Consequently, the abnormally low abundance of the the one observed in the gas-phase alkylation of toluene by C3H7+fragments, and the correspondingly high yield of the C3H7+ions produced by the strongly exothermic prothe C3H5+ion, measured in the mass spectrometer after tonation of C31& and c-C3&, and containing a considerable 10-4-10-5 s, must arise from a secondary decomposition, excess of internal energy.21 e.g. The peculiar selectivity of the C3X7+decay ions is therefore traced to their appreciable degree of vibrational C3H7+ C3H5+ + H2 (6) excitation (vide infra) and to the large excess of the arowhich is barely 18 kcal mol-' endothermic,Bof vibrationally matic substrate which does not allow thermalization of the excited C3X7-cations by unreactive third-body collision, excited daughter ions, as originally suspected by Wexler.8 before their reactive encounter with benzene or toluene. One can speculate about the source of such energy of the Consequently, the primary arenium ions from reaction 2 C3H7+decay fragments whose existence has been confirmed independently by the isomeric composition of are likely to contain an excess of vibrational energy, owing tritiated n-propyltoluenes and cymenes. Without invoking its origin to the vibrational excitation of the propyl cations, the peculiar effects of the nuclear transition, which are in addition to the intrinsic exothermicity of the alkylation discarded by theory and, in any event, should influence step. This allows, or enhances, secondary processes that as well the decay of other tritiated hydrocarbons, an exshift the kinetically controlled isomeric composition of the planation can be based on the recently suggestedz6 deprimary arenium ions toward a distribution controlled by formation energy of the decay ion. thermodynamic factors. Thus, the competition between benzene and toluene will tend to shift in favor of the latter In fact, owing to the sudden nature of the B emission (
