3326
J. Phys. Chem. 1980,84, 3326-3329
(57) P. A. Koiiman and I. D. Kuntz, J. Am. Chem. Soc., 94,9236 (1972). (58) L. Bertsch and H. W. Habgood, J . Phys. Chem., 67, 1621 (1963). (59) V. Grarniich and W. M. Meier, Z. Kristallogr., 133, 134 (1971). (60) W. Schirmer, H. Stach, H. Thamm, M. M. Dubinin, A. A. Isirikjan, N. I. Regent, and E. Ch. Anaktschjan, Z. Phys. Chem. (Leiprig), in press.
(61) R. J. Neddenriep, J. Colloid Interface Sci., 26, 293 (1968). (62) A. G. Bezus, A. V. Kiseiev, Z. SediBEek, and Pham Quang Du, Trans. Faraday Soc., 67,468 (1971).
(63) 0.M. Dzhigit, A. V. Kiseiev, K. N. Mlkos, G. G. Muttik, and T. A. Rahmanova, Trans. Faraday SOC.,67,458 (1971). (64) U. Lohse, H. Thamm, and H. Stach, "Adsorption of Hydrocarbons in Zeolites", Reprints of the Workshop, Academy of Sciences of the German Democratic Republic, East Berlin (1979). (65) M. M. Dubinin, A. A. Isirikjan, G. U. Rachmatkariev, and V. V. Serpinskij, Izv. Akad. Nauk SSSR, Ser. Khim., 1269 (1972). (66) V. Subramanian and K. Seff, J. Phys. Chem., 81, 2249 (1977). (67) N. Y. Chen, J. Phys. Chem., 60,60 (1976).
Gas-Phase ,6 Decay of Multitritiated Methane in Methyl Chloride and Bromide, and in Binary Mixtures of Methyl Fluoride, Chloride, and Bromide M. Coloslmo" and R. Bucci Istltuto dl Chimica Nucleare de/ C.N.R., C.P. IO, 00016 Monterotondo Stazlone, Rome, Ita& (Received: January 21, 1980)
The mechanisms of the gas-phase CX3+(X = H, T) electrophilic attack to methyl chloride and bromide are discussed on the basis of the radioactive end products. The observed chloride and bromide ion transfer processes support the formation of dimethylchloroniumand dimethylbromonium ions. CX3+attack in binary mixtures of CH3Y (Y = F, C1, and Br) is discussed in terms of the observed relative rates and the methyl cation transfer processes among the halomethanes. The existence of long-lived gaseous dimethylhalonium ions is supported.
Introduction The disagreement between the results obtained in the condensedl and in the gaseous2phase about the reactivity of methyl halides toward methylating agents led us to investigate the reactions between CX3+(H, T), produced by tritium 0decay,3p4eq 1,and CH3F at near atmospheric CX4
82%
+
CX3+ 3He + p-
+v
(1) pressure^.^ Labeled methyl fluoride was recovered as the only tritiated organic fluoro compound; thus, we concluded beyond doubt that the methyl ion attacked the fluorine n electrons and that the observed fluoride ion transfer involved participation of the symmetrical complex postuaccording to lated by Henis et
CX3+ + CH3F + CX3FCH3+ F= CX3F +CH3+ (2)
+
CX3+ CH3F
-
--
CX3FCH3+
CHX3 + CH2F+
(3a)
CHBX+ CX2F+ (3b)
Furthermore, we excluded that, if CX3+attack occurred on u electrons, it led to the formation of stable neutral fluoro compounds found in superacidic solutions.laVd The results agreed with those obtained in gas-phase y-radiolysis experiments on methyl fluoride: which supported the formation of ethyl fluoride in ion-molecule processes from the rearrangement of excited dimethylfluoroniumions and not from direct methylation of CH3F to the u bonds. In this article, we extend our investigation to methyl chloride and bromide and report on the behavior of these compounds toward CX3+ions during the reactions: CX3++ CH3Cl ... radioactive neutral compounds CX3++ CH3Br
--.,.
(4)
radioactive neutral compounds (5)
Then, the results are compared with those obtained in the CH3F system. In addition, we discuss the relative rates of CX3+attack in binary mixtures of CH3F, CH3C1, and 0022-3654/80/2084-3326$0 1 .OO/O
CH3Br, taking into account other data from the literat ~ r e . ~ ~ ~ The approach used in this work represents a sensitive method for determining the site of the attack of methyl ions. Moreover, the use of labeled ions at pressures higher than those commonly employed in conventional ionmolecule experiments provides a unique tool for the study of electrophilic reactions, filling the gap between solution chemistry and mass spectrometric methods.
Experimental Section Materials. Multitritiated methane, from stock solution of CT4 in CH4,578v9was freed from impurities through a well-established procedurea8Methyl fluoride and methyl chloride (Matheson Co., USA), methyl bromide (MerckSchuchardt, FRG), and oxygen (SI0 Co., Italy), added as a thermal radical scavenger, were used as received; gas chromatographic analysis confirmed the absence of interferring impurities in the reacting gases. Procedure and Analysis. Sample preparation was described in detail elsewhere? Duplicate experiments were carried out and at least seven analyses were performed. The total quantity of methane was 1.9 torr in every sample; thus, ion-molecule processes between CX3+ ions and methane were regarded as highly unfavored, and their contribution was disregarded. A previous work5 showed that radiolytic effects did not play a significant role during the formation of products. The flow-radiogas-chromatographiclo analyses through a Porapak Q column were carried out under the following conditions. (a) Pure methyl chloride and bromide systems were analyzed at 160 "C with 1.2 L h-l nitrogen flow; (b) the mixed methyl fluoride/methyl chloride and methyl fluoride/methyl bromide systems were analyzed at 60 "C with 1.2 L h-l nitrogen flow for 30 min, then the oven temperature was raised to 160 "C manually; (c) the mixed methyl chloride/methyl bromide systems were analyzed at 160 "C with a 1.2 L h-l nitrogen flow. Make-up nitrogen was added to the effluent gas from the gas chromatograph to produce a total flow of 10 L h-l at the exit of the ionization chamber. In ad@ 1980 American Chemical Society
B Decay of klultltrltiated Methane
The Journal of Physical Chemistry, Vol. 84, No. 24, 1980 3327
TABLE I: p Decay of CX4a in CH,Cl, CH,Br, and CH,F decay [CH,CII, [CH,Brl, [CH,F], [O,I, [CX,]," time, torr torr torr torr mCi/L days 660 4 2 154 660 4 2 153 660 4 2 152
yields of radioactive productsb
cx ,Cla 14(85)'
CX,BP -
CX ,Fa
ref this work this work 5
14(94)' 68( 7 5)'
Percent of radioactivity due to CX, group contained in a product relative to the radioactivity due to all a X = H, T. Calculated as t h e limiting values from t h e rate constants in ref 2. daughter CX,' ions, 11: p Decay of CX,a in CH,F/CH,Cl, CH,F/CB,Br, and CH,Cl/CH,Br -TABLE ---
a
[CH,F], torr
[CH,aI, torr
330 330
330
X = H, T.
[CH,Brl, torr
330 3 30 330 As in Table I, footnote b.
[O,l, torr 4 4 4
[CX4La mCi/L 2 2 2
dition, the analytical conditions were changed in order to detect any reasonable product of CX3+ attack on the substrates, i.e., radioactive hydrocarbons and alkyl halides within the range CI--C4. Radioactive hydrogen gas, presumably HT, could be separated at 610"C; it accounted for 1-2% of the radioactivity contained in the daughter fragments. Highly reactive labeled species are formed from 18% of the nuclear event^,^ and they are believed to account for the low yields of radioactive hydrogen.
Results The experimental conditions and the results obtained in the methyl chloride and bromide systems are shown in Table I, where also the data relative to methyl fluoride are reported for comparison. Table I1 presents the results concerning binary mixtures. The radiochemical yields are calculated as the percent of radioactivity due to the CX3 group contained in the products relative to the radioactivity due to aH1 CX3+ions produced by nuclear events, Le., 100 w = raclioactivity(CX3Y)/radioactivity(daughter CX3+). The data are mean values and are reproducible within a standard deviation of 15%. Discussion The main reaction channels for the CX3+ions are with methyl halides, which are by far the major components of the systems and the only source of halogen. The recovery of tritiated methyl chloride and bromide (Table I) is unambiguous proof of the attack of labeled methyl ions on the chlorine and bromine substituents, followed by the halide ion transfer. In fact, if the CXs group were bound to different poetitions before the collapse of the transients to the neutral compounds, other products would be formed. Halide ion abstraction can occur, for instance, in the one-step stripping mechanisms: CX:{++ CH3Cl
+
--
CX3C1 -t- CH3+
(6)
CX3+ CW3Br CX3Br 4- CH3+ (7) Electron impact experiments suggest that bonds are formed between methyl ions and methyl halides,2v7and the formation of the symmetrical complexes postulated by Henis' seems more likely: CX3+ + CH&I CX&lCH3+ == CX3CI + CH3+ (8) CX3++ CH3Br F= CX3BrCH3++ CX3Br + CH3+ (9) The approach utsed in this work does not verify whether intermediates or transition states are involved in reactions 8 and 9.
decay time, days
radioactive yieldsb CX,Fa
CX,CIa
29 30
47
358 359 357
CX,Bra 48 7
7
Only a minor part of the daughter methyl ions is recovered as CX3Cl and CX3Br. Because of the absence of radioactive compounds other than the methyl halides and the starting methane the balance of radioactivity, i.e. (activity of all daughter CX3+ ions) = C(activity of a product containiing CX3group), cannot be written. We are led to conclude that products not distinguished from the starting materials are formed. The formation of chloromethylium and bromomethylium ions from the attack of methyl ions to CH3Cl and CH3Br was established in the gas p h a ~ e . ~ r ~The J~J~ occurrence of processes 10 and 11agrees with the present CX3++ CH3Cl
-
-!=
CX3C1CH3+
85%5
CX3++ CH3Br ---
CHX3 + CH2Cl+ (loa) CH3X CX2C1+ (lob)
-
-
CX3BrCH3
-+
+
CHX3 + CH2Br+
(W
CH3X
+ CXzBr+
(1lb) results. In fact, methyl ions participating in the above reactions are turnled into radioactive methane and escape detection. The fate of chloromethylium and bromomethylium ions is uncertain. Guptzi et al.13 reported the occurrence of the reaction CX2C1+ CH3C1- CH3CIX' CXC1. (12)
+
+
In the present experiments CXC1. would be scavenged by oxygen; methylchloronium ion is known to react at almost the collision rate with bath m01ecules:~J~ CH,ClX+ $. CH3C1- CH3ClCH3+ + XC1 (13) Because of the greater amount of CH3C1 molecules in comparison with the CX3C1molecules produced, the contribution of reaction 14 to the CX3Cl yield should be negligible. On the other hand, Beauchamp et al.' ruled CH&lX+ CX3Cl- CX3ClCH3+ + XC1 (14)
+
out any further reaction between CX2Cl+ions and CH3Cl. No information is available on the reactivity of CX2Br+ ion; however, if the bromomethylium ion behaves with CH3Br as CX2C1+does with CH3C1,it should not give any significant contribution to the formation of CX3BrCH3+ and, at the end, of CX,Br.
3328
The Journal of Physical Chemistty, Vol. 84, No. 24, 1980
TABLE 111: Apparent Relative Rates of CX,' on CH,F, CH,Cl, and CH,Br
Y
Z
F F
c1 Br Br
CH,Y/ CH3Zb
CH,Y/ CH3Zc
a
Attack
Colosimo and Bucci
Scheme I CX3'
+
CH3CI
CX:
+
CH3Br
e C X J C I C H ~ ' e CXJCI +
CH:
ky/kzd
4.8 0.59 0.67 4.8 0.63 0.42 a 1.0 1.00 0.63 a X = H,T. Pure systems, from Table I. Binary mixtures, from Table 11. From the rate constants in ref 2.
CX3f
+
CH3F
e CX3BrCH; e CX3FCH:
.C-
$
CX3Br CX3F
+ CH3' + CH:
1
(B)
+CH3Z-CH3F
Table I shows that CH3C1 and CH3Br appear less reactive than CH3F toward CX3', contrary to what was found by Henis et Methyl ions formed in decay process 1can possess some vibrational energy, which arises from passing from the sp3 structure of methane to the sp2 structure of the planar methyl ion. This energy can reach a limiting value of 32.5 kcal mol-l, in the case of CH3+lla but a lower value in the case of tritiated and can be transferred to the dimethylhalonium ions, which are excited also by the exothermicity of CX3+ attack. The order of methyl cation affinities of CH3Y (Y= F, C1, and Br) is F < C1 5 Br;15 thus, CX3' attack on CH3Cl and CH3Br can involve more than 83 kcal mol-l, and on CH3F up to 76 kcal mol-'. As a consequence, dimethylchloronium and dimethylbromonium ions can be expected to be more excited than dimethylfluoronium ions. Reactions 2,8, and 9 are resonant and thermoneutral processes, if one takes into account the lack of isotope effects and that ionmolecule processes are assumed to occur without activation energy. On the contrary, reactions 3, 10, and 11 require the presence of an activation energy barrier due to the jump of H or X from one carbon atom to the other. The lower the energy content of CX3YCH3' ions, the less energy is available for them to undergo reactions 3, 10, and 11. As a consequence, in CH3F reaction 2 prevails over reaction 3,5 but in the present systems reactions 10 and 11become more important than reactions 8 and 9. Finally, the equal yields of CX3Cl and CH3Br show an equal apparent reactivity of CH3Cl and CH3Br toward the CX3+ ion at near atmospheric pressure. A further support to the previous interpretation is offered by the study of binary mixtures of CH3F,CH3C1,and CH3Br, where CX3+ ions have the same probability of attacking either component. It is reasonable to assume that a methyl halide molecule is attacked by a methyl ion through the same mechanisms both in the pure systems and in the mixtures, whose components have very similar character, and that the composition of the mixture modifies only the fate of the transients. Thus, the ratios among the yields of CX3Y can be used to indicate competition among the methyl halides for methyl ion.16 These values are presented in Table 111. First of all, we note that in the CH3C1/CH3Brmixture also the yields of CX3Cl and CX3Br are the same, and, remarkably, are the half of those found in the corresponding pure systems. The only relevant differences between the experimental conditions in Tables I and I1 are the number of the reacting molecules, which are in the ratio 1/0.5. The yields of CX3C1 and CX3Br in CH3C1/ CH3Br can be calculated by dividing by two the corresponding yields in Table I, if the mutual influence of these methyl halides on the long-lived transients is the same. In fact, dimethylchloronium and dimethylbromonium ions that survive decomposition processes 10 and 11 can undergo the methyl group transfer process: CX3ClCH3++ CH,Br + CX3BrCH3++ CH3Cl (15) The methyl cation affinities of CH3C1and CH3Br are very
Cx:
+
CH3Z
e CX3ZCH3+
CX3Z
+
+
CH3
Z = C1 and Br
close or even equa1,15and reaction 15 is roughly thermoneutral. As a consequence, the formation of dimethylchloronium and dimethylbromonium ions and, at the end, of CX3C1and CX3Br, is influenced by each component of the mixture in the same way; thus, the yields decrease by the same amount of the available reacting molecules of each kind in the system, A different situation occurs in the CH3F/CH3C1and CH3F/CH3Brsystems, where the yields should be 34% for CX3F and 7% for CX3C1and CX3Br if each methyl halide would influence the other in the same way, as in the CH3C1/CH3Brmixture. A sensible difference in methyl cation affinity exists between CH3F and CH3C1,and CH3F and CH3Br;15thus, the dimethylfluoronium ions that survive decomposition process 3 can transfer the methyl group according to CX3FCH3+ + CH3Cl- CX&lCH3+ + CH3F (16) CX3FCH3++ CH3Br
-
CX3BrCH3++ CH3F (17)
For these reactions we calculate AHl6= -7 kcal mol-l, and AHl7 = -12 kcal mol-l from the methyl cation affinitie~.'~ Statistically, reactions 16 and 17 should occur at every other collision, and the following theoretical yields should be obtained: CX3F, 17%; CX3CI and CX3Br,24 % ,l7 Since the experimental yields are quite different, some other effect must be taken into account. If one assumes that dimethylhalonium ions participating in reactions 2,8, and 9 are long-lived intermediates, they are allowed to undergo a large number of collisions with CH3C1and CH3Br; all such CX3FCH3+ions transfer the methyl group and are detected as CX3Cland CX3Br, and not as CX3F. In this way, the theoretical 34% yield of CX3Fdisappearance should add to the independent theoretical 7% yields of CX3C1and CX3Br, and 34% + 7% yields for CX3C1and CX3Br should be obtained.17 This value agrees with the experimental 47 and 48% found respectively for CX3Cl and CX3Br. In Scheme I, the reactions occurring in CH3Cl/CH3Br system are represented in A, while the situations occurring in CH3F/CH,C1 and CH3F/CH3Brsystems are shown in B. According to the proposed scheme, no CXBForiginates from CX3FCH3+ions in the binary mixtures. Another abundant species in the systems is the fluoromethylium ion, which is known to react with methyl ~ h l o r i d e : ~ CXzF+ + CH3Cl- CHXzF + CHZC1+ (18)
-
and, very likely, with methyl brornide:l8 CXzF+ CH3Br CHXzF + CH2Br+
+
(19)
In our previous work: we calculated that the yield of the labeled fluoromethylium ions formed in reaction 3 should amount to -32%, which also should be the yield of CXBF produced in reactions 18 and 19; if all CX2F' react with
0 Decay of Wiultitritiated Methane
The Journal of Physical Chemistry, Vol. 84,No. 24, 1980 3329
CH3C1 and CH3Br, 32% is in good agreement with the experimental yields of CX3F in Table 11. If one assumes that the kinetic data calculated under ion cyclotron resonance conditions hold also in the experiments o n the pure systems, the final product distribution shown in parentheses in Table I can be predicted. Contrary to what happens in CH3F, the present results compare badly with the calculations based on Henis's data.2 When a comparison is drawn among the ratios of the apparent rates of CX3+attack, interactions among the methyl halides are evident. In fact, we observe a similar apparent decrease of the reactivity of CH3F, when it is in the presence of CH3Cl and CH3Br; on the contrary, the relative reactivity of CH3C1and CM3Br is constant. Also the competition data compare badly with Henis's values shown in the fifth column of Table 111. This discrepancy can be expected, considering that our work provides apparent values, which depend on several factors due to the experimental condltions.l6 Nevertheless, all gas-phase works, including ours, agree about the nature of the ionmolecule processes occurring between the methyl ion and CH3F, CH,Cl, and CH,Br. Comparison with t h e Condensed Phase. When comparing the present results with those obtained in superacidic solution, the profound differences in the reaction environments1 pointed out in a previous article6 must be taken into account. Nevertheless, the use of "true" gaseous methyl ions shows that their attack on CHJ?,@CH3C1,and CH3Br occurs on the halide n basic center. In the gasphase, no evidence has been obtained about the particular behavior of methyl fluoride noted in supracidic solution.'
(2) J. M. S.Henis, M. D. Loberg, and M. J. Welch, J. Am. Chem. SOC., 96, 1665 (1974). (3) (a) F. Cacacw, Adv. Phys. Urg. Chem., 8, 79 (1970);(b) F. Cacace,
Conclusions The analyski of the radioactive end products shows that gaseous methyl ions attack methyl chloride and bromide on the halogen n basic center. The observed halide ion transfer process involves symmetrical complexes, namely, dimethylchloronium and dimethylbromonium ions. Comparison with the CH3F system, investigated under similar conditions, leads us to conclude that CH3F, CH3Cl, and CH3Br undergoes electrophilic CX3+attack through the same mechanisms, Le., the halide and the hydride abstraction processes, whose extent depends on the nature of the halogen. The study of the binary mixtures of methyl fluoride, chloride, and bromide establishes the occurrence of methyl group transfer among methyl halides according to their methyl cation affinities, and supports the formation of long-lived dimethylhalonium intermediates in the gas phase. References and Notes (1) (a) G. A. Olah, J. R. De Member, and R. H. Schiosberg, J. Am. Chem. Soc., 91, 2112 (1969);(b) ibid., 91, 2113 (1969);(c) G. A. Olah and J. R. De Member, ibid., 92, 718 (1970); (d) G. A. Olah, J. R. De Member, R H. Schlosberg, and Y. Halperii, ibu., 94, 156 (1972); (e) G. A. Olah, "Halonium Ions", Wiley, New York, 1975.
in "Interaction between Ions and Molecules", P. Ausloos, Ed., Plenum Press, London, 1975, p 527; (c) F. Cacace, G. Ciranni, and M. Schuller, J. Am. Chem. SOC.,97,4747 (1975). (4) A. H. Snell and F. Pleasonton, J. Phys. Chem., 02, 1337 (1958). (5) M. Coloslmo and R. Bucci, J . Phys. Chem., 82, 2061 (1978). (6) M. Colosimo and R. Bucci, J . Phys. Chem., 83, 1952 (1979). (7) J. L. Beauchamp, D. Holtz, S.D.Woodgate, and S.L. Patt, J. Am. Chem. Soc., 94, 2798 (1972). (8) F. Cacace and M. Schuller, J. LabeiledComp., 2, 213 (1975).The authors are Indebted to Professor F. Cacace of the University of Rome, Italy, and to Mr. M. Schuller of KFA, Jullch, FRG, who prepared and supplied labeled methane samples. (9) The authors are Indebtedto Professor G. Stoijcklin and Dr. A. Neubert of KFA, Julich, FRG, for isotopic composition analysis. (10) F. Cacace and Inam-Hul-Haq, Science, 131, 732 (1966). (1 1) (a) N. A. Mc Asklll, Aust. J. Chem.,22, 2275 (1969);(b) A. A. Herd, A. G. Harrlson, and N. A. Mc Askill, Can. J. Chem., 49, 2217 (1971). (12) Z.Luczynskl, W. Malicki, and H. Wincei, Int. J . Mass Spectrom. Ion Phvs.. 15. 321 11974). (13) S.K. dupta, E. G Jones, A. 0.Harrison, and J. J. Myher, Can. J. Chem., 45, 3107 (1967). (14) (a) J. E. Williams, Jr., W. Buss, L. C. Allen, P. v. R. Schlever. W. A. Lathan, W. J. Hehre, and J. A. Pople, J. Am. Chem. Soc.; 92, 2141 (1970);(b) V. 13. Nefedov, E. N. Slnotova, and G. P. Akulov, Mefcd Iztop. Indikatorov Nauch. Issled Prom. Proizvod., 346 (1971). (15) The methyl cation affinity of S is deflned, as in ref 7, as -AH for the reaction
-
CH,'
iS
+
CH3St
The values for CH3F, CH3CI, and CH3Br, reported In ref 7,are 44, 151,and >56 kcal mol-'. (16) The rate of formation of CX3Y can be expressed as d[CX3Y]/dt = k[CX3'] [CH3Y]
(a)
where k takes iinto account ail inverse reactions; the steady-state approxlmatlon on the CX3YCH3+ion is applied. At time t : [CXsyI t = k [CX,+I t[CH3YI
(b)
[CH3Y] Is a constant, because of the trace amounts of methyl halies consumed: [CX3Y], = k'[CX3']
(c)
The concentrations are proportional to the radioactiiitles; considering that (radioactivity A of CX3F)= w(radioactivity Bof all CX3+daughter ions): wA(CX,+) = k"A(CX,+)
or
w = k"
(4
For two substances, 1 and 2
w , / w2 = k i r f / k ; ' = k f f i 2
(e)
where w i and wp are the ylekls in Tables I and 11, and the k'fipvalues are the observed relative rates in Table 111. (17) I n binary mlxturesr CX3F yields should be: (a) the value In Table I reduced by 50 % from 68 % to 34% , because of the 50% decrease of the number of the CH,F molecules; (b) reduced by a further 50% from 34% to 17?6,because of the 1:l probability of undergoing reactions 16 and 17. On the other hand, the ylelds of CX3Ci and CX3Br should be: (a) the values in Table Ireduced by 50% from 14 to 7%, because of the 50% decrease of the number of the CH3CI and CH,Br molecules; (b) increased by the 17% contributlon given by CX3FCH," in reactions 16 and 17 from 7 to 24%. (18) Because of the uncertaln heats of formation of halomethyllum ions (cf. S.G. Lias and P. Ausloos, Int. J. Mass Spectrom. Ion Phys., 23, 273 (1977),a reliable calculation of At/,* cannot be carried out; anyway, it involves only few kilocalories per mole.
