y
Radiolysis of Tetrahalomethane-Arene Gaseous Mixtures
The Journal of Physical Chemistry, Vol. 82,No.
11, 1978 1207
Ionic and Radical Processes in the y Radiolysis of Tetrahalomethane-Arene Gaseous Mixtures Romano Cipolllni, * Istituto di Chimica, Universiti di Camerino, Camerino (Macerata), Italy
Gaetano Lilla, Nicola Pepe, and Maurizio Speranza Laboratorio di Chimica Nucleare del CNR, Montelibretti (Roma), Italy (Received b r c h 15, 1977; Revised Manuscript Received December 30, 1977) Publication costs assisted by the Unlversiti di Camerino
The reactivity and selectivity of radiolytically (60Coy rays) generated unsolvated trifluoromethylium (CF3+) ions toward selected aromatic substrates have been measured under conditions where nonionic processes can be safely ruled out. The same situation could not be attained for the trichloromethylium (CC13+)ion, owing to the overwhelming interference of CC13radicals, by far the major reactive species produced by the y radiolysis of gaseous carbon tetrachloride. The unsolvated CF3+ion is a moderate electrophile, whose attack on differently substituted aromatic substrates is rather indiscriminate; its positional selectivity toward a toluene molecule (para/0.5 meta = 4.3) appears intermediate between the one of i-C3H7+ions (para/0.5 meta = 2.7) and that displayed by the very mild t-C4H9+cation (para/0.5 meta = 35). Introduction Ionic processes in the gas phase have recently been investigated by new experimental kinetic methods that, unlike conventional mass-spectrometric techniques, allow identification of the final neutral products and determination of their structure and isomeric composition.l The establishment, a t the desired gas pressure, of a sufficient concentration of the selected ionic reagent(s) is the principal requirement of the kinetic approach, hitherto fulfilled by appropriate use of either nuclear-decay phenomena2 or radiolytic technique^.^ Radiolysis of gaseous mixtures is a widely employed source of ionic species, although the results, in some cases, may not be completely clear, because of the simultaneous formation of interfering free radicals and excited molecules. In particular, y radiolysis of gaseous CX4 (X = F or C1) mainly produces radicals (CX, and X) and ions (CX3+and X-) which are relatively unreactive toward the parent molecules and, therefore, tend to accumulate in considerable concentrations depending on the purity of CX4and on the nature of XV4 In a previous paper,5 reactivity features of the radiolytically produced CC13+ion and the mechanism of its attack to a benzene molecule have been briefly discussed. However, the formation of several products from the y irradiation of gaseous CC1,-benzene mixtures could not be ascribed to simple ionic reactions, suggesting the probable occurrence of extensive radical processes even in the presence of conventional radical scavangers. The aim of the present paper is the identification of radical reactions during the y radiolysis of CX,(X = F or C1) arene gaseous mixtures, the assessment of their extension, in comparison with ionic processes, and the measure of the reactivity features of CX3+ions toward aromatic substrates. Experimental Section Materials. Benzene and toluene were gas chromatographic standards from C. Erba Co., with an impurity content below 0.2 % Chlorobenzene (Merck-Schuchardt Co.) was purified by preparative GLC on a 3-m Silicone oil column operated a t 70 "C. The a,a,a-trifluoroxylenes isomers were prepared by decomposition of the corresponding a,a,a-trifluorotolylmagnesium bromides with dimethyl sulfate.6 The final products were purified by
.
0022-365417812082-1207$0 1.OO/O
preparative GLC on a 6-m DC 550 Silicone oil column operated a t 150 "C, and identified by spectrometric methods. Methane (Matheson Co.) was a ultra-high-pure product with a stated purity exceeding 99.99%; carbon tetrafluoride (Matheson Co.) was a Research Grade gas whose purity exceeded 99.5%. Carbon tetrachloride (Merck-Schuchardt Co.) was purified by preparative GLC on a 3-m Silicone oil column operated at 30 "C, until its purity, checked by electron capture GLC, exceeded 99.95%. Oxygen, isobutane, and the other used chemicals were Research Grade products employed without further purification. All the starting materials contained no detectable traces of the possible products of the trihalomethylation and the halogenation of the selected aromatics, as checked by GLC under the same conditions as the final analyses. Procedure. The gaseous systems were prepared by conventional techniques, using a greaseless vacuum line. The reagents and the additives were introduced into carefully outgassed 1-L. Pyrex bulbs, equipped with a break-seal tip. The bulbs were filled at the desired pressure with the appropriate gases, cooled a t liquid nitrogen temperature, and sealed off. The irradiations were carried out at 37.5 "C in a 220 Gammacell (Nuclear Canada Ltd.) at doses ranging from 1 to 10 Mrd (dose rate = 0.4 Mrd h-l), as determined by a Fricke dosimeter. The analysis of the irradiated mixtures was accomplished by injecting known amounts of the homogeneous gaseous system into a Hawlett-Packard gas chromatograph Model 5700 A, equipped with a FID unit. Analytical l/s-in. 0.d. stainless steel columns were used at temperatures from 60 to 170 "C, and the identity of the irradiation products was established by comparison of their retention volumes with those of authentic samples on at least two different columns. The yields were measured by integration of the peak area using individual calibration curves for each product. Results and Discussion The composition of the irradiated mixtures and the G(M) values of the final neutral products are reported in the Tables I and 11. The listed values represent the mean G values from several separate irradiations, carried out under @ 1978 American Chemical Society
1208
Cipollini et al.
The Journal of Physical Chemistry, Vol. 82, No. 11, 1978
TABLE I: Product Yields from the y Radiolysis of Gaseous CF,-Arene Mixtures System composition
CF, press, Torr 760 760 760 740 7 30 660 400 100 760 760 740 730 660 400 100
Substrate press, Torr
6
CH, press, Torr
Benzene, 2.0 Benzene, 2.9 Benzene, 2.2 Benzene, 2.5 Benzene, 2.4 Benzene, 2.4 Benzene, 2.3 Benzene, 1.9 Toluene, 1.8 Toluene, 1.5 Toluene, 1.0 Toluene, 1.2 Toluene, 0.8 Toluene, 1.4 Toluene, 1.6
GM valuesa
N,O
0,
press, Torr
press, Torr
4.0 4.0 30 100 360 660
18.0 4.0 4.0 4.0 4.0
30 100 360 660
4.0 18.0 4.0 4.0 4.0 4.0
CF3
0.86 0.93 1.12 0.74 0.52 0.49 0.39 0.34 1.27 1.33 0.82 0.33 0.27
0.68 0.79 0.73
0.70 0.11 0.02 0.02 0.02
0.04 0.04
ndb nd
0.003 0.00~ 0.18 0.37
nd nd nd nd nd nd nd
1.91 2.05 1.79
0.15
0.07
0.10
0.02
0.001 0.00~
nd nd
0.07
0.004 0.004 0.12 0.20
0.01
The GM values are defined as the number of the M molecules formed per 100 eV of energy absorbed by the system, nd = not detectable (% < 1 x
a
TABLE 11: Product Yields from the
7
Radiolysis of Gaseous CCl,-Arene Mixtures
System composition
cc1, press, Torr 100 100 100
100 100 100 100
100 100 100 100 100
100 100
100 100
100 100
100
Substrate press, Torr
6
Benzene, 2.9 Benzene, 2.0 Benzene, 2.7 Benzene, 1.8 Benzene, 1.9 Benzene, 2.1 Chlorobenzene, Chlorobenzene, Chlorobenzene, Chlorobenzene, Chlorobenzene, Chlorobenzene, Toluene 2.0 Toluene, 1.4 Toluene, 2.2 Toluene, 1.7 Toluene, 1.8 Toluene, 1.5 Toluene, 1.4
press, Torr CH,
GM valuesa
i-C,H,o press, Torr
press, Torr NO
30
30 660
CCI,
0.08 ndb nd nd nd nd nd nd
4.0 18.0 4.0 4.0 4.0
100
6 6 6' 0.20 0.21 0.37 0.12
4.0 18.0 4.0 4.0 4.0
100 660 1.9 1.6 1.8 2.0 1.6 1.9
press, Torr 0,
C
2.0 4.0 4.0
10 30
100 660
C
4.0 4.0 4.0
nd nd nd nd nd
CI
0.37 0.39 0.18 0.20 0.12
nd 0.39 0.41 0.25 0.32
0.31 0.08 C C
0.34 0.04
0.08 0.04
0.73 0.01 0.16 0.09
nd
nd
a The % values are defined as the number of the M molecules formed per 1 0 0 eV of energy absorbed by the system. The Disappearance of the nd = not detectable (GM < 1X methylated and ethylated products were not measured. starting toluene.
the same conditions, and their standard deviation is of the order of 10%. All the reported data have been measured a t a radiation dose of 4.8 Mrd. The conditions typical of the present experiments, in particular the low concentrations of the aromatic substrate and the total pressures used, ensure that the reacting species formed by y radiolysis attain the ground-state condition by collisional deactivation with the parent molecules before reacting with the substrate and that parasitic processes, induced by direct radiolysis of the aromatics, can be safely neglected. An analysis of the results confirms the expected4c products distribution from the y radiolysis of CX4-arene systems, indicating that the major reaction channels involve the displacement of an aromatic hydrogen by the CX3 group and the X atom. Besides, the G values of the
halogenated final products appear rather insensitive to the addition or variable concentrations of classical radical or electron scavangers, such as 02,NO, and N20, whereas they are dramatically affected by the presence of CH4 or isobutane. CF4-Arene Systems. An inspection of Table I shows that major fluorinated products from the y radiolysis of CF4-arene mixtures are the fluoro- and trifluoromethylsubstituted arenes, whose absolute yields are only slightly sensible to the presence of appreciable concentrations of either a thermal radical scavenger (0,) or an electron trapper (N20). The addition of variable concentrations of CHI, maintaining constant the total pressure of the system and the concentration of the additives, causes a noticeable decrease in the yields of both the trifluoromethylated and
The Journal of Physical Chemlstty, Vol. 82, No. 11, 1978
y Radiolysis of Tetrahalomethane-Arene Gaseous Mixtures
1209
1.8
1.6
1.4
1.2
1.0
.8
.6
I
KCH4
XCH4
Figure 1. Dependence of G M values for the CF, 4- benzene -k O2-t CH, system on the mole fraction of methane (Xw): (0)benzotrifluoride; (A)fluorobenzene; ( 0 )ethylbenzene.
fluorinated products. Benzene (Figure 1) and toluene (Figure 2) manifest a similar behavior, namely, the complete suppression of the single fluorination reaction channel, when the mole fraction of CH4 (XcHJ is I0.47, whereas still significant amounts of trifluoromethylated products are formed even at very high CH4 concentrations (XCH,= 0.87). T h e Trifluoromethylation Reaction. y radiolysis of gaseous CF4 essentially generates two different trifluoromethylating species, the CF3 radical and CF3+ions,4 which can both contribute to the formation of the final trifluoromethylated products? via the following processes: t A
CF, t C,H,RCF:
-AH
+B
+ C,H,R- -BH+
CF,C,H,R
(1)
CF,C,H,R
where R = H or CH3, A is an acceptor of H atoms, and B is a base.8 CF3+ions are inert toward ~ x y g e nunlike ,~ CF3 radicals, which are reported to be efficiently scavenged by it.9J0 The small effects of O2and N 2 0 on the total trifluoromethylation yield (henceforth symbolized by GtCF3), while confirming the unreactivity of CF3+ions toward the scavengers used, reveal also that the radical trifluoromethylation process (1) is not prevented by the presence of O2 or N20, presumably because their attack on CF3 radicals might lead to products with still good trifluoromethylating properties. On the other hand, CF3+ions are known to be absolutely unreactive toward methane,’l whereas CF3 radicals seem to be able to slowly abstract a hydrogen atom from methane.12 The remarkable decrease of the trifluoromethylation yield, even on addition of relatively small amounts of methane ( X c b = 0.04), is likely due to the joint trapping action of O2 and CH4 on the present radical species. Therefore, the dependence of GtCF, values as a function of XCHl (Figures 1 and 2) can be interpreted as
Figure 2. Dependence of GMvalues for the CF, -k toluene 4- O2 4CH, system on the mole fraction of methane (XCH): (0) a,cy,atrifluoroxylene isomers; (A) fluorotoluene isomers; ethyltoluene isomers.
(h)
the consequence of the superimposition of reaction channels 1 and 2, whose relative extent is strongly regulated by the presence and the concentration of CH4. The “ionic” trifluoromethylation yield, i.e., the G value of formation, only by process 2 of the trifluoromethylated product (henceforth symbolized by GicF) can be reasonably defined &s the trifluoromethylation yield when XC, 1 0.47. As a consequence, the quantity G’cF3 = G t c ~ ,- G i ~ ~ 3 represents the “radical” trifluoromethylation G value, Le., the product yield of reaction 1. From Figure 1,it results that the ionic and radical pathways contribute to almost the same extent to the trifluoromethylation of benzene (GICF, = 0.39; G’CF, = 0.54), whereas only -11% of the recovered a,a,a-trifluoroxylenes (Figure 2) can be ascribed to the attack of CF3+on toluene (GiCF, = 0.15; G’CF, = 1.18). A closer inspection of Figures 1 and 2 reveals that the GtCFs(s do not reach a constant value by increasing XcH4, as expected if methane exclusively affects the radical pathway and does not interact with the ions fromed from CF4. The smooth decrease of the trifluoromethylation yields, observed for both the aromatic substrates, increasing Xcb from 0.47 to 0.87, can be explained by taking into account that CHI modifies the concentration of CF3+ ions via the following competitive ionic processes induced by the radiolytically formed CH3+ ions:13 CH:
+ CH, .k-3
CH,’ t CF,
k, -f
C,H: CF:
t H,
(3)
+ CH,F
(4)
Thus, owing to the absolute unreactivity of CF3+toward CH411aand to the high efficiency of process 4,11bmethane, besides scavenging CF3 radicals, increases the actual concentration of CF3+ ions, in comparison with the one obtained by y irradiating pure CF4, at the same partial pressure. Increasing XCH4, process 3 overwhelms reaction 414 and, consequently, the ionic trifluoromethylation
The Journal of Physical Chemistry, Vol. 82, No. 1 1,
1210
Cipollini et al.
1978
TABLE 111: Gas-Phase Trifluoromethylation of Toluene System composition CF, press, Torr
Toluene press, Torr
0, press, Torr
Relative yields of isomers CH, press, Torr
% a,a,a-Trifluoroxylene a ,a,a-Trifluoroxylene a,a,a-Trifluoroxylene a,a,a-Trifluoroxylene a,a,a-Trifluoroxylene a,a,a-Trifluoroxylene a ,a,a-Trifluoroxylene
18.2 16.1 18.0 7.6 4.8 4.8 4.2 37.4 43.9 43.3 35.5
33.8 36.2 34.1 38.7 37.7 37.2 30.5 37.4 33.2 34.5 27.2
48.0 47.7 47.9 53.7 57.5 58.0 65.3 25.2 22.9 22.0 37.2
2.8 2.6 2.8 2.8 3.0 3.1 4.3 1.3 1.4 1.3 2.7
This work This work This work This work This work This work
5.4
94.6
35.0
Ref 3f, 1 8
4.0 18.0 4.0 4.0 4.0
30 100 360
100
1.6
4.0
660
1.3 1.3
1.5 2
760 E t 3 H 22.6
Xylene Et h yltolue ne Ethyltoluene Isopropyltoluenes
4.2
8
EtOH 8.1
tert-Butyltoluenes
pathway slowly depletes the surplus of CF3+ions generated by (4). This picture is further supported by the remarkable increase of ethylation yields (Figures 1and 2) from process 3, while the small amounts of methylated products remain relatively unchanged (Table I). When the extent of radical process 1 is minimized by the combined action of O2and CH4 (XCH4 I 0.47), it is possible to have a picture of the reactivity and selectivity of the CF3+ ion toward the selected aromatics. Its intermolecular selectivity can be estimated by comparing the GicF3 values obtained from toluene and from benzene ([GiCFsltoluene/ [GiCHsl benzene = ( h T / h B ) i ) . An appreciable substrate selectivity ( ( h T / h B ) i = 0.38, when X C H l = 0.47) is observed, in agreement with the apparent intermolecular selectivities between toluene and benzene found for ethyl16 (IZT/hB = 0.81) and isopropyl3d( h T / k B = 0.6-0.9) ions. In the present case, a conceivable explanation for the measured apparent substrate selectivity of CF3+ions can be provided by considering that CF3+ ions may abstract a side-chain hydride ion from toluene16 via the following exothermic process:"
+ C,H,CH,
k,
CHF, t C,H,CH;'
(5)
AH= - 64 kcal mol-'
The quantity [GI'CF~]toluene/[GrCFsl benzene = (hT/kB)r can be related to the substrate selectivity of CF3 radicals. The obtained value ( ( ~ T / / Z B )=~ 2.2) is in reasonable agreement with the rate constants ratio ( k T / k B = 1.6 f 0.2) measured from the direct attack of the CF3 radical, photolytically generated in condensed phase, on toluene and benzene.7b The electrophilic nature of the trifluoromethylating species is apparent from Table 111, where data concerning their positional selectivities in the toluene ring are compared with those of CH3+, C2H5+,i-C3H7+,and t-C4Hg+ ions. The positional selectivity of CF3+ions appears intermediate (para/0.5 meta = 3.1 (XCH,= 0.47); 4.3 (XCH = 0.87)) between that of i-C3H7+(para/0.5 meta = 2.7) and that associated to the very mild t-C4He+cation (para/0.5 meta = 35). Similarly to t-C4Hg+cation, CF3+shows a clear preference toward the para position of toluene. Finally, a rough estimate of the positional selectivity of the CF3 radicals toward toluene can be obtained by solving the following equations, for each of the three different position of the toluene ring: ("/.ItnGtcF, - (%)~G'cF, (%)Yn
Para:0.5 metaratio
meta para
1.8 1.5 1.0 1.2 0.8 1.4
CF:
%
ortho
760 760 7 60 7 30 660 400
C,H, 730 neo-C,H,, 710
%
Products, M
=
GrCF,
This work Ref 3d,e, 18 Ref 3d,e, 18
(XCH~ = 0.47). The resulting (%)rn values (ortho = 17.5%, meta = 36.1%, para = 46.4%), representing the percent of CF3 radical attack on the n position of toluene, indicate that CF3 radicals have a rather selective electrophilic character.' The Fluorination Reaction. As pointed out above, fluorination of the aromatic substrates represents the other major reaction channel from the y radiolysis of CFparene mixtures. Different from the trifluoromethylation pathway the single fluorination reaction appears completely supI 0.13). pressed even by a small addition of CH4 (XCH4 Oxygen and other conventional radical scavengers are reported to combine very slowly with halogen atoms,lg producing addition products which can easily lose halogen and again act as effective halogenating species.20 This behavior accounts for the small effect of O2 and N20 on the fluorination yields (Table I), whereas its remarkable dependence on X C H 4 (Figures 1 and 2) can be ascribed to the high reactivity of CH4toward halogen atoms producing hydrogen halide and methyl radicals, efficiently scavenged by 0 2 . A conceivable ionic mechanism, involving fluoride ions, to generate monofluorinated compounds can be safely ruled out on the ground of the absolute lack of reactivity, in the gas phase, of halide ions toward nonactivated aromatic molecules.21 In conclusion, the experimental results indicate that the monofluorinated products are likely originated by a single reaction channel: the attack of F atoms on the substrate. CC14-Arene Systems. The effect of additives on the GM values of the chlorinated products from the y radiolysis of gaseous CC4-arene mixtures is shown in Table 11. As in CF4 systems, the product yields appear relatively insensitive to the presence and concentration of radical scavengers, such as O2 and NO, while the addition of appreciable concentrations of CHI or isobutane causes the complete disappearance of both the monochlorinated and the trichloromethylated products (Figures 3 4 , indicating that y radiolysis of gaseous CC4generates a relatively high concentration of C1 and CC13 radicals with respect to the positively charged species. In fact, if appreciable concentrations of CC13+ions were formed, they would efficieutly attack the aromatic substrate, as demonstrated by mass-spectrometric techniques,21b since CC13+ ions are completely unreactive toward the other chemicals present in the mixture, such as CC14,22CH4,23and 02.3b On the contrary, CCl, radicals and C1 atoms can be eliminated by the joint action of and CH424or i ~ o b u t a n e ,as~ ~ observed for the analogous fluorinated species. Besides, the disappearance of the starting material when toluene is the substrate and the apparent intermolecular 024b319
where ( %'ot, is the percent of total trifluoromethylation on the n position of toluene, in the absence of CH4, and (%)in is the percent of CF3+ion attack on the same position
Sources
The Journal of Physical Chemistty, Vol. 82, No. 11, 1978
y Radiolysis of Tetrahalomethane-Arene Gaseous Mixtures
1211
TABLE IV : Intramolecular Distribution of Halogen Atoms within Selected Aromatics Product, M
System composition
CX, press, X Torr
F F F F F
69
0, NO CH, i-C4Hl0 press, press, press, press, Torr Torr Torr Torr
Substrate press, Torr Toluene, 1 . 8 Toluene, 1.5 Toluene, 1.0 Toluene, 1 . 2 Toluene, 0 . 8 Chlorobenzene, Chlorobenzene, Chlorobenzene, Chlorobenzene, Chlorobenzene, Chlorobenzene, Toluene, 2.2 Toluene, 1.7 Toluene, 1 . 8 Toluene, 1.5
c1 c1 c1 c1 c1 c1 c1 c1 c1 c1
760 760 760 730 660 100 100 100 100 100 100 100 100 100 100
C1 C1
CX,:toluene = 1 O : l liquid phase CX,:benzotrifluoride = 1:l liquid phase
c1
CX, :nitrobenzene = 1:1 liquid phase
C1
CX,:fluorobenzene = 1:l liquid phase
4.0 18.0 4.0 4.0
30 100
1.9 1.6 4.0 1 . 8 18.0 2.0 4.0 1.6 4.0 1.9 4.0
30 100 660
'
4.0 4.0 4.0 4.0
10 30 100
X
Relative yields of isomers
para: 0 . 5 meta ortho meta para ratio %
%
%
Sources
Fluor ot olue ne Fluorotoluene Fluorotoluene Fluorotoluene Fluorotoluene Dichlorobenzene Dichlorobenzene Dichlorobenzene Dichlorobenzene Dichlorobenzene Dichlorobenzene Chlorotoluene Chlorotoluene Chlorotoluene Chlorotoluene
36.1 35.6 35.0 36.6 40.9 31.9 30.5 31.2 30.9 31.2 29.8 40.4 39.0 38.7 41.2
30.1 29.8 30.2 31.4 27.3 24.3 27.3 24.2 24.9 24.8 24.8 21.8 21.9 21.5 18.0
33.8 34.6 34.8 32.0 31.8 43.8 42.3 44.6 44.2 44.0 45.4 37.8 39.0 39.8 40.8
2.2 2.3 2.3 2.0 2.3 3.6 3.1 3.7 3.5 3.5 3.7 3.5 3.6 3.7 4.5
This work This work This work This work This work This work This work This work This work This work This work This work This work This work This work
Chloro toluene Chlorobenzotrifluoride Chloronitrobenzene Chlorofluorobenzene
59.7 16.7
10.4 29.8 37.0 46.2
5.7 2.5
Ref 27a Ref 27b
23.6
21.0 55.4
5.3
Ref 27b
33.6
21.8 44.4
4.1
Ref 27b
selectivity of the trichloromethylating species (Table 11) support the idea that radicals assume a predominant role in the y radiolysis of gaseous CC4. In fact, the presence of appreciable amounts of benzyl chloride and the absence of a,a,a-trichloroxylenes isomers among the recovered products from the irradiation mixtures containing toluene as a substrate suggest that the radical species formed (C1 and CCl,) mainly abstract a side-chain hydrogen from initiating an energetically favored chain process responsible for the formation of benzyl chloride: CCl, (or C1) + C,H,CH, C,H,CH,
+
CC1,
-+
-+
CHC1, (or HCl)
+ C,H,CH,
(6) (7 1
C,H,CH,Cl t CCl,
whose GM value is not as high as expected, because of either the small concentrations of toluene present or the effective capture of the intermediate benzyl radical by oxygen. The lack of benzyl fluoride among the products from the irradiation of CF4-toluene mixtures is attributable to both the scarce tendency of CF3 to abstract a side-chain hydrogen7band the fact that CF, (or F) cannot mimic CC13 (or C1) in initiating a chain process, owing to the endothermicity of its second step (9) (AH= 13 kcal mol-'):
+
CF, (or F ) C,H,CH,
+
+ C,H,CH, CF,
-f
--f
CHF, (or HF)
C,H,CH,F
+ CF,
+ C,H,CH,
(8) (9)
The slight dependence of the products yields on the concentration of the added O2 lead us to believe that CC13 and C1 as the analogous fluorinated species, may attack 02 producing addition products which still possess trichloromethylating and chlorinating properties. The intramolecular distributions of the monohalogenated products from the gas-phase attack of halogen atoms on the selected substrates are reported in Table IV and compared with the positional selectivity exhibited by C1 atoms in liquid phase.27 As e x p e ~ t e dthe , ~ ~F ~atoms ~~ which result generally are more reactive than C1 atoms toward the selected substrate, since the fluorination G values ( G M = 0.79; 2.05) are always greRter than the chlorination values ( G M = 0.39, 0.41,0.34) (Table I and 11).
1
+
C"4
+
Figure 3. Dependence of GMvalues for the CCI4 benzene O2 CH, systom on the mole fraction of methane (XcH,): (0)benzotrichloride; (A) chlorobenzene.
+
This higher reactivity is accompanied by a relatively lower selectivity, which however confirms the electrophilic nature of the halogen atoms.29 Conclusions y radiolysis of gaseous mixtures containing CC14 as a bulk constituent induces mainly radical processes, while the irradiation of CF4 gives rise to ionic as well as radical reactions, whose relative extent is remarkably affected by the presence and the nature of the additives. in the latter
1212
The Journal of Physical Chemistry, Vol. 82, No. 11, 1978
'
Cipollini et al.
and intramolecular selectivity; in particular, they exhibits a clear preference toward the para position of toluene. As to the radical processes, fluorinated species appear markedly more reactive toward aromatics than the chlorinated ones; as a whole, their intermolecular and positional selectivity demonstrate a distinct electrophilic character.
OI
Acknowledgment. The authors thank Professor F. Cacace for stimulating discussion on the subject of this paper. References and Notes (1) (a) F. Cacace, Adv. fhys. Org. Cbem., 8, 79 (1970);(b) F. Cacace, In "Interaction between Ions and Molecules", P. Ausloos, Ed., Plenum Press, New York, N.Y., 1975; (c) P. Ausloos In "Ion-Molecule Reactions", J. L. Franklin, Ed., Plenum Press, New York, N.Y., 1970 (d) P. Ausloos, Annu. Rev. fbys. Chem., 17 (1966). (2) (a) For reviews, cf. ref la and l b ; (b) F. Cacace and G. Stocklln, J . Am. Cbem. Soc., 94, 2518 (1972);(c) S. H. Daniel and H. J. Ache, Radiocbim. Acta, 19, 132 (1973);(d) V. W. Jiang, K. A. Krohn, and M. J. Welch, J . Am. Chem. SOC.,97, 6551 (1975). (3) (a) For reviews, cf. ref IC and P. Ausloos, S.G. Lias, and A. A. Scala, "Ion-Molecule Reactions In the Gas Phase", American Chemical Society, New York, N.Y., 1970,p 264; (b) P. Ausloos, frog. React. Kinet., 5, 113 (1969);(c) F. Cacace, R. Cipollini, and G. Occhiucci, J. Cbem. Soc., ferkln Trans. 2, 84 (1972);(d) S. Takamuku, K. Iseda, and H. Sakurai, J. Am. Cbem. Soc., 93,2420 (1971);(e) F. Cacace and E. Possagno, ibid., 95,3397 (1973);(f) F. Cacace and P. Giacomello, ibid., 95, 5851 (1973); (9) F. Cacace and R. Cipollini, Radiochem. Radioanal. Lett., 16, 343 (1974);(h) F. Cacace and M. Speranza, J. Am. Chem. Soc., 94, 4447 (1972). (4)(a) J. E. Wilson, "Radiation Chemistry of Monomers, Polymers and Plastics", Marcel Dekker, New York, N.Y., 1974; (b) J. W. T. Splnks and R. J. Woods, "An Introduction to Radiatlon Chemistry", Wiley, New York, N.Y., 1964; (c) I.V. Vereshchinskii, Adv. Radiat. Cbem., 3, 75 (1972). (5) R. Cipollinl, Radiochem. Radioanal. Lett., 16, 193 (1974). (6) M. J. S.Dewar and A. P. Marchand, J. Am. Cbem. Sac., 88, 354
.3
2
1
0
0
1
XCH4
Figure 4. Dependence of GY values for the CCI, CH4system on the mole fraction of methane (Xw,): (A) chlorotoluene isomers.
+ toluene + O2+
(m) benzyl chloride;
G
(1966). I (7) (a) A. P. Stefani and M. Szwarc, J. Am. Cbem. Soc., 84, 3661 (1962); 6
(b) I.M. Whittemore, A. P. Stefani, and M. Szwarc, ibid., 84, 3799 (1962);(c) H. Komazawa, A. P. Stefani, and M. Szwarc, /bid., 85, 2043 (1963).
(8) In the gaseous mixtures used, A and B could be either the substrate itself and/or the glass walls of the bulb.
(9) L. W. Sieck, R. Gorden, Jr., and P. Ausloos, J. Res. Nat. Bur. Stand., Sect. A., 78, 151 (1974). 1101 L. M. Quick and E. Whittle. Trans. Faraday Soc., 67, 1727 (1971). i l li, fa1 . , E. Heckel and R. J. Hanrahan. J . Cbem. fbvs.. 62. 1027 i1975j:
.
:H4
Figure 5. Dependence of GMvalues for the CCI, -t- chlorobenzene -I- O2 4- CH4 system on the mole fraction of methane (XcH,): (A) dichlorobenzene isomers.
case the simultaneous presence of a conventional radical scavenger, such as 02, and of a halogen atom trapper, such as CH4,allows us to investigate the ionic processes induced by the attack of CF3+ions on selected aromatic substrates without interference from neutral species. Under these conditions, trifluoromethylium ions display a marked inter-
(b) R. J. Blint, T. B. McMahon,and J. L. Beauciamp, J. Am. Cbek. Soc., 96, 1269 (1974). (12) (a) R. E. Dodd and J. Watson-Smlth, J . Chem. Soc., 1465 (1957); (b) H. Carmichael and H. S.Johnston, J. Cbem. Pbys., 41, 1975 (1964);(c) C. L. Kibby and R. E. Weston, Jr., J . Am.-Cbem. Soc., BO, 1084 (1973);(d) R. A. Fass and J. E. Willard, J . Chem. Phys., 52, 1874 (1970). (13) (a) P. Ausloos, S. G. Lias, and A. A. Scala, Adv. Cbem. Ser., No. 58, 284 (1968);(b) P. Ausloos, S.G. Lias, and R. Gorden, Jr., J . Cbem. Pbys., 39, 3341 (1963);40, 1854 (1964). (14) The rate constants k , = 8.6X IO-'' cm3 molecule-' s-' and k , = 5.45 X lo-'' cm3 molecule-' s -I, measured at a total pressure of 0.01 Torr, indicate that methyl ions react with CF, almost as rapidly as with CHI, at that pressure (see ref 1 la). 1151 S. G. Lias and P. Ausloos. J . Cbem. fhvs., 37, 877 (1962). i16j V. Aquliantl, A. Gardini-Guidoni, and G. G. Volpi, Trans. Faraday Soc., 64, 3283 (1968). (17) The experimentalAH0,(CF3+)= 98.9 kcal mol-', according to R. E. Marcotteand T. 0. Tiernan, J. Chem. fbys., 54, 3385 (1971),while AH,O(C~H,CH,+) is reported to be equal to 213 kcal mol-' (F. P. Lossing, Can. J . Chem., 49, 357 (1971)). (18) F. Cacace, R. Cipollini, P. Giacomello, and E. Passagno, Gazz. Cbim. Ita/., 104, 977 (1974). (19)J. B. Levy and B. K. W. Copeland ( J . fhys. Cbem., 89, 408 (1965)), estimated a value of k = 10'' cms mol-' s-' for the rate constant of the following thlrd-order reaction:
'
F t O,+M+FO,+M When the halogen involved is CI, the same process proceeds with a k = 6.2 f 1.1 X lo', cm' moi-2s-' (J. E. Nicholas and R. G. W. Norrish, Proc. R . Soc. London, Ser. A , 307 391 (1968)). (20) R. R. Smardzewski and W. B. Fox, J . Am. Cbem. Soc., 98, 304
(1974). (21) (a) D. P. Therd and V. H. Hamill, J. Am. Cbem. Soc., 84, 1134 (1962); (b) L. i. Virin. Yu. A. Safin, L. I.Vikhrova, and R. V. Dzhagatspanyan, High Energy Chem. (fngl. Trans.), 4, No. 4, 296 (1970);(c) R. N.
Bimolecular Interactions of Triplet 3-Benzoylpyridine Compton and R. H. Huebner, Adv. Radiat. Chem. 2, 281 (1970). (22) J. R. Eyler, P. Ausloos, and S.G. Lias, J. Am. Chem. Soc., 96, 3673 (1974). (23) All the most llkely reactions between CCI3+and CH, exhibit a strong endothermic character (AHI 4-39 kcal mol-'). (24) (a) G. Polulet, G. Le Bras, and J. Combourieu, J , Chim. Phys., 71, 101 (1974); (b) S.Hautecloque, J. Chim. Phys. Physicochim. Biol., 67, 771 (1970). (25) F. S. Dainton and P. B. Ayscough in "Photochemistry and Reaction Kinetlcs", P. G. Ashmore, F. S. Dainton, and T. M. Sugden, Ed., Cambridge University Press, London, 1967.
The Journal of Physical Chemistty, Vol. 82, No. 11, 1978
1213
(26) (a) D. D. Newkirk, G. J. Gleicher, and V. R. Kock, Tetrahedron, 28, 449 (1972); (b) J. D. Unruh and G. J. Gleicher, J . Am. Chem. Soc., 93, 2008 (1971); (c) D. D. Tanner and N. Wada, ibki'., 97, 2190 (1975). (27) (a) G. Stocklin and W. Tornau, Radiochim. Acta, 6, 86 (1966); (b) ibid., 9, 95 (1968). (28) (a) K. Shobatake, J. M. Parson, Y. T. Lee, and S.A. Rice, J. Chem. Phys., 59, 1427 (1973); (b) K. Shobatake, Y. T. Lee, and S.A. Rice, ibid., 59, 1435 (1973). (29) (a) Z. Chweiner and J. Bednar, Radiochem. Radioanal. Left., 3, 263, 273 (1970); (b) A. H. Vasek and L. C. Sams, J. Fluorine Chem., 2, 257 (1972-1973); (c) ibid., 3, 397 (1973-1974).
Photochemical and Photophysical Behavior of Benzoylpyridines. 2. Bimolecular Interactions of Triplet 3-Benzoylpyridine in Aqueous Solution G. Favaro" and F. Masettl Istituto di Chimica Flsica, Universitg di Perugia, 1-06100 Perugla, Italy (Received May 3 I , 1977; Revised Manuscript Received November 11, 1977) Publication costs assisted by Consiglio Nazionale delle Ricerche, Roma
The triplet state lifetime for 3-benzoylpyridine has been determined in aqueous solution at different pH values. The lifetime was found to decrease with decreasing pH and with increasing concentration of 3-benzoylpyridine. These findings are interpreted in terms of a complex formation with hydroxonium ion and of self-quenching of the triplet. Kinetic parameters for these interactions have been determined. The results obtained are compared with those of benzophenone.
Introduction Interactions of triplet excited molecules with the solvent can play an important role in determining their photoreactivity and their efficiency as triplet excitation energy donors. Focusing our attention on photosensitization, solvent interactions can compete with the triplet-triplet energy transfer and thus reduce its efficiency. Aromatic carbonyl compounds have a high triplet yield and a relatively high triplet energy, so that the molecules are very often employed as triplet sensitizers. However, the different lifetimes, and the resultant different sensitization efficiency, of their triplets in various solvents are probably related to the tendency which they have to react with the solvent. Self-quenching processes also depend on the nature of the solvent. We present here a study of the triplet behavior of 3benzoylpyridine (3-BP) in aqueous solution as a function of pH. The results are compared with those obtained for benzophenone (B). The triplet behavior of B in aqueous solution has been extensively studied.14 A decrease in the phosphorescence emission intensity with decreasing pH was first discovered by Ledger and Porter,l who explained the quenching action of the proton as being due to an inversion of the relative positions of the (n,r*) and ( r , r * )lowest triplet states in the positive protonic field. Self-quenching has been found much more efficient in water1 than in b e n ~ e n e . ~ Rayner and Wyatt3 attributed the decrease with decreasing pH of the initial absorption intensities and lifetimes of the transients, produced from B by laser flash photolysis, to the establishment of acid-base equilibrium in the triplet state. The triplet state is in fact expected to be more basic (pK* -1)334 than the ground state (pK = -5 to -6I3r6 on the basis of pK* calculations by the Forster cycle. An alternative interpretation has been suggested by us4 relating the decrease in the emission intensity and triplet
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0022-365417812082-1213$01.OO/O
lifetime with decreasing pH at room temperature with the phosphorescence behavior in a rigid matrix. Low temperature emission spectra revealed the presence of a phosphorescence which originates neither from neutral B* nor from protonated BHt* and it was assigned to a hydrogen-bonded complex BH30+*of the excited carbonyl with the hydroxonium ion7 The pH region where this new emission appears (pH -3.5) is approximately the same as that at which the decrease in lifetime and emission intensity is observed at room temperature. We concluded that the same excited species, which emits phosphorescence a t intermediate acidities in rigid glasses, may also be responsible for the triplet behavior in fluid solution. In order to study further the phenomena associated with the triplet carbonyl in aqueous solution and to draw some generalization, the effect of variations in pH on the triplet lifetime should be examined for several substituted benzophenones. A regular substituent effect was found, following a Hammett type relationship, for a large number of benzophenone derivatives in benzene solution, suggesting that the excited triplet carbonyl behaves like an electrophilic reagent toward this solvent.6 The slight solubility in water of most of the benzophenone derivatives hinders a similar systematic study in this solvent. In previous we have found that 3-BP is not only reasonably soluble in water, but that it is also a good sensitizer in neutral as well in acidic aqueous solution, while the other isomers (2-BP and 4-BP) are only poor sensitizers. For these reasons 3-BP was chosen in this work for a detailed study of triplet behavior in aqueous solution. The configuration of 3-BP is described as being derived from an (n,r*) excitation, such as that of benz~phenone.~ It is thus to be expected that the photochemistry of 3-BP may parallel to some extent the known behavior of B. In addition, the heterocyclic nitrogen performs a role analogous to that of an electron-attracting substituent. This character is expected to be enhanced by nitrogen pro0 1978 American Chemical Society
