NOTES
749 Intramolecular Comparison of the Insertion into the C-H Bonds of Alkanes by Singlet Methylene Radicalsla
where reactant R1has been selected as a reference species and T is the temperature of the gas surrounding the particle. The sum over Pr,the products, is understood to include only those species which leave the particle surface. It is also possible to imagine the particle being comsumed in a photosensitized reaction. A similar expression for Fp would result. Approximate calculations, using eq 8 and the results of experiments for the photocatalytic oxidation of CO on ZnO given in ref 3, show that for 10% CO in air and 0.25-p particles of density of the order of 10 g/cc \PPIis of the order of the gravitational force on the particle. Speculation o a the possible effect of this particular example of photodiff usiophoresis on the distribution of atmospheric aerosols is not profitable without more information than is now available on aerosol chemistry in the atmosphere. We examine briefly now photodiffusiophoresis arising from radiation-induced variations over the particle surface of surfaae reactions and of vapor pressure. In these two examples, unlike the example we have just discussed, incident radiation plays only an indirect role in differentially heating a particle. Inasmuch as surface reaction rates and vapor pressure are both temperaturedependent properties, the differential heating by radiation would produce variations in either of these properties over the surface of the particle. In general, a force producing photodiffusiophoresis would arise from either of these physical property variations. Photodiff usiophoresis in all the examples discussed could occur either in the same or opposed to the direction of the incident radiation. Just as in photothermophoresis, one speaks then of positive and negative photodiff usiophoresis. Experimental study of photodiffusiophoresis would be of value. A very simple example for study would be the particle motion or force produced by a photocatalytic reaction. The observation of the force on ZnO particles participating in the photocatalytic oxidation of CO would be of particular interest inasmuch as the photocatalytic reaction for this sytem has been investigated already in some detail.a It is interesting to note that according to eq 8, in the free molecule region, the force producing photodiffusiophoresis can be used to obtain detailled information on the reaction through determinatioin of the phenomenological parameter Of. Acknowledgment. This study was supported by the National Center for Air Pollution Control, Bureau of Disease Prevention and Environmental Control, U. S. Public Health Service, through Grant APOO479-02.
by J. W. Simons, C. J. Mazac,lb and G. W. Taylor10 Chemistry Department, New Mexico State Univeraiiy, La8 Cruces, New Mexico (Received September 19,1967)
It has become evident that only in the presence of small percentages of radical scavengers such as oxygen ~
~-
Table I :a Insertion Ratios with n-Butane CHpSiHa-n-Butane-DM-Oa Mixtures (3660A) 180-
PCH88iHi
P(n.butane)
9.70 9.00 23.80 27.10 24.00 23.80 0.00 99.70 302.70 278.00 301.80
9.90 9.10 23.40 13.90 24.00 23.80 40.20 101.00 100.40 94.60 94.30
PDM
1.70 2.30 10.10 4.60 4.70 1.90 4.40 22.80 37.80 36.50 38.80
POa
11.70 15.60 21.10 19.50 22.20 17.20 9.00 20.30 9.40 42.90 108.90
pentane/ n-pentane
0.94 0.89 0.96 0.92 0.91 0.95 0.93 0.89 0.90 0.89 0.88
cis-Butene-2-n-Butane-Diazomethane-Oxygen Mixtures (3660A) 18G-
P(butene)
P(n.butane)
PDM
Poi
pentane/ n-pentane
31.90 2.48 26.90 16.82 0.88 0.27 6.72 0.56 36.90 0.18 0.36 0.21
31.60 0.56 8.25 4.69 0.29 0.10 2.05 0.19 10.90 0.06 0.11 0.07
7.50 0.34 4.14 3.50 0.13 0.06 0.93 0.13 3.90 0.04 0.07 0.05
12.00 0.48 6.00 6.00 0.21 0.07 10.00 0.18 8.80 0.05 0.08 0.07
0.84 0.93 0.86 0.87 0.87 0.87 0.91 0.95 0.98 0.88 0.87 0.88
0.17 0.21 5.52 0.56 0.37 2.47 60.30 0.16 36.38
0.05 0.07 1.59 0.18 0.11 0.72 18.00 0.05 11.05
(4358A) 0.03 0.05 0.89 0.11 0.05 0.32 8.77 0.05 9.73
0.05 0.07 8.80 0.16 0.07 0.36 7.60 0.07 12.40
0.87 0.92 0.92 0.89 0.93 0.94 0.89 0.91 0.83
' All pressures are in centimeters. Volume 76,Number 9 February 1968
NOTES
750 Table 11: Relative Rates per Bond for C-H Insertion by Singlet Methylene Radicals Methylene source
Alkane
Ketene (2800, 3130, 3340, and 3660 A) DM (4358, 4050, and 3660 A) Ketene (3130 b) Ketene (3130 b) DM (3660 and 4358
Isopentane
Ketene (3130 b) DM (4358 b)
A)
ktert/kpr
Ref
1.27, 1.22
1.52
11, 13
Isopentane
1.22
1.39
13
Isobutane %Butane n-Butane
1.32 1.35
12 12 This work
Propane Propane
1.29 1.20
12 10
ksec/kpr
1.20~
The average of two experiments which gave 1.0 and 1.4.
and nitric oxide do the photolyses of diazomethane (DM) and ketene give methylene radicals which react with hydrocarbons in a manner characteristic of purely singlet state specie^.^-^ A few gas-phase studies of the C-H insertion reaction of methylene radicals with alkanes have now been carried out with added nitric oxide and oxygen.’O-la Results obtained in this laboratory combined with these previous r e s ~ l t d ~ -lead ’~ to some interesting conclusions concerning the C-H insertion reactivity of singlet methylene radicals from ketene and diazomethane (DM) photolyses. During studies of the photolysis of methylsilanediazomethane-oxygen mixtures14 and of cis-butene-2diazomethane-oxygen mixtures1s at 4358 and 3660 A, it became useful to add varying proportions of n-butane to the reaction mixtures in both systems. The isopentane-n-pentane ratios obtained under the various reaction conditions are summarized in Table I. Important observations from Table I are that the ratio of secondary to primary C-H insertion in n-butane is independent of the total pressure in the range from 0.3 to 400 cm, of the photolysis radiation energy, of the identity and proportions of the added reactive gases, methylsilane and cis-butene-2, and of increased proThe average of portions of oxygen above -10%. those values in Table I is 0.90 f 0.03. The available data for oxygen or nitric oxide scavenged systems, which give intramolecular comparisons of insertion rates for different types of C-H bonds in alkanes, are summarized in Table 11. It is clear from the results in Table I1 that singlet methylene radicals insert slightly faster into secondary and tertiary C-H bonds than into primary C-H bonds and that the ratios ksec/kpr and ktert/kpr are relatively constant for DM and ketene methylene radical sources at different photolysis energies and for various alkane reactants. The small differences in selectivity for various alkanes could be within experimental error. It may be concluded from the results in Table I1 that the radical scavengers, nitric oxide and oxygen, reduce the selectivity of methylene radicals toward primary, seconThe Journal of Physical Chemistry
dary, and tertiary C-H bonds relative to that found in nonscavenged systems,lBJ7as has been pointed out p r e v i ~ u s l y , ~but ~ - ~a~ small selectivity still remains. Hersog and Carr deduced from their isopentane workla that this small remaining selectivity implies small activation energy or preexponential factor differences for singlet methylene radical insertion into primary, secondary, and tertiary C-H bonds. Although the selectivity is sufficiently small to be explained by either of the above factors, an activation energy difference provides the more attractive explanation because of the -2 kcal/mole greater bond dissociation energy for primary compared to secondary C-H bonds which are slightly stronger than tertiary C-H bonds. The larger selectivity exhibited in nonscavenged systems is best explained as due to the more selective abstraction of H atoms by triplet-state methylene (1) (a) This work was supported by the NSF under Grant No. GP6124; (b) NMSU Physical Science Laboratory Predoctoral Fellow: (c) NDEA Predoctoral Fellow. (2) J. W. Simons and B. 8. Rabinovitch, J . Phys. Chem., 68, 1322 (1964). (3) F. H. Dorer and B. 8. Rabinovitch, ibid., 69, 1952 (1965). (4) 8. Ho, I. Unger, and W. A. Noyes, Jr., J . Am. Chem. Soc., 87, 2297 (1965). (5) H. M. Frey, Chem. Commun., 260 (1965). (6) R. W. Carr, Jr., and G. B. Kistiakowsky, J. Phys. Chem., 70, 118 (1966). (7) B. S. Rabinovitch, K. W. Watkin, and D. F. Ring, J . Am. Chem. SOC.,87, 4960 (1965). (8) R. F. W. Bader and J. I. Generosa, Can. J. Chem., 43, 1631 (1965) (9) J. W. Simons and G. W. Taylor, to be submitted. (10) G.2.Whitten and B. S. Rabinovitch, J. Phys. Chem., 69, 4348 (1965). (11) R. W. Carr, Jr., ibid., 70, 1970 (1966). (12) M. L. Halberstadt and J. R. McNesby, J . Am. Chem. SOC.,89, 3417 (1967). (13) B. M. Herzog and R. W. Carr, Jr., J. Phys. Chem., 71, 2688 (1967). (14) C. J. Mazac and J. W. Simons, to be published. (15) J. W. Simons and G. W. Taylor, to be submitted. (16) W. Kirmse, “Carbene Chemistry,” Academic Press, Ino., New York, N. Y., 1964,Chapter 2. (17) H. M.Frey, Prow. Reaction Kinetics, 2 , 133 (1964). I
751
NOTES
radical^^^-^^ followed by recombination to give apparent insertion products. The variations in selectivity found for nonscavenged systems with different photolysis energies and methylene radical sources are best explained by variations in the proportion of tripletstate methylene produced by these conditions. It is well known that singlet state methylene radicals from the photolysis of diazomethane carry considerably more energy into an insertion or C=C addition reaction product than do those from ketene photolysis, and in both cases some fraction of any excess photon energy is also contained in the methylene r a d i ~ a l s . ' ~ JIn ~ view of the invariance in the selectivity between tertiary, secondar,y,and primary C-H bonds under varying conditions that are known to give methylene radicals of different energies, one might deduce that the 'excess energy carried into an insertion product by a methylene radical from ketene or diazomethane photolysis is not contained in the methylene radical's translational degrees of freedom but is all in its vibrational and possibly rotational degrees of freedom which do not contribute significantly to its insertion rate. This assumes, of course, that the invariance in selectivity implies invariance in rate, which would be the case if the selectivity is solely due to activation energy differences. Studies at different temperatures should provide some information on this point. It is quite possible that the initially formed singlet methylene radicals undergo a sufficient number of collisions prior to reaction in order to remove all of the excess translational energy but not a sufficient number to remove all of the excess vibrational energy. The possibility that the selectivity is primarily due to frequency factor differences which would not be altered by varying amounts of excess translational energy in the singlet methylene radicals cannot be ruled out.
CaH&NBr, we have determined the solubility of benzene in concentrated aqueous solutions of the tetraalkylammonium salts mentioned above.
Experimental Section Eastman (CH&NBr, (CzHJdNBr, (n-CsH&NBr, and (n-C4H9)4NBr were used. Solutions of known molality were prepared by weight and were agitated for 24 hr with excess benzene a t 25". Samples (1 ml) of the saturated solutions were diluted to 50 ml ((CH3)4NBr), 100 ml, or 250 ml (some of the ( T Z - C ~ H ~ )solutions) ~NB~ with 22% ethyl alcohol solution. The benzene concentration in the diluted solutions was determined from the absorbance a t 254 and 248 mp, using a Perkin-Elmer Model-4000A spectrometer. Solutions of the salts, diluted in the same way with 22% ethyl alcohol, were used in the reference cell. From these data, the molality of the benzene in the original solution was calculated on the assumption that the molar volume of benzene in the solutions is the same as that of pure b e n ~ e n e . ~The density data of Wen and Saito5were used in the calculations. Results and Discussion The results are given in Figures 1 and 2. The solubility of benzene in (CH&NBr solutions increases linearly with salt molality through the range of concentration studied (1 m to near saturation). With the other three salts, there are relatively sharp changes in the slope of the solubility curves. The change occurs a t around 5 m in (CzHJ4NBr solution, a t 2-3 m in
Solubility of :Benzenein Concentrated Aqueous Solutions of Tetraalkylammonium Bromides 4
by Henry E. Wirth and Antonio LoSurdo Department of Chemistry, Syacuse University, 18210 (Received October 86, 1967)
Syacuse, New York
6
e
10
m,,,, Figure 1. Solubility of benzene (mole/1000 g of water) in solutions of tetramethylammonium bromide and tetraethylammonium bromide.
Desnoyers, I'elletier, and Jolicoeur' have determined (1) J. E. Desnoyers, G. E. Pelletier, and C. Jolicoeur, Can. J . Chem., the solubility of benzene in aqueous solutions of (CH& 43, 3232 (1965). NBr, (C2HJ4NBr, (n-CaH7)4NBr,and ( T Z - C ~ H ~ )in ~ N B (2) ~ N. C. Den0 and C. H. Spink, J. Phys. Chem., 67, 1347 (1963). the concentration range 0.1-1.4 M . The salting-in (3) H. E.Wirth, ibid., 71, 2922 (1967). constants obtained were in excellent agreement with (4) T v s assumption was checked by direct determination of the densities of several solutions of (wCaH7)rNBr and (wC4Ho)pNBr those cited by :Den0 and Spink2 for the first three salts. saturated with benzene. The maximum difference between calcuIn view of the ~iuggestion~ that micelle formation is poslated and observed densities was 3 ppm. sible in concentrated solutions of (C&I6)4NBr and (n(5) W.Y.Wen and S. Saito, J. Phus. Chem., 68, 2639 (1964). Volume 71, Number 8 February 1968
