201
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Acknowledgment. I t is a pleasure to acknowledge support of this work under the Foundational Research Program of the Bureau of Xaval Weapons. NOa VI
(9)
S. Winstein and 11. E. Wood, J . A m . Chem. Soc., 62,548 (1940).
(10) F. A. L. Anet, ibid.. 84, 747 (1962).
(8)
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Isotopic Exchange of Iodine Atoms between Tetrabutylammonium Iodide and
No3
p-Nitrobenzyl Iodide in Carbon
IX
Tetrachloride-Nitrobenzene Mixtures’
6b2 Figure 2. R = CHs; R
=
CHa, C2Hr.
threo- molecule (dl-mixture) . For meso- or erythro- compounds, it seems clear, upon examination of Fig. 2a, that as the R groups become more bulky, the number of molecules with the trans conformation would increase and as a result the dipole moments would decrease ln magnitude. Similar consideration of dl- or threocompounds indicates that bulky R groups would tend to decrease the number of molecules with trans conformation and lead to increasing dipole moments. Thus the series of meso- or erythro- compounds would be expected to show decreasing dipole moments while the series of dl- or threo- compounds would be expected to show increasing dipole moments. It is seen from Table I that the moments of the meso- or erythro- compounds decrease while those of the dl- or thfeo- compounds increase in the predicted fashion. Additional support for the position taken here may be garnered from the data of Winstein and W0od.O Here again it may be seen that the moments of the meso- or erythro- compounds decrease while those of the dl- or threo- compounds increase. Spin-spin coupling constants between methine protons for the meso- and dl- forma of 2,3dibromobutane were recently measured by Anet.lo The data were interpreted as indicating the presence of larger amounts of rotational isomer with the bromines located trans In the dl- compound than in the meso- compound. This would mean that meso-:!,3dibrornobutane would have B larger dipole moment than dl-2,3-dibromobutane, which is a result in accord with the prevlous findings of Winstein and Wood.9 This explanation of the dipole moment differences between meso- and dl- isomers based on rotational isomerism gives added plausibility to the interpretation offered here, also based on rotational isomerism, for the dipole moment differences within:a homologous series of either erythro or threo type compounds.
by E. M. Morimoto and Milton Kahn Department o j Chemistry, The University of New Mezico, Albuquerque, N e v Mezico (Received July 8, 1963)
Frequently, in studies of the effect of solvent on the rate of a bimolecular ion-dipole reaction, an attempt is made to correlate the specific reaction rate with the dielectric constant of the solvent medium. The theoretical treatment of Laidler and Eyring2 predicts that a plot of the logarithm of the specific reaction rate a t zero ionic strength us. the reciprocal of the dielectric constant of the medium should be linear with a positive slope, provided nonelectrostatic effects are negligible. Because of specific solvent effects, this relationship would not necessarily be expected to hold for completely different solvents.s Even in solvent mixtures the results are not always in accord with this theory.4 The work presented here is concerned with the isotopic exchange of iodine atoms between tetrabutylammonium iodide and pnitrobenzyl’lodide in carbon tetrachloride-nitrobenzene mixtures a t 24.90’ over a dielectric constant range from 34.69 to 8.12; the concentrations of both reactants ranged from about 2.00 X 10-6 to 4.00 X 10-6 F . The exchange of iodine atoms between an organic iodide and iodide ion is a particularly suitable reaction for the study of solvent effects. Because iodine-131 activity is available in carrier-free form, it is possible, with proper precautions,6 to deter~~
This communication is based on work done under the auspices of the Atomic Energy Commission (Contract No. AT(11-1)733). 39, 303 (2) K. J. Laidler and H. Eyring, Ann. N. Y. Acad. Ai., (1940). (3) R. D. Heyding and C. A. Winkler, Can. J . Chem.,29, 790 (1951). (4) (a) E. Tommila and P. J. Antikainen. Acta Chem. S c a d . , 9, 825 (1955); (b) R. Puchs and A. Nisbet, J . A m . Chem. SOC., 81, 2371 (1959). (1)
Volume 68, Number I
January, 196.4
NOTES
202
mine second-order rate constants employing iodide M and, perhaps less.6 At concentrations as low as such low iodide concentrations these values of the specific reaction rates should be essentially equal to the hypothetical vaiues at zero ionic strength. Furthermore, at these iodide concentrations the formation of ion pairs should be minimized. Because hydroxylic solvents are thought to interact with simple anions via hydrogen bonding7 it is of especial interest to study such reactions in aprotic solvent mixtures.
Experimental Radioactivity. Iodine-131, obtained from Oak Ridge, F in KI and 5 was stored as a stock solution, 5 X X F in NaHSOs. Chemicals. pNitrobenzyl iodide was prepared as described previously8 and purified by seven recrystallizations from absolute ethanol. Eastman Kodak tetrabutylammonium iodide was purified via six recrystallizations from absolute ethanol. Both materials were stored in the dark over silica gel. J. T. Baker analytical grade nitrobenzene and Mallinckrodt analytical grade carbon tetrachloride were fractionally distilled through a 20-plate Oldershaw column in the dark, over alumina, under nitrogen, and stored under nitrogen a t 10”) in the dark; all solvents were used within 72 hr. of distillation. Erratic results were obtained when no attempt was made to remove oxygen and .exclude ,light from the solvents. Furthermore, such purified solvents yielded similar results 2 weeks subsequent to distillation even though special precautions were taken to exclude light and oxygen during the sampling process. Ordinary distilled water was redistilled from an alkaline permanganate solution in an all-Pyrex still, boiled, and flushed with nitrogen. Matheson prepurified grade nitrogen was further dried by passing it slowly through a liquid nitrogen cold-trap. All other chemicals employed were of analytical grade. Preparation of Reactant Solutions. p-h’itrobenzyl iodide solutions were prepared by the dissolution and volumetric dilution of accurately weighed amounts of the solid. Radioactive tetrabutylammonium iodide solutions were prepared as follows. Approximately 1 ml. of stock iodide tracer solution was treated with 2 ml. of a solution 0.2 F in KYCrz07 and 2 F in H&04, and extracted with benzene. After several washings with water, the benzene phase was shaken with an aqueous solution of tetrabutylammonium iodide ; the resulting active aqueous solution was evaporated to dryness under vacuum over a t least 4 hr. and the residue dissolved and The Journal of Phgsical Chemistry
volumetrically diluted with the desired solvent. The concentrations of the resulting tetrabutylammonium iodide solutions were calculated from the concentrations of the original aqueous solutions, the quantitative nature of all the steps in the activation procedure having been experimentally verified. The reactant solutions were stored in the dark under nitrogen at 10” and found to be stable for a t least 48 hr. after preparation. The solvent mixtures were prepared by weight and stored in the dark, under nitrogen, at 10”. All operations in the preparation of solvent mixtures and reactant solutions were conducted in semidarkness with minimum exposure to the atmosphere. Procedure for Exchange Experiments. The reactant solutions and reaction vessel (10-ml. centrifuge tube with glass stopper) were brought to reaction temperature in a constant-temperature ( .tO.Ol”) water bath. One-half milliliter of each reactant was then pipetted into the vessel and mixed. After a given interval of time, the reactants were separated and the radioactivity associated with each reactant determined. The procedure was repeated for various time intervals. Preliminary experiments indicated that light had a marked accelerating effect on the rate of the exchange reaction; therefore, all exchange experiments were conducted in the absence of light. Separation Procedure. The reactants in nitrobenzene-carbon tetrachloride mixtures were separated by shaking with 3 ml. of carbon tetrachloride and 4 ml. of water; benzene was substituted for the carbon tetrachloride when the reaction was conducted in pure nitrobenzene. The resulting two-phase system, with the pnitrobenzyl iodide in the organic phase and the tetrabutylammonium iodide in the aqueous phase, was centrifuged and separated. The organic phase was washed several times with water, the washings being added to the aqueous phase. The efficiency of the extraction of tetrabutylammonium iodide into the aqueous phase was determined through the use of “blank” reaction mixtures in which no p-nitrobenzyl iodide was present; corrections were applied whenever necessary. In general, less than 1% of the tetrabutylammonium iodide M. Kahn, “Iodine-131,” Chapter 9, “Inorganic Isotopic 9yntheses,” R. H. Herber, Ed., W. A. Benjamin, Inc., New York, N. Y., 1962. (6) E. R. Swart and L. J. le Roux, J . Chem. Soc., 2110 (1956). (7) (a) J. A. Ikary and M. Kahn, J . A m . Chem. Soc., 81, 4173 (1959); (b) E. A. S. Cavell, J . Chem. Soc.. 4217 (1958); (e) E. A. S. Cavell and J. A. Speed, ibid., 1453 (1960): (d) ibid., 226 (1961): (e) J. Miller and A. J. Parker, J . A m . Chern. Sac., 8 3 , 117 (1961); (f) A. J..Parker, J . Chem. Soc., 1328 (1961). (8) E. L. Purlee, M. Kahn, and J. L. Itiebsomer, J . A m . C h a . Soc., 7 6 , 3796 (1954). (5)
NOTES
203
was retained in the organic phase. Also, in each instance, no more than 0.2% of p-nitrobenzyl iodide was extracted into the aqueous phase. Measurements of Radioactivity. The liquid samples were counted with a well-type scintillation detector to expected standard deviations of 1% or less.
Results and Discussion The reaction investigated was p-NO2CeH4CHd
+ ((>4H9)4NI*
=
p-K02C6H4CH21*
+ (CdH,)&”
where the asterisks indicate radioactive atoms. The experimental data were evaluated with the aid of the logarithmic form of the first-order isotopic exchange law.9 The reaction was found to be first order with respect to both the p-nitrobenzyl iodide and iodide ion. In solvent mixtures of dielectric constant as low as 12.53, the absence of any systematic variation of the observed specific rate constant, k , suggests that the salt is completely dissociated; for the solvent mixtures of dielectric constant of 10.00 and 8.12, the values of k increase with decrease in tetrabutylammonium iodide concentration, indicating the presence of kinetically inactive ion psirs.1° Values of the degree of dissociation, a, calculated from the association constants for tetrabutylammonium iodide, l1 are in accord with the foregoing observations. In all instances, a was calculated employing the limiting form ‘of the Debye-Huckel activity coefficient expression12which should be
valid a t the low ionic strengths of the reaction mixtures. A semilogarithmic plot of k / a us. the reciprocal of the dielectric constant is shown in Fiq. 1. Over the range of dielectric constant from 34.69 to 12.53, the plot is linear with a positive slope, in agreement with the theory of Laidler and Eyring.2 At lower values of the dielectric constant the specific reaction rate is less than would be predicted from this theory. This departure from theory may be attributed to the accumulation of nitrobenzene around the iodide ion, causing the microscopic dielectric constant to be higher than the bulk v a l ~ e ~ ~ , ~ ~ and/or the increase of the “effective size” of the iodide ion with decrease in dielectric constant by virtue of “selective solvation” with nitrobenzene.11 G. Friedlander and J. W. Kennedy, “Nuclear and Radiochemistry,” John Wiley and Sons, Inc., New York, N. Y., 1955, p. 315. (10) C. C. Evans and S. Sugden, J . Chem. SOC.,270 (1949). (11) E. Hirsch and R. 31. Fuoss, J . Am. Chem. Soc., 82, 1018 (1960). (12) C. B. Monk, “Electrolytic Dissociation,” Academic Press, Inc., New York, N. Y., 1961, p. 30. (13) L. R. Dawson, J. E. Berger, and H. C. Eckstrom, J . Phys. Chem., 65, 986 (1961). (9)
?-Excitation of the Singlet and Triplet
States of Naphthalene in Solution
by B. Brocklehurst, G. Porter, and J. M. Yates I
I
f
I
1
I
90
Department of Chemistry, T h e UniTersity, Shefield 10, Englanil (Received August 1, 1963)
80
70 60 I
50 I
m
d
40 c
a‘ I&
30
20
II
20
40
60
I 80
f . . 100
I
I
120
140
108/D.
Figure 1 . constant.
Dependence of specific reaction rate on dielectric
The luminescence produced by high energy irradiation of organic solutions has been much studied but the mechanisms of excitation have not yet been fully elucidated. In the case of a dilute solution of a compound with excited states lying lower than those of the solvent, energy transfer processes are commonly involved, Le., energy absorbed by the solvent is transferred to the solute which emits light. In the field of radiation chemistry, similar eff ects-sensitized reactions and “protection”-are observed, The transfer of energy is usually discussed in terms of transfer of electronic excitation, but it is also possible that positive charge is transferred, followed by the recombination of the solute ion with an electron or negative ion to produce luminescence or chemical change. In frozen organic solutions (glasses) which have been bombarded with high energy radiation, the existence of trapped electrons and negative ions has been demonVolume 68, Number 1
January, 196.4
