NOTES
841
a t higher pH an in alkaline solution dichloramine-T does not exist. It is, therefore, obvious that in feebly acidic media (pEf 6 to ‘7)N-chloro-p-toluenesulfonamide is the predominant species of chloramine-T which reacts with hexaoyanoferrate(I1) in a slow and ratedetermining step as follows k’2
CH3C6H4S02N. EIC1 I- Fe(C“64- + reactive intermediate intermediate
+ 11-1-4-Fe(CN)d4- -+ CH3GGH4S02NH2 + 2Fe(CN)63-
(4)
Acknowledgment. Thanks are due to CSIR, New Delhi for the financial assistance to ivI. C. A.
k‘s
(5)
As a one-elecbon change is more likely in oxidation processes,16a reactive intermediate is formed in step 4 which immediately reacts with another molecule of hexacyanoferirate(1I) to give the products p-toluenesulfonamide and hexacyanoferrate(II1) (step 5 ) . Applying s teady-&ate conditions with respect to all the intermediates and taking kI-1 > Vz[Fe(CN)64-], the following rat e equation is obtained
a
-D-
(6) where
2k’z ~~
x
10-5
( K being 2.82 X
SOC., 447
Reactions of Fast Hydrogen Atoms with Ethane’”
Department of Chemistry, Amherst College, Amherst, Massachusetts 01002 (Received October 6 , 1970)
k [Fe(CN)e*-] [chloramine-TI [H+]
2.82
(15) W. C. E. Higginson and J. W. Marshall, J . Chem, (1957). (16) F. G. Soper, ibid., 125, 1899 (1924).
by J. E. Nicholas, F. Bayrakceken, and R. I3. Fink*Ib
Id [chloramine-T ] 2 dt
--[Fe(CN)s4-.] dt
pH 6 and 7. The “initial fast reaction” which under some conditions amounts to 30% of the total reaction cannot, therefore, be accounted for due to dichloramine-T only. It seems likely that the mechanism of the reaction is more complex than the one presented. It may‘be possible that the decreasing rate of reaction with time may be the result of inhibition by p-toluenesulfonamide leading to a repression of hydrolysis or disproportionation.
14)
The value of thc temmlecular rate constant, k , has been experimentally obtained as 14 X lo5 1.2 mol+ sec-’ and therefore tLe rate constant for the rate determining step may be cdculated as k’, = 19.7 1. mol-’ sec-l. This is a fairly high value and therefore well accounts for the low energy of activation. The negative entropy of activalion ( -16.5 eu) and the ionic strength effects also support the interaction between an uncharged molecule anti an ion in the rate-determining step. The derived rate law (6) suggests the first-order dependence in hex:tcyanoferrate(II), chloramine-?’, and €I+as ha,s been experimentally observed. Further, the initial fast reaction may be partially attributed to the fact that in acidic media some N chloro-p-toluenesuifo namide is converted to dichloramine-T (step 3 ) which is more reactive ipecies than chloramine-?’. From the work of Soperla and others14 it has been possible to compute the equivalent concentrations of *a11 the individual species as a function of pII and t o l d concentration. Such computations have revealed that, the disproportionation of N-chlorop-toluenesulfonamitle (step 3) is very little between
Publication costs assisted by the Diviswn of General Medical Sciences, ‘National Institutes of Health
Translationally excited atoms produced by photochemical techniques2-8 are particularly useful for studying epithermal or so-called “hot” chemical reactions as their initial kinetic energies are well defined. We wish to report the results of our studies of the reactions of photochemically produced deuterium atoms with ethane in a system where the initial energy of the deuterium is 2.0 eV and an efficient radical scavenger, bromine, is present. I n previous work with 2.8 eV deuterium atoms and ethane,3DBr was used as both the source of D atoms and as radical scavenger, and measurements were made of the ratio of D atoms undergoing all possible “hot” reactions with ethane t o those D atoms which were collision-moderated to thermal energies. The primary photolytic step is
A straightforward mechanism that can bc: written for such a photochemical system is (1) (a) This research was supported by National Institutes of Health Grant GM 13966. (b) Alfred P. Sloan Fellow, (2) R. J. Carter, W. H. Hamill, and R. R. Williams, Jr., J . Amer. Chem. SOC.,77, 6457 (1955). (3) R. M. Martin and J. E. Willard, J . Chem. Phys., 40, 3007 (1964). (4) A. Kuppermann and J. M. White, ibid., 44, 4532 (1966). (5) C. C. Chou and F. S. Rowland, J. Amer. Chem. Soc., 88, 2612 (1966). (6) R. G. Gann and J. Dubrin, J . Chem. Phys., 47, 1867 (1967). (7) R. G. Gann and J. Dubrin, ibid., 50, 535 (1969). (8) C. C. Chou and F. S. Rowland, ibid., 50, 2763 (1969).
The Journal of Physical Chemistry, Vol. 76,No. 6, 1971
842
The results of both the earlier work on ethane and this present work arc&interpretable on the basis of this mechanism. In the earlier work on the ethane system the substitution reaction 2 and the energy moderating reaction 4 do not seem to be explicitly considered in the kinetic derrvation. Bromine atoms formed in the primary phcrtochemical process and in subsequent reactions of DBr undergo termolecular recombination whiIe the other radicals are scavenged by DBr. The radical products of reactions H and 2 yield CzH5D and HD, respectnvely, and consequently the abstraction and substitution products cannot be distinguished. Thermalized D atoms are scavenged by DBr to give D2. This complete mechanism is consistent with results obtained in the DBr scavenged system for 2.8 eV deuterium atoms, It was tile object of the present work to further test the reac tion mechanism in the following respects : by changing the initial energy of the D atoms; by the addition of Brz to the system as a separate, more efficient radical scavenger, thus simplifying the reaction scheme leading to IdD, 2, and CzHjD products; and by deduction of jhe relative rates of some of the processes involved. Deuterium atoms with an initial kinetic energy of 2.0 eV were produced from DBr which was mixed with hydrocarblon and, where appropriate, Brz (at average total reagent presi,ures of 300 mm with constant Brz pressure OF 95 mm) in cylindrical quartz cells (z.5-cm i d . and 25-cm length) by irradiation with 2138-A light from a 1ovpressur.e Zn arc. Emission lines at 2025 and 2062 A were removed by a cis-butene3 filter (10 cm pressure, path length 3 em). All reagents were purified by vacuum distillation before use. After irradiation, EID and D., products were separated by freezing out =ondensable reactants and products in liquid Nz. I; as demonstrated as a separate series of experiments using 1moim amounts of H D and D2 that there were neither diffusion losses nor entrapment losses of there products using the separation scheme described above. The products were analyzed bp mass spectrometry. The mass spectrometer was a quadrupole resonance type’ whose electron source was operated under controlled conditions eliminating background mass peaks ac rnas’3es 4 and 3. It mas carefully calibrated before, durin5, and after sample analysis by direct use of HD :znd Dz mixtures for detection at the appropriate masses, Mixtures ol various DBr/ethane ratios were photolyzed at 296 f 2°K and the product ratio D*/HD was Tlre Journal of Physzcal Chemistry, Vol, 76, N o . 6, 1971
NOTEEI
I
1
I
1
2
3
REACTANT R A T I O ,
[DBr],”
[CIHb]
Figur: 1. Product ratios as a function of reactant ratios for 2138 A photolysis of DBr in CzHs. Closed circles: [Di]/[WD] for DBr scavenged system; open circles: [Dt]/[HD] for Brz scavenged system. Points shown are averages of at least three individual runs.
measured for both the DBr scavenged system and that to which Br2had also been added. Dark experiments, otherwise identical, were carried out to check background signals at each product mass. The percentage decomposition of DBr was kept below 1.0. For the DBr scavenged system the variation of the product ratio, D2/HD, with reactant ratio, DBr/CsH6, can be straightforwardly derived from the reaction sequence suggested above. Thus, if s1 is the average interaction cross section for the ith process in the energy range from the initial energy with which D* is formed to threshold, one may write
The result of plotting this product ratio against the reactant ratio is shown in Figure 1. As with the results for 2.8 eV deuterium atoms3 a straight line was obtained. The intercept corresponds to s5/(s1 sz), that is the ratio of collisions with ethane that result in moderation t o those that result in hot reaotion. With 2 eV deuterium atoms the result is 2.35 h 0.15 compared with 2.2 for 2.8 eV atoms. The slightly lower probability of hot reaction at the lower energy s e e m reasonable. (a sz)/(sl s~ s5) may adso be calculated. This gives the fraction of atoms that react on collision with ethane or “integral reaction probability” as it has been termedS6 The value obtained is 0.3 The slope of the line corresponds to js8 sd)/(sl sz) and so any variation with initial energy does not lead to such straightforward interpretation. It may be
+
+
+ +
-+
+
NOTES
843
expressed as the ratio of total probability of interaction of D* with DBr (hot reaction I- moderation) to the probability of‘ hot readion with ethane. The result was 1.15 4 0.10 for 2 eV atoms compared with 2.25 for 2.8 eV atoms. However, as described above (a sz s~)/(sI4-&!) may be deduced, and knowing (s3 sq)/(s~3 S Z ) , thct ratio (sb SI SZ)/(S, sd) may be calculated. This gives the relative total probabilities of interaction with ethane and DBr. The result at 2 eV is 2.9 f 0.4 and a4 2.8 eV from the earlier m701-k~a result of 1.4 may be deduced. A possible interpretation of this result is that the cross sections for total interaction with ethane and DBr approach each other with increasing Pnergy. I n the bromine sca~engedsystem the relative rates of reaction of radical with Brz are approximately ten times faster9 thrm with DBr. Having used a threefold higher pressure of Br? than DBr in all these samples, effectively all t ‘nermalieed species react to form bromides which are undetected. One can write (Dz)/(HD) = s3(DBr)/sl(C:zH6) which predicts a straight line through the origin for a plot of D,/HD vs. DBr/C2H6. As shown in Figure 1, such a result was obtained. The slope is given by S,/SI, which corresponds t o the relative probability of abstraction reaction of D* with r and ethane. The result obtained was 0.8 f 0.05. At this energy, 2 eTV, which is the only one for which this measurement he s been performed, the relative total probability of interaction is (C2Ho/DBr) = (2.9/1) and for the abstraction reaction only is (C?H,/DBr)/ (s~/sa) = 1.25 4: 0.08. The only threshold energy for primary I4 abslraction measured for hydrocarbon is -0.3 eW,? As regardls DBr, its bond energy (3.9 eV) is lower than CI3 in ethane (4.2 eV), and the corresponding abstraction reaction has a lower activation energy’O and presumable threshold energy. This might lead one to predtot dial, the cross section for abstraction from DBr would be higher than that for abstraction from ethane at fairly Iosv energies. However, competing with this postulated difference is the sterically enhanced probabjlity of abstraction of H from ethane rather than t h e well shielded D from DBr.I1 The relative weighting caf this factor in this specific system is unknon n.
+
+ +
+
+ +
A ~ ~ n , o w ~ eWe ~ ~wish ~ ~ ~to~ acknowledge t . the assistance of W. W. Phillips, C. H. Manstein, S. Topes, M. H. Nicholas, and C. Nicholas in completing this work. The correct, isotopic analyses of A. Kropf were also most helpful
(9) G. C. Fettis and J. €1. Knox, Progr. React. Kinet., 2, 30 (1964). (10) F. Bach, IC F. Uonhoeffer, and E. A. Moelwyn-Hughes, 2. Phys. Chem., 2?B, 71 (1934), (11) Detailed discussions of steric and bond energy effects are given by F. Schmidt-IBlesck and F. S. Rowland, Angew. Chem., 3 , 769 (1964): R. L.Wolrgang, i’rogr. React. K i n d . , 5 , 97 (1965).
Solvated Electron or Not? by T, R. Tuttle, Jr.,* and Philip Graceffa Depaitment of Chemistry, Brandeis Uniuersity, Waltham, Massachusetts 02164 (Receiaed October 19, 1970) Publication costs borne completely by The Journal of Physical Chemistry
We have observed that sodium metal dissolves in a mixed solvent consisting of tetrahydrofuran-& (THFdg) and naphthalene-& to give a green solution which is paramagnetic. Dilute sodium solutions yield electron spin resonance (esr) spectra such as those shown in Figure 1. At relatively high temperatures the esr spectrum consists of four equally intense equally spaced hyperfine components which undoubtedly arise from the interaction of the unpaired electron with a single Na23 nucleus. Such a spectrum is shown in Figure 1 (a). As temperature is lowered the separation between the hyperfine components is reduced in a manner similar to that observed in the esr absorption spectra of alliali metals dissolved in amine solvent^.^-^ Also, as temperature is lowered a single line absorbance grows in at the center of the hyperfine pattern as is shown in Figure 1 (b). The analogous single line absorbance which has been observed in esr spectra of alkali metal solutions in amines has been assigned to the solvated electron. l--9 The similarity between the esr spectra of amine solutions of alkali metals on the one hand and our solution of sodium in the somewhat exotic mixed solvent described above is both clear and striking, For this reason, except for certain prejudices, it is tempting to offer a common explanation €or the spectra of these different systems in terms of analogous sets of species. Unfortunately, we already have separate explanations for the spectra of metal solutions in amines on the one hand1+ and for sodium solutions in the THF-dsnaphthalene-& solvent on the other. lo Nevertheless, we may profit by examining each explanation in turn as it is applied to the system for which it was not intended.
K. D. Vos and J. L. Dye, J . Chem. Phys., 38, 2033 (1963). K. Bar-Eli and T. R. Tuttle, Jr., ibid., 40, 2808 (1964). K. Bar-Eli and T . R. Tuttle, Jr., ibid., 44, 114 (1966). J. L. Dye and L. R. Dalton, J . Phys. Chem., 71, 184 (1967). R. Catterall and M. C. R. Symons, J . Chem. Soc., 6656 (1965). R. Catterall, M. C. R . Symons, and J. J V ~Tipping, J . C‘hem. SOC.A , 1529 (1966). (7) L. R. Dalton, J. C. Rynbrandt, E. M. Hansen, and J. L. Dye, J . Chem. Phys., 44, 3969 (1966). (8) R. Catterall, M. C. R. Symons, and J. W. Tipping. J . Chem. SOC.A , 1234 (1967). (9) V. A. Nicely and J. L. Dye, J . Chem. Phys., 53, 119 (1970). (10) N. M. Atherton and S. J. Weissman, J . Amsr. Chem. Soc., 83, 1330 (1961). (1) (2) (3) (4) (5) (6)
The Journal of Physical Chemistry, Vol. 76, No. 6, 1971
