NOTES
tion, leading to the intermediate shown (or possibly one with a different pentacoordinated structure) provides an effective mechanism for accomplishing the structural changes, which would be consistent with the (relatively) short relaxation times. The rate constants for the twostep mechanism can be obtained from eq. 5 and 6 by dropping k31 and k13, respectively. Without additional evidence, however, it is not possible to carry a kinetic analysis of this mechanism any further. Some of thc thermodynamic data give an indication that a one-step mechanism is not to be entirely discarded. The standard entropy change for reaction I is 44 e.u. Crouthamel, et al., have estimated that the difference in the entropy of aquation between paraperiodate and metaperiodate (vix., AX,,(H410a-) AX,,(I04-)) is 23 e.u. The authors conclude that a considerable part of this large difference might be due to a difference in the solvent-orienting abilities of the two ions. Specifically, the greater charge localization at the two unprotonated oxygen atoms in paraperiodate ion produces more solvent structure organization at these two places than a t the corresponding four sites
3873
on metaperiodate ion. Since significant solvent organization is thereby implicated in reaction I, the possibility of an orientation favorable enough to perrnit the simultaneous hydrolysis of two associated water molecules should not be omitted. In this case, the rate constants k D and k H would be given, simply, by lcsl and k13. However, this process seems to be less likely than the two-step mechanism on the grounds that such unique configurations would not occur as frequently as H-bond forniation and subsequent proton transfer are known to occur.12 Further work on acid-base catalysis in nonaqueous solvents will be undertaken in order to distinguish, experimentally, between the two mechanistic possibilities.
Acknowledgment. This investigation was supported by Public Health Service Research Grant GM-08893-03 from the National Institute of General Medical Scienoe, Public Health Service. (11) F. A. Cotton and G. Wilkinson, “Advanced Inorganic Cheniistry,’’ Interscience Publishers, Inc., New York, N. Y.,1962,p. 307‘. (12) M.Eigen, Angew. Chem., 75, 489 (1963).
N0T.ES Competitive Scavenging of Methyl Radicals by Galvinoxyl and Iodine1 by Robert H. Schuler Radiation Research Laboratories, Mellon Institute, Pittsburgh, Pennsylvania (Received J u n e 89-1964)
Bartlett and Funahashi2 have recently studied the relative rates for reaction of galvinoxy13and iodine with cyanoisopropyl radicals and found that the former rate is ten tinies greater. This result necessarily implies that, a t least for the cyanoisopropyl radical , iodine is an inefficient scavenger and that the reaction is activation controlled. Work from these laborat o r i e ~ ,in~ which the absolute rate of scavenging of methyl radicals by iodine is determined by comparison with the rate of bimolecular combination of radicals, has indicated that the rate for reaction 1
CH3.
+ I2 +CH31 + I .
(1)
is in the range of 108-109 1. mole-‘ set.-' and is essentially diffusion controlled. In view of these facts a direct determination of the relative rates for the scavenging of methyl radicals by galvinoxyl and iodine seemed desirable. Since Bartlett and Funahashi have demonstrated the mutual conipatability of these two scavengers such a direct determination is readily passible.
Experimental Methyl radicals are generated by the radiolysis of 2,2,4-trimethylpentane for which G(CH3.) had previ(1) Supported, in part, by the U. S. Atomic Energy Commission. (2) P. D. Bartlett and T. Funahashi, J . Am. Chem. Soc., 84, 25159 (1962). (3) Formally known as 2,6-di-t-butyl-a-(3,5-di-t-buty1-4-0~0-2,5cyclohexadiene-1-y1idene)-ptolyloxy: G. M. Coppinger, ibid., 79, 501 (1957). (4) R.H.Scliuler and R. R. Kuntz, J . P h y s . Chem., 67,1004(1963).
Volume 68, Number 12 December, 196.4
3874
NOTEP,
cyanoisopropyl radicals is considerably less than for reaction with methyl radicals, If the present result is combined with the previous estimate of 3 X lo*for IC1 then an absolute rate of 2 X lo7 is obtained for ICZ. Since, because of the high extinction coefficient for g (E431 150,000),2 studies can be carried out a t low concentrations, this low value for ICz suggests that it should be possible to make a direct comparison of ICz with the rate for bimolecular combination of radicals a t moderate radiation intensities. Studies of this nature are in progress.
-
Acknowledgment. The author wishes to thank Mr. G. Buzzard for his assistance with the methyl iodide determinations reported here.
0.01 0.1
100
10
1 [Golvinoxyl]/
[I*]
the System Water-Sucrose-Glycine at 25"'
Figure 1. Ratio of scavenging of methyl radicals by galvinoxyl [Go(CHJ) - G(CHJ)] and by iodine [G(CHJ)]. Solid curve corresponds to simple competition (slope = I), with the rates equal a t [g]/[I2] = 16. Error limits indicated are 10.01in AG.
ously been demonstrated to be 0.69. Reaction with iodine is measured directly from the formation of methyl iodide and with galvinoxyl (g) from the decrease in G(CH31) at a particular ratio of (g)/(L). Since iodine concentrations were always in excess of M complications due to abstraction of hydrogen from the solvent should be absent. Galvinoxyl concentrations were varied from 0.3 to 6 X M and M . The iodine concentrations from 0.01 to 3 x methyl iodide yields were determined by the radiochemical methods previously d e ~ e l o p e d . ~
Results and biscussion The rate for reaction of methyl radicals with galvinoxy1 CH3.
+g
-
products
(2)
relative to (1) is given in Fig. 1. It is seen that a simple competition seems to exist with the rate for (2) only as large as that for (1). This somewhat decreased rate is not too surprising in view of the delocalization of the electron in the galvinoxyl radical and in view of possible steric coniplications in reaction with such a large scavenger molecule. Assuming that reaction 2 is a t all rapid (Le., kz > lo7 l./mole-' sec. -I) the present result confirms the previous conclusion that reaction 1is essentially diffusion controlled. Apparently, the efficiency of iodine for reaction with The Journal of Physical Chemistry
Diffusion Coefficients for One Composition of
by Peter J. Dunlop2 and Louis J. Gosting Chemistry Department and Institute j a r E n z y m e Research, University of Wisconsin, M a d i s o n , Wisconsin 65706 (Receieed J u n e 20, 1964)
Although a number of papers which present data for isothermal diffusion in ternary systems have appeared in the literature during the past decade, only a few3-6 of these report data for two nonelectrolytes in water. As part of a program for studying diffusion in aqueous ternary systems by nieans of the Gouy diffusiometer it was decided to obtain some data for the system watersucrose-glycine, This note reports values for the four diffusion coefficients for one composition of this system, together with auxiliary data for the partial specific volumes and the refractive index derivatives. Measurements were made a t rather high solute concentrations in the hope that the cross-term diffusion coefficients would be large and capable of being determined quite accurately.
Experimental We use the numbers 0, 1, and 2 to denote the components water, sucrose, and glycine, respectively. (1) This investigation was supported in part by National Science Foundation Research Grant GP-179 and by National Institute of Arthritis and Metabolic Diseases (U.S.P.H.S.) Research Grant AM05177 and Career Award K6-AM-16,715 (to L. J. G.). (2) Department of Physical and Inorganic Chemistry, Adelaide University, South Australia. (3) P. J. Dunlop, J . P h y a . Chem., 61, 1619 (1957). (4) F. E. Weir and M. Dole, J . Am. Chem. Soc., 80, 302 (1958). (5) P. N. Henrion, T r a n s . Faraday Soc., 60, 75 (1964).
