PERIODATE OXIDATION OF ETHYLENE GLYCOL : KINETICS
July 20, 1954
have slopes of kz" as a function of C1-, and intercepts with values of kl" a t various C1-. A plot is then made of kl" vs. C1- and k2" vs. C1-. These plots yield straight lines which have slopes of k1 and k2 and intercepts of kzKl and k4K1, respectively. The rate constants were calculated and the average values are shown in Table 11. TABLE I1 RATEAND EQUILIBRIUM CONSTANTS 40" [Ti+a] = 0.015 M ,[Clod-] = 0.5 M X 109 k, X 103 8.89 f 0.9 3.59 f0.4 ki
ki X 10'
kr X
IO2
3.4040.3 1.36f0.2
The rate constants were determined a t both 40 TABLE I11 ACTIVITY ENERGIES AND ENTROPIES 40' ki X 101 4FZ3~3,kcal. 4H:, kcal. 4S:, e.u.
k, X 101 4 F & kcal. AHi, kcal.
as:,
e.".
k3 >' 10' AF:sis. kcal.
4 H f , kcal. e.u.
AS:.
kr X 102 1.36 f 0.2 4Fzai3, kcal. 21.0 f 0.1 4 H i , kcal. 10 f 50% A s J , e.u. -36 f 50%
3.5 i 0.4 20.4 f 0 . 1 f 14% 25 I50% 17
KI
2.18
AH, cal.
AS, e.u.
* 0.2
17.8 i 0 . 1 2500 -50
i 50% i 100%
3.40 i 0.3 20.4 f 0.1 6 f 50% -46 i 50%
[CONTRIBUTION No. 321
+ Clod-
Ti+3 f ClOa-
+H
+ ClOaTiCl++ + Clod-
TiCl++
[complex] +T i O + +
+ C103 + clos
+ Z [complex] +Ti(OH)+s
[complex] + TiO++ c1os f C1- [or TiOCl+]
+
4-H + J _ [complex] --f Ti(OH)+3
8.89 f 0.9 19.9 +e 0.1 26 f 15% 20 f 50%
A F m , kcal.
and 50'. The energies of activation and the entropies of activation for the various steps in the reaction were then calculated. The results are shown in Table 111. The reaction is thought to proceed through the formation of an activated complex, the disproportionation of which is the rate-determining step. The following four equations have been proposed to explain the kinetics of the reaction Ti+a
Kl 2.18 f 0 . 2
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+ c l o s + C1-
[or TiOHCl+a]
From the standpoint of the free energies involved and the probability of reaction most likely to occur, the first and third equations above would seem t o be the most important. The radical C103 is rapidly reduced further to the final product C1-. It may well be true that the affinity of the Ti(1V) for OH- and O= accounts for the low activation energy of this reaction as compared with reductions of perchlorate with Fe(II), Sn(II), Cr(I1) and others. AMES,IOWA
FROM THE
INSTITUTE FOR ATOMICRESEARCH AND DEPARTMENT OF CHEMISTRY, IOWASTATE COLLEGE^ ]
The Kinetics of the Periodate Oxidation of Ethylene Glycol and a Series of Methylated Ethylene Glycols BY FREDERICK R. DUKEAND VERNONC. BULGRIN RECEIVED FEBRUARY 8, 1954 The specific periodate oxidation of the homologous series of glycols, ethylene glycol through pinacol, shows a discontinuity in mechanism between trimethylethylene glycol and pinacol. The latter is very slowly oxidized in an acid-catalyzed bimolecular step, while the former, as with the other glycols studied, proceeds rapidly in a non-catalyzed unimolecular step. The effect of stepwise methyl group substitution in ethylene glycol is determined, both in the free energy of complex formation with periodate and on the unimolecular rate constant.
The mechanism of the periodic acid cleavage of ethylene glycol may involve an intermediate coordination compound of the oxidant and reductant, the rate-determining step being the disproportionation of the intermediate.2 The rate of the reaction has been demonstrated to be nearly independent of hydrogen ion concentration in the pH range three to even.^^^ Waters4 attributed the constancy of the rate in this region to the fact that hydrogen ion catalysis is balanced by decreasing ionization of HJOs to the reactive HJOe-. Crouthamel, Martin and co-workers, in a spectrophotometric study of periodate species in solution,jS6showed that the
concentration of negative ion, predominantly IO4-, is nearly constant in the PH range three to seven. Price3found the rate of oxidation of pinacol to be a peculiar function of hydrogen ion concentration, having a sharp maximum a t p H 2 and a minimum for the range four to six. A kinetic study of the oxidation of the series ethylene glycol through pinacol was undertaken, in an effort t o observe the effect of methyl substitution on the rate and equilibrium constants and to determine a t which point in the series the change in reactivity exhibited by pinacol begins to show up. Experimental
(1) Work was performed in the Ames Laboratory of t h e Atomic Ethylene and propylene glycols obtained from the MatheEnergy Commission. son Co. were twice distilled before using. meso-2,3-Butane(2) F.R. Duke, THIS JOURNAL, 69, 3054 (1947). diol, which had been prepared by the fermentation of corn (3) C.C.Price and M. Knell, i b i d . , 64, 552 (1942). using Aerobacter aerogenes,? was purified by recrystallization (4) W. A. Waters in H. Gilman, "Organic Chemistry," Vol. IV, from dry isopropyl ether.* levo-2,3-Butanediol was preJohn Wiley and Sons, Inc., New York,N. Y., 1953, Chap. 12 (7) E. R.Kooi, E. I. Fulmer and L. A. UnderkoBer, I n d . Eng. Chon., (5) C.Crouthamel, H.V. Meek, D. S. Martin and C. V. Banks, THIS 40, 1440 (1948). JOURNAL, 71, 3031 (1949). (6) C.Crouthamel, A. Hayes and D. S . Martin, ibid.. 73, 82 (19.51). ( 8 ) C . E. Wilson and K. J Lucas. T i i r s J O U R N A L , 58, 2396 (1936).
3804
FREDERICK
I
