5334
EDMUND
R. MALINOWSKI AND LEONARD s. LEVITT
[ C O N rRIBUTION FROM THE CHEMISTRY
DEPARTMENT, STEVENS INSTITUTE
Vol. 80 O F TECHNOLOGY]
Mechanism of Organic Oxidation in Aqueous Solution. 11. Effect of Initial Persulfate Concentration on the Rate of Oxidation of Isopropyl Alcohol1 BY EDMUND R. MALI NOW SKI^
AND
LEONARD S. LEVITT
RECEIVED FEBRCARY 10, 1958 The rate of oxidation of isopropyl alcohol by potassium persulfate a t 60’ in aqueous solution of PH 8.0 and ionic strength 0.5 is first order with respect to the measured disappearance of persulfate. The rate constant is proportional to initial alcohol concentration a t low concentrations but becomes independent of it a t higher concentrations. The variation of the pseudo first-order constant with initial persulfate concentration, using 0.02 M isopropyl alcohol and initial concentrations of persulfate ranging from 0.00125 to 0.06 M, has now been investigated, and it is found, rather surprisingly, that a limiting initial rate is also attained a t the higher initial persulfate concentrations, similar to that observed when initial alcohol concentration is varied. The rate equation -dP/dt = k P = [kma,AoPo/(b Ao)(c Po)]P(where b and G are empirical constants) relates the observed rate constant k to the initial alcohol and persulfate concentrations (A0 and ?@).This equation is in excellent agreement with the data for 18 kinetic runs. The analogy between persulfate oxidation kinetics and enzyme kinetics is pointed out.
+
Introduction
We have found previouslya that the rate Of oxidation of isopropyl alcohol K 2 S 2 0 8 in solution of pH 8.0 and ionic strength 0.5 is first order with respect to the measured disappearance Of persulfate* A limiting rate was reached at higher initial concentrations o€ alcohol, where the rate became independent of the initial alcohol concentration. Similar results have been observed in the persulfate oxidation of mercaptan^,^,^ of diethyl and diphenyl and of thiodiglycol sulfoxide.s The variation of the pseudo first-order rate constant with varying initial persulfate concentration has now been investigated a t 60” using 0.02 M isopropyl alcohol and initial concentrations of persulfate ranging from 0.00125 to 0.06 M . Experimental All kinetic runs were carried out at 60.0’ with an initial concentration of 0.02 M isopropyl alcohol (Eastman Kodak Co. “spectro” grade). The PH was maintained a t 8.0 by a phosphate buffer, and the ionic strength was brought up to 0.5 by the addition of K2S.04. The course of the reactions was followed by iodometric titration. The buffer solution and solution for titration of the samples were prepared in the same manner as that previously described,s and the experimental procedure also was identical.
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for the rate constant in duplicate experiments wxs very s a t i s f a ~ t o r y . ~ There are points of inflection in the straight lines (Fig. 2 cunres A and B) a t approximately the concentration for which i t would be expected, from the stoichiometry of the oxidation, that all the alcohol should be oxidized t o ketone. The rate constant measured for the second portion of these is designated k f . Table I gives the rate constants k observed for the oxidation of 2-propanol a t different initial persulfate concentrations. Also included in this table are the corrected rate constants k, obtained by subtracting from k the value 0.018 hr,-l, which is the first-order rate constantfor the disappearance of persulfate when no alcohol is present in the solution.6 F~~ the two in which the persulfate concentration was greater than that of the alcohol, Table I also shows the corrected rate constants k,‘ for the portion of reaction after the inflection points. That this represents the rate of oxidation of the resulting acetone is supported by a separate run with 0.02 M acetone as the substrate, for which k, was found to be 0.084 hr.-I. These data are presented graphically in Fig. 3, where k, is plotted as. [ S ~ O S ’ ] ~ . TABLE I PSEEDO FIRST-ORDER RATECONSTANTSFOR
THE PERSUL0.02 M 2-PROPANOL AT 60” IN AQUESOLUTION OF pH 8.0 ASD IOXIC STHEXGTK 0.5
FATE OXIDATION O F
Results
OUS
It is evident from the accompanying graphs (Figs. 1 and 2 ) of the logarithm of the persulfate concentration vs. time that the reactions are first order with respect to the measured disappearance of persulfate. The rate constant for a given run was calculated in the usual manner from the estrapolated half-time obtained from the logarithmic plot of the data. The reproducibility of the value
[KzSnOnla
(1) This paper was given a t t h e N e w Vork Meeting of the A.C.S.. September, 1967. (2) Taken, in part, from the M.S. Thesis of E . R. Malinowski. (3) L. S. Levitt and E. R. IIalinowrki, Taxs J O U R N A L , 7 7 , 4517 (1955); 78, 2018 (195F). ( 1 ) R . L . Eager and C. A. Winkler, Can. J. Rc-s.,B26, 527 (1948). 6)L. S.Levitt, ibid., 31, 915 (1953). (6) E . Howard, Jr., and I,. S. Levitt, THISJOURNAL, 76, 6170 (1953). (7) L. S. Levitt and E. IIoward, Jr.. A h t . of Papers, Dallas Meeting A.C.S., 1956, 51-N, (8) L. S. Levitt and D. G. Ziebell, Ahst. of Papers, Cincinnati Meeting A.C.S., 1953, 41-N.
1112,
hr.
0.00125
1.45
,00250
1.25
.0050o ,0400
1.03 0.82 .79
. ofioo
.81
.0200
k!
hr.
-1
0.48 .55 .67 .84 .88
.85
fi., hr. - 1
ko‘, hr. -1
0.46 .53 .65
...
.82 .86 .83
...
... ... 0.073 0,092
Discussion It is obvious from Fig. 3 that a limiting initial rate is also attained above a certain initial persulfate concentration (approximately 0.02 M ) . This is similar t o the effect observed when initial alcohol concentration is varied and the persulfate concentration is held constant. At lower initial persulfate (9) T h e same values for k, within 2’35, under the same experimental conditions, have been observed by Mrs. B. W,Levitt in this Laboratory; Abst. of papers, Chicago Meeting, A.C.S.. 1958, 5-P.
Oct. 20, 3958
I\IECII..ZNISM O F O R G A S I C OXIDATION IN
I 0.81 0.6
AQUEOUSSOLUTION
5335
/'-
I- ,f
A
B 0
0.01
0.02
0.03
0.04 0.05 0.06 (h0. Fig. 3.--Variation of k, with initial concentration of persulfate; temp. 60" ; initial isopropyl alcohol concentration 0.02 M. [sz08-10
,
4-
,
,
,
l
c
!
60 80 100 120 140 Time (min.) Fig. 1.-Rate of persulfate oxidation of 0.02 M isopropyl alcohol a t 60' with different initial persulfate concentrati m s : A, 0.005 df; B, 0.0025 -If; C, 0.00125 M. 0
50
20
40
I-----\
varies, however, with the square of the initial persulfate concentration a t low values of the later and with the first power of the initial persulfate concentration a t high values. With respect to concentration, then, the reaction is either second order or first order in persulfate, or somewhere between these two extremes for intermediate persulfate concentrations. The fundamental difference between the order with respect to time and that with respect to concentration has been pointed out by Letort."' It is seen from eq. 1 that a t high initial persul]~ c fate concentrations for which [ S Z O ~ '>> k
=
(2)
kmalr(p)
and, therefore, the rate constant is independent of initial persulfate concentration. At low initial persulfate concentration IS,O,-]
