Application of Electron Spin Resonance to the Determination of Hydroperoxides SIR: Electron spin resonance (ESR) is extensively used in the studies of free radicals as well as in the studies of paramagnetic salts. However, there are few examples in which ESR has significant importance as an analytical technique. The experimental result, which is reported in this paper, will find an application to the determination of hydroperoxides in a solution. The solutions of I,1-diphenyl-2-picryl hydrazyl (DPPH) in oenzene and, in the case of tert-buty I hydroperoxide (BHPO) in several otha?r solvents, were prepared. To these solutions, BHPO (containing 10% of tert-butyl alcohol), cumene hydroperoxide (CHPO), peracetic acid (PAA) or meto-chloroperbenzoic acid (CPBA) was added and after a definite interval of time the ESR spectra were observed. The ESR measurements were ma3e on an X-band spectrometer a t room ,;emperature and the spectra were recorded as the second derivatives. The solvents used were reagent grade and were not treated to remove any dissolved oxygen. EFFECT OF ADDITION CF PAA OR CPBA
The semilogarithmic plots of the concentration of DPPH, after the hydroperoxides are added, us. time are shown in Figure 1. The curve obtained after the addition of CPBA shows that the D P P H is decomposed exponentially with respect to time. I n the case of PAA, the curve deiiates from the straight line. This may be attributed to the gradual decomposition of PAA in the
solution. From the experimental relation for the decay of the concentration of DPPH, it can be concluded that the hydroperoxides are not themselves consumed in the decomposition. During the reaction the ESR spectra observed are always the same as that of a solution of D P P H in benzene to which no hydroperoxides are added. However, when a large excess (beyond saturation) of one of these hydroperoxides is added to a benzene solution, the spectrum observed 10 hours after the addition of the hydroperoxide consisted of three lines. OF
Figure 1. Semilogarithmic plot of concentration of DPPH in benzene after hydroperoxide is added, vs. time CPBPU rnetachloro perbenzoic acid. PAPU peracetic acid. Initial coiicentration of DPPH i s 2 mrnoles per 1 as shown in flgure
-.
-0.5
2
+ ..
E U
6-1.0 0 -
CUMENE HYDROPEROXIDE
When CHPO is added to a solution of D P P H in benzene, both decay and transformation of the ESR spectrum take place even when the concentration of CHPO is small. The original five spectral lines caused by D P P H are gradually replaced by a triplet. When the concentration of CHPO is 4 mmoles per liter, a considerable amount of the triplet lines is observed, as shown in Figure 2. As seen in Figure 2, the chemical species giving rise to the triplet is not perfectly stable. When different solvents are used, the rates of conversion of the original quintet into the triplet are different, as shown in Table I. I n Table I, the original concentration of the solution of D P P H is 2 mmoles per liter except in the solution of D P P H in cyclohexane, n-here the solubility of D P P H is about 0.5 mmole per liter a t 20" C.
When BHPO is added to a solution of D P P H in benzene an effect similar to that observed by CHPO is found. However, the reaction of BHPO with D P P H is faster than that of CHPO with DPPH. The decay and transformation of D P P H is, as is shown in Figure 2 and Table I, not complete. Therefore, the
c
0.0
-1.5 EFFECT
EFFECT O F ferf-BUTYL HYDROPEROXIDE
-1
0.3
Table I.
0
500
1000
Time(min)
Figure 2. Semilogarithmic plot of concentration of DPPH and R in benzene after peroxide is added vs. time 0 or D,C, 4: DPPH concentration after addition of CHPO (curnene hydroperoxide, 4 rnrnoles per liter) or R,C,4: R concentration after addition of CHPO (4 rnrnoles per liter) 0 or D,B,4: DPPH concentration after addition of BHPO (ferf-butyl hydroperoxide, 4 rnrnoles per liter). A or R,B,4: R concentration after addition of BHPO (4 rnrnoles per liter)
0
spectrum obtained when the reaction is still taking place is a hybrid of the quintet and the triplet. The slight difference in g-factor between the triplet and the quintet makes this hybrid spectrum asymmetric. The concentration of the transformed free radical R (giving rise to the triplet) can be estimated by an analysis of this spectrum. The relation between the concentration of added BHPO and the percentage of decomposed and transformed DPPH, is shown in Figure 3. It can be seen that the rate of decomposition of D P P H shows a greater dependence on the concentration of BHPO than the rate of the transformation of DPPH.
The Percentage Conversion of Quintet into Triplet after 10 Minutes
Solvents Hydroperoxides, mmole/l. 40 CHPO 10 BHPO
Benzene
Toluene
Cyclohexane
CCI,
13
14 12
37 40
22
16
VOL. 35, NO. 13, .DECEMBER 1963
9
2213
may account for the fact that an exchange interaction for the species R is not observed.
2.C
-.
RATE CONSTANTS FOR BOTH REACTIONS
1.5
On the basis of these results, the reaction between DPPH and hydroperoxides will be formulated as:
r.-0
vr
DPPH
L
?/ 1.0
+ 0.2 HPOZ
+
X
-+
A
+
c5
0.2 HP02:kl
DPPH
$-
1.5 HPOz + X:kz
(2)
v
oi
where, HPOs denotes the hydroperoxide, denotes some decomposition intermediate, and -4 denotes the final nonparamagnetic species, and kl and IC2 are rate const'ant,s,k1 being the overall rate const,ant for the reaction,
-0
X
0.5
i 0 g . c (mM/ I ) Figure 3. Logarithmic plot of percentage conversion of DPPH to A and R at 10 minutes after addition of BHPO vs. concentration of BHPO (initial concentration of DPPH in benzene 2 mmoles per liter)
DPPH -+ A (3) The proportions of HPOz appearing in Equation. 1 and 2 were determined from the slopes of the curves in Figure 3. The values of kl for various hydroperoxiden were estimated from the initial slopes of the curves in Figure 1 and Figure 2. The initial velocities for the reaction 1 and 2 will be, (DPPH) dt
=
-kl (UPPII), (HP02),0.* ('L')
The intensities of the ESR spectra of the solutions of DPPH, a t various concentrations and before and after t.he addition of BHPO, are compared when the concentration of BHPO is the same in each case-i.e.; 2.5 molea per liter. It is shown in Figure 4. ..I solution of D P P H exhibits ail eschange interaction phenomenon when the concentrat,ion is greater than some value between 2 and 9 mnioles per liter ( 1 ) . However, it can be seen from Figure 4 that \\.hen the concentration of D P P H is 30 illmole,$ per liter, the addition of BHPO decreases the intensity of the ESR spectrum to less than 1/100 and it is this decrease in concentration which
Table
If! droperoxides kl
k*
2214
II.
The Values
dt
=
-kz (DppH)01.5
(2')
If the ratio of initial rates of disappearance of D P P H in reaction 1 and 2 is approximated by the ratio, I/?, of the amount of D P P H which disappeared in reactions 1 and 2 after the reaction for the first 10 minutes, then r
k2
= -~ (HPO2)'I.J
(4)
ki
Thus L s can be estimated from kl and T. The values of kl and kz derived in this manner are list'ed in Table 11. In the earlier communication (2) the authors stated t'hat the transformation
kl and kz for
PAA CPBA 2.1 x 10-8 1.8 x 10-3 less than
ANALYTICAL CHEMISTRY
(DPPH!
Various Hydroperoxides
BHPO
x 1.6 x
4.7
10-2 10-4
.O
' 5
: -v/1>
(1)
C
2
'
!:;
CHPO 4 . 0 x 10-3 5 8 X
Figure 4. Logarithmic plot of ESR intensity vs. concentration of DPPH in benzene
0:When hydroperoxide is not added, 0 : When BHPO is added to 2.5 rnrnoler per liter of DPI'H owurred only in the pre-ence of tertiary hydroperoxides. This habeen shon-n incorrect, though the rate of tranJformat'ion of D P P H in the pre>ence of per acid- is very small. ANALYTICAL APPLICATION
I.
1he t\-pical tcchiiique for the cleterrninatioii of hjdroperoxides is iotlomet'ry. Iodomet'ry, hon.e\.er, does not determine different types of hJ-droperoxides separately from their misture. In some instance> it will be desirahle to know the amount of a particular t>ye of hydroperoside in a misture of inany different hydroperosides such a> primary, secondary, tertiary, etc., a i the reactivity of a hydroperoxide depends greatly on its structure. The different. reaction rate constants presented here will be useful in such cases, although no atoichiometric relation betxeen D P P H and hydroperoxides has been found. LITERATURE CITED
(1) Hutchison, C. A., Pastor, It. C., Kowalskj-, -4.G., J . Chrm. Phys. 20,
534 (1952).
(2) Veda, H., Kuri, Z.,Yhids, S ,lbzd., 36, 1616 (1962). HI'54sHl U E D A '
Department of Physics Duke University Durham, Y.C. l Present address, Department of Chemistry, The University of Texas, Austin, Texas. RECEIVEDfor review April 10, 1963. Accepted October 1, 1963. Nishina Memorial Fellow 1961. This work was supported by the ilir Force Office of Scientific Research, Air Research and Developnient Command, Giant SOrlF-AFOSR-62-327.
