A Polarographic Study of Organic Peroxides E. J. KUTAl and F. W. QUACKENBUSH Department of Biochemistry, Purdue University, lafayetie, Ind.
b An improved procedure is outlined for the polarographic study of organic peroxides in a nonaqueous electrolyte solution. Polarograms were observed for 23 commercial organic peroxide compounds having the following functional peroxide groups: hydroperoxides, peroxy acids, peroxyesters, six-membered bicyclic peroxides, diacyl peroxides, other diacyl peroxides, and ketone peroxides. Twenty-one of the compounds showed one or more characteristic reduction waves; they were placed in five groups based on different half-wave potentials. A linear relationship existed between diffusion current and concentration for all of the peroxides investigated except peroxyacetic and peroxybenzoic acids. Two of the peroxides failed to show evidence of reduction under the conditions used (0.0 to -2.0 volts).
ture of benzene (Merck, thiophenefree) a n d absolute methanol (Baker) was made 0.3M with lithium chloride for t h e supporting electrolyte. T h e lithium chloride was recrystallized from methanol before use. When 0.002% of methylene blue and 0.01% of ethylcellulose were added t o this solution, the capillary constant was unaltered. However, when the ethylcellulose was increased to 0.3% and methylene blue to 0.05%, the m and t values m r e 1.83 and 3.71 mg. per second, respectively, thus changing the capillary constant m2/3t1/*to 1.86 mg.2/3second-ll2. Procedure. Five milliliters of t h e polarographic solvent containing a 1 m M concentration of the organic peroxide were transferred into t h e cell and a stream of nitrogen (oxygenfree) x i s bubbled through t h e solution for 10 minutes. One milliliter of mercury was placed in t h e cell t o serve as the anode; the nitrogen
D
Table 1.
StrUCtUral diff erences i n organic peroxides has become increasingly important in the fields of fats, oils, rubber, gasoline, polymers, and combustion products. Paralleling this importance is the use of the polarograph to determine these various organic peroxides. This paper describes a n improved polarographic procedure and its application to differentiate between certain organic peroxide structures b y their half-wave potentials. The solvent used was a n equal-volume mixture of methanol and benzene to which lithium chloride and ethyl cellulose were added. IFFERENTIATION Of
stream was continued into t h e solution for 2 minutes and then raised to flow over the surface of the solution during polarography. All of the polarograms were recorded a t a constant rate of potential increase (0.154 volt per minute). Following preliminary tests to compare ethylcellulose preparations (Table I), ethylcellulose [ethoxy content 48.7%, viscosity 42 c.p.s. (centipoises per second), Hercules Powder Co.] was added in all the experiments: 0.01% for hydroperoxides, alkyl peroxyesters, and trans-annular and ketone peroxides; 0.30% with 0.015% of methylene blue for peroxy acids, diaroyl peroxides, and diacyl acid peroxides; 0.30% with 0.002% methylene blue for diacyl aliphatic peroxides. The organic peroxide samples (Table 11) were used as received without further purification. Total peroxide was determined by the iodometric method of Wheeler (10).
Effect of Ethylcellulose Preparations on Current-Voltage Behavior of tert-Butyl Hydroperoxide and Autoxidized Mineral Oil
Peroxide-Containing Substance, % RIineral tert-Butyl oil, hydro(P.V: peroxide = 28)
Ethylcellulose Preparation Ethoxy, Viscosity, C.P.S. 7i
Ethylcellulose Concn. E1'2 ( i d ) b
0
1.3
45.8
98
-0.60(0.12) -1.27(0.20)
-0.51 (0.01) -1.24(0.21)
0
1.3
47.8
100
-0.60(0.13) -1.16(0.19)
-0.58(0.07) -1.16(0.28)
0
1.3
50.1
96
-0.71 (0.20) -1.10 (0.06)
-0.69 (0.13) -1.08 (0.13)
0
1.3
48.2
-0,66(0.13) -1.12(0.18)
-0.60 (0.04) -1.22(0.21)
0
1.3
48.7
42
-0.67(0.12) -1.12 (0.17)
-0.60(0.06) -1.15(0.24)
0
1.3
47.8
100
-0.65(0.13) -1.16 (0.19)
-0.58(0.06) -1 l S ( 0 . 2 8 )
0
1.3
48.5
277
-0.70(0.13) -1,14(0.15)
-0.62(0.10) -1.22 (0.29)
0.04 0.04 0.04
0 0 0
45.8 47.8 50.1
98 100 96
-1.27(1.44) -1.24(1.53) -1.15(1.38)
-1.31 (1.23) -1.27(1.36) -1.17(1.20)
0.04 0.04 0.04 0.04
0 0 0 0
48.2 48.7 47.8 48.5
4.5 42 100 277
-1.20(1.37) -1.18 (1.48) -1.22(1.53) -1.20(1.33)
-1.23 (1.25) -1.22(1.28) -1.27 (1.36) -1.24(1.22)
4.5
EXPERIMENTAL
Apparatus. A Sargent Model XXI recording Polarograph with a waterjacketed cell (30" + 0.2" C.) containing a n internal mercury pool anode with a n approvirnate surface area of 6.6 sq. em. was used. A marine barometer capillary with open circuit a n d a drop time t of 3.80 seconds was used as t h e dropping mercury electrode in the solvent system. T h e value m (rate of flow of mercury) was 1.85 nig. per second; therefore, r n W / 6 was 1.88 rnga2I3 second-1/2. Triple-distilled mercury was used in the cell assembly. Reagents. An equal-volume mix-
0.5070
0.0170
Peroxide value, in moles Oz/kg. mineral oil.
Present address, General Foods Corp., 555 South Broadway, Tarrytown, N. Y.
* Half-n-ave potential in volts with corresponding diffusion current in microamperes. VOL. 32, NO. 9, AUGUST 1960
1069
X laboratory ozonizer (S), which generated f1.57~ozone, was used t o ozonize 50-mg. portions of olefinic samples dissolved in 20 ml. of distilled ethyl acetate. The ozonides were concentrated to 2 ml. under reduced pressure and polarographed. RESULTS
Ethylcellulose Effects. Several ethylcellulose preparations were dissolved a t t h e desired percentage concentration in t h e electrolyte solution. These stock solutions were used to dilute weighed samples of tert-butyl hydroperoxide and autoxidized mineral oil. Polarograms were then recorded (Table I). Autoxidized mineral oil, in the absence of ethylcellulose, showed a halfwave potential of -0.80 volt. On addition of ethylcellulose this wave was resolved into two waves, at -0.67 and -1.15 volts (Figure 1). These two waves were clearly defined and reproducible; however, with increasing ethoxy content of the ethylcellulose, the distance between them narrowed (Table I). Only a t the highest percentage of ethoxy content was the diffusion current affected. Viscosity had little effect on either half-wave potential or diffusion current. All of the ethylcellulose preparations tested resolved the reduction wave a t -0.80 volt into two waves. tert-Butyl hydroperoxide showed a half-wave potential of - 1.04 volts; on the addition of ethylcellulose (4801, ethoxy, viscosity 42 c.p.s.) , the wave appeared at - 1.15 volts. However, with increasing ethoxy content the halfwave potential shifted toward zero. The point of maximum sensitivity, as measured by the maximum diffusion current, was about 48y0ethoxy. Various viscosities a t constant ethoxy content had little effect on half-wave potential and diffusion current. Increasing the concentration of ethylcellulose fiftyfold did not alter substantially the polarograms of either the mineral oil or the tert-butyl hydroperoxide. Ethylcellulose was added in all subsequent experiments. Solvent Effects. Methyl oleate and methyl linoleate, autoxidized at 100' C. to peroxide values of 150 and 375, respectively, were subjected t o polarography in various solvents (Table 111). I n the usual solvent system of methanol-benzene (1 to 1) and with ethylcellulose present, the two reduction waves a t about -0.7 and -1.1 volts were observed with both esters. Replacement of t h e benzene with other aromatic solvents, toluene or xylene, gave essentially the same results. However, on replacement with nonaromatic solvents a single reduction wave was obtained in this range of potentials. An aromatic solvent 1070
ANALYTICAL CHEMISTRY
Figure 1 . Comparison of polarogramsof autoxi-
V)
dized mineral oil (Peroxide value = 28)
A.
With ethylcellulose
E.
Without ethylcellulose
(0.01%)
-1 2
-0 6
-1
e
APPLIED POTENTIAL. VOLTS
Table II.
Polarographic Behavior of Organic Peroxide Compounds
Compounds
Peroxide Content,
% (11)
Peroxide Structure
Bis(2,4-dichlorobenzoyl )peroxide*
0 /I
-O-O-C-CH~-CH~
1
dOOH
Acetyl peroxidec
96.8
0.00
I
0
8
e
CHI -0-0-
-CH3
CeH18-
Peroxyacetic acid
CHI -0OH
48.4
-0,28
94.2
-0.00 -1.20
OH I
bI -O-O-b-C,H,,
Bis( 1-hydroxylheptyl)peroxide*
H
H
i
Peroxybenzoic acid
23.6
CHa
0.00 0.00
@o-ooH
CHI
I
Methyl ethyl ketone peroxide*
0.00
HOOb
OH
GROUP2
99.6 ci
CI
Succinic acid peroxideb
Half-Wave Potential, Volts
1
CH3CH2-h-O-O-C-CH2CHs
I
8
0 I
H
H
I
49,3
-0 60 -1.26
97.8
-0 66 -1 08
49.8
-0.70 -1.05
?
Phenylcyclohexane hydroperoxided
Di-tert-butyl perphthalateb 0
--
Figure 2.
4
m
- 40
A.
U
n
A
4 0
u L
27
-
i 2 - 2
D
x
36
-5
Polarograrns
Methyl ethyl ketone per-
oxide (commercial) in methyl phthalate 6. Dimethyl phthalate
di-
I
4
U
0
-1 2
-06
APPLIED
Table 11.
POTENTIAL.
-1 6
VOLTS
Polarographic Behavior of Organic Peroxide Compounds (Continued)
Peroxide Content, Compounds GROUP3
tert-Butyl peracetate
Peroxide Structure
% (11)
Half-Wave Potential, Volts
0
I!
CH&O-O-C(
terl-Butyl perbenzoateb
CH3)a
@-C+CICH3)3
GROCP4 p-Menthane hydroperoxided
CH3-@zH3 I OOH
Cumene hydroperoxided
(-JpH3
98.8
-1.02
92.6
-0.95
44.1
-1.06
96.5
-1.08
26.9
-1.06
90.3
-1.08
97.7
-1.10
OOH
tert-Butylisoprop ylphenylhydroperoxidee Pinane hydroperoxided
@-W3 OOH
Diisopropylphenylhydroperoxided
CH3 HC
OOH
tert-Butyl hydroperoxideb
(CH3)3-C-O-O-H
56.8
-1.15
Hydrogen peroxide
H-0-0-H
22.8
-1.16
...
-1.22
GROUP5 Ascaridole cH b
GROUP6 Di-tert-butyl peroxideb 1-Phenylmethyltert-butyl peroxide
eH I/CH3 H3
yc
(CH3),-C-O-O-C(CH3),
Eastman Kodak Co. Wallace and Tiernan, Inc. Buffalo Electrochemical Go., Inc. Hercules Pom-der Co. Phillips Petroleum Co. 1 Bios Laboratories, Inc. 0 No reaction. a
* Lucidol Division,
0
Not reduced N o t reduced
0
seemed to be necessary for the ethylcellulose to resolve the single reduction wave into two ITaves. Maximum Suppression. Autoxidized methyl linoleate a n d other peroxidic substances showed maxima in t h e region of zero voltage irrespective of the presence of ethylcellulose. For use as a maximum suppressor, methylene blue was investigated in concentrations as high as 0.015$&. d solution of 0.3’% ethylcellulose and 0.0157, methylene blue in the absence of peroxides gave a small reduction wave at -0.30 volt; this wave disappeared at concentrations of methylene blue below 0.00570. -4 smaller amount (0.00270) was effective in suppressing the maximum shown by autoxidized methyl linoleate and some other maximum-producing peroxidic substances. I n the absence of the maximum, a reduction wave was then observed at a voltage near zero for such compounds. The effects exerted by ethylcellulose and methylene blue are not fully understood. Studies with aqueous media have suggested that maximum suppressors exert their effects by complex formation ( 1 ) and by influencing adsorption on the mercury drop ( 4 ) . However, neither explanation is fully satisfactory (6). Polarographic Behavior. Twentytwo different organic peroxides obtainable from commercial sources were compared polarographically (Table 11). These compounds fell into six different groups, based on their structures and behavior in t h e polarographic cell. T h e first group of eight compounds showed reduction waves a t or near zero voltage. Included were diaroyl and diacyl peroxides and peroxy acids. The diaroyl peroxides [benzoyl and bis(2,4-dichlorobenzo~l)p e r o x i d e s ] showed linear relationships between diffusion current and concentration in the range 9 X 10+ to 4 X 10-4M, and the diacyl peroxides (acetyl, lauroyl, and succinic acid peroxides) between 1 x 10-2 and 1 X 10-4M. The acetyl peroxide was received in dimethyl phthalate because of its instability, and this diluent, present during polarography, gave a n additional reduction wave at -1.72 volts (Figure 2). The succinic acid peroxide also showed a second reduction wave, a t - 1.44 volts, which was attributed to the free acid group ( 5 ) ; a polarogram of succinic acid showed a reduction wave a t approximately the same potentials. Peroxyacetic and peroxybenzoic acids gave reduction waves at 0.00 voltage in the presence of ethylcellulose and methylene blue in concentrations stated above, but they did not demonstrate a linear relationship between diffusion current and concentration. The acids evidently reacted slowly with the methaVOL. 32, NO. 9, AUGUST 1960
1071
Table 111.
Effect of Solvent on Half-Wave Potential of Autoxidized Methyl Oleate and Methyl Linoleate
No. of Reduction Waves for Autoxidized Esters
Ells Autoxidized Esters Solvent" Methyl oleate Methyl linoleate6 Benzene -0.68; -1.05 -0.70; -1.08 2 Toluene -0.70; -1.16 -0.70; -1.04 2 Xylene -0.76; -1.44 -0.72; -1.08 2 Skellysolve B -0.66 -0 72 1 Ethylene chloride -0.66 -1.04 1 Cyclohexane -0.98 -0.88 1 Cyclohexanol -0.84 1 Ethyl acetate -'i .'17 -1.23 1 In mixture with methanol (1 to 1, volume). In all cases it contained 50y0 methanol and 0.05% ethylcellulose. b Showed large current maximum at zero voltage, while oleate showed none.
no1 in the solvent ( 8 ) , as a continuous decrease in diffusion current was observed with increased time of contact. Peroxyacetic acid showed a n additional wave at a half-wave potential of - 1.41 volts, probably because of the presence of acetic acid, whose half-wave potential was observed to be 1.44 volts. Bis( 1-hydroxyheptyl) peroxide gave two reduction waves at 0.00 and -1.20 volts. A linear relationship existed between the concentration (1 x to 1.3 X 10d4M) and diffusion current at half-wave potential of - 1.20 volts, but not a t 0.00 voltage. I n the second group two reduction waves were obtained for each of the three peroxides, the first a t -0.60 to -0.70 volt, the second a t -1.05 to -1.26 volts. Methyl ethyl ketone peroxide in dimethyl phthalate showed three half-wave potentials, one of which (-1.82 volts) was attributed to the phthalate e&er (11) (Figure 2). The first reduction wave (0.60 volt) was observed only when the concentration of peroxide was below O.OlM, and the relationship between diffusion current and concentration was nonlinear, since diffusion current showed a maximum a t a concentration of 2.1 X 10-3M. The second reduction wave (half-wave potential, - 1.26 volts) demonstrated a linear relationship between diffusion current and the above concentrations. The samples of phenylcyclohexane hydroperoxide and di-tert-butyl perphthalate showed, for both reduction waves, a linear relationship between diffusion current and concentration in the range of to 10-4M. Group three consisted of two peroxy esters (tert-butyl perbenzoate and tertbutyl peracetate) which gave a single reduction wave at about -1.0 volt. Both compounds showed a linear relationship between diffusion current and concentration in the range of to 1 0 - 4 ~ .
The fourth group of seven hydroperoxides also showed a single reduction
1072
ANALYTICAL CHEMISTRY
wave, and at a slightly more negative potential than the third group. The group consisted of diisopropylphenyl, tert-butylisopropylphenyl, p-menthane, cumene, pinane, and tert-butyl hydroperoxides, and hydrogen peroxide. Five of the more complex members of the group reduced in the range of - 1.02 0.02 volts. All gave a linear relationship between diffusion current and concentration in the range to 10-4M. I n a class by itself (and therefore designated in Group 5 ) was the transannular peroxide ascaridole, which reduced at -1.22 volts. It showed a linear relationship between diffusion current and concentration in the range 9.2 X lO-'to 1.6 X 10-3M. Two peroxides, di-tert-butyl peroxide and I-phenylmethyl-tert-butyl peroxide, were not reduced in the voltage span of 0.00 to -2.00 volts.
tained for di-tert-butyl perphthalate. Careful measurement of the half-wave potentials would be required to differentiate the hydroperoxides from the peroxyesters. Some evidence has been reported (9) to show that hydroperoxide may be reduced to hydroxyl at the dropping mercury electrode; the reduction products of peroxyester are not known. The relatively high potential required to reduce ascaridole suggests that the bicyclic structure may help stabilize this type of peroxide. Outstanding among the observations was the stability of the dialkyl peroxides. Criegee (2) and Milas (7) have shown that the neutral ozonide molecule is in equilibrium with two zwitterions. hlilas showed that when olefins are ozonized in the presence of carbonium ions, the resulting hemiperacetal peroxides decompose to form hydroperoxides and carbonyl compounds. When the ozonides of diethyl fumarate, diethyl maleate, methyl oleate, and methyl linoleate were dissolved in the electrolyte, they may have reacted with methanol, giving a hemiperacetal peroxide which reduced near the range of bis(1-hydroxyheptyl) peroxide, - 1.20 volts. ACKNOWLEDGMENT
The authors acknowledge the generous supply of commercial peroxides contributed b y the Lucidol Division of Wallace and Tiernan, Inc., Hercules Powder Co., Buffalo Electrochemical Co., and Phillips Petroleum Co. Thanks are expressed to The Dow Chemical Co. and Hercules Ponder Co. for supplying various ethylcellulose preparations.
DISCUSSION LITERATURE CITED
The arbitrary grouping of the different peroxides represents only an attempt to compare those structures similar in polarographic behavior. The procedure does seem to have merit. All compounds falling into Group 1 correspond to peroxy acids or their dimers except the bis(1-hydroxyheptyl) peroxide, and this compound distinguishes itself by giving two waves a t potentials corresponding in some respects to other groups. Group 2 is perhaps the least homogeneous from the structural standpoint and no attempt will be made to rationalize structural relationships to the similarity of polarographic behavior. The methyl ethyl ketone contains some of the corresponding hydroxy monohydroperoxide, which presumably is also one of the products of reduction. Hydroperoxides reduced a t potentials near - 1.0 volt, and all seven of those tested behaved similarly. Only one reduction wave was obtained for tert-butyl perbenzoate, while two waves were ob-
(1) Coe, R. H., Rogers, L. B., J . Am. Chem. SOC.70, 3276 (1948). (2) Criegee, R., Lohaus, G., -4nn. 583, 6 (1953).
(3) Horning, E. C., "Organic Syntheses " Vol. 111, p. 673, Wiley, New Yo&, 1956. (4) Keilin, B., J . Am. Chem. SOC.7 0 , 1984 (1948). (5) Korshunov, I. A., Kuznetsova, Z. B.,
Schchennikova, M. K., J . Phys. Chem. (U.S.S.R.) 23, 11 (1949). (6) Meites, L., Meites, T., J . Am. Chem. SOC.73, 177 (1951). (7) Milas, N. A,, Davis, P., Xolan, J. T., Ibid., 77,483 (1955). (8) Parker, W. E., Ricciuti, C., Ogg, C. L., Swern, D., Ibid., 77, 4037 (1955). (9'1 Skoog, D. A., Lauwzecha, 9.B. H., ANAL.CHEM. 28,825 (1956). (10) Wheeler, D. H., Oil & Soap 9, 89 (1932).
(11) Whitnack, G. C., Gantz, E. St. C., ANAL.CHEW25,553 (1953).
RECEIVEDfor review July 21, 1958. Resubmitted January 4, 1960. Accepted June 9, 1960. Journal paper No. 1259 of the Purdue University Agricultural Experiment Station.
