Nuclear Magnetic Resonance Spectra of Organic Peroxides Daniel Swern,l Anthony H. Clements,z and T. M. Luong3 Fels Research Institute and Department of Chemistry, Temple Uniuersity, Philadelphia, Pa. 19122 The N M R spectra of approximately twenty highly purified organic peroxides (hydroperoxides, peroxy acids, dialkyl peroxides, t-butyl peroxyesters, and diacyl peroxides) and selected nonperoxidic analogs have been determined. The signal of the hydroperoxy proton is considerably downfield from that of the hydroxyl proton, and its position is also concentration dependent. The upfield shift of the signal of the hydroperoxy proton with dilution is of much smaller magnitude than that in alcohols. The proton of the peroxycarboxylicacid group is even more deshielded than the hydroperoxy proton but its chemical shift is independent of concentration. The correlation between increasing acidity and downfield shift of the acidic proton has now been extended to include hydroperoxides and peroxy acids. The N M R spectra of the other classes of peroxides studied are generally not significantly different from those of their nonperoxid ic ana logs.
ALTHOUGH nuclear magnetic resonance spectroscopy (NMR) is well established for determination of structure and analysis of organic compounds, it has been used to only a limited extent in the qualitative and quantitative analysis of organic peroxides. The unavailability of pure reference peroxides and the difficulties and hazards in preparing, storing, and handling them account for the neglect of this area and the paucity of data. Organic peroxides are intermediates in many important chemical processes, and systematic study and classification of their NMR spectra are essential for their detection and identification by that technique. In pioneering work on alkyl hydroperoxides (n-propyl, n-butyl, n-octyl, and isobutyl), Fujiwara, Katayama, and Kamio ( I ) reported large differences in chemical shift among hydroperoxides, alcohols, and carboxylic acids for the proton attached to oxygen and to a lesser degree for the a-methylenic protons. The hydroperoxy proton (600-R ca. 9 ppm) was shown to be considerably deshielded in comparison with the hydroxyl proton (80-H ca. 4 ppm downfield from tetramethylsilane = 0) and produced a signal in the same general region as the carboxyl proton (6coz-R ca. 10 ppm). The resonances of a-methylenic protons in hydroperoxides are only slightly more deshielded (ca. 0.4 ppm) than the analogous protons in alcohols. Davies, Hare, and White ( 2 ) studied the proton magnetic resonance spectra of t-butyl and isobutyl groups in boron derivatives and also used NMR to follow the autoxidation of isomeric tributyl borons. t-Butyl, t-butoxy, t-butylperoxy, isobutyl, isobutoxy, and isobutylperoxy groups attached to boron can be readily differentiated. Brill (3) used NMR to identify isomeric hydroperoxides.
We have been preparing organic peroxides of high purity for study of their carcinogenic, mutagenic, and teratogenic properties and, during the course of that study, we have determined their NMR spectra. Classes of organic peroxides examined are hydroperoxides, peroxy acids, dialkyl peroxides, diacyl peroxides, and t-butyl peroxy esters. We have also compared the NMR spectra of some of the peroxides with those of their nonperoxidic analogs and, in the case of one hydroperoxide and peroxy acid, we have also studied the effect of concentration on the chemical shift of the hydroperoxy and peroxycarboxylic acid protons, respectively. EXPERIMENTAL
Apparatus and Procedure. NMR spectra were obtained at 37 "C with a Varian A-60 Spectrometer; carbon tetrachloride, deuteriochloroform, or hexadeuterioacetone solutions at concentrations of approximately 100 mg of peroxide per ml were employed except in those cases where solubility limitations required lower concentrations. When the concentrations were known exactly the information is given in the tables below. Spectra were obtained on freshly prepared peroxides. Analysis by Microanalysis Inc., Wilmington, Del. Compounds. All peroxides should be regarded as hazardous compounds. Vacuum distillation was conducted behind safety shields. Liquid heating baths (water or oil) were used, never electric heating mantles. Solvents for recrystallization were dried and purified by conventional means. Nonperoxidic compounds of analytical purity were obtained by standard methods and their preparation is not described. The purification of the peroxides is briefly described. t-BuTYL HYDROPEROXIDE (I). The commercial product, purity 90 %, obtained from the Lucidol Division, Wallace and Tiernan, Inc., was fractionally distilled. The central portion of the main fraction, bp 34 "C (16 mm), was retained. It was a colorless liquid, nDz5 1.3994; peroxide oxygen: calcd 17.8%, found i 7 . 8 Z . Its purity was >99.5%. 2,5-DIMETHYL-2,5-DIHYDROPEROXYHEXANE (11). A Commercial sample, purity 70%, obtained from the Lucidol Division, Wallace and Tiernan, Inc., was dissolved in ether and the solution was dried over anhydrous magnesium sulfate for 24 hours. The ether solution was filtered and the solvent evaporated. The residue was recrystallized three times from a mixture of olefin-free hexane and ether. The pure peroxide was a white solid, mp 103-104 "C; peroxide oxygen: calcd 18.0%, found 17.9%. Its purity was >99%. CUMENE HYDROPEROXIDE (111). A commercial sample, purity 83 %, obtained from Hercules Inc., was fractionally distilled. The central portion of the main fraction, bp 54 "C (0.05 mm), was retained. It was a colorless liquid, nD25 1.5232; peroxide oxygen: calcd 10.5 %, found 10.3%. Its purity was 98.5 %. 4-HYDROPEROXY-2,6-DI-I-BUTYL-4-METHYLCYCLOHEXA-2,5-
To whom inquiries should be addressed. * Present address, Unilever Research Laboratorium, Vlaardingen, The Netherlands. Present address, Laboratoire de Lipochemie, C.N.R.S., Thiais, France. (1) S. Fujiwara, M. Katayama, and S. Kamio, Bull. Chem. SOC., Jap., 32, 657 (1959). (2) A. G. Davies, D. G. Hare, and R. F. M. White, J. Chem. SOC., 1961, 341. (3) W. F. Brill, J . Amer. Chem. Soc., 87, 3286 (1965). 412
ANALYTICAL CHEMISTRY
DIEN-~-ONE (IV). This compound was prepared by a slight modification of the procedure of Bickel and Gersmann ( 4 ) ; mp 115-116 "C; peroxide oxygen: calcd 6.35%, found 6.40%. Its purity was 99%. Exposure to light caused the compound to darken and decompose. HYDROXY-^ '-HYDROPEROXYDICYCLOHEXYL PEROXIDE (V). A commercial sample, purity 85 %, obtained from the Lucidol Division, Wallace and Tiernan Inc., was recrystallized at - 10 "C from a mixture (1 :3) of ether and olefin-free petroleum (4) A. F. Bickel and H. R. Gersmann, Pvoc. Chem. SOC.231 (1957).
ether (bp 32-40 "C). The pure peroxide was a white solid, mp 93-97 "C; peroxide oxygen: calcd 13.0%, found 13.1%. Its purity at the time of preparation was estimated to be >99% but the compound deteriorates rapidly, as discussed later, and its NMR spectra are of doubtful value. PEROXYBENZOIC ACID(VI). This compound, mp 41.5-42.5 "C, was prepared by the procedure of Silbert, Siegel, and Swern ( 5 ) ; peroxide oxygen: calcd 11.6%, found 11.6%. Its purity was >99.5 when freshly prepared but it slowly decomposes at room temperature; it was stored in the dark at - 10 "C. PEROXYPELARGONIC ACID(VII). This compound, mp 35.637.5 "C, was prepared by the procedure of Parker and coworkers (6); peroxide oxygen: calcd 9.18%, found 9.18z. Its purity was >99.5% when freshly prepared but it decomposes at room temperature; it was stored in the dark at - 10 "C. PEROXYLAURIC ACID(VIII). This compound, mp 51-52 "C, was prepared by the procedure of Parker and coworkers (6); peroxide oxygen: calcd 7.40z, found 7.35%. Its purity was >99%. This peroxy acid is stable for long periods in the dark at 0 "C. CYCLOHEXANEPEROXYCARBOXYLIC ACID(7) (IX). Commercial cyclohexanecarboxylic acid was purified by recrystallization from hexane-ether; the pure compound had mp 4344 "C. Hydrogen peroxide (13 ml, 70 %) was added drop-wise at -5 to 0 "C to a stirred solution of cyclohexanecarboxylic acid (12.8 grams) dissolved in methanesulfonic acid (64 grams) in an open beaker. Stirring was continued at -5 to 0 "C for two hours after addition was complete, followed by addition of ice and water (200 ml) at such a rate that the temperature never exceeded + 5 "C. The diluted reaction mixture was rapidly extracted with cold ether, washed several times with cold saturated ammonium sulfate solution and then with cold water. The ether solution was dried over anhydrous magnesium sulfate in a refrigerator followed by vacuum evaporation of the ether from the filtered solution at a maximum temperature of 20 "C. The light yellow oily residue was recrystallized three times from olefin-free petroleum ether (bp 30-40 "C) to yield cyclohexaneperoxycarboxylic acid, a white crystalline solid, mp 22-23 "C (yield 78%); peroxide oxygen: calcd 11.1%, found 11.2%. Its purity was >99% when freshly prepared. It decomposed slowly even when stored in the dark at - 10 "C. ~NITROPEROXYBENZOIC ACID(X). This compound, a pale yellow solid, mp 133 "C, was prepared by the procedure of Vilkas (8); peroxide oxygen: calcd 8.702, found 8.30%. Its purity was 95 (impurity p-nitrobenzoic acid). The compound is stable at room temperature. DI-&BUTYL PEROXIDE (XI). A commercial sample, purity 99 %, obtained from the Lucidol Division, Wallace and Tiernan, Inc., was fractionally distilled. The central portion of the main fraction, bp 30-32 "C (30-35 mm) was retained. It was a colorless liquid, nD2j 1.3866. Its purity was >99.5%. This compound is stable at room temperature and can be stored safely in the dark for long periods without decomposition, although as a routine procedure in our laboratory it is stored in a refrigerator. 2,5-DIMETHYL-2,5-DI(I-BUTYLPEROXY) HEXANE (XII). A commercial sample, purity 90 %, obtained from the Lucidol Division, Wallace and Tiernan, Inc., was fractionally distilled. The central portion of the main fraction, bp 55-57 "C (7 mm) was retained. It was a colorless liquid, nD25 1.4200. Anal. calcd for C~H3404: C 66.17; H 11.80; 0 22.03. Found: C 66.22; H 11.62; 0 (direct analysis) 21.98. Its purity was >99.5%. This compound has about the same stability (qualitative observation) as di-t-butyl peroxide.
of Campbell and Coppinger (9) from highly purified 2,6-di-tbutyl-4-methylphenol, t-butyl hydroperoxide, and cobalt naphthenate catalyst. t-Butyl alcohol was used as solvent. Recrystallization of the crude reaction product (after evaporation of all volatiles) from isooctane at -40 "C yielded an off-white, crystalline solid in almost quantitative yield; mp C 74.02; 87.5 "C [lit. 74 "C (9)]. Anal. calcd for C19H3203: H 10.38; 0 15.60. Found: C74.12; H 10.30; 0 (direct analysis) 15.57. Its purity was >99.5%. Exposure to light and air caused the compound to darken. DICUMYL PEROXIDE (XIV). A commercial sample, purity >go%, obtained from Hercules Inc., was recrystallized from olefin-free petroleum ether at -25 "C to yield the pure peroxide, a colorless crystalline solid, mp 40.5-41 "C. Its purity was >99.5%. ASCARIDOLE (XV). A commercial sample, obtained from Eastern Chemical Corp., was fractionally distilled. The central portion of the main fraction, a pale yellow oil, bp 42-43 "C (0.01-0.05 mm) and nD25 1.4716, was retained. It gave one major spot on TLC and several minor spots. Its purity was not assessed quantitatively. 1,l '-DIHYDROXYCYCLOHEXYL PEROXIDE (XVI). A commercial sample, purity 95 %, obtained from the Lucidol Division, Wallace and Tiernan, Inc., was recrystallized from a mixture of diethyl ether : olefin-free petroleum ether (2:3) at -5 "C. The pure peroxide was a crystalline solid, mp 71-73 "C; peroxide oxygen: calcd 6.93 %; found 6.80%. Its purity was 98 %. &BUTYL PEROXYACETATE (XVII). A commercial sample, 75 % solution in benzene, obtained from the Lucidol Division, Wallace and Tiernan, Inc., was fractionally distilled. The central portion of the main fraction, bp 25-26 "C (0.5 mm) was retained. It was a colorless liquid, nD25 1.4035; peroxide oxygen (10): calcd 12.12%; found 11.98%. Its purity was >99 %. BUTYL PEROXYBENZOATE (XVIII). A commercial sample, purity 98 %, obtained from Lucidol Division, Wallace and Tiernan, Inc., was fractionally distilled. The central portion of the main fraction, bp 78-79 "C (0.2 mm), was retained. It was a colorless liquid, nD25 1.4984. Anal. calcd for C10H1403: C 68.02; H 7.27; 0 24.71. Found: C 68.21; H 7.24; 0 (direct analysis) 24.82. Its purity was >99.5 %. BENZOYL PEROXIDE (XIX). The purest commercial samples obtained from various chemical supply houses, were purified by recrystallization from cyclohexane : ether (2:l) at - 15 "C. The pure peroxide was a white crystalline solid, mp 105-107 "C; peroxide oxygen: calcd. 6.60%, found 6.59z. Its purity was >99.5%* DILAUROYL PEROXIDE (XX). A commercial sample, purity 96.5 %, was recrystallized from diethyl ether at - 10 "C. The pure peroxide was a white crystalline solid, mp 55-56 "C; peroxide oxygen: calcd 8.04%, found 8.07%. Its purity was >99.5 %. DISCUSSION
Hydroperoxides. Table I summarizes the significant portions of the NMR spectra of hydroperoxides. In two cases, the NMR spectra of the nonperoxidic analogs, the corresponding alcohols, were also determined and compared with those of the hydroperoxides. The hydroperoxy proton appears as a broad singlet considerably downfield from tetramethylsilane (TMS) as an internal standard. The chemical shift is concentration dependent, as Fujiwara, Katayama, and Kamio also showed (1); at 4-t-BUTYLPEROXY-2,6-DI-t-BUM-4-METHYLCYCLOHEXA-2,5- approximately 1 M in chloroform (or deuteriochloroform) the DIEN-~-ONE (XIII). This was prepared by the procedure hydroperoxy proton gives a signal at about 8 ppm downfield. (5) L. S. Silbert, E. Siegel, and D. Swern, Org. Syn., 43, 93 (1963). The corresponding alcohols at the same concentration have (6) W. E. Parker, C. Ricciuti, C. L. Ogg, and D. Swern, J . Amer. an 0--N signal at about 1.6-2.0 ppm. Although aromatic Chem:Soc., 77, 4037 (1955). (7) T. M. Luong, Dr. Sc. Thesis, University of Paris, Paris, France, (9) T. W. Campbell and G. M. Coppinger, J. Amer. Chem. SOC.,74, 1965.
(8) M. Vilkas, Bull. SOC.Chim. Fr., 1959, 1401.
1469 (1952). (10) L. S. Silbert and D. Swern, ANAL.CHEM., 30, 385 (1958).
VOL. 41, NO. 3, MARCH 1969
413
protons absorb in the same downfield region as hydroperoxides do, there is no problem in identifying the 0-0--H signal as the hydroperoxy proton readily undergoes deuterium exchange. Methyl protons p to the hydroperoxy function are unaffected by it and show a typical sharp singlet at the same position (+0.04 Hz) as that of the nonperoxidic analogs. Table I1 shows the concentration dependence of the ~
Table I. NMR Spectra of Hydroperoxides, ROOH’ Proton NonConcn, chemical shifta peroxidic M -0-0--H p-CH, analogc Hydroperoxide r-Butyl hydroperoxide*(I) 1.0 7.97 1.25 (1.0) (1.62) (1.23) ?-Butyl alcohol*) 2,5-Dimethyl-2,5dihydroperoxyhexane*(II) 0.62 8.12 1.20 Cumene hydroperoxide(II1) 7.87 1.55 4-Hydroperoxy-2,6-di-t7.58 1.40 butyl-4-methylcyclohexa(2.01) (1.42) (4-Hydroxy analog *) 2,5-dien-1-one* (IV) 1-Hydroxy-1’-hydroperoxy- 0.41 9.16d dicyclohexyl peroxide(V) a In CCl, solution; * indicates CDC13. * In ppm (tetramethylsilane = 0). c Nonperoxidic analogs and their chemical shifts shown in parentheses. d See text. Table 11. Concentration Dependence of 0-0--H Signal of t-Butyl Hydroperoxide in CHCl t-Bu OOH -0-OH Concn M Signal, ppm 1.51 8.15 1.00 7.97 0.50 7.78 0.01 7.33 0.00 7.1-7.2 Table 111. NMR Spectra of Organic Peroxy Acids, RCOgH‘ Proton chemical shift* NonConcn, a-C-H2 peroxidic Peroxy acid M 0-0--H (orC-H) analogc Peroxybenzoic(V1) 11.75 (13.15) (Benzoic acid) Peroxypelargonic(VI1) 0.67 11.20 2.40 (0.67) (11.84) (2.35) (Pelargonic acid) 0.50 10.90 2.40 Peroxylauric(VII1) (0.50) (11.84) (2.34) (Lauric acid) 11.40* 2.30* Cyclohexaneperoxycarboxylic(1X) (11.54)* (2.40)* (Cyclohexanecarboxylic acid) In CC1, solution; *indicates CDCI,. * In ppm (tetramethylsilane = 0). c Nonperoxidic analogs and their chemical shifts shown in parentheses. Q
Table IV. Concentration Dependence of Peroxy Acid and Carboxylic Acid Proton Signals Peroxypelargonic Pelargonic Concn, M acida acida 0.67 11.20 11.84 0.067 11.370 10.70h Signal of acid proton in ppm (TMS = 0). CC14 solution. * These values are approximate owing to the broadness of the peaks at high dilution. Q
414
ANALYTICAL CHEMISTRY
0-0--H signal of t-butyl hydroperoxide in chloroform. The signal shifts upfield from 8.15 at 1.51M to 7.33 ppm at 0.01M; at infinite dilution the signal is at 7.1-7.2 ppm (extrapolation from 0.01M). The concentration dependence is presumed to result from a decrease in intermolecular hydrogen bonding with dilution ( I / , 12). In comparison with alcohols, hydroperoxides are not extensively hydrogen bonded. The large downfield shift of the signal of the hydroperoxy proton, compared with the hydroxy proton of alcohols, cannot therefore be attributed to an unusual hydrogen bonding effect but is a reflection of the large deshielding effect of the peroxy group. The mechanism by which this deshielding effect operates is not clear. 1-Hydroxy-1 ’-hydroperoxydicyclohexyl peroxide (V, Table I) was of interest as it can undergo various kinds of inter- and intramolecular hydrogen bonding. Unfortunately, it is in facile equilibrium with other species and it decomposes rapidly, especially in solution, so it was not possible to study the concentration dependence of the 0-0--H and 0--H protons. For example, cyclohexanone was identified by IR and GLPC in dilute solutions of the peroxide used for NMR studies. It is tentatively assumed that the signal at 9.16 ppm is derived from the hydroperoxy proton which is deshielded even more than usual because of its proximity to the peroxide and hydroxyl groups. The assignment, however, is tentative.
U
V
U
Peroxy Acids. Table I11 summarizes the significant portions of the NMR spectra of organic peroxy acids and their nonperoxidic analogs, the carboxylic acids. The acidic proton of peroxy acids appears as a broad singlet at 10.90-11.75 ppm (TMS=O), always upfield relative to the signal of the carboxylic acid proton at high concentrations (above 0.5M). The reason for the upfield shift is not clear and is in contrast to the chemical shifts of hydroperoxides and alcohols (Table I). A possible explanation is that the intramolecular hydrogen bonding and skewed structure universally found in peroxy acids (13) constrains the acidic proton of peroxy acids to the zone of positive shielding of the carbonyl group. The effect of dilution on the chemical shift of the peroxy acid and carboxylic acid protons was investigated (Table IV). As the Table shows, the signal of the peroxy acid proton shows little or no shift with dilution, a result consistent with the intramolecular structure of peroxy acids (13), whereas that of pelargonic acid moves substantially upfield. The upfield shift of the acidic proton of pelargonic acid upon dilution in carbon tetrachloride is in itself interesting in view of the failure of acetic acid to show the same effect (14). Acetic acid remains largely associated as the dimer in carbon tetrachloride but dissociates to the monomer in more polar solvents, such as acetone, in which an upfield shift of the proton signal is then observed. The signal of the a-methylene protons of aliphatic peroxycarboxylic acids is a typical triplet (6 = 2.40 ppm) which may (11) L. M. Jackman, “Applications of Nuclear Magnetic Resonance Spectroscopy In Organic Chemistry,” Pergamon, New York, N.Y., 1959. (12) J. C. Davis, Jr., K . S . Pitzer, and C. N. R. Rao, J. Phys. Chem., 64, 1744 (1960). (13) D. Swern and L. S. Silbert, ANAL.CHEM.,35, 880 (1963). (14) C. M. Huggins, G. C. Pimentel, and J. N. Shoolery, J. Phys. Chem., 60, 1311 (1956).
Peroxide
Table V. NMR Spectra of Dialkyl Peroxides, ROOR“ Proton chemical shiftb CH, -0-0-C-CH,
I I
-0-O-C-CH3
I
Di-t-butyl peroxide(X1) 2,5-Dimethyl-2,5-di(t-butylperoxy)hexane(XII)
CH,
1.17
(1.18)* 1.17
Nonperoxidic analogc
-
(1.12)* 1.15
(&Butylisopropyl ether)
4-r-Butylperoxy-2,6-di-t-butyl-4-
methylcyclohexa-2,5-dien-l-one(XIII) 1.19* Dicumyl peroxide(X1V) Ascaridole(XV) In CCI, solution; * indicates CDCl,. b In ppm (tetramethylsilane = 0). c Nonperoxidic analog and chemical shifts shown in parentheses.
be slightly deshielded (-0.05 Hz) in comparison with the analogous protons of carboxylic acids. The signals of the remaining aliphatic protons in peroxy acids are identical with those of their nonperoxidic analogs (15). In cyclohexaneperoxycarboxylic acid, the proton CY to the peroxy function is slightly shielded (+O.l Hz), compared with that in cyclohexanecarboxylic acid. The only NMR solvent suitable for use with p-nitroperoxybenzoic acid (X) was hexadeuterioacetone; unfortunately the peroxycarboxylic acid proton exchanged with deuterium of the solvent. Dialkyl Peroxides. Table V summarizes the significant portions of the NMR spectra of dialkyl peroxides and that of one nonperoxidic analog, t-butyl isopropyl ether. This group of compounds was notable for the lack of any unusual influence due to the peroxide group. The NMR spectrum of di-t-butyl peroxide (XI), for example, exhibits a single peak (sharp singlet at 1.17 ppm) at the same chemical shift as that of the t-butyl group of t-butyl iso-propyl ether. The NMR spectrum of 2,5-dimethyl-2,5-di(t-butylperoxy) hexane (XII) shows three sharp singlets at 1.17 (18H), 1.15 (12H), and 1.52 (4H) ppm, assigned to the protons of the two t-butyl groups, the remaining four methyl groups and the two methylene groups, respectively. Compound XIII, the peroxydienone, shows four singlets assigned as follows: 1.19 (-00-Bu-t), 1.24 (-C-Bu-t), 1.32 (-CH3 at 4-position), and 6.55 ppm (vinyl protons). In dicumyl peroxide (XIV), the methyl groups appear as a sharp singlet at 1.53 ppm, slightly downfield from the methyl group signals in the nonaromatic compounds studied. This shift is attributed to deshielding by the aromatic rings. The NMR spectrum of ascaridole (XV) was complex, but consistent with the known structure. The main features were (a) the singlet (3 H) of the methyl group attached to the peroxy carbon (1.31 ppm), (b) the doublet (6 H) of the methyls of the isopropyl group (1.08 and 0.95 ppm), (c) the vinyl double doublet (2 H) (centered at 6.4 ppm), and (d) a complex multiplet (5 H) centered at 1.78 ppm. The peroxy group may be exerting a small long range deshielding effect on the vinyl protons, as a nonperoxidic analog [2.2.2] bicyclooctene also shows a double doublet centered at 6.26 ppm. The double doublet of the vinyl group of ascaridole has coupling constants of J c=c 8 and J long range 1 Hz.
A\H
The NMR spectrum of a 10% solution of 1,l’-dihydroxy(15) C.Y.Hopkins “Progress in the Chemistry of Fats and Other Lipids,” R. T. Holman, Ed., Vol. 8, Pergamon, New York, N.Y., 1966.
1.32* 1.53 1.31
Table VI. Chemical Shifts of Acidic Protons and pKQValues Chemical shifts, ppm Compound (TMS = O)Q pKa0 Carboxylic acids (RC0,H) 11-13.5~ 3-6 Peroxycarboxylic acids (RC0,H) 10.9-1 1.8 7-8 7.6-9.2. 11.6-12.8 Hydroperoxides (ROOH) Alcohols (ROH) 0.5-5.5~ -16 a Chemical shift values are given for approximately 10% solutions in CC14 or CDC1,. 0 Typical values. c Upfield shift on dilution, especially in polar solvents which encourage dissociation of dimers to monomers. The chemical shifts of alcohols are especially sensitive to dilution. cyclohexyl peroxide (XVI) in deuteriochloroform shows two multiplets at 1.8 (8 H) and 2.35 ppm (12 H), assigned to the ring protons. N o clearly defined hydroxyl signal could be detected. A weak, broad singlet at 9.78 ppm is tentatively assigned to the hydroxyl protons on the assumption that they should be extensively deshielded, but the assignment is uncertain. t-Butyl Peroxyesters. In the two pure t-butyl peroxyesters studied, t-butyl peroxyacetate (XVII) and benzoate (XVIII), there appears to be a small upfield shift of the signals of the t-butyl protons to 1.28 and 1.36 ppm, respectively. The t-butyl signal of t-butyl acetate is at 1.95 ppm. (In a slightly impure sample of t-butyl peroxy-2-ethylhexanoate,the t-butyl signal is at 1.30 ppm). The methyl protons of the acetate and peroxyacetate groups are at the same position (2 i 0.02 ppm). Diacyl Peroxides. There are no special features to the NMR spectra of dibenzoyl (XIX) and dilauroyl peroxides (XX) ;their NMR spectra are essentially identical with those of related nonperoxidic analogs. Also, the a-methylene protons of dilauroyl peroxide give the same signal and chemical shift as those in peroxylauric acid (VIII). General Comments. The organic peroxides studied can be divided into two groups, those with a proton attached to the peroxy group (hydroperoxides and peroxy acids) and those without (dialkyl peroxides, peroxy esters, and diacyl peroxides). The chemical shifts of the acidic protons and the pKa values of typical carboxylic acids, peroxycarboxylic acids, hydroperoxides, and alcohols are shown in Table VI. The pKa values were obtained from the literature (16, 17). Chemical shifts of carboxylic acids and alcohols were taken mainly from (16) A. J. Everett and G. J. Minkoff, Trans. Faraday SOC.,49, 410 (1953). (17) P. Sykes, “A Guidebook To Mechanism In Organic Chemistry,” 2nd ed., Wiley, New York, N.Y., 1965, pp 38-48. VOL. 41, NO. 3, MARCH 1969
415
the literature (11, 12, 14, 18) but also from the present study. With increasing acidity, the chemical shift of the proton moves downfield. Trifluoroacetic acid, for example, in dimethyl sulfoxide shows a signal at about 15 ppm but the proton signal of most carboxylic acids in nonpolar solvents is within the range shown in Table VI. It is thus possible to distinguish peroxy acids from hydroperoxides by NMR, not only from the differences in their chemical shift values but also by studying the effect of dilution. Peroxycarboxylic acids show no change on dilution whereas hydroperoxides do. In peroxides without acidic protons, chemical shifts are generally not significantly different from those of their nonperoxidic analogs. Thus, although NMR cannot be used to (18) J. R. Dyer, “Applications of Absorption Spectroscopy of Organic Compounds,” Prentice-Hall, Englewood, N.J., 1965, pp 58-132.
distinguish between these peroxides and analogous nonperoxides, the NMR spectra of pure peroxides are indeed useful in their identification. ACKNOWLEDGMENT
The authors thank Dr. Joseph DiPaolo of the National Cancer Institute for encouragement and suggestions, and R. Rodebaugh and T. M. Santosusso for determination of some of the NMR spectra. T. M. Luong thanks Dr. C. Paquot of the Laboratoire de Lipochemie, C.N.R.S., Thiais, France for the opportunity to conduct postdoctoral work at Temple University. RECEIVED for review November 5, 1968. Accepted January 9, 1969. The authors are grateful to the U.S. Public Health Service for partial support of this work under Grant Contract PH 43-66-68 of the National Cancer Institute.
Isotopic Determination of Calcium-48 by Deuteron Activation Tielman J. de Waal,’ Max Peisach, and Ren6 Pretorius2 Southern Universities Nuclear Institute, P.O. Box 17, Faure, C.P.,South Africa The biological use of the stable isotope calcium-48 for tracing required the determination of isotopic concentrations in calcium samples. The separated calcium was converted to fluoride and distilled in vacuum onto tantalum disks. After activation with 5-MeV deuterons the activities of 8.8-min 49Ca gave a measure of the calcium-48 content and the 4-hour activities of scandium-43 and -44 a measure of the natural calcium content. By using the W a and 43Caof natural calcium as internal standard, the isotopic concentration of %a was determined without the need for measuring fluxes, yields, or sample weights. Milligram quantities of calcium fluoride were analyzed with a relative standard deviation of ~k2.075.
THEBIOLOGICAL half-life of calcium ( I ) is so long, 1.6 X 104 days, that the use of the radiocalcium tracers, 46Caor 47Ca,is frowned upon by the medical profession for studying calcium metabolism in humans. This is especially true when healthy patients such as young children or pregnant women are involved, because the effect of long-term exposure to even relatively small amounts of radiation is not yet fully understood. The availability of stable isotopes of calcium has made it possible to carry out such investigations without exposing the patient to any radiation hazard. A calcium preparation enriched in a selected isotope is administered and thereafter the isotopic concentration in samples drawn from the patient has to be determined. The following conditions (2, 3) will determine whether a 1 Formerly from the Department of Chemistry, University of Stellenbosch, C.P., South Africa. Present address, South African Titan Products (Pty) Ltd., Umbogintwini, Natal 2 On leave at Activation Analysis Research Laboratory, Texas A & M University, College Station, Texas 77843
(1) Health Phys., 3, 161 (1960). (2) M. Peisach and R. Pretorius, Proc. Symp. Med. Electron. Nucl. Instr., Johannesburg, 1965, Leech 37, 11 (1967). (3) W. F. Bethard, R. A. Schmitt, D. A. Olehy, S. A. Kaplan, S. M. Ling, R. H. Smith, and E. Dalle Molle, “Nuclear Activation Techniques in the Life Sciences,” I.A.E.A., Vienna, 1967, p 533. 416
ANALYTICAL CHEMISTRY
stable isotopic tracer can usefully be applied: The isotope should have a relatively low abundance in nature; it should be available in adequate enrichment; it should be reasonably priced; and a method should be available whereby its isotopic concentration could be determined at the dilution pertaining to the experiment. The heavier isotopes of calcium occur in nature in such low isotopic concentrations that each can serve as a stable isotopic tracer. The extent to which each meets the first three of the above requirements may be judged from Table I. It is clear that calcium-46 has a far higher enrichment factor than the other isotopes and thus has a wider potential use. However, although calcium-43 and -48 have lower enrichment factors than that of calcium-46, they can nevertheless be diluted over a considerable range and their much lower cost makes their use as tracer more practicable. Calcium-43 cannot be activated with thermal neutrons because neutron capture leads to a stable product. Neutron activation has, however, been used for the determination of calcium-48 (4, 5 ) even though the method was not applicable to the isotopic determination of calcium-48 unless the total calcium content was known or separately determined. This disadvantage has been overcome by using proton activation (6) or by measuring the prompt neutrons emitted during proton irradiation (7) when the isotopic concentration of calcium48 was determined directly, by expressing the count obtained from this isotope relative to that obtained from some other stable calcium isotope representing the total natural calcium in the same sample. By using an internal standard in this way, there are further advantages because there is then no need to know the irradiation flux, the reaction yield, or the sample weight. ~~
(4) E. Junod and J. Laverlochere, “Proc. 3rd Intern. Colloquium on Biology,” Saclay, 1963. (5) F. W. E. Strelow and H. Staerk, ANAL. CHEM., 35, 1154 (1963). (6) M. Peisach and R. Pretorius, ibid., 38, 965 (1966). (7) W. R. McMurray, M. Peisach, R. Pretorius, P. van der Merwe, and I. J. van Heerden, ibid., 40,266 (1968).
