Application of Nuclear Magnetic Resonance to Quantitative Analysis of Mixtures of Organic Peroxides, Hydroperoxides, and Alcohols George A. W a r d and Robert D. Mair Research Department, Hercules Incorporated, Wilrnington, Del. 19899
ALTHOUGH a variety of chemical and instrumental methods has been applied to the analysis of organic peroxides, hydroperoxides, and alcohols, the complete analysis of mixtures of structurally related peroxy compounds and alcohols is often a difficult problem ( I ) . Nuclear magnetic resonance (NMR) spectral data for a few alkyl peroxides and hydroperoxides have been reported in the literature (2, 3), and N M R has been used in the qualitative identification of unknown peroxy compounds ( 4 , 5), but thus far no study of the application of N M R to the quantitative analysis of peroxy compound mixtures has been reported. N M R has some distinct advantages in such applications for it is rapid, nondestructive, and applicable t o a wide variety of compounds. In addition, it does not require the preparation of pure calibration standards for quantitative purposes, often a problem with unstable compounds in other instrumental techniques. This paper describes a brief study of the application of N M R to the quantitative analysis of peroxy compounds. Spectral data for a number of compounds that have not previously been reported are given, and examples of the quantitative aspects of the method are included to point out its advantages and limitations.
Figure 1. NMR spectrum of methyl protons of (left to right) a,a-dimethylbenzyl peroxide, hydroperoxide, and alcohol in acetone
EXPERIMENTAL
Apparatus. Spectra were run at ambient instrument temperature, 46 “C., using a Varian A-60A N M R spectrometer. Tetramethylsilane was used as an internal reference in all chemical shift measurements. Reagents. Solvents used were commercially available reagent grade materials. Peroxy compounds and alcohols were samples prepared at the Hercules Research Center and characterized by microchemical analysis and IR and N M R spectra. Assay for peroxide and hydroperoxide content was carried out by procedures described by Mair and Graupner (6). RESULTS AND DISCUSSION
Spectral Data. The area and chemical shift of N M R peaks for labile -0OH and -OH protons vary with the sample concentration and solvent composition, due to the effects of solvation, hydrogen bonding, and chemical exchange. For this reason the N M R peaks of these protons are not suitable for quantitative analysis. In the present work, the nonexchanging protons in positions a or p to the -OOR, -OOH, (1) R. D. Mair and R. T. Hall in “Treatise on Analytical Chemistry,” Part 11, Vol. 14, I. M. Kolthoff and P. J. Elving, Eds., in Press.
(2) S. Fujiwara, M. Katayama, and S. Kamio, Bull. Chem. SOC. Jap., 32, 657 (1959). (3) A. G. Davies, D. G. Hare, and R. F. M. White, J . Chem. SOC., 1960, 1040. (4) A. G. Davies, D. G. Hare, and R. F. M. White, ibid., 1961, 341, ( 5 ) W. F. Brill, J . Amer. Chem. SOC., 87, 3286 (1965). (6) R. D. Mair and A. J. Graupner, ANAL.CHEM., 36, 194 (1964).
538
ANALYTICAL CHEMISTRY
and -OH groups were studied. These proton groups have different chemical shifts in related compounds, depending on the variation of the magnetic environment of the proton with the stereochemistry and inductive properties of the neighboring functional groups. Table I presents a list of the chemical shifts of P-CH protons in some related peroxides, hydroperoxides, and alcohols which have not previously been reported. These data were obtained using approximately 5 % solutions of the various compounds in CDC13, and are subject t o some variation with both concentration and nature of the solvent. As an example of the effect of solvent, Table I1 gives chemical shift data for mixtures of related peroxides, hydroperoxides, and alcohols in several solvents. One would expect the chemical shift differences between protons a to the -OOH, -OOR, and -OH groups t o be significantly greater than those given in Tables I and I1 for p protons, for the effect of functional groups on the electronic environment of neighboring protons is rapidly attenuated as the number of bonds separating the group increases. Fujiwara, Katayama, and Kamio reported the chemical shifts of the a protons in several alkyl hydroperoxides (2). Their data were obtained with an N M R spectrometer operating at 27.03 MHz, and a tabulation of their results, converted to the shifts expected for a conventional 60 MHz spectrometer, is given in Table 111. The chemical shifts of the a-protons in the corresponding alcohols, taken from A.P.I. reference spectra, are also given. The chemical shifts given in this table refer to the separation, in Hz, of the a-proton peak and the methyl proton peak in the same compound for no internal reference was used
+ 3.3 H r
Table I. Chemical Shifts of Protons 0 to the -OOR, -OOH, and -OH Groups All compounds run as dilute solutions in CDCl, v(Chemica1 shift of indicated orotons in Hz vs. TMS) ROH RROOR ROOH -75.2 (CH313C-73.0 -76.3 ~
-91.8
~
- 87.8
-92.0
-
-90.0
-88.3
-91.4
-90.0
-92.2
-91.4
CH3
-91.4 CH 3
Table 11. Effect of Solvent on 0-Proton Chemical Shift v,Hz RSolvent ROOR ROOH ROH (CH3)3CCDC13 -73.0 -76.3 -75.2 DMSO -70.3 -69.5 -68.3 Pyridine -74.2 -81.0 -82.2
Figure 2. NMR spectrum of (left to right) t-butyl hydroperoxide, alcohol, and peroxide in CDCI, Table 111. Chemical Shift of a-Protons in Hydroperoxides and Alcohols v, Hz R ROOH ROH CH3-CHZ-CHZ-176 -152 CH3-(CHz)Z-CHZ-176 -155 -176 CH3(CH2).5CH2-155 CHdcH2hCHzCH3-CH-CH2-169 - 146 CH3
c_H3
CDCl3 DMSO Pyridine Acetone
-91.4 -89.4 -95.5 -92.1
-92.2 -91.4 -91.5 -91.5 - 102.4 -102.4 -91.3 -89.7
by Fujiwara, Katayama, and Kamio. Confirmation of the ~ 2 Hz 0 chemical shift difference between the a-protons of related hydroperoxides and alcohols was obtained in our laboratory by synthesizing benzyl hydroperoxide from benzyl alcohol. The a - C H 2 group in the hydroperoxide was found to give a sharp peak at - 290 H z in CC14, compared to - 271.5 for the alcohol. From these data it is apparent that using a spectrometer such as the Varian A-60A, or equivalent, with a resolution of 0.5 H z or better, one would have no difficulty in resolving a-proton peaks for hydroperoxides and alcohols. Even though the a-proton peaks in some of these compounds are split into multiplets by coupling with the @-protons,the groups should be well resolved, for the vicinal coupling constants are in the 6-9 Hz range. For the @-protons,however, the data given in Tables I and I1 show separations of only 1 t o 4 Hz, and resolution of the peaks is more difficult. Figure 1 shows the methyl group spectrum of a mixture of a,a-dimethylbenzyl peroxide, hydroperoxide, and alcohol in acetone. In this spectrum the separation between the peroxide and hydroperoxide peaks is approximately 0.9 Hz, and that between the hydroperoxide and
Table IV. Quantitative Analysis of Mixtures of t-Butyl Alcohol and t-Butyl Hydroperoxide in Dioxane Theoretical "/o alcohol 25.0
50.0
:4 Found 24.3 25.6 av = 25.2 24.7 25.8 s = 0.6 25.4 50.7 50.8 av = 50.8 51.0
75.0
90.0
50.8 s 50.8 71.7 71.5 av 73.2 72.7 s 73.0 88.6 87.4 av 87.8 88.3 s 88.9
=
0.1
=
72.4
=
0.7
=
88.2
=
0.6
alcohol peaks is 1.4 Hz. With the line width at half-height for the peaks ( v l l 2 ) of approximately 0.5 Hz, the peaks are not completely resolved, although the peak heights and areas are easily measured graphically. VOL. 41, NO. 3, MARCH 1969
539
In Figure 2 , the spectrum of a mixture of tert-butyl hydroperoxide, alcohol, and peroxide is shown. Again, the resolution is adequate for qualitative identification of the components and quantitative measurement of the peak height or area. Quantitative Measurements. As a n example of a typical quantitative application of the method, a n experiment was carried out to determine the accuracy and precision obtainable in quantitative measurements on the closely spaced N M R peaks for @-protonsin peroxide systems. As a model system, mixtures of tert-butyl hydroperoxide and tert-butyl alcohol were prepared in dioxane, in which the methyl peaks are separated by approximately 0.9 Hz. I n this system, the resolution is similar to that obtained in Figure 1 and overlap of the peaks prevents the use of electronic integration as a measure of the peak areas. Although the peak area could be measured by means of a planimeter, it was found that satisfactory quantitative results could be obtained by using the peak height as a measure of the relative concentration of the components. This procedure is not usually recommended for ordinary quantitative N M R work involving multiplets or peaks of varying width but it is often satisfactory when the peaks to be measured are very sharp singlets with a constant line width (7). The results of repetitive analysis of several mixtures are given in Table IV. Quantitative agreement with the known composition of the mixture and precision of the analysis, as shown by these data, are both reasonable for such closely spaced peaks. The authors' experience with other mixtures of similar peroxy compounds has shown that these data are (7) A. Mathias, ibid.,38, 1931 (1966).
CURVE
MOLE 96 I-BuOOH
3.5 8.2 15.7
Figure 3. NMR spectrum of mixtures of t-butyl peroxide and hydroperoxide typical for mixtures in which the N M R peaks are separated by 0.8 Hz or more. In such systems a relative error of approximately 3 and a relative standard deviation of less than 3 are typical. The lower limit of detection of a minor component in the presence of a larger component was estimated by preparing mixtures of t-butyl peroxide and t-butyl hydroperoxide in CDC13, in which the methyl peaks are separated by ~3 Hz. Figure 3 shows spectra of samples containing 0, 3.5, 8.2, and 15.7 % of the hydroperoxide. From these results it is apparent that 2-3% of the minor component would give a detectable peak in this system. Based on these results, N M R appears to be a useful tool for the analysis of peroxides, hydroperoxides, and alcohols, mixtures of which are often difficult to analyze by chemical or other instrumental techniques. It should be applicable to a wide variety of compounds, and provides a rapid and quantitatively accurate and precise measurement in a variety of solvents.
x
RECEIVED for review October 11, 1968. Accepted December 11, 1968.
I CORRESPONDENCE Explosion Hazard When Decomposing Organic Matter with Nitrates SIR:The recent note by Bowen ( I ) is of particular interest here, especially in light of an explosion which occurred while a student was ashing a sample of sodium citrate which had been treated with nitric acid. The force of the explosion was sufficient to shatter the refractory lining of a muffle furnace, break the brick furnace door, and fracture the metal supports which held the door closed. The student had treated 50 ml of 1M sodium citrate with 5 ml of concentrated nitric acid and evaporated the mixture t o dryness before placing it in the furnace. The explosion took place somewhere below 500 "C. Because it is common practice to destroy organic matter by igniting with nitrate salts either with or without pretreatment with nitric acid, it was decided to investigate the reactions between various organic materials and salts of nitric acid. Aqueous solutions of the nitrates were mixed with the organic materials and evaporated to dryness on a steam plate before ashing. After drying, the porcelain casseroles containing the samples were placed on a piece of fire brick in the furnace. The brick held the casseroles about 10 cm above the oven floor. The furnace door was removed and a fire brick placed in the opening so as to allow about 5 cm of open space at the top for observation. The charges used were about ' / l o that used in the original explosion (about 1 gram of organic matter
(1) H. J. M. Bowen, ANAL.CHEM., 40, 969 (1968). 540
ANALYTICAL CHEMISTRY
and 8 meq of nitrate salt solution). The ratio of nitrate t o organic matter was kept constant and roughly equivalent to that used in the original explosion. The ratio of nitrate to organic matter used was considerably lower than that recommended by Bowen. The rate of heating was such that the furnace temperature rose from ambient to 500 "C in about 35 minutes. The results are shown in Table I. The data in Table I show that the likelihood of explosion increases in the order of Mgzl, Caz+, Na-, which is also the order of increasing melting points of the of the nitrates of these elements. It is possible that explosions occur at the instant when the nitrate salts liquefy and come in contact with organic materials at high temperature. Salts melting at low temperatures are less likely to produce an explosion. Inasmuch as Bowen used a mixture of sodium and potassium nitrates which melts at lower temperature than either salt alone, he decreased the danger of explosion. The vigor of the reaction is also influenced by the ease of oxidation of the organic matter present. Compounds requiring high temperatures for oxidation are more likely to explode when oxidation finally begins (see acetates). Pretreatment with an excess of nitric acid sometimes lessens the danger of explosion. Of analytical interest is the fact that samples which underwent a flaming ignition vigorous enough for the flames to reach the roof of the furnace appeared no different after ignition than those which ashed more slowly without flames.
