Structure of Amines by Nuclear Magnetic Resonance Spectrometry SIR: Nuclear magnetic resonance (NhIR) spectrometry is very useful in elucidation of structure of amines. A proton on a nitrogen atom may undergo rapid, intermediate, or slow exchange. If the exchange is rapid, the N H proton(s) is decoupled from the N atom and from protons on adjacent carbon atoms. The K H peak is therefore a sharp singlet, and the adjacent C H protons are not split by X H . Such is the case for most aliphatic amines. At an intermediate rate of exchange, the IiH proton is partially decoupled, and a broad NH peak results. The C H protons are not split by the NH. Such is the case for N methyl-p-nitroaniline ( 2 ) . If the X H exchange rate is slow, the X H peak is still broad because the electrical quadrupole moment of the nitrogen nucleus induces a moderately efficient spin relaxation and thus an intermediate lifetime for the spin states of the nitrogen nucleus. The proton thus sees three spin states of the nitrogen nucleus (spin number = 1) which are changing a t a moderate rate, and the proton responds by giving a broad peak. I n this case, coupling to the adjacent protons may be observed. Such is the case for 4-chloroindole ( I O ) , 4-(p-carbethoxyanilino) butan-2-one ( I ) , and 4( p - carbethoxyanilino) - 3 - buten - 2one (1). Only limited information can be obtained from a n NMR spectrum of aliphatic amines. Since there is no coupling, the number of protons on the nitrogen atom cannot be determined by evamination of the adjacent protons, nor can it be obtained by integration of the S H peak which is usually buried under the absorption of the aliphatic hydrogen atoms when the spectrum is run in carbon tetrachloride or deuteriochloroform (11). Tertiary amines, of course, show no N I I peak. The adjacent protons-Le., those of the acarbon(s)-are usually shifted downfield far enough so t h a t they can be integrated, and their splitting by the protons on the p-carbon(s) can be ascertained. Thus, the number of protons on the a- and p-carbons can be determined if the proton:, on the a-carbons can he recognized. The problems in amines are similar to, but more complex than, those encountered in the spectrum of alcohols. The hydroxylic proton can be made t o couple in deuteriochloroform or carbon tetrachloride by removing the traces of
acids usually present in these solvents (3). However, in these solvents, the hydroxylic proton peak is frequently buried under the upfield absorption. The utility of acetone, or dimethyl sulfoxide in particular, for permitting coupling of the hydroxylic proton and shifting the hydroxylic proton absorption downfield was pointed out by hlcGreer and Mocek (7), and applied to the classification of a wide variety of alcohols by Chapman and King (4). Aliphatic amines do not show coupling in dimethyl sulfoxide. However, we have observed that amine hydrochloride
and trifluoroacetate salts in either deuteriochloroform or dimethyl sulfoxide (6) gave a broad N+H peak downfield; furthermore, the protons on the a-carbon were coupled to the N + H protons ( 8 ) . Unfortunately, deuteriochloroform is not a good solvent for amine salts. Dimethyl sulfoxide proved to be only a fair solvent for most of the amine salts we studied and a poor solvent for others; in addition, its proton absorption obliterated the region of interest. Even the protons in commercially available deuteriated dimethyl sulfoxide interfered seriously with ob-
ji
L
r6.6
T7.0
I
II
r 7.I
m
H 2 0 CPS
H 20cps
T 5.6
IE
H 2Ocpr
H 20cps
~ 6 . 5
ma
H 20 cps Figure 1 .
H tocpr
IUb
k-4 2ocps
Splitting of protons for some primary, secondary, and tertiary amine
salts I CsH6N'H
ez
I1 RCHzCH(Me)N 'H Me2 RCHzCHlMe)N+HzMe -. CsHsC_HzN'Hs (RCHzCH2)zN 'Hz RCH -~~H +H ~N ~ a. N 'Ha irradiated b. P - C K irradiated The protons under observation are underlined.
111 IV V VI
VOL. 37, N O . 1 1 , OCTOBER 1965
1417
servation of coupling of the proton on the a-carbon of aliphatic amines. Direct addition of excess trifluoroacetic acid to the amine under study turned out to be a simple, satisfactory procedure for preparing a solution of amine salt. Thus, the N M R spectrum of an amine can be run in a suitable solvent (carbon tetrachloride or deuteriochloroform), the solvent evaporated with a stream of nitrogen, and trifluoroacetic acid added directly to the residue in the N M R tube. If the trifluoroacetic acid is added all at once, the heat of salt formation is controlled by dilution. Examination of the amine salt spectra yielded useful information. The broad absorption of the protons on the nitrogen atom was found downfield in the aromatic region, and the number of protons could be obtained by integrating the spectrum. This permitted classification of the amine as primary, secondary, or tertiary. The protons on the acarbon could be recognized by the downfield shift (12) from their position in the free amine spectrum, and by their additional splitting. Splitting of the protons on the a-carbon(s) of amine salts is a function of the number of protons on the p-carbon(s) and those on the nitrogen atom. In the compounds examined, the J values were very similar 1 rule was applicable. and the S The splittings are shown in Figure 1 for some primary, secondary, and tertiary amine salts. Spin decoupling can be used very effectively in amine salts. By de-
+
coupling the N + H protons from the nitrogen atom (irradiating the nitrogen atom), splitting by the protons on the a-carbon(s) would become apparent. This would give an additional check on the number of a-protons in the event they are buried under other absorption. Decoupling of the a-protons by irradiating the N + H protons gives a simplified a-proton spectrum : this provides additional confirmation for the identity of the a-proton peaks, and gives the number of 0-protons. The a-proton splitting can also be simplified by irradiating the 0-protons; in simple cases, the resulting a-proton pattern will depend on the number of N + H protons thus providing another parameter for classification of amines. Spin decoupling was carried out on a primary amine salt (Figure 1, VIa and VIb) to illustrate its utility in these situations. Thus, irradiation of the N +H3 protons collapsed the a-proton sextet to a triplet (VIa), and irradiation of the 0-protons resulted in a quartet (VIb). Of course, complications are encountered as the number of a- and 0-carbons increases. I n some cases, double decoupling experiments may be useful in simplifying complex splitting. It is of interest to note that the geminal methyl groups in compound I1 are nonequivalent because of asymmetry a t the a-carbon. Reynolds and Schaefer (9) pointed out that methylene proton nonequivalence is observed in protonated benzylamines which lack a plane of symmetry along the CsH5CH2h- bond axis. Thus, the methylene
protons of protonated N-methyl-dibenzylamine shows the A B part of a n ABX s p e c t r u m 4 . e , eight peaks. Following submission of this paper, a paper appeared (6) in which the shift of N methyl groups in perdeuterioacetic acid and in trifluoroacetic acid was used for detection of N-methyl groups. A large number of shift values and J-values were given. LITERATURE CITED
(1) Acton, E. M.,, Stanford Research
Institute, unpublished work.
(2) Bhacca, N. S., Hollis, D. P., Johnson,
L. F., Pier, E. 4;, “High Resolution NMR Catalogue, Vol. 2, spectrum 489. Varian Associates, Palo Alto, Calif., 1963. ( 3 ) Bruce, J. M., Knowles, P., Proc. Chem.
SOC.1964, 294. (4) Chapman, 0. L., King, R. W., J . Am. Chem. SOC.86, 1256 (1964). ( 5 ) Freifelder, M., Matoon, R. W., Ng, Y. H., J . Org. Chem. 29, 3720 (1964’1. (6) Ma, J. C. N., Warnhoff, E. W., Can. J . Chem. 43, 1849 (1965). (7) . , McGreer. D. E.. Mocek. M. M., J . Chem. Educ. 40, 358 (1963). (8) Reynolds, W. F., Schaefer, T., Can. J . Chem. 41. 2339 f 1963’1. (9, Ibid., 4i,2119(1964). (10) Silverstein, R. RI., Bassler, G. C., “Spectrometric Identification of Organic Compounds,” p. 145, Wiley, New York. 1963. (11) Ibid., p. 137. (12) Sudmeier, J. L., Reilley, C. N., ANAL.CHEM.36, 1698 (1964). \ - - - - ,
WILLIAM R. ANDERSON, JR. ROBERTM.SILVERSTEIN Stanford Research Institute Menlo Park, Calif.
Coulometric Titration of Hydrogen Peroxide with Electrogenerated Iodine SIR: Coulometrically generated cerium(1V) has been used to titrate 50 peq. of hydrogen peroxide with an error of 2y0 (10, 1 1 ) and generated manganese(II1) has been employed in the titration of 0.7 to 5.5 mg. of hydrogen peroxide with average errors of 0.3 to 0.4% (12). The present paper describes the coulometric determination of hydrogen peroxide by iodometry. Following reaction with iodide in the presence of a molybdenum catalyst, excess thiosulfate is added and the excess is determined by titrating with electrolytically generated iodine. The thiosulfate titer is determined coulometrically under the same conditions. A pH range of 0 to 7 . 5 has been investigated. The method has been applied to the determination of 2.6 mg. (150 peq.) to 0.9 pg. (0.05 peq.) of hydrogen peroxide with an average relative error ranging from less than 0.1 to 4%. 1418
ANALYTICAL CHEMISTRY
Swift and coworkers (8, 9) and Bard and Lingane (1) have described coulometric iodometry, and pardue (6) combined precision null-point potentiometry with electrolytic generation of iodine for microrange iodometry. They determined 36 to 3.6 pg. of hydrogen peroxide over a p H range of 2 to 4.5 with an error of 0.05 pg. at all concentrations. EXPERIMENTAL
Reagent grade chemicals were used without further purification. All solutions were prepared using freshly boiled, distilled, and deionized water. Solutions of 0.05N hydrogen peroxide were standardized by titrating a 10-ml. aliquot iodometrically with standard sodium thiosulfate (5). A 10-ml. buret was used for hydrogen peroxide titrations. The sodium thiosulfate was standardized coulometrically at p H 3.0 (9)*
The generating solution consisted of
0.1,M potassium iodide which was made O.OO1lvin sodium carbonate to decrease air oxidation of the iodide. The phosphate buffers employed (ionic strength = 0.2M) were prepared as previously described (4). All pipets and burets were calibrated. Micropipets were made from polyethylene as described by Mattenheimer ( 7 ) and they were calibrated coulometrically (3)with precision of 0.2y0or better. Coulometric titrations with generating currents of 4.825 ma. or -greater were made with a Sargent coulometric current source, Model IV. For currents less than these, a ChrisFeld Microcoulometric Quantalyzer, Model 6 (ChrisFeld Precision Instruments Corp., Beltsville, Md.) was used. The generating anode and cathode were platinum foils of 1.5 sq. cm. and 0.8 sq. cm., respectively. The cathode compartment was a glass tube fitted with a sintered glass frit end. A 3%
