V O L U M E 2 3 , NO. 5, M A Y 1 9 5 1
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O F URANIUM
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P E R C E N T OF URANIUM
Figure 3. Calibration Curie for Establishing Relative Weight Per Cent of Uranium and Lead
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Figure 4.
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G R A M S / LITER
Family of Cranium Calibration Curves for Lead as an Impurity
the ratio of the uranium La peak to the lead La peak is plotted against weight per cent or uranium in Figure 3. These specimens wrre prepared from salt solutions of uranium and lead as usual but only the uranium and lead were considered as contributing t o the weight per cent. Next, a set of standards with equal weights of lead and uranium was prepared for uranium concentrations ranging from 0.1 gram per liter to 1 gram per liter. These standards give the calibration curve in Figure 4,which also shows the uranium calibration curve from Figure 1. Similar curves would result from standards with different relative amounts of lead. However, for less than 10% of lead by weight, no correction of the pure uranium Calibration curve was found necessary.
In practice, when heavy elenierit impurities were suspected, the procedure n-ould be first to determine the impurity element from its x-ray spectrum, then, from a curve similar to Figure 3, determine the ratio of ULa to, say, PbLa. This value would indicate the appropriate curve of the family in Figure 4. The uranium content in grams per liter would then be obtained directly from the counting rate. ilbout 2 minutes would be required to getermine the ratio, once t,he impurity element was known. Reading the data from Figures 3 and 4 would require less than 1 minute. KuLidiuni is typical of elements in class 2. However, because of the different separation in wave length of the cy and 0 lines of the K series as compared to the L series, none of these elements overlaps both the U L a and ULB. Therefore, if overlapping of one uranium L line should occur, the other uranium L line would be used to determine the uranium content Silver was chosen as typical of t h e elements in class 3. The second-order K line from 1 gram per liter of silver was too weak to be detected under the experimental conditions, and it was concluded that no interference was to be expected from such elements. Other elements of common occurrence, such as iron, have no effect on the st,andard 10070 uranium calibration curve of Figure 1. For example, with equal weights of iron added to each of the uranium st,andards, the measured uranium content was within 5% of that for t,he pure uranium standards. The effect of impurities on the x-ray fluorescence analyses should be compared with their effect on x-ray absorption analyses in Bartlett's work ( 1 ) . Bartlet,t found that, in x-ray absorption analysis, the interference from impurities increased uith atomic number because of the increasing absorption coefficient. He prepared 1% solutions of various elements and determined the apparent uranium concentration which each would indicate in the absence of uranium. For example, a 1% solution of calcium indicated 1.5 grams per 1it.er of uranium: a 1% solution of bromine indicated 5.0 grams per liter; a 1% solutim of lead indicated 8.8 grams per liter. With the fluorescence method, no elements in such low concentration would have any effect on the uranium determination. LITERATURE CITED
(1) Bartlett, T.I\'., U. S. Atomic Energy Commission, .4ECD R e p t . 2765, 2766 (19491. (2) Birks, L. S., and Brooks, E. J., . 1 ~ . 4 1 . .CHEX,22, 1017 (1963). (3) Birks, L. S., Brooks. E. J., Friedman, H., and Roe, R. XI., Ibid.. 22, 1258 (1950). (4) Friedman, H., and Birks, L. S.,Rea. Sci. Instruments, 19, 323 (1948). RECEIVED Sovember 14, 1950.
Characterization of Benzene Ring Substitution by Infrared Spectra C. Tv. YOUNG, R . B. DuVALL, A N D NOR1IAN WRIGHT, T h e Dow Chemical Co., .Widland, .Click
F
OR a number of years the authors have been employing infrared spectra for characterizing benzene ring substitution. The spectral interval from 5 to 6 microns is peculiarly useful for this purpose. .4romatic compounds have usually a much more intense and a richer absorption pattern in this region than do other kinds of compounds. Fortunately, application of the method is not limited to a few benzene derivatives, but has almost universal validity for aromatic compounds. The niethod has stood the test of experience m-ith a large number and variety
of compounds. Inasmuch as the certainty of the method is now well established by experience, the authors are bringing it to the attention of chemists generally nho will, they believe, find it of considerable value. INFRARED SPECTRA
Figures 1, 2 , 3 , and 4 are collections of spectral fragments in the *5- to Gmicron interval of compounds which illustrate the method. Three examples of each typical pattern have been chosen, for the
710
ANALYTICAL CHEMISTRY
The general problem of determining the type of substitution in aromatic compounds is greatly simplified by the information obtainable from infrared spectra as described in this paper. The spectra of henzene derivatives exhibit absorption patterns in the 5 - to 6-micron region which are characteristic of the number and location of substituent groups on the benzene ring. These patterns, rather than specific frequencies, offer the key to the interpretation of the substituent configurations. The atomic constitution or chemical functionality of the substituent groups has only a minor influence on the appearanceof the patterns. This new method represents a useful addition to the analytical methods that are available for use by the structural organic chemist.
most part, in order to illustrate t'he essential similarity of typical patterns for a given substihent configuration. The spectra were obtained on a Baird Associates double-beam spectrophotometer with a sodium chloride prism. Materials liquid at, room ternperahres were examined in a 0.1-mm. cell. Solids were dissolved in carbon tetrachloride a t a concentration of 100 mg. per ml., and spectra were obtained in a 1.0 mni. cell. Weak Iiands of carbon tetrachloride were eliminated with a compensatitig cell of suitable thickness. A few compounds of low solubility were examined in t,hicker cells. These methods of obtaining the spectra correspond to the authors' customary procedures in obtaining spectra. In practice this is more convenient, thun attempting comparison on an equivalent molar basis. Although a double-beam instrument was employed for the spectra reproduced, single-beam instruments have been found t o be perfectly satisfactory. Water vapor background causes no difficulty under ordinary conditions of operation of singlebexm infrared spectrometers.
from the typical. For these variant compounds, there is no danger that the kind of substitution will be mistaken from the 5to 6-micron patt'ern inasmuch as the variant patterns do not resemble any of the typical patterns. It is a general rule that if the pattern of a compound deviates from the normal type for a specific configuration as given in Figure 5, such a pattern will resemble none of the typical patterns. This simply means that on rare occasions compounds are encountered for which no information about substitut,ional configuration can be obtained by the method expounded herein. Di-, tri-, and tetrasubstituted benzenes appear to be very regular with respect to typical patterns. Occasionally a more marked splitting of doublet bands mag occur than is generally typical, but such effects of wave length shifts of the individual bands making up the typical pattern are not troublesome. o-Sitrotoluene has a typical ortho pattern (Figure l),although nitrobenzene is atypical. Generally, groups causing atypical mono-patterns do not, except possibly rarely, cause deviations when t,he degree of substitution increases. The penta- and hexasubstituted compounds cannot be uniquely characterized by this method. I n the first place, only a few esaniples of such Compounds have been available to this laboratory. Consequently, what are listed as typical patterns in Figure 5 are riot, statistically significant. In the second place, the last three patterns of Figure 5 show that uniqueness of pattern tends to be lost with increasing substitution. The patterns of the penta- and hexasubstituted compounds have been included merely for the sake of completeness and for intercomparison with the patterns of the less substituted compounds. Compounds with substituent groups which contribute a fundamental band in the 5- to 6-micron interval will be expected to mask seriously the substitution patterns. In practice, carbonyl compounds make up the only significantly large class for which the effect of a fundamental (carbonyl bands) is notable. If the 5.5- to 6-micron portion of Figure 5 is masked off, a fair picture is given of the rather severe limitations placed on the applicability of the method to carbonyl compounds. THEORETICAL APPLICATIONS
ASALYTICAL APPLICATIONS
If no distinction is made among various kinds of organic groups, considering each only in its property as a substituent attached to the benzene ring, there are twelve different configurations: one monosubstituted, three disubstituted (ortho, meta, para), three trisubstituted (1,2,3-, 1,2,4-, 1,3,5-), three t'etrasubstituted (1,2,3,4-, 1,2,3,5-, 1,2,4,5-), and one each of penta- and hesasubstituted. Each configuration is identified by means of a unique spectral pattern between 5 and 6 microns. Figure 5 is a chart of these typical patterns. The chart represents generalized conceptions of spectra which, although they correspond exactly to no specific compounds, are immediately identified with the appropriat,espectraof Figures 1,2, and 3. For mono-substituted compounds, Figure 5 shows that the characteristic pattern for the phenyl group consist's of a series of four bands showing a diminishing relative intensity with increasing wave length. In Figure 4,spectral patterns of monosubstituted compounds are to be found which do not follop- the rule, a t least closely enough to be entirely typical. Fluorobenzene and the ethers illustrated appear to form almost a special class. Their patterns resemble somewhat the more typical phenyl pattern of Figure 5, but only if the entire spectrum were shifted to shorter wave 1engt.hs. Styrene, in Figure 4, illustrates a different variation. Inasmuch as styrene contains the vinyl group, an intense band characteristic of this group appears at, about 11 microns. The enhanced intensity of the 5.5-micron band hence occurs as the result of a superposition of the overt,one of the 11-micron band on the usual phenyl pattern. Kitrobenzene, also illust,rated in Figure 4, deviates in a marked wav
There seems little question from the background picture contributed by normal mode treatments of the vibrat,ional spectra of benzene and substituted benzenes, from existing information, that the spectral patterns in the 5- to 6-micron region for aromatic compounds are made up of overtone and combination bands. -41~0,empirical examination of infrared spectra confirms this spect'ral pattern composition. (Naturally, this does not apply to carbonyl bands, etc., which appear for compounds containing substituent groups giving rise to fundamentals in this region.) I t is doubtful that any convincing assignmentof particularfundamental frequencies could be made generally a t present which, together with appropriate selection rules, would place the int8erpretation of these patterns on a satisfactory theoretical basis. However, esamination of Figure 5 elicits aome interesting spec,ulations. On the whole, the patterns simplify as substitution increases, or, in other words, as the number of benzene hydrogens decreases. Also, the more symmetrical para- and 1,3,5-trisubstituted forms have simpler patterns, suggesting the operation of more rigorous selection rules-at least, in respect to effect on relative intensities. The entire picture indicates the importance of ring hydrogen frequencies, but also shows that ring skeletal frequencies play as important a role in contributing to the patterns by way of higher harmonics. The examples of nitrobenzene and fluorobenzene, which deviate from type, indicate that several perturbing effects can be operative. The regularity found generally is little short of amazing. Retention of the relative intensity pattern is perhaps the most unespected feature of all. It is not,, therefore, surprising that' certain groups-for example, the vinyl group in styrene-may
V O L U M E 23, NO. 5, M A Y 1 9 5 1
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&-o.Dichlorobenzene e - T o l u e n e
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(Left), Chlorobenzene (Center), Phenol (Right)
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(Left) ndhlorotolueno (Center), rn-Xylen; (Right)
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Figure 1.
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ANALYTICAL CHEMISTRY
712 1,P,3-1
,!2,3.Triehlorobenrano (Left), P 3-Dichloro-1 -Ethylbenzene (Center), P,6-Xyienol (Right)
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Figure 2.
Infrared Spectra
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V O L U M E 2 3 , NO. 5, M A Y 1 9 5 1
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--Hexamethylbenzene (Lek) Hexrchlorobmzene (Rightj
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Figure 3.
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ANALYTICAL CHEMISTRY
714
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HEXAintroduce buperposing overtones on the typical pattern. Other substituent groups may a t times be expected to introduce overtone or combination bands which disturb the typical pattern? and which must be explained in less obvious ways (nitrobenzene). It is plausible to assume that certain groups may influence the electronic structure of the benzene ring so that normal mode irregularities would make themselves felt as disruptions of the typical patterns. These typical patterns suggest an interesting side issue. Unless there are truly astounding coincidences, the 5- to 6-micron patterns must be explained as analogous transitions for similarly substituted benzene$. This affords an extra requirement on
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normal mode analyses of such compounds inasmuch as the assignments of fundamentala must lead to reasonable explanations of the typical patterna. RECEIVED June 18. 1950. Presented in part before the Division of Analytical Chemistry at the 117th Meeting of the AYERICAS CHEMICAL S O C I E ~ . Houston. Ter.
