Spectra-Structure Correlations of AI kylpyridines H. E. PODALL' P urdue Universify, West Lafayette, Ind.
b The alkylpyridines have characteristic infrared absorption maxima in the region from 1 1 to 15 microns, and characteristic band patterns in the region from 5 to 6 microns, which depend primarily upon the positions of the alkyl groups on the pyridine ring, The alkyl groups on the pyridine ring also have characteristic bands in the 7- to 9-micron region, depending upon whether the alkyl group is methyl, isopropyl, or fert-butyl. The ultraviolet absorption maxima have been similarly correlated with the substitution positions of the alkyl groups and are essentially independent of whether the alkyl group is methyl, ethyl, isopropyl, or terf-butyl. Twenty-nine alkylpyridines were studied.
S
useful spectra-structure correlations have been found for the alkylbenzenes, anti similar correlations h a r e been proposed for the alkylpyridines ( 2 ) . Hon-ever. the latter are based on a considerably fewer number of compounds and are consequently not so well eqtablished. The generality of the proposed correlations n-as tested on several alkylpyridines, for many of which the spectra had not previously been reported. EVERAL
or the carbon disulfide solutions were used. The reported absorption maxima are considered comparable to within 0.1 micron. Ultraviolet Spectra. T h e ultraviolet spectra of t h e pyridine bases were determined in 0 . 1 s hydrochloric acid and in 0 . 1 s sodium hydroxide solutions a t concentrations of about 1 X -11 with respect to the pyridine base. The spectra were generally measured first with a Cary recording spectrophotometer and then with a Beckman llodel DU spectrophotometer for more detail. The spectra ivere always measured a t 25" -;t 2" c. DISCUSSION
Infrared Correlations. Infrared spectra-structure correlations have been found for t h e alkylbenzenes in t h e regions of 5 t o 6, 7 t o 9, and 11 t o 15 microns. T h e absorption bands i n t h e region from 11 t o 15 microns have been attributed t o aromatic C-H out-of-plane bending, a n d are characteristic of t h e substitution position on the benzene ring (1, 6, 9). The same situation appeared to be true for some
Table I.
Principal Peaks" of Alkylpyridines in Region from 1 1 to 15 Microns
EXPERIMENTAL
Materials. T h e pyridine bases used n ere generally of 99% purity or better, except for 2-ethyl-6-tert-butylpyridinej 96.5y0 pure; 2.6-diisopropylpyridine, 98% pure; 3-tert-butylpyridine, 987, pure; and 2-methyl-3-isopropylpyridine, 98yGpure. T h e pyridine bases were all carefully dried over calcium hydride before use. Infrared Spectra. T h e spectra of the pure liquids \?-ere determined in 0.06m m . sodium chloride standard cells with a Perkin-Elmer recording infrared spectrophotometer, Model 21, Series 110. T h e spectra of t h e 2methyl-3-alkyl- and 2-methyl-5-alkylpyridines n ere determined in carbon disulfide solution (0.200 gram per 5 ml. of carbon disulfide) in 0.1-mm. matched sodium chloride cells, using carbon disulfide a s blank. KO difference was found in the spectra of the dimethylpyridines in the region from l l to 15 microns when either the pure liquids
Present address, Ethyl Corp., Baton Rouge, La.
of the pyridine compounds reported in the literature (2). Therefore, it was desirable to check this correlation for the alkylpyridines available in this study. I n Table I are listed the two strongest bands in the 11- to 15-micron region for pyridine and 29 alkylpyridines. Like the alkylbenzenes, each group or class of alkylpyridines has a t least one characteristic band in this region of the spectrum. The 2-alkylpyridines all have a band a t 13.3 + 0.1 microns, the 3-alkylpyridines have a band a t 14.1 microns, and the 4alkylpyridines have their strongest band a t 12.4 =k 0.2 microns. d 2,s-dialkylpyridine has its strongest characteristic band 0.4 micron smaller than the corresponding 2,3 isomer. As in the case of the alkylbenzenes, the characteristic absorption maximum decreases as the bulk of the alkyl substituent increases. Hon-ever, the decrease is greatest in going from methyl to ethyl, and no further change is apparent in going to isopropyl or to tert-butyl. The band patterns in the region from 5 to 6 microns are also Characteristic of the positions substituted on the ben-
lle
Et Iso-Pr tert-Bu Ve Et Iso-Prb tert-Bub
(Pyridine, 14.2, 13.4) 2-Alkyl 3-Alkyl 13.4 14.1,12.8 13.3,12.6 14.1,12.4 13.4,12.8 14.1,12.4 13.4,12.7 14.1,12.4 2-Me-3-alkyl 12.75,13.78 12.50,13,56 12.52,13.58 ...
...
4-Alks1 12.6,13.8 12.2,12.9 12.2,13.3 12.2,14.1
2-Me-5-alkyl 12.30,13.81 12.10,13.66 12.10,13.63 12.08,13,60
2,4-Dialkyl 12.3,ll.O
Dimethyl
2,fi-Dialkyl Dimethyl 13.0,13.8 Ne-tert-Bu 12.7,13.4 Et-tert-Bu 12.3.13.4 Iso-Pr-tert-Bu 12.3: i3.4 Diiso-Pr 12.3,13.3 Di-tert-Bu 12.3,13.4 Trialkyl
2,3,6-Trimethyl 12.2,13 4 2,4,6-Trimethyl 11 9,13.9 2,6-Dimethyl-3-iso-Prb 12 2,13.4 a I n order of decreasing intensity. Assigned structures are based in part upon this infrared correlation.
VOL. 29, NO. 10, O C T O B E R 1957
1423
zene ring (IO). The infrared patterns for the alkylpyridines in this region were not so distinct as desired. However, the data indicate t h a t the alkylpyridines, like the alkylbenzenes, also possess characteristic and distinguishable patterns which depend mainly upon the positions substituted, irrelevant of the alkyl group. Generalized patterns are shown in Figure 1, The region from 7 to 9 microns is known to be characteristic of the type of alkyl group (primary, secondary, or tertiary) present on the aromatic nucleus ( 1 ) . This region of the spectra for the alkylpyridines vvas therefore next considered. A methyl group was generally characterized b y a sharp peak at 7.30 to 7.35 microns, a n isopropyl group by a split peak a t 7.2 to 7.3 microns, and a krt-butyl group by a split peak at 7.25 to 7.35 microns (the lower ware length component being n-eaker), a peak a t 8.0 microns, and usually a weaker peak a t 8.3 microns. These data are shown in Table 11. Cook and Church (6) recently reported similar infrared spectra-structure correlations for pyridine compounds. A greater variety of monosubstituted pyridines \$-ere studied ; however, their study included fewer disubstituted pyridines, particularly fewer isomeric dialkylpyridines. Their correlations are essentially in agreement with the findings given here. COMPARISOh. WITH ALKYLBENZESES. Previous attempts have been made t o correlate the infrared or Raman spectra of pyridine compounds with corresponding benzene compounds. Thus, Xanzoni-Ansidei (8) observed that the Raman spectrum of 2,4,6-trimethylpyridine was similar t o that of mesitylene, Kohlrausch and coworkers ( 7 ) observed that not only was the Raman spectrum of 2,4,6-trimethylpyridine similar to that of mesitylene, but that the Raman spectrum of pyridine had certain similarities t o t h a t of benzene, those of 2- and 3-methylpyridine t o that of toluene, and those of 2,4- and 2,6-dimethylpyridine to that of mxylene. Recently, Cannon and Sutherland ( 4 ) compared the infrared spectrum of pyridine in the region of 900 t o 650 cm.-l (11 to 15 microns) to the spectra of monosubstituted benzenes, 3-substituted pyridines to metadisubstituted benzenes, 4-substituted pyridines t o para-disubstituted benzenes, and 2,6-disubstituted pyridines to those of 1,2,3-trisubstituted benzenes. I n Table I11 the methylbenzenes are compared with the analogous pyridines, using generally the strongest peaks in the 11- to 15-micron region. This type of comparison amounts to comparing molecules of about the same shape and is equivalent to that used by Kohlrausch. The mean deviation is 0.32 micron, which is considerably
1424
ANALYTICAL CHEMISTRY
greater than the experimental error. It appears, however, that excellent agreement is obtained when comparing molecules of similar symmetry-Le., 4-methylpyridine vs. toluene, 2,6-dimethylpyridine vs. m-xylene, and 2,4,6trimethylpyridine us. mesitylene. The most intense bands in the 11to 15-micron region of the infrared spectra of the methylpyridines were then compared with those of the methylbenzenes having the same number of unsubstituted hydrogen atomsi.e., a Cannon-Sutherland type of cor-
Table It.
relation. This comparison amounts to treating the nitrogen atom in the pyridines as if i t were a substituent. The comparison (Table IV) shows t h a t the agreement is excellent. Therefore, this type of correlation appears to be more inclusive than that of Kohlrausch. I n general, however, the best agreement is obtained when comparing molecules of the same number of aromatic hydrogens and of the closest symmetry (Table V). The patterns in the 5- to 6-micron region of the infrared spectra of the
Peaks Characteristic of Alkyl Groups in Alkylpyridines"
Methyl 2-Me 3-Me 4-Me 2,3-di-?*Ie 2,4-di-Rfe 2,5-di-Me 2,4,6-tri-Me
7.33 7.30 7.30
7.29,7.36 7.32 7.35 7.35 tert-Butyl ii20(W),7,35
2-iso-Pr 3-iso-Pr 4-iso-Pr 2,6-diiso-Pr
Isopropyl 7 . 3 (split, equal intensity) 7.2(S),73(W) 7 2(W),7.3(S) 7 , 3(split)
8.0,8.3(s) 2-tert-Bu 7.3 8 . 0 , 8 . 3(S) 3-tert-Bu 7.3 8 . 2 , 8 . 3(W) 4-tert-Bu 8 . 0 , 8 . 3(IT) 2-Me-6-tert-Bu 7.25 (IT),7.35 7 . 3 (S),7 . 4 8.0,8 .3(11-) 2-hle-5-tert-Bu 7.25 (\T), 7 . 3 5 8 . 0 , 8 . 3(S) 2-Et-6-tert-Bu 8.0,8.3 (W) 2,6-di-tert-Bu 7.25 ( W ) , 7 . 3 5 a S signifies peak of relatively strong intensity; \I7 signifies peak of relatively weak intensity compared to wave length which has no letter following it.
2.4 -DIALKYL
2-ALKYL
3-ALKYL
2 , 5 -0IALKYL
4-ALKYL
2,6-DIALKYL
2.4 ,S-TRIALKYL
2.3- DIALKYL
MICRONS
MICRONS
Figure 1. Generalized infrared band patterns for alkylpyridines in region from 5 to 6 microns
alkylpyridines were also compared with those of the alkylbenzenes. T h e patt,erns for the monoalkylpyridines were in poor agreement with those for the nionoalkylbenzenes and in somewhat better agreement with those for the corresponding dialkylbenzenes. On the other hand, the patterns for the dialkylpyridines \\-ere in much bett'er agreeinent with those for the trialkylbenzenes. Similarly the pattern for 2,4,6-triniethylpyridine seemed to be in better agreement with those for the 1,3,5-triulkylbenzenes than those for the 1.2.3.5-tetraalkylbenzenes.It appears. therefore, that t,he character of the aromatic nucleus as well as t'he manner of its substitution affects the infrared spectrum in this region, and that the more heavily substituted it is, t'he lees effect the aromatic nucleus has upon the resulting pattern. More distinct patterns in the region from 5 to rj niicrons are required for the alkylpyridines in order to establish thls point. Ultraviolet Spectra. T h e niajor ultraviolct absorption maxima a n d corresponding extinction coefficients of pyridine and of some alkylpyridines, previously unreported, are listed in Table VI. Table VI1 summarizes the niajor ultrayiolet' absorption maxima of pyricline aiid all t'he alliylpvridines studied in this laboratory. These data show that the ultraviolet absorption niaxiniuni depends primarily upon the position of the alkvl group on the ring and is essentially independent of whether the alkyl group is a methyl, ethyl, isopropyl, or tert-butyl. I n some series there appears to be a slight shift in the absorption maximum as one goes from a methyl- to a tert-butyl-substituted pyridine. However, as some of the pJ-ridines used contained slight impurities, it is difficult' to assess the significance of these shifts a t the present time. If one calculates the shift in the ab.;oiytion iiiaxiniuni in going from pyridine to 2-> 3-, or 4-methylpyridine (in :icitl as well a s in base) and assumes these shifts to be characterist'ic of the position suhstitut~edand to be additive, one may calculate the absorption niaxim:i for the di- and trinlkylpyridines (shoivii in Table VJI). The predicted and experiment,al values were thereby found to be in excellent agreement with each other; the deviation generally n-as \T-ithin I mp, except for t'he 2,3tlialkylpyritlinea, where the deviation \vas 2.3 to 3 mp. In the latter case the deviation niay be due to the proxiinity of t,he alkyl groups. APPLICATIONS
OF
CORRELATIONS
Table 111.
Kohlrausch-Type Correlation
Benzene Compound
Pyridine Compound
Benzene Toluene
Wave length, microns 14.70 13.75
a-Xylene m-Xylene
13.5 13.05
p-Xylene Mesitylene 1,2,4-Triniethylbenzene
12.6 12.0 12.45
Kave length, microns 14.25
Diff., Micron 0.45 13.3 0.45 14.1,12.8 0.35 0.05 13,sa 12.8 0.7 12.3 0.75 13.0 0.05 12.35 0.25 11.98 0.02 12.3 0.15 1Iean 0.32
Pyridine 2-Methylpyridine 3-Methylpyridine 4-Methylpyridine 2,3-Dimethylpyridine 2 4-Dimethylpyridine 2,6-Dimethylpyridine 2,5-Dimethylpyridine 2,4,6-Trimethylpyridine 2,3,6-Trimethylpyridine ~
Strongest band is actually at 12.6 microns. Table IV.
Cannon-Sutherland Correlation
Benzene Compound Wave length,
1Ie 1,2-di-Me 1J-di-lle 1,4-di-Me 1,2,3-tri-lIe
microns 13 75 13 5 13 0 12 6 13.10
l,2,4-tri-lIe
12.45
1,2,3,5-tetra-hle
11 98
Table V.
Pyridine Compound Wave length, microns Pyridine 14 25 2-Me 13 3 3-Me 12 8,14 1 4-1Ie 12 6 2,3-di-JIe 12 8 2,6-di-Me 13.0 2,4-di-lIe 12 30 2,5-di-XIe 12 35 2,4,6-tri-lIe 11 98 Mean
Comparison of Molecules of Same Number of Aromatic Hydrogen of Similar Symmetry
Benzenes
Pyridines
Wavf length, microns
1,2,3-tri-lIe 1,3-di-Me-2-Et lj3-di-Me 1,2-di-?*Ie-3-Et lJ-di-Me-4Et 1,4-di-bIe-3-Et 1,2,3,5-tetra-Me 1,3,5-tri-JIe Table VI.
Diff., 3Iicron 0 5 0 2 0 2 0 0 0 3 0.1 0 15 0 1 0 0 0 03
13 13 13 12 12 12
Wav? length,
microns 13 0
2,B-di-lIe
1 0
0 8 2
2,3-di-hIe 2,4-di-RIe 2,5-di-hIe 2,4,6-tri-LIe
4 11 9
12 0
12 8 12 3 12 35 11 9
Extinction Coefficients of Alkylpyridines at Major Ultraviolet Absorption Maxima in Acid and in Base
0.10M HCI Wave length.
Pyridine 3-Me &-Ne 2,3-di-lIe 2-Me-3-Et 2-Me-3-iso-Pi2,5-di- h1e 2-Me-5-E t 2-lIe-5-iso-Pr 2-Me-5-tert-Bu 2,4-di-Me 2,6-di-M e 2,4,6-tri-Me 2,6-di-JIe-3-iso-Pr Somewhat low.
mM 255.5 262 5 262 5 252 5 267 0 267 0 267 5 260 5 269 5 269 n 269 0 259 0 270 0 267 5 275 5
E
0.10W SaOH
x
10-3
5.39 6 63
5 47
4 51
7 49 7 12 7 36
6 6 7 7
82 83
n8
3i
6 25 8 73
7 62 2 41"
Wave length. - ,
mM 2.57 n 262 0 263 0 255 0 265 0 265 0 265 0 268 0 268 0 267 5 267 0 259 0 268 0 264 5 271 0
x 10-3 2 3 3 2 4 3
99
56
11 09 22 77 4 04 3 71 3 62 .1 8.1
3 92 2 97 3 71 3 96
1 344
The structures of three previously VOL. 29, NO. 10, OCTOBER 1957
1425
unknown alkylpyridines (2-methyl-3isopropyl-, 2-methyl-5-tert-butyl-, and 2,6-dimethyl-3-isopropylpyridine) and of another alkylpyridine 2-methyl5-isopropylpyridine which m s described only once before but not fully characterized, were assigned on the basis of these correlations. It was later found that the chemical and physical properties, elemental analyses, and neutral equivalents were also in accord with these assignments. The infrared correlations have been used for determining the pyridine base impurities present in the alkylpyridines here studied and in following the course of their purification. Thus, by determining the pyridine base impurities present in commercial samples of 2methyl-:-ethyl- and 2,4,6-trimethylpyridine (S), it mas possible to choose methods for selectively separating these close-boiling pyridine base impurities. Finally, the infrared spectra ha\-e been used as a criteria of purity for some of these pyridine bases, and have agreed with the results obtained by other analytical methods, such as cryoscopic methods. ACKNOWLEDGMENT
The author wishes to thank Verne R a l s h and Heino Susi for the infrared measurements, Robert Curry for the ultraviolet measurements, Herbert C. Brown for suggesting the infrared correlation, and Charles Snioot for the discussions concerning these correlations. The author is also indebted to Xavier 3lihm and Bernard Kanner for use of their spectral data on the higher monoalkylpyridines and on the
Table VII.
Additivity of Ultraviolet Absorption Maxima of Alkylpyridines
In Acid Pyridine 2-3112 2-Et 2-iso-Pr 2-tert-Bu 3-?.le 3-Et 3-iso-Pr 3-tert-Bu 4-hIe
4-Et 4-iso-Pr 4-t~rt-Bii
2-iIe-3-lXe 2-Me-3-Et 2-Me-3-iso-Pr 2-RIe-5-hle 2- hIe-5-E t 2-Me-5-iso-Pr 2-lle-6-tert-Bu 2,4-di-Me 2,6-di-hIe 2-Me-6-tert-Bu 2-E t-6-tert-Bu 2-iso-Pr-6-tert-Bu 2,6-di-tert-Bu 2,4,6-tri-Me 2,6-di-lIe-3-iso-Pr
In Base
Xobsd.
Xoalcd.
Xobad.
285.5 262.5 263 263.3 263.3 262.5 262.3 262.3 261.2 252.5 252 251.7 252.5 267 ~. 267 267,5 269.5 269,5 269 269 259 270 271 271 27 1 271 267.5 275.5
255.5 262.5 262.5 262.5 262.5 262.5 262.5 262.5 262.5 252.5 252.5 252.5 252.5 269.6
257 262 262 261.5 261 263 262.3 262 261.3 255 255 255 255 265 265 265 268 268 267.5 267,O 259 268
2,&dialkylpyridines, respectively, and to Russel L. Hudson of the Ethyl Corp. for his helpful suggestions in the preparation of this manuscript. LITERATURE CITED
(1) Barnes, R. B., Gore, R. C., Stafford, R. Williams, V. Z., ASAL. CHEW20, 102 (1948). (2) Bellamy, L. J., “Infrared Spectra of Complex hlolecules,” p. 232, Wiley, NeTy Tork, 1954. (3) Brown, H. C., Johnson, S., Podall, H., J . Am. Chem. SOC.76, 5556 (1954). (4) Cannon, C. G., Sutherlanct, G. B.
269.5
269.5 269.5 269.5 269.5 269.5 259.5 269.5 269.5 269.5 269.5 269.5 266.5 276.5
Xcalcd.
257 262 262 262 262 263 263 263 263 255 255 285 255 268 268 268 268 268 268 268 260 267
...
...
264: 5 271.0
265 273
... ...
B. M., Spectrochim. Acta 4, 37395 (1951). (51 ColthruD. S . B.. J . Oat. Soc. Ant. 40, 397 (i&o). Cook. G. L.. Church. F. AI.. J . Phus. .. Ch&. 611 458 (1957). ’ Herz, E., Kahovec, L., Kohlrausch, K. W.F., 2. physzk. Chenk. (Leipzzg) B53, 124-46 (1943). hlanzoni-Xnsidei. R., Boll. sci. fucoltd chim. ind., Bologna 1940, 137-42. Thompson, H. W,,J . Chena. SOC. I
.
1048.328.
Y & ~ g , ’ C ~ kDuvall, ., R. B., Wright, N., h A L . CHEV. 23,709 (1951). RECEIVED for review October 22, 1956. Accepted Rlay 11, 1957.
Color Reaction between Thorium and Quercetin and Separation Scheme for Interfering Ions OSCAR MENIS, D. L. MANNING, and GERALD GOLDSTEIN Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Jenn. The color reaction between thorium and quercetin i s the basis of a precise method for the spectrophotometric determination of thorium. The yellow complex exhibits maximum absorption from 420 to 425 mp when measured against a reagent blank. The absorbance of the complex at 422 mp i s constant in solutions ranging from a pH of 2.7 to 3.5, and adheres to Beer’s law over a range of 10 to 150 y of thorium in a 25-ml. volume. The ratio of quercetin to thorium in the
1426
ANALYTICAL CHEMISTRY
complex i s 2 to 1, while the dissociation constant i s calculated to be approximately 1.2 Major interferences are discussed. The separation of thorium from interfering substances i s effectively accomplished b y a combination of ion exchange separation and thenoyltrifluoroacetone extraction.
x
I
COKNECTION with a survey of organic compounds t o be used as colorimetric and fluorometric reagents pi
for thorium, the properties of the thorium-quercetin complex and its applicability to the colorimetric determination of thorium were studied. Of the previously described colorimetric methods for the determination of thorium, the thoron method is perhaps the most widely used. The method has been utilized extensively in the analysis of monazite sands (2, 7 , 8, 20, 21, 24) and thorium compounds (4, 15, 35, 36). Other reagents which have been utilized successfully
