458
G. L. COOKAND F. M. CHURCH
vigorous mulling caused a clear-cut polymorphic transition without any detectable changes of the first kind. Pelleting brought about the same conversion, but produced some amorphous regions a t the same time. Heating the pellets quickly eliminated the amorphous phases and then slowly aided change to the second structure. Since the original crystal structure was subsequently found to be unstable at normal conditions, the effectiveness of mulling in aiding transition is easily understood. Less readily explainable are the differences in the spectrum of stable 2-aminobenzoic acid given in Fig. 7. The f i s t and second spectra are from a hand ground mull and a machine ground mull, respectively, and they show very marked differences in the bands indicated by arrows. Since the bands which show increased intensity after vigorous mulling have not broadened, the changes may be due to polymorphism. If so, then pelleting in KBr causes transition to still a different crystalline structure as shown in the fourth spectrum. The third spectrum is from a XI pellet ground for 10 sec. and is very similar to the first mull; the only changes are a slight broadening and decreased resolution of some bands, but the changes are much less than those produced by vigorous mulling. Similar changes have been observed sufficiently often in a number of other compounds such as 2,bdinitrophenylhydrazine, salicylic acid, etc., that any comparison of solidstate spectra, whether mulls or pellets, should allow for possible crystal changes.
Conclusion Although the pellet technique has some serious limitations, it is still eminently suited for obtaining
Vol. 61
spectra of microsamples and of amorphous polymer or resin samples. With some precautions, crystal distortions usually can be minimized in all but the most sensitive samples, and the pellets, while occasionally giving spectra not quite as good as mulls, will yield spectra which are good enough qualitatively for most problems. I n particular, cautious grinding followed by appropriate heat treatment will give many excellent spectra from pellets which would otherwise give very poor spectra. On the other hand, rather than trying to avoid crystal changes, these have been induced purposely t o solve problems encountered in polymorphism. This is usually easier and faster than recrystallization or cocrystallization. Rather than being relatively rare, polymorphism occurs very frequently in organic preparations, especially if stabilizing impurities are present or if different solvents are used in recrystallization. For the reasons outlined above, the pellet technique should have a place beside the mineral oil mull in every laboratory. I n combination, these two methods of obtaining spectra will yield more information about a sample than will either one alone.
CORRELATIONS OF THE INFRARED SPECTRA OF SOME PYRIDINES BY G. L. COOK‘AND F. M. CHURCH Petroleum and Oil-Shale Experiment Station, Bureau of Mines, Laramie, Wyoming Raceivsd October 16, 1056
The spectra of pyridine and 33 substituted pyridines are compared to establish correlations of the absorption bands with: the molecular structure of the compounds. Five groups of compounds are considered, namely, 2-, 3- and 4monosubstituted, and di- and tri-substituted types, Absorption bands that are generally characteristic of an alkylpyridine system are found near 1600, 1570 and 1000 cm.-1. For 3-alkylpyridines and 2,5-dialkylpyridines, the latter peak is removed to 1021-1034 cm.-l. Peaks in the regions 1280-1330 cm.-‘ and 1222-1253 cm.-l are strong confirmatory evidence for the alkylpyridine system. The separation of the absorption bands near 1600 and 1570 cm.-1 is approximately.40 cm.-l .fqr 4-monoaWpyridines and 20 cm.-l for the other two types of monosubstituted pyridines. The 4-monosubstltuted pyridines also have a band in the region 1067-1072 cm.-l. The 2-monosubstituted pyridines have bands a t 1050 ern.-“ and in the region 11461152 cm.-l. The 3-monosubstituted pyridines have bands a t 1117-1131 cm.-1 and 1180-1196 cm.-l. Disubstituted pyridines all have a peak in the region 1099-1136 cm.-l. No correlations were found in this region for trisubstituted pyridines. Out-of- lane deformation vibrations for 2-monoalkylpyridines were found in the region 743-750 cm.-I; for 3monoalkylpyric!nes a t 789-810 cm.-1 and 712-715 em.-’; for 4-monoalkylpyridines at 785-822 cm.-1; for disubstituted pyridines a t 816-833 cm.-1 and 725-743 cm.-l; and for trisubstituted pyridines at 724-732 cm.-l. For monoalkylpyridines the relative intensity and location of absorption bands in the 1667-2080 cm.-1 region differonlyif the position of substitution differs. Typical absorption atterns in the region are given for 2-, 3- and Pmonoalkylpyrid~nes. Patterns are also Suggested for 2,3-, 2,4-, 2,5- and32,6-disubstituted pyridines. The patterns for the disubstituted compounds may need some modification as additional compounds become available.
Correlation of the infrared absorption bands of simple compounds with their structures can provide a means for determining the structure of more complex compounds. This paper presents correlations based on the infrared spectra of pyridine and 33 substituted pyridines. The infrared absorption spectra are discussed in relation to each of five classes. These are 2-, 3- and 4-monosubstituted, disubstituted and trisubstituted classes. A sufficient number of compounds of each monosubstituted type were examined to indicate the general applicability of the correlations. The disubstituted and trisubstituted compounds were principally methyl-substituted. Hence the ranges given for the correlations of the spectra of the latter (1) Bureau of Mines, Regign 111, Laramie, W ~ D .
compounds may have to be broadened as additional compounds become available. Most of the previous work has been based on the spectra of ~ y r i d i n e ~and - ~ the methylpyridines.B Bellamys has reviewed work through 1953 concerning the assignment of vibrations for pyridine, the monomethylpyridines, 2,6-dimethylpyridine and various alkaloids containing the pyridine (2) C . €I. Kline, Jr.. and J. Turkevich, J. Chem. P h y s . , 12, 300 (1944). (3) L. Corrsin, B . J. Fax and R. C . Lord, ibid., 21, 1170 (1953). (4) H. M. Randall, R . G. Fowler, N . FusOn and J. R . Dangl, “Infrared Determination of Organic Structures,” D. Van Nostrand Co., Ino., New York, N. Y., pp. 7, 17, 39, 42, 91, 225. (5) L. J, Bellamy, “The Infrared Spectra of Complex Molecules,” John Wi1ey +nd Sons, Inc., New York, N. Y., pp. 232-239 (22 reference&
459
CORRELATION OF INFRARED SPECTRA OF PYRIDINES
April, 1957
TABLE I 1600-1400 C M . - ~ABSORPTION BANDSIN
THE
SPECTRA OF PYRIDINES Absorpt,ion bands. cm. -1
Compound
Pyridine 1600 1600 2-Methylpyridine 1603 2-Ethylpyridine 2-n-Pentylpyridine 1600 1597 2-Hexylpyridine 2-( 5-Nonyl)-pyridine 1597 1597 2-Benzylpyridine 3-Methylpyridine 1605 1-(3-Pyridyl)-4-methylaminobutane 1585 (dihydrometanicotine)" 1-(3-Pyridyl)-4-methylamino-l-butene 1585 (metanicotine)" 1592 3-( l-Methyl-2-pyrrolidyl)-pyridine(nicotine)" 1590 3-( l-Methyl-2-pyrryl)-pyridine (nicotyrine )a 1590 3-( 2-Pyrrolidyl)-pyridine (nornico tine)" 1597 4-Methylpyridine 1605 4Ethylpyridine 1613 4-n-Propylpyridine 4-Isopropylpyridine 1600 4-Pentylpyridine 1610 1608 4-(5-Nonyl)-pyridine 4Benzylpyridine 1600 2,3-Dimethylpyridine .. 2,4-Dimethylpyridine 1610 2,5-Dimethylpyridine 1608 1597 2,6-Dimethylpyridine 3,PDimethylpyridine 1600 2-Methyl-5-e thylpyridine 1603 1605 2-Methyl-4-ethylpyridineb 1600 3-Ethyl-4-methylpyridine 2-Methyl-6-ethylpyridineb .. 2,3,6-Trimethylpyridine 1600 1610 2,4,6-Trimethylpyridine 1610 2,4,5-Trimeth~lpyridine~ 1605 2,4-Dimethyl-6-ethylpyridine 1608 2,6-Dimethyl-4-ethylpyridine a C. R. Eddy and A. Eisner, Anal. Chem., 26, 1428 (1954). Edinburgh, Scotland.
nucleus. Besides indicating the generality of the correlations already established, this paper introduces several new correlations within the five classes of pyridines. Experimental A Perkin-Elmer model 21 spectrophotometer equipped with sodium chloride optics was used to obtain the spectra. The spectra were run, using slit program 4 (slit opening 0.017 mm. at 2.0 N ) . Fixed cells 0.034 and 0.086 mm. thick were sufficient to bring out the features of the spectrum between 5 and 15 p . The 26 compounds used to obtain the spectra in this Laboratory were of commercial origin. The manufacturers placed the minimum purity of each at 95% or greater. As small impurity peaks could be expected to appear in the spectra, only relatively strong peaks for the most part were used for the correlations. The source of spectra, including literature references where appropriate, are given in Table I.
Results and Discussion The absorption bands from 650-2080 cm.-l are grouped into frequency ranges to facilitate the discussion of the spectra. Table I, in addition to indicating the compounds used, includes data for one of these frequency ranges-1600 to 1400 cm.-l. The' sodium chloride optics have low disper-
1582 1577 1577 1577 1577 1577 1575 1587
1501 1492
1572
..
1477 1474 1477
1458
..
..
1460 1443 1439 1468 1437 1437 1437 1453
1475
..
1443
1486
..
.. .. ..
..
..
.. ..
1570 1475 1478 1577 .. 1486 1568 1538 1479 1577 .. 1490 .. 1555 1506 1570 .. 1508 1567 .. 1495 1555 .. 1504 1570 .. 1501 1565 .. 1497 1563 .. 1567 .. .. 1567 1492 .. 1572 1492 .. 1585 .. 1473 1564 .. .. 1567 1490 .. 1588 .. 1479 1567 1497 .. 1587 1499 1466 1582 .. 1471 1572 1534 .. 1565 1555 1495 1570 .. .. 1570 1541 .. Spectra furnished by H.
.. ..
..
..
..
1458
1429
1460
..
.. ..
..
..
.. .. 1403 1418
.. ..
.. .. .. 1424
..
..
..
1420 1416 1418 1416 1427 1406 1422 1420 1412 1418 1418 1416 1420 1400
1437 1466 .. 1468 .. .. 1464 .. 1468 .. 1471 .. 1433 1456 .. 1451 1439 .. .. 1451 .. .. 1451 .. 1414 .. 1453 1412 1456 .. .. .. 1451 .. 1412 .. 1449 .. 1410 1458 .. .. 1439 .. .. .. 1443 *. 1410 1439 .. 1468 1391 .. 1451 .. 1416 1441 1462 1431 1450 B. Nisbet, Heriot-Watt College,
sion in the 3000 cm.-l region. Hence, this region is omitted from the discussion. 2080-1667 Cm.-1 Region.-Young, Duvall and Wrighta presented average absorption patterns for benzenes in the 1600-2000 em.-' region that were typical with respect t o the position and number of substituent groups. It has been suggested3 that many of the vibrations of pyridine are quite close to those of benzene. Therefore, similar correlations would be expected to exist for the pyridines. Figure 1 presents typical absorption patterns for the 2-, 3- and 4-monosubstituted pyridine types. The pattern for pyridine is included in the figure for comparison. The patterns differ enough in position, number and relative intensity of the absorption peaks to be readily differentiated. The variation from the typical patterns is indicated in Fig. 2 , which shows the spectra of five 2monosubstituted pyridines. The substituent alkyl groups varied from methyl to 5-nonyl. Apparently, the spectra of the five compounds can be nearly (6) C. W. Young, R. B. Duvall and N. Wright, Anal. Chem.. 23, 709 (1951).
G. L. COOKAND F. M. CHURCH
460
,..- ..
LM.
Vol. 61
I
7
0 10 20 30 40
50 0
IO 20 30 r' 40
0
E
5
50 60
% e 70 c
rw
E
O 10 20
30 40 0
10 20 30 40 4.0
50 60 "4.8
5.0
5.2
5.4
5.6
5.0
6.0
MICRONS.
Fig. 2.-Spectra 5.0
52
5.4
M ICRONS
.
5.6
sa
co
of five 2-monosubstituted pyridines in the 2000-1667 cm.-1 region.
maxima near 1600 cm.-l. This double maximum appears to be a general characteristic of the pyridine nucleus and as such is similar to the phenyl superimposed. Similar results were obtained for the corn pound^.^^^ One of the exceptions noted, 2methyl-6-ethylpyridine, exhibited an additional 3- and 4-monosubstituted pyridines. Figure 3 presents absorption patterns for 2,3-, peak a t 1529 cm.-l, which was not included in 2,4-,2,5- and 2,Gdisubstituted pyridine types. Table I. This vibration is probably related to the The pattern for each type of substitution is quite double-bond system of pyridine, as are those in distinctive, Only one or two compounds of each of the 1600 ern.-' region. For monosubstituted pyridines the average the types illustrated were available. However, the remarkable similarity of the patterns (Fig. 2) for separation of the two bands can be used to difthe 2-monosubstituted compounds as the chain ferentiate 2- or 3-monosubstituted pyridines from length was increased suggests that the typical ab- 4-monosubstituted pyridines. For 2- and 3-mOnOsorption patterns given for the disubstituted com- substituted pyridines the average separation of the pounds will require little change as more compounds bands is 20 cm.-l, but for 4-substituted pyridines the separation is 40 cm.-l. The spectra of pyridines become available. 1600-1400 Cm.-' Region.-Two of the bands in with a variety of monoalkyl substituents were exthe 1600-1400 cm.-l region have been assigned t o amined, as shown in Table I, and the correlation interactions between C=C and C-N vibrations was found to be valid for all. I n addition to the two bands in the 1550-1600 of the pyridine rings.6 The first of these bands occurs near 1600 cm.-l and the second near 1500 cm.-l region, all the 4-monosubstituted pyridines cm.-l. Bellamy6 states that the 1600 cm.-l band showed an absorption band near 1500 em.-'. The may show a double maximum, the second maxi- 2-benzylpyridine and the 3-methylpyridine also mum occurring a t a lower frequency than the 1600 exhibited peaks near 1500 cm.-l, but the other 3ern.-' band. With the exception of 2,3-dimethyl- monosubstituted pyridines, the nicotines, did not pyridine and 2-methyl-6-ethylpyridine1 all of the have such a peak. The 1501 em.-' for 2-benzylcompounds given in Table I have absorption bands pyridine probably results from a C=C vibration a t 1588-1613 cm.-l and at 1555-1588 em.-'. The of the phenyl ring.6 Hence, the presence of this bands in these two positions are assigned to the peak in the 2-benzylpyridine spectrum does not C=C and C-N groups, as suggested by Bellamy. invalidate the correlation of the peak with 4With the two exceptions noted, the disubstituted substitution. The appearance of the peak in the and trisubstituted pyridines had two absorption spectrum of 3-methylpyridine suggests that ' the Fig. 1.-Typical
absorption patterns for pyridine and three monosubstituted pyridines.
April, 1957
CORRELATION OF INFRARED SPECTRA OF PYRIDINES
correlation must be qualified to include possible interference from 3-alkylpyridines. One or more additional absorption bands were found near 1475 and 1440 or 1460 cm.-l in the spectra of the 2- and 3-monosubstituted compounds. I n Table I these bands were grouped into columns, taking into consideration shape and intensity as well as position of the bands. On this basis, the 1468 cm.-l peak for 2-n-pentylpyridine is placed in the sixth column of the table in preference to the fifth, Peaks in these three regions occur near the position assigned to vibrations of alkyl groups.6 With the exception of 4-methylpyridine, the 4-monosubstituted compounds showed similar maxima near 1465 cm.-l. All of the 3- and 4-monosubstituted compounds and 2-methyl- and 2-ethylpyridine had peaks in the 1403-1424 cm.-' region. The origin of this vibration is not clear. The disubstituted and trisubstituted compounds absorbed near 1450 or 1440 em.-' and near 1410 cm.-l. The latter peak was found in the spectra of 22 of the 34 compounds shown in Table I. Hence, its presence in a spectrum can be used as additional evidence for the presence of the pyridine system, but the reverse is not necessarily true. The peaks near 1500 cm.-l in the spectra of the disubstituted compounds could interfere in differentiating 2- or 3-monosubstituted compounds from 4-monosubstituted compounds. However, the 1667-2080 em.-' region supplies a basis for the determination. 1400-650 Cm. -l Region.-Table I1 presents the prominent peaks in the 1400-650 cm.-' region shown by the spectra of the 34 pyridines. The compounds were grouped for the purpose of presentation in the table into 2-monosubstituted, %monosubstituted, 4-monosubstituted1 disubstituted and trisubstituted types. The methyl vibrations occur in the expected position, near 1380 cm.-1.6 For the 4-isopropylpyridine the split vibration of the methyl groups, although not shown specifically in the table, appeared at 1385 and 1364 cm.-l. The two bands were of nearly equal intensity. A peak a t 1280-1330 em.-' appeared in the spectra of 32 of the 34 pyridines. The peak varied in intensity between compounds. It has been suggested3 that this vibration in pyridine is due to an overtone of the fundamental that occurs near 650 cm.-'. The variable intensity of the peak reduces its value for characterization studies. A peak in the 1,220-1250 cm.-' region can probably be attributed to bending vibrations of the hydrogen atoms on the pyridine ring. An additional vibration a t 1145-1152 em.-' for 2-monosubstituted pyridines probably arises from the same source. The latter vibration, which was especially constant in position, was found in the spectra of all the 2-monosubstituted pyridines. Both of these vibrations were medium to strong compared to the general level of absorption in the region. The spectra of the 3-monosubstituted pyridines showed an absorption band in the 1180-1196 crn.-l region. This peak probably arises from hydrogen bending vibrations and is analogous
461
CMI!
7
0
IO 20
30 40 50
60 70
0 IO
20
30
- 40
c n. 50 e
3 60 4 O IO 6 20 c
E V n.
30 40 50
0 IO 20
30 40 50
60 70 804.8
Fig. 3.-Typical
5.0
5.2
5.4 5.6 MICRONS,
5.8
6.0
absorption patterns for disubstituted pyridines.
to the 1146-1152 cm.-l peak of the 2-monosubstituted pyridines. Other bands in the hydrogen bending region, which were constant in position, were found a t 1050-1052 cm.-' for the 2-monosubstituted pyridines, a t 1117-1131 ern.-' for 3-monosubstituted pyridines, a t 1067-1 072 em. -l for 4-monosubstituted pyridines, and a t 1099-1136 cm.-l for the disubstituted pyridines. These were generally medium to strong bands in the spectra. Bellamy6 gives 1100-1000 cm.-l as the range in which a ring vibration for pyridines2J has been found. As shown in Table 11, this band occurred in the very narrow range of 988-1001 cm.-' for all of the pyridines except 3-monosubstituted and 2,5-disubstituted compounds. For the exceptions, a strong band appeared in the 1021-1033 em.-' region. All of the 2-monosubstituted pyridines showed a strong band in the 743-750 cm.-l region. This has been assigned t o out-of-plane deformation vibrations of the ring hydrogens.6 The vibrations are analogous to those of the phenyl hydrogens.6 For the 3-monosubstituted compounds this band appeared in the 789-810 cm.-l region, accompanied by a second band near 712 em.-'. For the
EDWARD A. HEINTZAND DAVIDN. HUME
462
Vol. 61
TABLE I1 1400-650 CM.- l INFRARED ABSORPTION PEAKS OF PYRIDINES Compound 2-Monosubstd. 3-Monosubstd. 4-Monosubstd. Disubstd. Trisubstd.
13771383 13771383 13771383 13771383 13771383
12991311 13121318 12991311 13001330 12801330
1225a 12391253 12221253 12261250 12221231
11461152 11801196
Frequencies of absorption regions in cm.-' 10501052 111710211131 1034 10671072 1090102Sb 1136 10321040
a The posit'ion of this band was constant; hence, no range is given. dines. Not present in spectra of 2,5-disubstituted pyridines.
b
995"
9951000 988998' 9961001
743750 789810 785822 816833
712715
725. 743 724-, 732
Present only in spectra of 2,5-disubstituted pyri-
4-monosubstituted compounds the band appeared at 785-822 em.-'. The disubstituted pyridines showed two such bands in the 650-1000 cm.-' region. These occurred a t 816-833 cm.-' and 725The trisubstituted compounds ex743 em. -l. amined had only one strong peak (725-732 cm.-l) in the low-frequency region. I n each instance the out-of-plane vibration bands were the strongest observed in the 650-1000 cm.-l region.
would have to be made from other regions of the spectrum. As no correlat,ions were made in this region for trisubstituted pyridines, the characterization of these must also be made from other spectral regions. Out-of-plane deformation vibrations for 2-monoalkylpyridines were found in the region 743-750 em. -l; for 3-monoalkylpyridines in the regions 789-810 crn.-I and 712-715 em.-'; for 4-mOnOalkylpyridines in the region 785-822 em.-'; for Summary and Conclusions disubstituted pyridines in the regions 816-833 Peaks near 1600, 1570 and 1000 cm.-l are char- cm.-l and 725-743 cm.-l; and for trisubstituted acteristic of an alkyl-pyridine system. I n 3- pyridines in the region 724-732 cm.-l. alkylpyridines and 2,5-dialkylpyridines the 1000 The location and relative intensity of the peaks cm.-l peak is removed to 1021-1034 em.-'. Peaks in the 1667-2080 cm.-l region are characteristic of in the regions 1280-1330 cm.-l and 1222-1253 position of substitution for monoalkylpyridines. cm.-' are strong confirmatory evidence of the Changes in size and branching of the substituent presence of an alkylpyridine. group have little effect on the over-all appearance The separation of the two peaks near 1600 and of the spectrum in this region. Assignments of 1570 cm.-l is approximately 40 cm.-l for 4-mOnO- typical substitution patterns for some disubalkylpyridines. This distinguishes the 4-mono- stituted compounds are included. These may need alkylpyridines from the other monosubstituted modification as additional compounds of the types pyridines; in these spectra the two peaks are become available. separated by only 20 cm.-l. Acknowledgment,-This work was performed a t I n the region 1050-1200 cm.-' peaks a t 1050 the Bureau of Mines, Petroleum and Oil-Shale cm. -l and a t 1146-1 152 cm.-l characterize 2- Experiment Station, Laramie, Wyo., under the monoalkylpyridines; peaks a t 1117-1131 cm-l and general direction of H. P. Rue, H. M. Thorne and 1180-1 196 cm.-I indicate 3-monoalkylpyridines; J. S. Ball. The authors wish to thank G. U. and a peak a t 1067-1072 crn.-l indicates 4-mOnO- Dinneen and H. H. Heady of this Laboratory for substitution. All the disubstituted pyridines ex- their review of the manuscript. The work was amined have peaks in the 1099-1136 em.-' region. done under a cooperative agreement between the This overlaps the vibration of 3-monoalkylpyri- University of Wyoming and the Bureau of Mines, dines, The distinction between these two types U. S. Department of the Interior.
INTERFACIAL TENSION AND COMPLEX FORMATION BYEDWARD A. HEINTZ AND DAVIDN. HUME Contributionfrom the Department of Chemistry and Laboratory for Nuclear Science of the Massachusetts InStitute of Technology, Cambridge 39, Massachusetts Received October 1 Y . 1966
The interfacial tension method, proposed by Kazi and Desai for the study of complex formation in aqueous solution, has been examined critically and the effect of experimental variables on the results determined. The drop volume technique originally proposed was found to give results which were too erratic and unreliable to allow correlation with solution composition. Measurement of interfacial tension by the ring method, which was found to be much more accurate, showed no evidence of changes in interfacial tension due to complex formation alone.
During the last few years several workers have reported, in a long series of a r t i ~ l e s , l -studies ~ on (1) H. J. Kazi and C. M . Desai, J . I n d i a n Chem. SOC.,30, 287, 290, 291, 421, 423, 424, 426, 872, 873 (1953); 31, 163, 165, 329, 331, 332, 415, 416, 418, 633, 636, 638, 640, 769 (1954); Science and Culture, 19, 259 (1953).
complex formation in aqueous salt solutions carried out by measurement of the interfacial tension and Organic liquids "having between the (2) H. J . Kazi, J . I n d i a n Chem. SOC., 31,763 (1954); 3 2 , 6 4 (1955). (3) C . M. Desai, ibid., 81, 957 (1954). (4) H. V. Barat and C . M. Desai, ibid., 32, 61 (1955).
