INFRARED STUDIESOF AMINE-HALOGEN INTERACTIONS
Nov., 1960 PP = 2PO
cos (V2)
(1)
where po is the moment of the monomer molecule, and 6 is the angle between the average moments of the monomer units as directed along the asymmetric carbon-carboxyl carbon bond. To evaluate the average polarization of the whole polymer, we assume as a first approximation that the average moment pave per monomer unit of the ensemble of monomer pairs in the polymer chain is the root mean square of the weighted moments of the pairs n
n f i pia
(Pave)’
1
= ___ = 42fi
fi COS’
P’
(‘~Si/2)
1
TABLE IV STERICFACTORS IN THE CONFORMATIONS OF ISOTACTIC AND SYNDIOTACTIC POLYMETHYL METHACRYLATE Conformation symbol
Isotactic species, d-d Dipole-dipole angle, fi degrees
I I1 I11 IV V VI VI1 VI11
IX
5 fi 1
where (‘n” is the number of different conformations possible to the given polymer species (e.g., 9 for the isotactic or the syndiotactic species), pi is the moment of the ith pair and the fi’s are the probabilities of the individual pair conformations of moment pi as shown in shorthand form in Fig. 3 above. The values of fi were 0 5 fi S 1. Inspection of the molecular models in the form of pentamers (e.g., Fisher-Taylor-Hirschfelder models) permitted an estimate of the relative probability of the various conformations and the dipoledipole angles of the monomer pairs. The results of this analysis are recorded in Table IV. The values of 2fl c0s2(6/2)/2fi for the two species, calculated from the above data are: 0.767 and 0.570, for the isotactic and syndiotactic, respectively. The values for the polymer average moments, pave = po ( 2 f i c0s2/(6/2))’/2/Zfi, using the value uo = 1.69 Debyes, as for a similar simple ester, methyl pr0pi0nate.l’~ are 1.48 and 1.27 Debyes, respectively. The observed moments for the isotactic and syndiotactic species, expressed as the
1705
Syndiotactic species, 14 dipole-dipole angle, fi degrees
50 45
140
160 120 120
60 100 115 120 55
50
105 160 60 20
55 175
apparent moment per monomer unit were 1.43 and 1.265 Debyes, respectively. This is in the order and magnitude expected for such molecules when calculation of the average moment is made as above using consideration of the steric repulsions and basic moment for the ester units. The closeness of the calculated to the observed values of the average unit moments is regarded as rather fortuitous, but the calculation is expected to show the relative sizes of the two species’ moments to a fair approximat ion . (estimated precision, 3 ~ 5 %in the ratio
Pave pave
(d (1 - 4 ”’>
-
The method of calculation given has had a measure of success in predicting the relative moments for the isotactic and syndiotactic forms of several other related polymers. It is planned to describe this more fully in a forthcoming discussion of those polymers. Acknowledgment.-The authors wish to acknowledge with appreciation the helpful discussions with Dr. Keniti Kigasi.
INFRARED STUDIES OF AMINE-HALOGEN IXTERACTIONS’ BY RALPHA. ZINGAROAND W. B. WITMER Department of Chemistry of the Agricultural and Mechanical College of Texas, College Station, Texas Received Mav 1 1 , io60
The marked changes which are brought about in the 1000 cm.-l region of the infrared spectrum of pyridine upon the addition of iodine have been found to be generally characteristic for amine-halogen solutions. Infrared studies on a series of solutions made up from five different halogens and inter-halogens in pyridine and various pyridine derivatives, all reveal corresponding infrared shifts. Several new solid amine-halogen complexes have been isolated, and, in every case, the frequency shifts observed in the solutions can be correlated with infrared bands characteristic of the solids. Comparison of the spectra with those of substituted benzenes gives a reasonable interpretation of the data.
Introduction It has been recently demonstrated2 that the marked changes which are obeerved in the infrared spectrum of pyridine upon the addition of iodine3 can he correlated with the spectra of solid compleyeq of the tyne (Py1)X and ~ P ~ ~ J These ~X. solids possess infrared bands which hare the same (1) Presented at the Southwest Regional Meeting of the American Chemical Society, Baton Rouge, Louisiana, December 4, 1959. (2) R. A . Zingaro and W. E. Tolberg. J . Am. Chem. SOC..81, 1353 (19.59). (3) D. L. Glusker and H. W. Thompson,
J. Chem.
Soc., 471 (1955).
location, and which are of the same intensity as the new bands which are found in the infrared spectrum of pyridine following the addition of iodine. The present investigation represents an extension of these studies and includes a variety of solutions made up of different halogens and interhalogens in a number of pyridine derivatives. The amines were chosen so that both steric and electronic effects could be observed. The fundamental purpose of this study was to determine whether the rather profound infrared shifts which are observed in iodinepyridine solution could be observed as a phenom-
Vol. 64
RALPHA. ZINGAROAND W. B. WITMER
1706
enon generally characteristic of halogen-amine systems. Such a generalization, if established, should contribute substantially to ~t better understanding of the nature of amine halogen interactions. Experimental Amines.-The pyridine was of the same quality as described previously* and was purified in the same manner. The othw starting materials were Eastman white label grade in the case of 2-chloropyridine, 2-bromopyridine and quinoline. The starting materials in the case of the amyland benzylpyridines were either Eastman yellow label or Matheson technical grade products. All were dried over sodium hydroxide for several days and distilled from calcium oxide under reduced pressure. Middle fractions, only, were used. The four amyl- and benzylpyridine middle fractions were subjected t o at least one, or more, additional fractionations. Because of the limited amount of data available on the physical constants of these compounds, the boiling points and refractive indices of the amines used are listed in Table I.
Instrumentation.-A Beckman IR-4 was used for all measurements. The instrument was calibrated regularly by means of a standard polystyrene sample. Amine-Halogen Complexes.-Several solid 1 :1 addition compounds, previously unreported in the literature, were prepared. In the case of the 4-n-amylpyridine derivatives, equimolecular quantities of 4-n-amylpyridine and the halogen, or interhalogen, were combined in chloroform at Dry Ice temperature. The major portion of the chloroform was evaporated a t reduced pressure and the solid, after removal by filtration in a dry atmosphere, was washed with anhydrous carbon tetrachloride and sodium dried ethyl ether. The quinoline-bromine complex was prepared by direct combination of solutions containing equimolecular quantities of bromine and quinoline in carbon tetrachloride. The solid, which separated out immediately, was separated and washed as described in the paragraph immediately preceding. Analysis of the compounds for “free” halogen was performed by titrating the iodine liberated upon treatment of the complex with potassium iodide solution. The compounds showed an analogous solubility pattern. They were only very slightly soluble in carbon tetrachloride, ether or ligroin, but soluble in chloroform and alcohol. The relevant data is summarized in Table 11.
TABLE I PHYSICAL PROPERTIES OF SOMESUBSTITUTED PYRIDINES Pyridine substituent
B.p., O C .
-Refractive Found
indexReported
2-Benzyl 98.5 (4.0)“ 1.5771 (26)* 4-Benzyl 110.0 (6 .O) 1.5814 (25) 2-n-Amyl 63.0 (2.0) 1.4861 (26) c 4-n-Amyl 78.0 (2.5) 1.4892 (25.4) 1.4908 (20)‘ 2-Chloro 49.0(7.0) 1.5322(20) 1.5322(20)6 2-Bromo 49 0 ( 2 7 ) 1.5713(20) 1.5713(20)6 Figure in parentheses gives the pressure in mm. Figure in parentheses gives the temperature. No literature values were found. Halogens and Interhalogens.-The bromine and iodine were Mallinckrodt “Analytical Reagent” grade and were used without further purification. Iodine monochloride and iodine monobromide were prepared according to published directions.* Their purification was accomplished by cooling and separation of the crystalline interhalogen from the liquid. The solids were subjected to this crystallization process several times. Bromine monochloride was used in the form of a mixture. To liquid chlorine, at -70°, was added an equimolecular quantity of bromine. The container was then warmed to -10 to -15’ and complete mixing accomplished by rapid stirring of the mixture. Samples for Infrared Study.-The chloroform used was “-4nalvtical Reagent” grade. Standard solutions of the halogen, interhalogen o r of the amine in chloroform, were prepared separately by weighing out the appropriate amount of solute and diluting to volume. The solutions were prepared as near t o the time of measurement as possible, and mixing of the two solutions to achieve the desired relative concentrations was done just preceding the measurement. In no case was any change in the spectrum noticed during the first several hours, thus precluding any complications which may have arisen as a result of any extensive chemical reaction 1 aking place during the actual time of measurement of the s p e ~ t r u m . ~Potassium bromide discs were prepared in the usual manner.2 (4) J. P. Wibaut and J. W. Hey, Rec. trau. chim., 72, 513 (1953). (5) H. C. Brown and X. R. Mihm, J . A m . Chem. SOC.,77, 1723 (1955). (6) “Inorganic Syntheses,” Vol. I, McGraw-Hill Book Co., Inc., New York, N. Y.,1939,p . 165. (7) The possibility that there may occur a rapid chemical reaction between the halogen and the amine is very real, but unavoidable. However, the concmtration of reaction products. if formed, is so small that they are not detectable by infrared methods. The formation of small amounts of reaction products may be of much greater importance when methods such a8 ultraviolet spectroscopy or conductivity are used. Not only are such methods more sensitive to the presence of impurities, but may themselves be causative. For instance, ultraviolet radiation is known to catalyze free radical halogenations, and the electrode surfaces may also function similarly.
TABLE I1 AMINE-HALOGEN ADDITIONCOMPOUNDS Coordinating Halobase gen Color 4-n-Amylpyridine Bre Orange 4-n-Amylpyridine BrCl Pale yellow 4-n-Amylpyridine IBr Yellow Quinoline Brz Orange
Total halogen, % Found Calcd. M.p., OC. 51.0 51.7 85-87.5, sl. d 43.3 43,b 105-105.5 5 7 . 5 58.1 100.5-101 55.7 55 3 79-79.5
Positive Bromine and Iodine Salts.-The compounds, (BrPy2)NOj and ( BrPy2)C1O4, were prepared according to methods described in the literature.8.Q Dipyridinebromine( I ) acetate, (BrPY&2H302, was prepared as a new compound in the followlng way. I n a dry, glass stoppered ehrlenmeyer flask, 0.055 mole of pyridine and 0.02 mole of silver acetate were combined with about 50 ml. of chloroform. In a second flask, 0.026 mole of bromine was mixed with 30 ml. of chloroform. Both solutions were cooled in Dry Ice after which the cold bromine solution was added in small portions to the pyridine-silver acetate solution. Silver bromide was removed by filtration and the clear filtrate was poured into cold, dry ether. A precipitate did not form a t once so the solution was stored in a stoppered flask in a Dry Ice chest. The pale yellow solid which formed overnight changed in color to a light orange on separation from the solution. The material was washed with cold, dry ether and further dried by pulling dry air through it. This compound decomposes very rapidly in presence of water. The complex melts a t 45-50’ with decomposition. Calcd. for C12H1302S2Br: Br, 26.9. Found: Br, 26.7. The yield was 20%. Mono-2-bromopyridineiodine(I) benzoate was prepared in very small quantities. Into 100 ml. of anhydrous chloroform, 11.5 g. (0.05 mole) of silver benzoate, 9 g. of 2bromopyridine (1.5 g. in excess of 0.05 mole) and 12.7 g. (0.1 mole) of iodine crystals were combined. The suspension was vigorously stirred for 20 minutes in an anhydrous atmosphere. The dissolved iodine gave a violet solution and only a very small quantity of the iodine was consumed. The solution was filtered and to the filtrate was added 250 ml. of petroleum ether. After storage for 48 hours a t Dry Ice temperature, the very small quantity, ca. 1 g., of orange-brown crystals was removed and washed with cold petroleum ether and dried in vacuo over sulfuric acid. Analysis for iodine gave 31.6%. For the indicated complex, the calculated value is 31.3%. The compound decomposed over a wide range from 55-90’, with liquefaction occurring over the range 79-90‘.
Results The characteristic infrared absorption system in the 1000 cm.-l region for the eight amines investi(8) H. Carlsohn, Be?., 68, 2209 (1935). (9) M. Ushakov, X. Chistov and N. Zelinskii, ibid., 68, 824 (1934)
INFRARED STUDIES OF AMINE-HALOGEN INTERACTIONS
Nov., 1960
1707
TABLE I11 INFRARED ABSORPTIONOF VARIOUSPYRIDINES AND SOME OF THEIR HALOGEN COMPLEXES
-
Location of band, em.-\
Phyaical state
Compound
1 Pyridine" (1.0 F ) 2 Iodineb 3 Bromine (1.0 F ) 4 BrCl (1.0 F ) b 5 (1Py)Br" 6 (1Py)CP i (1PY)F" 8 (PYH+)I-~ 9 (BrPy)Br 10 (BrPy,)NO, 11 ( BrPy,)acetate 12 (BrPy2)C104 13 2-n-Amylpyridine (2-AP), 1.O F 14 Iodine 15 Bromine (1.0 F) 16 IBr (1.0 F I 1'7 IC1 (1.0 F ) 18 BrCl (1.0 F ) 19 4-n-Amylpyridine (4-AP), 1.0 F 20 Iodine, satd. 21 Bromine, 1.0 F 22 IBr, 1.0 F 23 IC1, 1.0 F 24 BrCl, 1.0 F 25 [Br(4-AP)]Br 26 [I(4-AP)]Br 27 [Br(4-hP)]C1 28 2-Benzylpyridine (2-BaP), 1.0 F 29 Iodine, satd. 30 Bromine, 1.0 F 31 IBr, 1.0 F 32 ICl, 1.0 F 33 BrC1, 1.0 F 34 4-Benzylpyridine (4-BzP), 1.0 F 35 Iodine, satd. 36 Bromine, 0.4 P 37 Bromine, 1.0 F 38 IBr, 1.0 F 39 IC1, 1.0 F 40 BrC1, 1.0 F 41 Quinoline (Q) 1.0 F 42 Iodine, satd. 43 Bromine, 1.0 F 44 [Br(&)lBr 45 2-Chloropyridine (CP), 1 0 P 46 Bromine, 0 4 F 47 Bromine, 1.0 I: 48 Bromine, 2.0 F 49 2-Bromopyridine (BP), 1.0 F 50 Brominr, 0.3 P 51 Bromine, 2.0 F 52 Bromine, 3.0 F 53 Bromine, 4.0 F 54 [I(BP)]benxoxte
CHC1, soln. Satd. soln. in Py $1.0 F Py in CHC1, +1 .O F Py in CHCl, Solid Solid Solid Solid Solid Solid Solid Solid CHC13 soh. +1.0 F 2-AP in CHCl3 +l .O F 2-AP in CHCl, 1 . 0 F 2-AP in CHCl, +1 . O F 2-AP in CHC1, +1 .O F 2-AP in CHCl, CHCla soh. +1.0 F P A P in CHCI, +1 .O F P A P in CHC1, +l.OFPAPinCHCl, +1.0 F 4 A P in CHCI, +1 .O F 4 A P in CHCl, Solid Solid Solid CHC1, soln. + l .0 F 2-BzP in CHCl, 1 . 0 F 2-BzP in CHC1, +1 .O F 2-BzP in CHCls 1 . 0 F 2-BzP in CHC1, +1 .O F 2-BaP in CHC1, CHCl, 1. 0 F 4 B z P in CHCls 1.0 F 4 B z P in CHC13 +1 .O F 4 B z P in CHCl, +1.0 F 4-BzP in CHCl3 +1 .0 F 4-BzP in CHCl, +l .O F 4 B z P in CHCl, CHCl, soh. +l.0 F Q in CHCl, +1.0 F Q in CHCl, Solid CHCla soh. +1.0 F CP in CHCll +1.0 F CP in CHC1, $1.0 F CP in CHCl, CHCl, soln. +1 . O F BP in CHC13 $1 .O F BP in CHC13 $1 .O F BP in CHCl, + l .O F BP in CHCl, Solid
989
995 1008 1015 1009 1016 996 1006 1010 1010 1013 1016
+
+ +
+ +
1027
1005 1010 1013 1011 1012 1014
995 1011 1018 1018 1021 1026 1026 1015 1024 997 997" L003c
995 995" 995"
939 939" 939'
1005 1013 1009 1014 1013 1001"J 1002c.d 1001c.d 1002"J 1002"J 1002e,d 1002"-d 952" 943 947 952
991 9916 1002c 9918 1002" 9916 1001' 988 988' 999" 988* 999' 988" 999' 9889 999'
1070 1031" 1060 102g6 1039O 1065 1032' 10406 1065 1031" 1057 1033c 1054 1027" 1032" 1056 1030 1051 1035 1044e 1058 1034 1058 1039" 1059 Obscured by perchlorate 1051 1017" 1051 10440 1056 1057 1058 1075C 1057 1 0 W 1050c 1068 1023" 1065 1065 1065 1066 1066 1066 1062 1057 102gd 1050 1028d 1052 102gd 1043" 1055 1028d 1055 1024d 1058 102gd 1058 1028d 1048" 1069 1011 1028d 1049" 1064 1015 1028d 1048" 1064 1019 102gd 1064 1018 1024d 1064 1020 1028d 1065 1018 102W 1065
1012 1012 1009" 1015" 1015" 1007r 10070 1006
1025h
1043 1043 1043 1043 1041 1041 1043 1043 1043 1043
From data of Zingaro and Tolberg.z * From data of Glusker and Thompson.3 Band of weak intensity; all other bands are strong to very strong in intensity. Large Probably ring vibrational frequency associated with benzyl group. Shoulder. * Benzoate absorption band. One of the referees decrease in band intensity. f Increase in band intensity. questioned the meaning of a 1.0 F BrCl solution since BrCl is an equilibrium mixture of bromine and chlorine. Experimentally, such a solution is 0.5 F with respect to bromine and chlorine and the total halogen concentration is 1.0 F. In the presence of the amine donor the equlibrium mixture is assumed to exist almost quantitatively in the form of the complex B:BrCl, where B is the donor molecule. This assumption is sup orted by the existence of stable solids such as Py:BrCl and hysical properties of the solution, e.g., the non-volatility of txe halogens, which differ radically from the properties of the gee equilibrium mixture. 5
@
RALPHA. ZINGAROANI W. B. WITMER
1708
gated are listed in Table 111. The highly characteristic band (quinoline excluded) and one of considerable interest in this study is the very sharp and intense absorption near 1000 cm.-l which is found a t a low frequency of 988 cm.-' for 2-bromopyridine and a t a high frequency of 997 cm.-1 in the case of the 2-benzyl derivative. The pair of intense bands a t 1027 and 1070 cm.-' characteristic of pyridine does not persist through the series. With the exception of 2-bromopyridine and 4-namylpyridine in which the two bands remain, but with the band of lower frequency greatly diminished in intensity, this pair of bands is replaced in the substituted compounds by a single intense absorption a t 1060 f 15 cm.-l. These observations are in excellent agreement with those of other investigators.lOJ1 The change from a three to a two band system in this region appears to be characteristic of monosubstitution in the pyridine ring. Both benzyl derivatives show strong absorption a t 1029 cm.-l, which undoubtedly is due to aromatic ring vibrations in the benzyl group. The 4-benzylpyridine also possesses a weak absorption a t 1002 em.-' probably due also to the benzyl group. Neither of these bands is affected by the addition of halogen. Examination of Table I11 shows that the disappearance of the 1000 cm.-' band and the simultaneous appearance of a new, intense band a t a frequency of up to 31 cm.-' higher (Table 111, 24, 25) is characteristic not only of solutions of halogens and interhalogens with pyridines, but also of the crystalline, solid derivatives (Table 111, 5-12, 25-27, 44). It has been noted2 that the 990 cm.-' band of pyridine is replaced by a band a t successively higher frequencies in the series PyIz (solution), (IPy)Br, (1Py)Cl and (1Py)F. Similar trends were observed in the present investigation for the series BI2, BIBr and BIC1, where B represents the coordinating pyridine base. Also, as shown in Fig. 1, and in Table 111, the addition of BrCl invariably results in the 1000 cm.-' band being located a t a higher frequency than is observed on the addition of bromine alone. The effect of changing the basic strength of the coordinating amine can be seen by examining the data in Table I11 (45-54) and Fig. 2. Although numerous attempts were made to prepare solid derivatives of 2-chloropyridine and 2-bromopyridine, only one solid was obtained, the mono-(2bromopyridine)-iodine(1) benzoate, characterized in the 1000 cm.-' region by a pair of bands a t 1006 cm.-' and a t 1043 em.-'. Examination of Fig. 2 shows that the 988 cm.-l band of 2-bromopyridine diminishes gradually in intensity as the bromine concentration is increased. It persists as a shoulder even when the bromine is present in a molar ratio of four to one. The 988 cm.-' band of the free base is eventually replaced by a new band at 999 cm.-'. Also, comparison of Figs. 1 and 2 shows that the new band a t 999 cm.-l in the case of the 2-bromopyridine solution, is broader and less intense than that of the free donor, whereas the new band is as
90 /
80
70
*s 3 60
i
c.$r
50
40
30
I
VoI. 64
I
10 A, CI.
Fig. 1.-Infrared absorption in 1000 cm.-l region: a, 2-namylpyridine (1.0 M ) in CHCla; b, same ria a Br2 (1.0 M ) ; c, same asma,,+ BrCl (1.0 M ) .
+
(10) G. L. Cook and F. M. Churoh, THIS JOURNAL, 62.458 (1957). (11) A . R. Katritzky, J. N. Gardnerand A. R. Hand& J . Chsm.Soc., 2198 (1958).
Nov., 19GO
INFRARED STUDIESOF AMINE-HALOGEN INTERACTIONS
sharp and as intense, if not more so, in the case of the halogen complexes formed with the strong donor. It would be desirable to be able to compare solution spectra with those of solids in the case of every amine studied. This was done wherever possible. Considerable effort was devoted to the preparation of coordinated halogen(1) salts using silver salt metathesis reactions, and to the preparation of crystalline molecular addition compounds, but it was possible to isolate only very viscous oils in many cases. For example, both iodine and bromine reacted rapidly and quantitatively with silver salts in the presence of 2-amyl-, 2-benzyl- or 4-benzylpyridines, but crystalline derivatives were not obtained. The highly viscous, non-crystallizable products that were isolated possessed the chemical properties characteristic of halogen(1) salts, e.g. rapid oxidation of iodide. Also, their infrared spectra were examined and they showed bands typical of the solids, i e . , the lower frequency absorption a t 1000 cm.-' region replaced by a band at a frequency of 15-35 cm.-' higher than that found for the free base. These data are not tabulated since these oils could not be characterized as definite compounds. Their formation and properties, however, do corroborate the conclusions which will be drawn concerning the nature of the amine-halogen interactions. It is to be noted also that in every case, the characteristic absorption at lOGO f 15 cm.-' is retained, even following halogen, or interhalogen interaction.
1709
F, em.-1.
lo00
90
80
TO
60 &+.
.-$
'iP 50 9
i;
Discussion In the first paper of this series2 the interpretation 40 of the data was based on the assumption that the 990 cm.-' absorption of pyridine was an in-plane hydrogen deformation mode. The new band observed on halogen complexing was interpreted as a shift in this band due to the influence of the large, 30 polarized iodine atom on the adjacent ring hydrogens. A similar argument can be made for the effects presently observed. Such an interpretation, while plausible, requires that the bromine interaction be stronger than that of iodine since bromine has been found consistently to bring about a larger shift in the 990 cm.-' band of the various amines in20 vestigated than does iodine. It is difficult to conceive of the bromine molecule being more highly polarized than iodine. This argument is also iiiconsistent with the effects that would be expected when the relative electronegativities and the sizes of bromine and iodine are compared. Part of the difficulty in the interpretation was 10 due to the lack of agreement in the assignment of the 990 cm.-l band which is highly characteristic 10 10 of pyridine and monosubstituted pyridines. For A, P. instance, Bellamy12 reports that this band is due to either a ring vibration or a hydrogen deformation Fig. %-Infrared absorption in 1000 cm.-l region: a, 2mode. In independent studies this band has been bromopyridine (1.0 M) in CHCl,; b, same aa a + Br2 (0.3 assigned to a r-CH modell and to a symmetrical M); e, same rn a Br2 (1.0 M ) ; d, same as a Brz (2.0 ring vibration.'O In earlier studies on deuterated M); e, same as a Brz (4.0 M). pyridines,13-14this absorption was assigned to an A1
I L ++
(12) L. .J. Bellamy. "The Infrared Spectra of Complex Molecules " John Wiley and Sons, Inc.. New York, N. Y . ,second edition, 1958, p. 277.
+
(13) L. Corrsin, R. J. Fos and R. C. Lord, J. Cir
