III N 131 225 STUD-25$ ON EODINE COMPLEXES 0F SUBSUTUTED PYREDENES THOSE ‘50.!" fin Dawn 0% M. S. MICBEGAN STATE UNEE’ERSEY‘Z Ming Keong Wang 1965 MICHIGAN STATE UNIVERSITY EAST LANSING. MICHIGAN wESIS ,. 4 Emu ABSTRACT STUDIES ON IODINE COMPLEXES OF SUBSTITUTED PYRIDINES by Ming Keong Wong Halogen and interhalogen compounds can act as Lewis acids and form molecular complexes with.ebctron donors. Aromafic nitrogen- containing organic compounds such as pyridine or substituted pyridines, are stronger electron donors than their carbon analogs, and hence they form stronger molecular complexes with Lewis acids. Of all the halogens iodine forms relatively weak molecular com— plexes with organic bases. On the other hand it is not as strong a halogenating agent as the stronger Lewis acids such as iodine mono— chloride or iodine monobromide. It is, therefore, easier to work with since the probability of side reactions is minimized. The mole ratio method was used for the determination of formation constants of iodine with h-picoline, 2,h-lutidine, 3,5—lutidine, 2,3,6— collidine, h-phenylpyridine, and 3,5—dichloropyridine. Spectrophoto- metric measurements were made on solutions of iodine and the respective pyridine bases in carbon tetrachloride solutions. From plots of amine/iodine mole ratio, the molar absorptivities of the undissociated complexes were obtained. These values were then used for the calcula- tion of degrees of dissociation and formation constants of the com— plexes. For very weak complexes (such as the iodine - 3,5-dichloro- pyridine complex) Ketelaar's modification of Benesi-Hildebrand method was used. In the case of h—phenylpyridine, neither method gave satis— factory results. This is probably due to the formation of two complexes, Py'IZ and Py-2IZ. Ming Keong Wong The strength of the iodine—pyridine base complexes decreases in the order 3,5-lutidine > 2,b—lutidine > h—picoline > 2;%6-collidine > 3,5—dichloropyridine. The order of basicity of the compounds is as follows: 2,3,6—collidine > 2,b-lutidine > 3,5—lutidine > L—picoline > L—phenylpyridine > 3,5-dichloropyridine. STUDIES ON IODINE COMPLEXES OF SUBSTITUTED PYRIDINES By Ming Keong Wong A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Chemistry June 1965 ACKNOWLEDGMENT I wish to express my very sincere appreciation and thanks to Dr. A. I. POpov, for his constant help and guidance throughout the course of my studies here and during the preparation of this thesis. Acknowledgment is given to the Asia Foundation and the Singapore Government for the scholarship which made this study possible. ii TABLE OF CONTENTS Page HISTORICAL INTRODUCTION . . . . . . . . . . . . . . . . . . . 1 EXPERIMENTAL PROCEDURE . . . . . . . . . . . . . . . . . . . 9 EXPERIMENTAL RESULTS . . . . . . . ... . . . . . . . . . . . 15 DISCUSSION OF RESULTS . . . . . . . . . . . . . . . . . . . . 38 BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . A2 LIST OF TABLES Table Page \] l pKa values of some substituted pyridines . . . . . . . 2 Formation constants of some substituted pyridine - iodine complexes . . . . . . . . . . . . . . . . . . . 5 3 Spectrophotometric data of mole ratio method on the pyridine - iodine system in carbon tetrachloride . . . 17 b Spectrophotometric data of mole ratio method on the 2,h—lutidine - iodine system in carbon tetrachloride . 21 5 Spectrophotometric data of mole ratio method on the 3,5-lutidine - iodine system in carbon tetrachloride . 2b 6 Spectrophotometric data of mole ratio method on the 2,3,6—collidine - iodine system in carbon tetrachloride 27 7 Spectrophotometric data for Ketelaar's Plouson the b-phenylpyridine - iodine system in carbon tetrachloride . . . . . . . . . . . . . . . . . . . . 31 8 Spectrophotometric data for Ketelaar's Plots on the 3,5-dichloropyridine - iodine system in carbon tetrachloride . . . . . . . . . . . . . . . . . . . . 3h 9 Degree of dissociation and formation constants of iodine-substituted pyridine complexes . . . . . . . . 37 iv LIST OF FIGURES Figure page 1 Absorption spectra of b-Picoline - Iodine system in carbon tetrachloride . . . . . . . . . . . . . . . 16 2 Mole Ratio method on the h—Picoline — Iodine system in carbon tetrachloride . . . . . . . . . . . . . . . 18 3 Absorption spectra of 2,b-Lutidine - Iodine system in carbon tetrachloride . . . . . . . . . . . . . . 20 A Mole Ratio method on the 2,L—Lutidine — Iodine system in Carbon tetrachloride . . . . . . . . . . 22 5 Absorption spectra of 3,5—Lutidine - Iodine system in carbon tetrachloride . . . . . . . . . . . . . . 23 6 Mole Ratio method on the 3,5-Lutidine - Iodine system in carbon tetrachloride . . . . . . . . . . 25 I Absorption spectra of 2,3,6-Collidine - Iodine system in Carbon tetrachloride . . . . . . . . . . 26 8 Mole Ratio method on the 2,3,6-Collidine — Iodine system in carbon tetrachloride . . . . . . . . . . 28 9 Absorption spectra of h-Phenylpyridine - Iodine system in carbon tetrachloride . . . . . . . . . . 30 IO Ketelaar's Plousfor h—Phenylpyridine — Iodine system in carbon tetrachloride . . . . . . . . . . 32 ll Absorption spectra of 3,5—Dichloropyridine - Iodine system in carbon tetrachloride . . . . . . . 33 12 Ketelaar's Plodsfor 3,5-Dichloropyridine - Iodine system in carbon tetrachloride . . . . . . . . . . 35 HISTORICAL INTRODUCTION The Lewis acid character of certain halogen and interhalogen com- pounds has long been amply illustrated by the large number of complexes which they form with electron donors. Among the halogens, iodine is the strongest Lewis acid, due to the ease of which it expands its octet and accommodates extra electron pairs. The donor strength of Lewis bases as well as the influence of molecular structure on the donor strength can be conveniently illustrated by the relative strength of their halogen complexes. Because of the reactivity of the halogens, such studies must be made in chemically inert solvents. Since the classic Spectrophotometric studies of complexing re- action of iodine with benzene and some other aromatic hydrocarbons by Benesi and Hildebrandl, a large amount of work on complex formatiOn involving halogens and interhalogens has been reported by many in- vestigators. A large number Of organic Lewis bases were investigated and their relative basic strength determined. Aromatic nitrogen—containing organic compounds are known to be more basic than their carbon analogs. Consequently, they form stronger complexes with halogens and interhalogens. This is evidenced by the fact that relatively stable solid complexes of organic bases with halogens had been reported many years before the work of Benesi and Hildebrand. The existence of a solid molecular addition compounds of iodine with pyridine was reported in 1933 by Chateletz, and the composition (C5H5N)2—I2 was assigned to the crystals obtained. Chatelet also 2 reported isolation of two hydrated complexes which he identified as IZ-Py(HZO)6 and Iz—Py4(HZO)24. Most of the evidence for the formation of addition compounds of iodine with organic bases come from spectral studies. Zingaro, Vander- Werf and Kleinberg3 have investigated the behavior of iodine in pyri- dine solutions and postulated the reaction to be: 2 12 + Py > IPy+ + I,‘ Infrared studies of iodine with pyridine and 2-picoline in carbon tetrachloride by Mulliken, Gluster, and Thompson4 showed a definite shift of absorption band from those of the individual compounds upon the addition of iodine to the respective organic base. The shift of absorption bands was attributed to the formation of halogen-amine complex. Mulliken and Reid5, in their study of the pyridine-iodine system in heptane, reported the existence of a 1:1 molecular complex. Upon the addition of pyridine the 520 mu peak of iodine in heptane was shifted to D22 mu. This peak was attributed to the absorption by the complex, and the association constant for the complex was calculated to be 290 at 16.70. Since iodine is a relatively weak Lewis acid, its molecular complexes with organic bases are likewise weak, and with few exceptions, spectrOphotometric studies by mole ratio method and method of contin- uous variation do not give satisfactory results. The strongest iodine charge—transfer complex reported in literature is the molecular complex between iodine and triethylamine6 with equilibrium constant 5 of about b.61 x 103 (liter/mole) at 250. However, other studies using 3 stronger interhalogen Lewis acids such as iodine monochloride and iodine monobromide, do show the formation of 1:1 molecular complex7. Moreover, in determining the equilibrium constants of iodine—organic base molecular complex, consistent values are obtained by calculations at different wavelengths based on the assumption of 1:1 complex. Mulliken5>8 suggested the existence of two types Of complex for the pyridine—iodine system: an "outer complex" (A) and an "inner complex" (B) with the structures /I I // ' :- : + (A) (B) Glusker and Miller9 isolated two different addition compounds formed by h-picoline and iodine: Compound I has an ionic structure and compound II is a molecular addition compound, analogous to the addition compound of pyridine and iodine prepared by many workers. Their studies have led to the postulation that compound II is an inter- mediate in the formation of compound I, and the mechanism of the reac— tion is as follows: CH3 CH3 CH3 I2 + I ———> I <._———————> I + I9 / Te») 1“ N6 1 solution , ]I&3 I 1 I Compound II CH3 CH3 CH3 \. \\ + -———————> I :jj: Q) N H I l " t I I63 I Compound I X-ray studies by Hassel, Roemming and Tuftelo on the 1:1 addition compound of h-picoline and iodine showed that the I-I-N arrangement in compound II is linear with the I-I distance of 2.83 A while the I-N distance is 2.31 X. Investigations by Hassel and Hope11 on the reaction product of pyridine and iodine prompted them to postulate the cation of compound I has a similar linear structure as that of (H3C—<\ />N - I - N<\ /> -CH3) instead of the d-addition arrangement as proposed by Glusker and Miller9. compound II: The strength of the molecular complexes formed by iodine and organic bases depends primarily on two factors, the donor strength of the base and on steric effects. Brown, Schlesinger and Cardonlz studied the reactions of boron trifluoride with pyridine and 2,6—1uti- dine and reported that pyridine forms a stronger molecular complex than 2,6—lutidine, though the latter is a stronger base than pyridine. Studies by POpov and Rygg7 on iodine and interhalogen complexes of substituted pyridines also show the effect of steric hindrance. They reported the order of stability of the complexes in each of the iodine or interhalOgen series with the three pyridines as follows: 2—Picoline > pyridine > 2,6—1utidine. The order of basic strength is: 2,6—Lutidine > 2-picoline > pyridine. S S. Basu and J. N. Chandhuri13 investigated the charge-transfer interaction between iodine and a number of substituted pyridines in chloroform solutions. They found the logarithms of equilibrium con- stants of the complexes to be approximately proportional to the pKa values of the respective amine. The equilibrium constants for 2,6- lutidine and 2,h,6—collidine were found to be much less than those expected from the corresponding pKa values. They attributed the dis- crepancies to steric effect of the.methyl groups at 2 and 6 positions. Halleuxl4 reported that in the reaction between phenol and pyri- dine bases < CSHSOH + B C6H50H ——- B the strength of the hydrogen bonds formed varies in the following order: 3,5—lutidine > h—picoline > 2,6—lutidine > 2—picoline > pyridine. The basic strength of the compounds is, 2,6-lutidine > 3,5—lutidinerv b—picolinelv 2-picoline > pyridine. The above mentioned investigations showed that steric hindrance is the main factor in the observed discrepancies between basic strength and strength of the complexes. In the determination of basic strength, very small proton is involved, and there is little steric effect present. Brown and Mihm15 reported absence of any important steric effects in the addition of the proton to pyridine, 2,6—1utidine and monoalkylpyridines. Sacconi, Paoletti and Ciampolini16 studied the heats of neutralization in water of a series of pyridine bases which include pyridine, picolines and lutidines. They found that for pyridine, picolines and lutidines, a linear relationship between the heats of neutralization and basicity constants is followed, indicating the absence of steric effect in the reactions. For large iodine or O interhalogen molecules, the presence of substituent groups adjacent to nitrogen atom will invariably hinder the combination of the Lewis acid with the base, and thus weakening the molecular complex formed. This present study was intended to determine, by spectrophotometric measurements, the relative strength of the complexes formed in carbon tetrachloride solutions between iodine and the following pyridine bases: h—Picoline, 2,h—lutidine, 3,5-lutidine, 2,3,6—collidine, h— phenylpyridine, and 3,5-dichloropyridine. Such studies should yield interesting data on the effects of the basic strength of the organic compounds, and of the steric configuration, on the strength of the molecular complexes formed. The base ionization constants of some methylpyridines have been investigated by several workers.12,14‘2° Their values are not always in exact agreement, but comparison of available data on some of the methylpyridines show that there is a basic agreement as to the order of basicity. The pKa values of methylpyridines quoted in Table I are mostly taken from reference 18. The sources of other data are listed immediately after the name of the compound. It should be emphasized that these data do not allow precise comparisons since some of the values have been obtained by different investigators using different methods and different solvents. The pKa of 3,5-dichloropyridine is not available in the literature. However, it is reasonable to conclude from the structure that the de— crease in pKa value of pyridine by 3,S-dichloro-substitution will be about twice that caused by 3-chloro-substitution. 7 Table I. pKa values of some substituted pyridines Compound pK Pyridine 5.30 2-Picoline b-Picoline 3,5—Lutidine 2,6-Lutidine 2,b-Lutidine 2,3,6-Collidine h-Phenylpyridine(2l) 2—Chloropyridine(22) 3-Chloropyridine(22) 3,5-Dichloropyridine mOUI-xl0\0\0\0\\n oo O From the pKa values, it is seen that the substitution of phenyl group in the h-position of pyridine ring causes a slight decrease in basicity, and according to values given by Favini21, the decrease in basicity is more pronounced when phenyl group is in positions 2 and 3 of the ring. Favini also attributed the observed bathochromic shift in ultraviolet spectra of L—phenylpyridine to a quinoid structure contribution. Indeed the substitution of a phenyl group in the para position has changed greatly the nature of the pyridine ring, so that the ultraviolet absorption spectrum of the compound is similar to the spectrum of biphenyl.23,24 Since benzene is known to form weak complex with iodine, it is expected that for h-phenylpyridine, two reaction sites will be avail- able to coordinate with iodine in the phenylpyridine molecule. It has long been regarded that the linearity of the Benesi—Hildebrand plots provides evidence for the presence of only one 1:1 molecular complex25‘28. Johnson and Bowen29, by using synthetic data, showed that linearity of the Benesi—Hildebrand plot is not by itself sufficient for asserting 8 the presence or absence of more than one molecular complex in solution. , They examined the system tetracyanoethylene-naphthalene-carbon tetra- chloride and found a systematic variation of equilibrium constant with wavelength. Thus they emphasized the necessity to examine the variation of equilibrium constant with wavelength before asserting the presence or absence of equilibria other than 1:1 stable charge-transfer complexes. The h—phenylpyridine-iodine—carbon tetrachloride system will provide interesting data for further examination of the above mentioned rela- tionships. The substitution of electron-withdrawing chlorine atom to the pyridine ring decreases markedly the basicity, the effect being greatest at d-position and smallest at y-position. The formation constants of some of the organic base-iodine complexes as reported by Popov and Rygg are listed in Table II. The 2-chloro- pyridine—iodine system in n-heptane has been investigated by Witmer and Zingaro30, and the reported Kf for the 1:1 complex is also listed: Table II. Formation constants of some iodine-substituted pyridine complexes. Complex Kf Pyridine-iodine lOl (25°) 2-Picoline-iodine 150 (25°) 2,6—Lutidine—iodine 50-8(25°) 2-Chloropyridine-iodine 3.3(280) EXPERIMENTAL PROCEDURE Reagents Carbon Tetrachloride. Carbon tetrachloride used in this study was purified by refluxing it with alkaline potassium permanganate for 2h hours, washing with water, further refluxing with barium oxide for 10 hours, and then distilling two times from fresh barium oxide. The final distillation was done using one meter fractionating column packed with glass beads. Iodine. Iodine obtained from J. T. Baker Chemical Co. was puri- fied by sublimation from a mixture of twenty grams of iodine, four grams of calcium oxide, and four grams of potassium iodide. The iodine thus Obtained was resublimed without adding calcium oxide and potassium iodide. b-Picoline. Aldrich Chemical Company 95% pure h—picoline was stored over sodium hydroxide pellets for ten days and then twice dis- tilled from fresh sodium hydroxide, using one meter packed fractionat- ing column. The fraction boiling at 11430 at 760 mm was used for sub— sequent spectrophotometric measurements. Reported boiling point of the liquid is lh2—lh3°.31 It was also reported that a sample of picoline of 99.88 moles % purity had a boiling point of th.356° at 760 mm."52 2,A~Lutidine. One hundred grams of 2,h—lutidine (Aldrich Chemical Company) was refluxed over barium oxide for three hours, and then lO fractionally distilled through a one meter fractionating column. The middle fraction, boiling at 156.50 at 755 mm (reported value is 157.10 at 760 mm32) was collected and used for subsequent measurements. 3,5-Lutidine. The purification procedure was analogous to that used for 2,h-lutidine. The fraction boiling at 1710 at 756 mm was collected and used. The reported boiling point is 171.60 at 760 mm.32 2,3,6-Collidine. One hundred grams of collidine (Aldrich Chemical Company) was refluxed with barium oxide for two hours and then distilled through a one meter fractionating column. The fraction boiling at 1690 at 733 mm was collected and used. Literature value for the boil— ing point is 168—1700 at 730 mm.33 h—Phenylpyridine. This chemical was obtained from L. Light and Co. Ltd., England. Initially recrystallization from n-heptane was used in an attempt to purify the compound.32 However, after two recrystal— lizations the crystals obtained were still slightly yellow. The com- pound was finally purified by recrystallizing it from hot water.34 After two recrystallizations, the crystals obtained were pure white and their melting point was 7h.5° (uncorrected). (Lit. val. 7&035 and 77-78032.) 3,5—Dichloropyridine. This compound (Aldrich Chemical Company) was purified by two recrystallizations from 95% ethanol. The product was dried in a desiccator over calcium chloride for 6-8 hours. The solid compound seemed to have a fairly high vapor pressure at room temperature. The pure white crystals obtained had a melting point of 65° (uncorrected). (Lit. val. 6b-65036 and 66-67037.) 11 Preparation of Solutions Stock solutions of iodine of concentrations ranging from 1.0 x 10-3 to 2.0 x 10—3 mole/liter were prepared by dissolving a known weight of iodine in carbon tetrachloride in volumetric flasks. The exact concen- tration of the solutions was determined by titration with standard sodium thiosulfate solution. Stock solutions of h-picoline, 2,h-lutidine, 3,5—lutidine, 2,3,6— collidine, b-phenylpyridine and 3,5—dichloropyridine were prepared by dissolving a known weight of the respective substance in an appropriate volume of carbon tetrachloride. The solutions for Spectrophotometric measurement were prepared by mixing proper aliquots of the two desired stock solutions and then diluting to the desired concentration with the solvent. Spectrophotometric Measurements All spectrophotometric measurements were made on a Cary recording spectrophotometer, Model lb, using silica cells of 1.00 i 0.01 cm. path-lengths. The absorption spectra were measured between BOO mu and 600 mu. All the measurements were made at room temperature vary— ing between 23—2OOC. The absorption of each solution was measured against a reference cell containing the pure solvent. Applications of Spectrophotometric Data Since all the pyridine bases studied form relatively weak complex with iodine, the formula of the molecular complex in solution cannot be.determined by mole ratio method. However, it is feasible to determine 12 the degree of dissociation and formation constant of the individual molecular complex by the mole ratio method, assuming 1:1 complex is being formed in the solution. Measurements and Calculations of Equilibrium Constants It has been fairly well established in a number of previous in- vestigationsl‘lo, that in reasonably dilute solutions (f 10"2 M) heterocyclic amines form l:l complexes with halogens and interhalogens. Thus, the measurements of the complex formation constant were reduced to the determination of the equilibrium concentrations of the complex, I of the free base and of the free halogen in a solution containing known v‘ infijal amounts of the two reagents. At a given wavelength the total absorbance (A) of a halogen-amine solution is A = EBb[B] + €X2b[X2] + EBX2b[BX2] where b is the pathlength of the cell, EB , 6X2 , and EBXZ are the molar absorptivities of the base (B), the halogen (X2), and the complex (BXZ) respectively. If CB = initial concentration of the base, CX = initial concentration of the halogen, and 2 CC = equilibrium concentration of the complex, then A = eBb(cB — cc) + EX2b(CX2 — CC) + EBXZbCC. If we can express the concentration of free base and that of free halogen 13 by OC, where d 2 degree of dissociation of the complex. The equi— librium becomes: A = EBbaC + de + X2bC(l - G). ex 2 EB Since 6 B Of the pyridine bases is negligible between 300-600 mu, the wavelength region selected for this study, the equation reduces further to: A = :XZde + eszbCa—a). The molar absorptivity for the undissociated complex, EBXZ’ is ob- tained from the mole ratio method plots, the molar absorptivity of the iodine, 6X2 , is readily measured, and C is known. Thus, the value of the degree of dissociation, d , can be determined from the above equation. The formation constant was calculated from the equation: OZC For extremely weak complex such as 3,5-dichloropyridine—iodine system, the limiting value of molar absorptivity could not be reached even when excess of more than two thousand mole ratio of the base was used. Consequently,the molar absorptivity of the complex, as well as the formation constant, was calculated using Ketelaar's modification of the Benesi-Hildebrand method. For the reaction in this present study: A + B ———> AB <.__ Ketelaar, Stolpe, Goudsmit and chubas38 have shown that if CB(total) >> CIZ(total)’ then CB T’CB(total)’ the following equation can be derived: 1A 1 l l l —— : — X __ + 6t ‘ 6x 6c ‘ Ex KfCB EC ‘ Ex where 6t is the apparent molar absorptivity of iodine (i.e. measured absorbance of the solution divided by the total concentration of iodine), EX is the molar absorptivity of iodine, EC is the molar absorptivity of the complex, and CE is the equilibrium concentration of the base. The unknowns are E and K 1 C f . A p101; Of Fm VS. 1 C_ gives a straight line, of which the intercept yields the value of EC and the slope, that of Kf . EXPERIMENTAL RESULTS Spectrophotometric Measurements on the Individual Components The absorptions of b-picoline, 2,h-lutidine, 3,5—lutidine, 2,3,6- collidine and 3,5—dichloropyridine were practically zero between 300 mu to 600 mu. At 300 mu, h-phenylpyridine had a molar absorptivity of about 70; at 310 mu, the molar absorptivity decreased to about 7, and above 325 mu, the absorption was practically zero. The solution of iodine in carbon tetrachloride showed an absorp- tion maximum at 517 mu, with a molar absorptivity of 930. The result agrees with the values given by Benesi and Hildebrand (E = 93O)1, and Popov and Rygg (E = 92O)7. Spectrophotometric Measurement on the Molecular Complexes In all the runs of the mole ratio method on the iodine-pyridine base systems, the concentration of iodine was held constant and the mole ratio of pyridine base to iodine was increased until the limit- ing absorbance was obtained. The absorption spectra of all the solu- tions were obtained during a period of one to eight hours after prep- aration. h-Picoline - Iodine Complex. The absorption spectra showed a maximum at b08 mu and an isosbestic point at L72 mu. The molar absorp- tivity of the complex at h08 mu was 1710. The absorption spectra, with the isosbestic point, are shown in Fig. 1. The absorbance and molar absorptivity values, at four wavelengths, for the different runs are shown in Table 3 and the plots of the molar absorptivity values vs. the mole ratio of h-pic/IZ are shown in Fig. 2. 15 16 .AHHH csmv mowpoficomupmp cogomo cw Empmhm 0mm 03 NH I seafloowdiq mo muwoodm coHpQOOmQ< Aiev cpmcofim>m3 om: 00: .a a: 00m 1“ l H.O m.O m.o 2.0 m6 aouquosqw 1 7 Table 3. Spectrophotometric data of mole ratio method on the 8-picoline - 7 12 system in CC14 Conc. M.R. of 808 mu 820 mu 850 mu 517 mu of I2 eric/IZ .As e As e As 6 As 6 Run I 0.0 0.007 18 0.016 31 0.086 165 0.882 923 1.0 .092 177 .095 182 .118 227 .837 836 .0005215 M 5.1 .309 593 .308 583 .215 813 .331 635 20.3 .610 1170 .591 1133 .385 662 .176 337 50.7 .738 1810 .712 1365 .396 760 .110 211 96.0 .790 1515 .770 1877 .825 815 .083 159 192 .833 1598 .800 1535 .835 835 .065 125 287 .83 1600 .797 1530 .825 815 .057 109 383 .85 1680 .811 1556 .825 815 .053 102 880 .869 1668 .820 1572 .832 826 .053 102 752 .863 1657 .810 1553 .825 815 .053 102 1028 .878 1686 .820 1572 .825 815 .053 102 1688 .892 1711 .810 1553 .390 787 .087 90 Run II 0.0 .009 17 .018 27 .081 156 .883 930 1.0 .089 171 .091 175 .118 228 .880 886 .0005196 M 38.0 .685 1320 .667 1283 .372 716 .180 270 85.0 .776 1892 .750 1882 .810 788 .090 173 200 .831 1600 .795 1530 .823 815 .065 125 500 .868 1672 .816 1571 .828 828 .058 108 800 .872 1678 .811 1561 .813 798 .050 96 1000 .887 1706 .823 1585 .815 798 .089 98 1200 .896 1723 .820 1578 .800 770 .087 90 1800 .908 1780 .822 1583 .800 770 .087 90 Run III 0.0 .008 18 .008 29 .087 169 .260 935 1.0 .030 108 .032 115 .057 205 .287 887 .0002781 M '31.8 .302 1085 .293 1052 .175 628 .110 395 95.8 .395 1820 .382 1378 .216 776 .065 238 223 .838 1562 .819 1508 .225 809 .085 162 353 .883 1595 .826 1531 .231 830 .038 137 705 .860 1655 .838 1578 .233 837 .038 122 1058 .866 1675 .880 1581 .230 826 .032 115 1810 .871 1695 .883 1595 .230 826 .030 108 1765 .877 1715 .882 1588 .230 826 .030 108 2115 .878 1723 .838 1578 .219 787 .025 90 2820 .877 1715 .831 1550 .205 737 .022 79 Run Iv 0.0 .010 18 .018 26 .113 163 .686 930 1.0 .188 207 .186 210 .170 288 .578 832 .0006951 M 38.2 .978 1807 .950 1367 .527 758 .169 283 189 1.119 1610 1.078 1550 .578 832 .088 127 887 1.170 1683 1.113 1603 .585 881 .072 108 786 1.195 1720 1.108 1595 .550 792 .068 92 882 1.200 1727 1.108 1595 .580 777 .068 92 1100 1.215 1788 1.113 1603 .585 788 .058 83 l8 .mowooficomopmb conumo cm Eowmzm NH I ocHHoowdiq och :o poops: oflpmm 0H0: .m .mwm NH 6» seaaoeaa-a do oaemm sac: OQCH coma OOmH OQOH 00w 000 004 00m 00 fl O P \J O . is Nam 11 r. o o o b.0m 0 .oo: . I 2.00 z. U . as as A. d o I o _ a .000H 0 \fooma o \. Lpofi. C o I 'NIrA TO SI 18 won \ IOOwH l9 2,8-Lutidine - Iodine Complex. The absorption spectra showed a maximum at 812 mu, and the molar absorptivity of the complex at this wavelength was 1680. The isosbestic point showed by all the runs was at about 873 mu. The absorption spectra, with the isosbestic point, are shown in Fig. 3. The absorbance and molar absorptivity values, at four wavelengths, for the different runs are given in Table 8 and the plots of the molar absorptivity values vs. the mole ratio of 2,8-lut/I2 are shown in Fig. 8. 3,5—Lutidine - Iodine Complex. The maximum absorption wavelength of the complex was at 809 mo, and the molar absorptivity at that wave- length was 1690. The isosbestic point was at about 872 mu. The ab— sorption spectra, with the isosbestic point, are shown in Fig. 5. The absorbance and molar absorptivity values, at four wavelengths, for the different runs are given in Table 5 and the plots of the molar absorptivity values vs. the mole ratio of 3,5—1ut/I2 are shown in Fig. 6. 2,3,6-Collidine - Iodine Complex. The absorption spectra of the complex showed a maximum at 822 mu, and the isosbestic point was at about 879 mu. The molar absorptivity at 822 mu was 1600. The ab- sorption Spectra, with the isosbestic point, are shown in Fig. 7. The absorbanceewfl.molar absorptivity values, at four wavelengths, for the different runs are given in Table 6 and the plots of the molar ab- sorptivity values vs. the mole ratio of 2,3,6-coll/I2 are shown in Fig. 8. 20 .AH cumv oowuoficomupmp conumo cw Empmzm NH I ocwowpzquqam mo mopoodm comeDOmb< 0mm 00m Anev cpmcofim>m3 6m: 03 .m .mE 00m 1 1.] d m6 aouquosqv 21 Table 8. Spectrophotometric data of mole ratio method on the 2,8—1utidine - 12 system in CC14 Conc. M.R. of 800 mu 8l2 mu 880 mu 517 mu of I2 2,8—Lut/I2 As 6 As 6 As 6 As 6 Run I 0.0 0.000 O 0.001 8 0.028 86 0.256 920 1.0 .033 119 .037 133 .086 165 .280 862 .0002781 M 5.7 .137 892 .150 580 .120 832 .189 680 53.8 .352 1265 .385 1388 .278 985 .070 252 188 .395 1820 .830 1585 .302 1085 .088 158 370 .820 1510 .857 1688 .318 1128 .038 122 555 .819 1508 .858 1650 .321 1158 .030 108 780 .827 1537 .859 1658 .320 1150 .027 97 Run II 0.0 .008 8 .013 13 .091 88 .950 915 1.0 .310 298 .350 337 .307 295 .766 737 .001039 M 89.5 1.505 1850 1.627 1565 1.186 1108 .189 188 188 1.578 1520 1.705 1680 1.185 1180 .118 110 288 1.627 1565 1.738 1673 1 200 1155 .105 101 308 1.632 1571 1.737 1672 1.177 1133 .100 96 810 1.655 1592 1.767 1700 1.175 1130 .098 98 513 1.683 1620 1.770 1705 1.180 1135 .095 92 Run III 0.0 .006 11 .010 19 .089 98 .888 933 1.0 .111 212 .122 233 .121 231 .831 826 .0005230 M 87.7 .733 1801 .790 1510 .558 1058 .088 168 209 .808 1585 .868 1610 .595 1137 .060 115 318 .815 1558 .878 1670 .596 1139 .055 105 818 .819 1565 .879 1679 .596 1139 .053 101 523 .835 1595 .886 1693 .595 1137 .050 96 Run Iv 0.0 .008 6 .008 13 .058 87 .575 922 . 1.0 .189 239 .163 261 .157 252 .898 793 .0006236 M 80.0 .850 1362 .925 1882 .650 1082 .120 192 175 .950 1522 1.028 1682 .713 1185 .070 112 263 .960 1538 1.035 1660 .710 1138 .067 107 382 .970 1555 1.088 1675 .708 1135 .063 101 512 1.000 1602 1.067 1710 .708 1130 .058 93 577 .992 1590 1.066 1709 .710 1138 .057 91 22 00m ovweoacomppop conpmo cw Eopmzm NH I oompwpsquam och co 605002 owpmm 0H0: .3 .mflm NH 68 ecaeaesa-a.m we oaamm 6H6: 8a 0% 8m 6% 8m 8m 62 o . a . . . 1 . O O - as .1. Hm DC a .. ooN .62 O .. C .1 . 00w . oooa O nu IIAV as on; .ooma . .ooaa 3 q is I o co: .1) .ooea anI as was 1x .oowa .A>H czmv opmooacomwpmw conpmo cw Empm%m NH I OCHUHpDAImxm mo mupoodm compdoomn< Aiev chmcofio>m3 00: com 23 \\ l .m a: 0.0 H.O N.O m.o 4.0 m6 aoueqdosqv 28 Table 5. Spectrophotometric data of mole ratio method on the 3,5-1utidine - 12 system in CC14. Conc. M.R. of 809 mp 830 mu 860 mu 517 mu of I2 3,5-Lup/IZ As 6 As e As 6 As 6 Run I 0.0 0.006 12 0.022 88 0.138 268 0.870 927 1.0 .137 273 .125 289 .158 316 .801 800 .0005018 M 5.0 .819 838 .350 698 .212 823 .263 525 20.0 .672 1380 .552 1100 .257 513 .125 250 50.0 .779 1555 .635 1267 .283 568 .080 160 101 .815 1627 .660 1316 .288 575 .068 128 201 .888 1685 .675 1386 .285 568 .052 108 302 .882 1680 .670 1338 .275 588 .086 92 802 .836 1670 .660 1316 .275 588 .085 90 Run II 0.0 .005 10 .021 82 .128 256 .866 932 1.0 .130 260 .120 280 .153 306 .801 802 .0005005 M 5.1 .818 828 .350 700 .218 828 .271 582 20.5 .693 1386 .565 1130 .270 580 .138 268 101 .820 1680 .665 1330 .288 576 .065 130 202 .835 1670 .677 I358 .291 582 .058 108 303 .886 1692 .676 1352 .289 578 .051 102 808 .882 1688 .667 1338 .278 556 .086 92 505 .886 1692 .667 1338 .280 560 .086 92 Run III 0.0 .012 18 .080 88 .218 258 .773 930 1.0 .300 361 .270 325 .273 328 .625 752 .0008318 M 25.6 1.275 1535 1.038 1289 .860 553 .188 178 156 1.807 1692 1.118 1385 .868 568 .088 101 312 1.826 1715 1.113 1380 .851 582 .077 93 390 1.838 1730 1.123 1352 .858 586 .075 90 868 1.867 1763 1.130 1360 .860 553 .075 90 Run IV 0.0 .009 32 .017 61 .076 275 .260 935 1.0 .056 201 .058 198 .085 305 .280 862 ,9002781 M 6.1 .193 693 .163 585 .111 800 .168 608 153 .860 1652 .370 1330 .161 578 .080 188 257 .865 1670 .378 1358 .163 585 .038 122 350 .867 1678 .372 1338 .159 572 .031 112 867 .876 1711 .378 1358 .160 575 .030 108 583 .888 1755 .388 1381 .163 585 .030 108 25 .mcmuoHcOmupop conpwo cw Eopwxm NH I mumpwpsquxm Nap NH 68 esseMDsDIm.m so oases was: so access oassm mass .6 .maa QB afim ASN Ada o a . T A. O Q )0! Av! 18 me a com . . . u_ooa — as 06: < c .86 O. \ ”cow 0 .oooH .003 O o o L; . as ems Leona O OH «0 IA; .1 a I I 18 003 C cows 26 .AHHH csmv mcwpoHcomppop conumo cm 0mm Espmxm NH I NCHUHHHoquxmAN Ho mspomdm compasomn< com Aiev CpmcmHm>w3 ems III 004 .s 6E 00m . 0.0 Lfi O aoueqaosqv 27 Table 6. Spectrophotometric data of mole ratio method on the 2,3,6-collidine - I2 system in CCl4 Conc. M.R. of 800 mu 822 mu 850 mu 517 mu of I2 (kfllidine/Iz As 6 As 6 As 6 As 6 Run I 0.0 0.005 10 0.018 27 0.086 165 0.882 923 1.0 .022 82 .037 73 .100 192 .871 903 .0005215 M 5.0 .070 138 .105 201 .188 288 .880 888 9.9 .121 232 .176 337 .198 380 .803 777 50.0 .380 652 .857 875 .385 738 .250 878 96.3 .818 803 .570 1092 .870 900 .187 359 289 .525 1008 .706 1355 .555 1063 .118 226 883 .560 1075 .787 1833 .581 1115 .108 200 701 .590 1131 .767 1872 .592 1135 .095 182 1000 .618 1176 .786 1510 .595 1180 .088 169 1850 .663 1271 .809 1552 .598 1188 .083 159 1982 .653 1252 .805 1582 .592 1135 .081 155 3820 .776 1890 .858 1686 .592 1135 .073 180 Run II 0.0 .008 8 .027 26 .170 168 .970 933 1.0 .062 60 .103 99 .218 210 .919 885 .0010393 M 28.9 .670 685 .916 882 .772 782 .886 868 96.8 1.010 972 1.337 1287 1.060 1020 .330 318 802 1.210 1165 1.588 1890 1.180 1135 .180 173 810 1.370 1320 1.613 1552 1.185 1180 .160 158 1215 1.885 1830 1.685 1620 1 200 1155 .157 151 1620 1 627 1565 1.730 1665 1.180 1135 .186 181 2025 1.828 1780 1.785 1718 1.182 1137 .188 139 Run III 0.0 .001 8 .008 28 .086 165 .262 981 1.0 .006 22 .013 87 .089 176 .258 913 .0002781 M 37.2 .108 378 .185 521 .180 503 .187 672 130 .187 672 .272 977 .230 827 .122 838 583 .270 970 .371 1335 .302 1085 .078 282 905 .299 1075 .395 1820 .308 1108 .057 205 1510 .380 1221 .817 1500 .315 1133 .050 180 1880 .360 1293 .830 1585 .322 1157 .089 176 2255 .359 1290 .829 1582 .320 1150 .087 169 2570 .388 1251 .825 1530 .320 1150 .087 169 Run Iv 0.0 .006 11 .015 27 .090 162 .518 928 1.0 .028 50 .082 76 .106 191 .503 903 .0005561 M 83.3 .383 617 .875 853 .810 736 .278 500 130 .500 900 .678 1220 .550 988 .172 309 260 .571 1028 .755 1358 .600 1078 .132 237 758 .663 1198 .838 1508 .688 1158 .100 180 1138 .800 1880 .890 1598 .652 1171 .098 169 1520 .828 1890 .900 1618 .650 1168 .087 156 2085\ .832 1896 .907 1630 .687 1162 .086 188 28 l. .NUHHoHcomHHNH conHmo CH Empmxm NH I NCHUHHHooIoamam 05H co Boxwoa owpmm 6H0: .w .mmm NH 6N NeasaHHOQI6Am.N Ho oases 8H6: 00m H OOQH 00m H OOOH OOm 000 00.4 00m 0 .IIJ']. 1 1 . Iq . A d . 0 V l I I0 Lb _ as Nam l. I. P ) ._ VI 1C .18 Oman. .OONH D. l as 004 QIIIQ‘. 18mm: .1 C) 1 OOOH 29 8-Phenylpyridine — Iodine Complex. The absorption spectra of the complex seemed to depend markedly on the concentrations of iodine and 8-phenylpyridine. From Runs II to V, the maximum absorption peak was at about 810 mu, and the isosbestic point at 873 mu. The Ketelaar's plots showed that a straight line generally could not be obtained. In the wavelength region between 810 mu and 830 mu, a fairly straight line might be drawn through the points. However, the formation con- stants calculated at 810 to 830 mu differ widely, ranging from about 80 to 200 (liter/mole), and the values were reproducible for runs (IV and V) with approximately equal iodine concentration. The absorption spectra, with the isosbestic point, are shown in Fig. 9. The data for the Ketelaar's plots, at six wavelengths, are given in Table 7 and the Ketelaar's plots are shown in Fig. 10. 3,5—Dichloropyridine — Iodine Complex. The absorption spectra of the complex showed a maximum at 880 mu, and the isosbestic point was at 885 mu. The molar absorptivity at 880 mu was 1320. The absorption Spectra of the complex are shown in Fig. 11. The data for Ketelaar's plotg at four wavelengths, for the different runs are shown in Table 8 l — E x and the Ketelaar's plots of E t vs. are shown in Fig. 12. .1. CB Detailed study of change in absorbance of the solutions with time was not carried out. About half of the solutions of every complex were rerun after a period of twenty—four to thirty hours. With the excep— tion of 8—picoline and 8-phenylpyridine, only slight changes (from 0.3 to 8 per cent) in absorbance were observed. For 8-picoline and 8— phenylpyridine, greater changes in absorbance (from 5 to 8 per cent) were observed for several solutions. No correlation of change in 3O .AHH csmv NBHHoHcomHHNH cooomo CH Eopmxm NHIocHUHHHQHzcochq Ho mppooam coHHQHomnd Ajay cpmcoHo>m3 0mm 08 om: cos .a 6.: 00m ‘ 4 .0.0 H.O m.0 4.0 m0 aouquosqw 31 Table 7. Spectrophotometric data for Ketelaar's plots on 8—phenyl— pyridine - 12 system in CCl4 Conc. M.R. of _l_ l 4 of I2 B/IZ CB at - exx 10 390 800 810 820 830 850 mu mu mu mu mu mu Run I 20 100 9.65 8.30 7.92 8.36 9.76 17.60 50 80 6.72 5.99 5.82 6.19 7.15 12.99 .0005010 M 200 10 5.03 8.83 8.75 5.09 5.90 10.10 399 5 3.88 3.61 3.98 8.37 5.22 9.20 898 8 3.08 3.28 3.61 8.12 8.93 8.77 Run II 73 89.5 9.88 7. 83 6.88 7.13 8.12 15.15 185 28.8 8.08 6. 61 6.11 6.82 7.36 13.65 .0002781 M 298 12.2 7.88 6. 23 5.79 6.08 6.95 12.82 882 8.1 7.19 5. 97 5.60 5. 89 6.79 12.67 735 8.9 6.85 5. 82 5.50 5. 78 6.73 12 67 Run III 198 28.7 8.08 6.60 6.11 6.38 7.20 13.30 587 8.16 6.98 5.85 5.88 5.70 6.52 12.02 .0002086 M 980 8.88 6.12 5.26 5.08 5.31 6.18 11.15 1372 3.89 5.63 8.92 8. 73 5.02 5.79 10.88 Run IV 192 15.1 7.86 6.32 5.78 6.01 6.95 12.75 319 9.02 7.02 5.88 5.52 5.76 6.63 12.09 .0003880 M 887 6.85 6.88 5.76 5 88 5.70 6. 58 12.09 578 5.02 6.87 5.53 5.16 5.52 6. 82 11.78 Run v 171 16.9 7.67 6.38 5. 90 6.17 7.07 13 03 256 11.3 7.30 6.12 5. 78 5.98 6.92 12.68 .0003865 M 381 8.85 6.98 5.82 5. 88 5.78 6.68 12.05 512 5.68 6.55 5. 56 5.30 5. 50 6.88 11.75 683 8.23 6.12 5. 23 5.10 5. 82 6.35 11.52 x 104 ’— L/ "'4 l 32 18 13- 1 /e’ ,r' C) H1 / / H/ 12. ,z’ 45” /0/ / / O V, ”’ 1. 850 mo 11” 2. 390 mu 3. 830 mu 8. 800 mu 10’ 5. 820 mu 6. 810 mu 9. 8, LL 1 1 J 41 J .— 0 3 6 1 9 l2 15 10 CB Fig. 10. Ketelaar's Plotsfor 8-Phenylpyridine—Iz system in carbon tetrachloride (Run V). 33 .AH comv NUHHOHcomHHNH conumo CH Empmzm NHINCHUHHHQQOHQHCOHQImNm Ho mppoodm coHHQHOmnd Anev cpmcoHN>m3 08 0mm 00m 0m: 004 dl .2 a; 00m 0.0 m6 aoueqaosqv 38 Table 8. Spectrophotometric data for Ketelaar's plots on 3,5-dichloro- pyridine - 12 system in CCl4 ___l___ 3 Conc. M.R. of .3; 6t - E X 10 of I 2 B/I 2 c X B 800 mu 820 mu 880 mu 860 mu Run I 282 9.97 6.62 3.60 3.11 8.83 362 6.65 8.67 2.63 2.38 3.32 .0008157 M 883 8.97 3.85 2.22 1.96 2.77 1078 2.23 2.62 1.89 1.33 1.89 1615 1.89 2.27 1.30 1.17 1.68 Run II 1.0 -- -- -- -- ~- 361 10 5.67 3.16 2.88 8.03 .0002781 M 722 8.97 3.71 2.08 1.85 2.67 1070 3.36 3.03 1.70 1.58 2.21 1775 2.03 2.38 1.39 1.25 1.79 2820 1.89 2.16 1.25 1.18 1.65 2880 1.27 2.18 1.22 1.11 1.62 Run III 158 15.60 7.58 8.29 3.80 5.88 231 10.80 5.56 3.16 2.80 3.98 .0008171 M 308 7.80 8.85 2.53 2.28 3.22 388 6.28 3.88 2.19 1.97 2.81 615 3.90 3.00 1.73 1.57 2.22 35 0 2 8 6 1 8 10 CB Fig. 12. Ketelaar's Plotsfor 3,5-Dichloropyridine - I2 system in carbon tetrachloride (Run III). 12 18 36 absorbance with concentrations of iodine and pyridine bases could be observed. However, there was a tendency that greater changes were observed for solutions of high concentration of pyridine base (high pyridine—base/IZ mole ratio). The degree of dissociation and formation constant values for the five complexes are listed in Table 9. For 8—phenylpyridine-iodine system, because of non—linearity of Ketelaar's plots and variation of formation constant value with wavelength, no meaningful values of formation constant can be given. Table 9. Degree of dissociation, formation constants of Iodine - sub- stituted pyridine complexes in carbon tetrachloride. Complex Run ancquf 0 K Kf 2 Average I .0005215 0.908 225 . . II .0005196 .908 226 L‘P1C°llne ‘ 12 III .0002781 .985 222 223 IV .0006951 .882 218 I .0002781 .923 325 . . II .0010390 .813 290 2’8"L”t1d1“€ ‘ 12 III .0005230 .871 325 317 IV .0006236 .852 327 I .0005018 .883 881 . . II .0005005 .888 822 3’5‘LUt1dlne ' 12 III .0008318 .780 835 838 IV .0002781 .898 855 I .0005215 .9g8 i2.8 . . II .0010390 .9 3 9.7 2,3,0-C0111d1n6 - 12 III .0002781 .987 148.1 50.9 IV .0005561 .972 53.2 I .0008157 -- 3.63 3,5—Dichloropyridine - I2 II .0002781 -— 3.89 3.77 III .0008171 -- 3.78 DISCUSSION OF RESULTS The complex formation of iodine with certain substituted pyridines have been studied in carbon tetrachloride solutions. In each case a 1:1 complex between the amine and the halogen was expected7'10. As will be shown below, in the case of 8-phenylpyridine some evidence was obtained for the formation of higher complexes. In most cases the molar absorptivity of the complex was obtained by the extrapolation of the mole-ratio plots.7 It should be noted that in the case of very weak complexes, such as 2,3,6—collidine-iodine complex, very large excess of the base had to be added (up to 2000 fold excess) before the absorbance became reasonably constant. Under these conditions the solutions were quite concentrated in the base and small deviations in the molar absorptivity were observed, which could be due to the change of the refractive index of the solution7,34. In any case the change in the extrapolated molar absorptivity was quite small and should not affect the calculated value of the complex formation constant. The data obtained in this investigation indicated that the posi- tions of the absorption maximum as well as those of the isosbestic . point are somewhat related to the strength of the complex. Thus for 3,5-lutidine—iodine complex, which is the strongest complex in the series, both maximum absorption peak and isosbestic point are at shorter wavelengths than that of the other complexes, while 3,5—dichloropyridine has the longest wavelengths for its maximum absorption peak and its isosbestic point. 38 39 The 8—phenylpyridine-iodine complex offers an interesting example of a case where more than one complex can be expected to form in solu- tion. It has long been regarded that the presence of only one absorb— ing complex was evidenced by a straight line resulting from the Benesi- Hildebrand plot.25728 Recently, however, Johnson and Bowen29, using synthetic data, showed that a straight line could be obtained from the Benesi—Hildebrand plot even though two different complexes might be present in solution. They concluded that constancy of formation constant calculated at different wavelengths was a better criterion for assert— ing the presence of only one complex. This present study of 8—phenyl- pyridine-iodine complex showed that at low iodine concentration a fairly straight line could be obtained between 810 mu to 830 mu, using Ketelaar's modification of Benesi—Hildebrand plots. At other wave— length regions studied, reasonably straight lines generally could not be obtained. However, even in the region that gave fairly straight lines, the resulting formation constant calculated at different wave- lengths differed widely, ranging from 80 to 200 (l/m). From Runs IV and V, with approximately equal concentration of iodine and excess 8- phenylpyridine, the results were reproducible. The values varied with iodine concentration and also were affected by the concentration of the base. In Run I, at higher iodine concentration, the maximum ab- sorption peak shifted from 810 mu at 8-phenylpyridine/iodine mole ratio of 20 to about 375 mu at mole ratio of 500. This drastic shift of maximum absorption was not observed in the other compounds studied. The presence of more than one absorbing complex was further affirmed by the high molar absorptivities obtained from Ketelaar's plots. It 80 is likely that at relatively low iodine/base mole ratios, the complexa- tion occurs primarily through the nitrogen of the pyridine ring. At higher concentration of iodine, the complexation of phenyl group with iodine becomes more extensive and thus causes a change in molar ab- sorptivity and shift of maximum absorption peak to lower wavelength. Due to lack of time the study of the two 8-phenylpyridine-iodine complexes were not pursued further. From Table I, it is seen that 2,8-lutidine is slightly more basic than 3,5-lutidine. However, due to the presence of one methyl group adjacent to the complexing nitrogen atom, the complex formation is slightly hindered. Thus 2,8—1utidine forms a weaker complex than 3,5—1utidine. The effect of steric hindrance is more pronounced for 2,3,6—collidine, in which two methyl groups are adjacent to the com— plexing nitrogen atom. Thus though 2,3,6—collidine is the strongest base, it forms the weakest complex among the four methyl pyridines studied. Comparison between the formation constants of iodine with 3,5—dichlor0pyridine and 2—chloropyridine also reveal the steric effect of the chlorine atom adjacent to complexing nitrogen atom. Al- though 2-chlor0pyridine is more basic than 3,5-dichloropyridine, steric hindrance again renders the 2—chlor0pyridine-iodine complex to be slightly weaker than that of the 3,5-dichloropyridine. The strength of the iodine-pyridine base complexes follows the order 3,5-lutidine > 2,8—lutidine > 8—picoline > 2,3,6-collidine > 3,5—dichloropyridine. The order of basic strength is as follows: 2,3,6—collidine > 2,8—1utidine > 3,5-lutidine > 8-picoline > 8—phenyl- pyridine > 3,5-dichloropyridine. 81 In summary, the investigations of the interaction of iodine with some substituted pyridines have shown the following: (1) The Lewis acid—base interaction of pyridines with iodine does not parallel their basic strength. This is probably due to steric effects. (2) Iodine—8—phenylpyridine system cannot be explained on the basis of a simple 1:1 complex formation. It is possible that the results are indicative of the presence of more than one complex. la. 1b. 2b. 10. ll. 12. 13. 18. 15. 16. 17. 18. H. BIBLIOGRAPHY .A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 19, 2832 (1988). Ibid., 7;, 2703 (1989). M. Chatelet, Compt. rend. $96, 1821 (1933). Ibid., 196, 1607 (1933). R. O. H. . Hassel, Chr. Roemming and T. Tufte, Acta Chem. Scand., 15 . Sacconi, P. Paoletti and M. Ciampolini, J. Am. Chem. Soc., 82 A. Zingaro, C. A. Vander Werf and J. 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L. Kotlyarevski, E. K. Vasil'er and L. I. Vereskchagin, C.A. 56, 9916a (1960). N. Sharpe and S. Walker, J. Chem. Soc., 2978 (1961). Forsyth and F. L. Pyman, J. Chem. Soc., 2912 (1926). Heilbron, Editor, Dictionary of Organic Compounds, Vol. II, Oxford University Press, New York, N.Y. Beilstein Handbuch der Organischen Chemie, Band XX, 231, II 152, J. Springer-Verlag, Berlin, Germany, 1935, 1958. A. A. Ketelaar, C. v.d. Stolpe, A. Goudsmit and W. chubas, Rec. trav. Chim., Z}, 1108 (1952). CHEMISTRY LIIM