July 20, 1955
3883
~-METHYLPYRIDINIUILI IODIDE
[CONTRIBUTION FROM
THE
MALLINCKRODT CHEMICAL LABORATORIES OF HARVARD UNIVERSITY]
Additions to Pyridinium Rings. I. 1-Methylpyridinium Iodide BY EDWARD M. K O S O W E R ~ V ~ RECEIVEDJANUARY 12, 1955 The ultraviolet spectra of aqueous solutions of methylpyridinium iodide have been examined, and Hantzsch’s statement that these d o not obey Beer’s law has been confirmed. Quantitative optical measurements indicate t h a t a new species is present, which is formulated as the product of addition of iodide ion to the pyridinium ring at the 2-position.
A precise and intimate knowledge of the pyridinium ion would be of considerable value for the interpretation of certain reactions, including those in the biochemical field involving diphosphopyridine nucleotide (DPN). For example, the oxidation of 1-methylpyridinium iodide to 1-methyl-2-(1H)pyridone with alkaline potassium ferricyanidea is presumed to proceed by way of an intermediate “pseudo-base” I.4
I CH3
While analogous addition products are known for other quaternary heterocyclic nuclei, e.g., 9-phenyl10-methylacridinium hydroxide,6 direct evidence is lacking in the pyridine series. This article presents such evidence and, in particular, demonstrates the presence of an equilibrium between the dissociated ions of 1-methylpyridinium iodide (IIA, ITB) and another species, for which the structure I11 seems most probable, but which may conceivably be IV or V. IIA C S P
I +
IIB
I n 1911, Hantzsch6 investigated the spectra of 1methylpyridinium salts and other heterocyclic salts. He found that many of them did not obey Beer’s law for ultraviolet absorption in solution; the various color changes associated with certain salts were considered to be due to “chromoisomerism.” This behavior on the part of l-methylpyridinium iodide’ led Hantzsch to propose that two (1) Department of Chemistry, Lehigh University, Bethlehem, Pa.
( 2 ) Postdoctoral Fellow of t h e National Institutes of Health, U. S. Public Health Service 1952-1954. (3) H. Decker, Ber., 26,444 (1892). (4) See discussion by H. S. Mosher, “Heterocyclic Compounds,” Vol. I, R. C. Elderfield, editor, John Wiley and Sons, Inc., New York, N. Y . , 1950, Chapter 8, pp. 415-417. ( 5 ) A . Hantzsch and A I . Kalb, Ber., 32, 3109 (1899). (6) A. Hantzsch,, ibiif., 44, 1783 (1911).
(7) Hantzsch relied t o a great extent upon the spectrum of a chloroform solution which showed two bands not present in aqueous or alcoholic solution. Triiodide ions8 have been shown t o possess very strong absorptions a t wave lengths very close t o those of the “new” bands, and the experiments described in the present work indicate t h a t it
“chromoisomers” of the salt existed, one, the normal benzenoid form, the other, a “quinoid” form for which he favored the structure VI,l0 and which he described as a salt, I
/==\
. S-CH3
\==/
VI
Experimental I-Methylpyridinium iodide was prepared from redgent grade pyridine and methyl iodide in absolute ethanol. The rellowish product was recrystallized from absolute ethanol t o yield white crystals, m.p. 116-117” (reported 116”,” 11701*). Weighing and transferring operations were facilitated if the crystals were fairly large since the solid is hygroscopic. Crystals of convenient size were obtained by slowly cooling a n absolute ethanol solution after seeding; when filtering the salt, it is important to avoid excessive air flow through the solvent-wet solid or a brown tinge indicative of iodine or tri-iodide will appear. Freshly prepared concentrated aqueous solutions of 1methylpyridinium iodide are light yellow in color, but on standing in contact with air this color deepens and thin films actually become brown. Almost all the color is removed by addition of a small quantity of sodium thiosulfate. Consistent and reproducible ultraviolet spectra were not obtained until dilute aqueous sodium thiosulfate solution wai used as solvent for the iodide. A control using a much lower concentration of thiosulfate than t h a t in most of the solutions showed only a slightly lower absorption, indicating that thiosulfate ion was not responsible for a n y important amount of the observed a b ~ o r p t i 0 n . l ~ Aqueous solutions of 1-methylpyridinium iodide 1% ere prepared in the concentrations listed underneath (the letter5 correspond to the curves) Fig. 1. The solvent was aqueous sodium thiosulfate solution, 9.36 X 10-3 N,prepared from Mallinckrodt A.R. grade crystals. The ultraviolet absorptions were measured with a Cary Recording Spectrophotometer, Model 11 M. would be extremely difficult t o prepare and keep I-methylpyridinium iodide solutions which were completely free of triiodide. I n a later article,* Hantzsch reports some additional d a t a on iodides, none of which excludes t h e trivial explanation t h a t triiodide ion is present. Thus, although the low dielectric constant of chloroform might be e r pected t o favor nun-ionic species a t the expense of ionic ones, the evidence t h a t t h e new bands d o represent a “chromoisomer” is far from convincing. I t also appears unlikely t h a t strong absorption a t 3600 3700 A. would be expected for any molecule closely related t o the 1methylpyridinium ion. Chloroform solutions have not been examined a t t h e present time because ionic association would make t h e method used in this paper difficult t o apply, even were it pocsihle til prepare such non-aqueous solutions of the necessary purity. (8) 1’. L. Gilbert, R. R. Goldstein and T. M. Lowry, J . Ciicm. S o c , 1092 (1931). (9) A. Hantzsch, Ber., 51, 1544 (1819). (10) I n the second article9 on this subject, Hantzsch rejected VI and favored a non-ionic structure with five bonds t o nitrogen, RIN-I This may be excluded on t h e basis of the known inability of nitrogen t u expand its valence shell. (11) A. Hantzsch, Ber., 42, 81 (1900) (12) A. B. Prescott, THISJOURNAL, 18, 91 (1896). (13) T h e weak ultraviolet absorption rnf the thiosulfate ion has been measured by H Ley and IC. Konig, %. p h y s i k Chain., B41, 305 (1938). T h e optical density, D ,for t h e concentration used in this work wonld be less t h a n 0.16 a t 2700 A. and absorption falls off rapidly toward longer wave lengths
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