M. J. Minch and S. Sadia Shah University of the Pacific Stockton, California 95211
II AIntroductory Merocyanin Dye Preparation for the Organic Laboratory
Because students enjoy preparing compounds with interesting or unusual properties, we suggest t h e following synthesis of a compound t h a t exhibits pronounced color changes with changes of solvent polarity. For example, solutions of l-methyl-4-[(oxocyclohexadienylidene)ethylidene]-l,4dihydropyridine (MOED)are yellow, red, violet, or blue in water, ethanol, acetone, or pyridine, respectively. T h e effect is most striking when aqueous solutions are diluted with various portions of a n organic co-solvent such as pyridine; the color varies across the whole spectrum from golden yellow t o a brilliant hlue.
Color of Various (MOED) Solutions hMAX(MOED)a inm)
Solvent
Color
Water Methanol Ethanol 2-Propano1 DMSO
Yellow Red-orange Red violet Blue-vlolet Blueviolet Blue
Acetone Pyrldlne
a Wavelength of maximum absorbance for the lowest frequency peak of 5 X IO-~M(MOED).
(nu (MOED)
The preparation of (MOED) involves the N-methylation of 4methylpyridine, followed by condensation of the salt (I) with 4-hydrmyhenzaldehyde in ethanol-piperidine. The resulting henzyl alcohol (11) is dehydrated and deprotonated by gentle heating in aqueous KOH. CHJ
+~
C
H
,
C H , - + ~ c K , I-
(11
As a variation, one of the authors (M.J.M.) assigned the syntheses of higher N-alkyl homologs as special projeets in a second semester "open-ended" organic laboratory at Tulane University (3).The appropriate alkyl halides were synthesized and purified by established procedures (4).
Procedures 1.4-Dimethylpyridiniu Iodide ( I ) Methyl iodide (28.4 g, 0.20 mole) is slowly added to a cold solution of Cmethyl pyridine (18.6g, 0.19 mole) in 20 ml dry 2-propanol and the mixture heated at reflux for 2 hr. The cold solution yields yellow crystals. Two recrystallizations from ahs alcohol yield 28.4 g (60.4%) product; mp 144%.
-
We have found that this synthesis is followed very beneficially hy a brief discussion of electronic spectroscopy and solvent effects. A discussion in terms of the relative importance of the zwitterionic and nonionic resonance structures as a function of solvent (I) generally suffices for sophomores. More sophisticated theoretical aceounb have been published (2). Dilute solutions of (MOED) (5 X 10-5M) in 0.01 Maqueous sodium or tetramethylammonium hydroxide give spectra intense enough to be easily measured on an inexpensivecalorimeter. Because the colors are vivid, students acquire a good appreciation of the relationship between observed color and the absorption spectrum. (MOED) is a weak base and added hydroxide ion is needed in water and most organic solvents to ensure that (MOED) does not exist as the conjugate acid form (111). Without added hydroxide, the absorption spectrum of (MOED) is characterized by two peaks, the higher frequency one corresponding to (111).
1-Methyl-4-[(oxocyclohexadienylidene)ethylidene] 1.4dihydropyridine (MOED)( 5 ) 1.4-Dim~thylpyridiniumiodidr (28.4g, 0.12 m d ~ lfreshly , rerrva. tallized (KtOH-H70,2:1j1-hydroxyhrnrald~hyderl? Sg.0.12 molr!, and . ~nioeridineYl0 ~ - , ,~~ml. 0.10 mole, are dissulved i n 150 ml drv rthand and heated at reflua ior 24 hr. ~oolingthe reaction mixru; yleldsa red precipiutc which is removed try filtration.This solid is rwpended in 7W ml of 0.2Af KOH and hentpd (uithuut boiling! 30 mi". The rwl solution yields hlue-red crystals which are recrystallizedthree times from hot water. Yield: 22 g (86.30%),mp 220°C (lit ( 6 )220). ~7~~~~~
~
Literature Cited (1) B7ooker.L. G.S.,Keyes, G. H., and Heseltine, D. W.,J. Amar. Chem. Soc., 73,5350 (1951);Bmoker, L. G. S., Craig,A. C., Hese1tine.D.W., Jenk1ns.P.W., and Lincoln. L. L., J , Amor. Chom Soc, 87,2443 (1955). (2) McRae.E. G.. J P h y s . Chem.. 61.562 (1957). (3) Nugent,M.J..J.CHEM. EDUC.,49.491(1972). (4) Blalt A. H., (Editor), Org Syn.Coil Voi. 11, 246 (1943). (5) Phil1ips.A. P.,J. Org. Chem., 14,302 (19491. (6) Dauidson, S.J., and Jeneks, W. P., J,Amer Chem. Soc, 91,225 (1969).
Volume 54. Number 11, November 1977 / 709
