Solvatochromism is used to describe a pronounced change in the position of a UV-Vis absorption band with a change in solvent polarity. Betaine-30 (2,6-diphenyl-4-(2,4,6-triphenyl-pyridinio)phenolate) has one of the largest effects ever observed. The compound is red in methanol, violet in ethanol, blue in isoamyl alcohol, green in acetone, and yellow in anisole, thus covering the whole visible range (1). This corresponds to a range for lambdamax of some 357 nm. The solvatochromism probably derives from solvent stabilization of the polar ground state relative to the less polar charge-transfer excited state.
Betaine-30 makes an impressive demonstration, and a picture of the dye in various solvents has recently been included in an organic textbook (2). The multistep synthesis provides students with experience working on a small scale. If only the demonstration is of interest, betaine-30 is available from Aldrich.
The syntheses and solvatochromic properties of betaine-30 and related compounds came out of the PhD theses of Reichard et al. and have been described in the primary (3-5) and secondary (1) literature.The synthesis is convergent, which offers two advantages. Each branch can be run on a modest scale, and a reasonable amount of betaine-30 can still be obtained. The synthesis can be run as a team project, with different students working on each branch. The overall synthesis of betaine-30 is shown in Figure 1.
Figure 1. Convergnet synthesis of betaine-30.
The first branch involves the synthesis of 2,4,6-triphenylpyrilium hydrogen sulfate. This is prepared by an acid-catalyzed condensation/cyclization with chalcone and acetophenone. Chalcone is commercially available but can also be made by a crossed aldol condensation (6). The mechanism of the second reaction provides an interesting challenge, especially because it involves an unexpected oxidation/aromatization step at the end. Our version of the mechanism is shown in Figure 2. The second branch is the synthesis of 4-amino-2,6-diphenylphenol. This can be synthesized from 2,6-diphenylphenol by nitration followed by reduction.
Figure 2. Mechanism for the formation of 2,4,6-triphenylpyrilium hydrogen sulfate.
The convergent step in the synthesis is a mechanistically interesting opening/recyclization reaction to produce betaine-30. We have shown our version of the mechanism in Figure 3. The reaction is interesting to run as well because the mixture becomes ink-black. Recrystallizing dark crystals from a dark solution is a new experience for most.
Figure 3. Mechanism for the convergent step in the synthesis of betaine-30.
The synthetic scheme can be expanded to include the condensation to produce chalcone (6) and the synthesis of 2,6-diphenylphenol. Including these reactions emphasizes the importance of condensation reactions in organic synthesis.
We have used this as an independent study project and also as a group project in an advanced synthesis lab. In both cases, students were able to get reasonable yields and reproducible results. NMR spectra and melting points confirmed the structures of products.
Demonstration
The solvatochromic effect can be easily demonstrated by simply dissolving betaine-30 in a variety of solvents. Quantitation is not critical. An effective method is to put a pinch in the bottom of test tubes or Erlenmeyer flasks and then add the (colorless) solvents to them with mixing.
Experimental Procedure
Caution: All of the reactions reported were carried out on the bench top. In addition to chemical safety goggles, students should wear protective gloves and a lab coat because both 2,4,6-triphenylpyrylium hydrogen sulfate and betaine-30 are highly colored dyes and can be messy without proper protection. The toxicity hazards of 2,4,6-triphenylpyrylium hydrogen sulfate, 2,6-diphenylphenol, 4-nitro-2,6-diphenylphenol, 4-amino-2,6-diphenylphenol, and betaine-30 are not known. Use extreme caution when using concentrated nitric and sulfuric acids. It is also advisable to use glacial acetic acid in a fume hood.
2,4,6-Triphenylpyrylium Hydrogen Sulfate (7)
A 25-mL Erlenmeyer flask was charged with chalcone (6) (4.28 g 0.0206 mol), acetophenone (1.24 g, 0.0103 mol), and conc. H2SO4 (3.2 g). The contents were heated on a steam bath for 3 h. Then 20 mL of water was added. A precipitate formed that dissolved on further heating. In the process of heating, a black oil separated. It was removed by gravity filtration. The filtrate was set aside and yellow crystals formed. The black oil in the filter paper was rinsed with 4 mL of hot water, and the filtrate was treated with 0.20 mL of conc. H2SO4. Upon cooling additional product was collected. The combined yield of 2,4,6-triphenylpyrylium hydrogen sulfate was 2.43 g (58%). (The two batches were not mixed together; the initial batch looked purer and gave a better mp, 266Ð268 ° (lit. (4) 271-273.5.)
4-Nitro-2,6-diphenylphenol (8)
A nitric acid solution was prepared by adding 6.0 mL of 65% HNO3 and 6.0 mL of water. To this solution was added 2.00 g (8.12 mmol) of 2,6-diphenylphenol (Aldrich). After a few minutes the solution became yellow and finally turned orange. Stirring was continued overnight, and the crude product was suction-filtered and washed with water. The crude product was dissolved in hot ethanol and treated with activated carbon. After hot filtration through Celite the solution was concentrated by evaporization and allowed to cool. After cooling in an ice bath, a total of 1.09 g of 4-nitro-2,6-diphenylphenol was collected by suction filtration; 46% yield, mp 135 ° C (lit. (8) 135 ° C).
4-Amino-2,6-diphenylphenol (8)
To 50 mL of hot 5% NaOH was added 1.16 g (4.00 mmol) of 4-nitro-2,6-diphenylphenol. The deep-red solution was stirred vigorously, and solid sodium dithionite (Na2S2O4) was added in small portions until the solution turned yellow. A small excess of Na2S2O4 was added, and the solution was heated for an additional 15 min. As the hot mixture was adjusted to pH 5 with glacial acetic acid, the product precipitated. After cooling, the product was filtered and rinsed with cold water. A total of 0.86 g of 4-amino-2,6-diphenylphenol was recovered; yield 82%, mp 145 °C (lit. (8) 147-148 ° C).
Betaine-30 (Reichardt's Dye, Dye ET-30) (8)
To a small reflux set-up was added 0.172 g (0.658 mmol) of 4-amino-2,6-diphenylphenol, 0.251 g (0.618 mmol) of 2,4,6-triphenylpyrylium hydrogen sulfate, 0.245 g of anhydrous sodium acetate, and 3.1 mL of ethanol. The mixture was refluxed for 3 h, after which 1.5 mL of 5% NaOH was added. The resulting dark-blue crystals were filtered from the warm solution and rinsed with water. The crystals were dried in a vacuum desiccator, and they turned green. The green crystals are the dihydrate form of the dye; yield: 0.232 g (64%), mp 202-280 ° C (lit. (8) 200-275 ° C). The broad melting point has been described by others (9).
Literature Cited
1. Reichardt, C.Solvents and Solvent Effects in Organic Chemistry,. 2nd Ed.; VCH, Weinheim,1988, p. 288.
2. Streitwieser, A.; Heathcock,C. H.; Kosower, E. M., Intr oduction to Organic Chemistry, 4th Ed., MacMillin, New York, 1992, Essay 4, p. 621d.
3. Dimroth, K, Reichardt, C., Siepmann, T., Bohlmann, F., Liebigs Ann. Chem., 1963 661, 1-37.
4. Reichardt, C., Angew. Chem. Internat. Edit., 1965 4, 29-40.
5. Reichardt, C., Angew. Chem. Internat. Edit., 1979 18, 98-110.
6. Durst, H. D., Gokel, G. W. Experimental Organic Chemistry, 2nd Ed., McGraw-Hill, New York, 1987.
7. Chadwick, T.C., Anal. Chem., 1974, 64, 1326.
8. Kessler, M. A.; and Wolfbeis, O. S. Synthesis,1988 8, 635-636.
9. Johnson, B. P., Gabrielsen, B., Matulenko, M., Dorsey, H. G., Anal. Lett., 1986 19, 939-962.
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