Developing an Invisible Message about Relative Acidities of Alcohols

In the Classroom edited by

Ed Vitz Kutztown University Kutztown, PA 19530

Developing an Invisible Message about Relative Acidities of Alcohols in the Natural Products Henna, Turmeric, Rose Petals, and Vitamin A Brahmadeo Dewprashad* and Latifa Hadir Department of Science, Borough of Manhattan Community College, City University of New York, New York 10007 *[email protected]

Introducing chemical concepts to students by using concrete examples and interest-arousing demonstrations is a wellaccepted pedagogy. Demonstrations and activities pertaining to invisible inks are popular (1). Natural indicators obtained from common fruits, vegetables, teas, and flowers have been used in a number of instructor demonstrations and simple student experiments (2-9). These activities are engaging and attract students' attention (1-9). We have developed a demonstration utilizing invisible inks and solutions of three naturally occurring dyes as developing agents. The demonstration not only attracts students' attention but also illustrates the concepts of relative acidities of alcohols, resonance stabilization, and keto-enol tautomerization. A core concept in undergraduate organic chemistry is that the acidity of a functional group increases if the anion formed (from loss of Hþ) is stabilized. Both resonance and inductive effects are considered when evaluating the relative acidities of different molecules. However, many students rely more on recall of memorized facts rather than on deductive reasoning when predicting relative acidities. This may be because many students have difficulties writing resonance structures. These students may feel that they can arrive at a “correct” answer to predict relative acidities (or relative reactivities) without drawing resonance structures. The demonstration can be used to introduce exercises that provide students with additional practice drawing resonance structures and predicting relative acidities. The demonstration uses dilute basic solutions as invisible inks. The inks are made visible using naturally occurring dyes that undergo pH-dependent structural and color changes. One of the developing agents is an ethanolic solution of the 2-hydroxy-1,4-naphthoquinone, which gives henna, a natural dye used in hair colorants and in body decorations, its characteristic color. Resonance theory predicts that 2-hydroxy-1,4-naphthoquinone should lose the hydrogen atom of its vinyl alcohol under mild basic conditions as the corresponding anion formed is resonance stabilized (Scheme 1). Delocalization of the electrons (of the anion) results in a shift of the wavelengths absorbed by the compound and a resultant change in color. A solution of 2-hydroxy-1,4-naphthoquinone is yellow under acidic conditions but changes to reddish orange under basic conditions. A message written on a filter paper with a basic solution such as dilute Na2CO3 or NaOH becomes invisible on evaporation of the solvent. When a solution of 2-hydroxy-1,4-naphthoquinone is sprayed onto the filter paper, the deposited base reacts with the 36

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dye and the anion formed is reddish orange, displaying the invisible message. If a dilute HCl solution is sprayed onto the displayed message, it becomes blurred and disappears due to reformation of 2-hydroxy-1,4-naphthoquinone. The displayed message and its disappearance are shown in Figure 1. An alternative is to use an ethanolic extract of henna as a developing solution. The message developed has a greenish-orange tint. The green color probably arises from the chlorophyll extracted along with the 2-hydroxy-1,4-naphthoquinone from the henna, which is made by pulverizing leaves from the plant Lawsonia inermis. Another developing agent is an ethanolic extract of turmeric, a spice commonly used in Asian and African food. The extract is yellow owing to the presence of curcumin (Figure 2), the compound that gives curry its characteristic yellow color and reputed health benefits. Curcumin exists predominantly as the enol and is yellow under acidic conditions but changes to a red color under basic conditions (10). Delocalization of the electrons of the anion (shown in the supporting information) results in a

Figure 1. Appearance (left and center) and disappearance (right) of messages on filter papers sprayed with a solution of 2-hydroxy-1,4naphthoquinone. (The identifying cuts are not shown.) Scheme 1. Reaction of 2-Hydroxy-1,4-naphthoquinone with a Base and the Resultant Resonance-Stabilized Product

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Vol. 87 No. 1 January 2010 pubs.acs.org/jchemeduc r 2009 American Chemical Society and Journal of Chemical Education, Inc. 10.1021/ed800014k Published on Web 12/18/2009

In the Classroom

Scheme 2. Predominant Reaction of Cyanidin Diglucoside with a Weak Base and the Resultant Resonance-Stabilized Product

Figure 2. Stuctures of curcumin and vitamin A.

shift in wavelength absorbed, which changes the color of the compound. On spraying with dilute HCl solution the message disappears. A natural extension is to discuss the structural changes in cyanidin, a pigment molecule responsible for the red, blue, and purple color of many plants. Another developing agent used is an ethanolic extract of rose petals. The compound that gives roses their red color is cyanidin diglucoside. This compound (which has a cyanidin moiety substituted with two glucose units) is red, but under mildly basic conditions (pH 8-9) it loses an Hþ to give, predominantly, the anion that has a blue color (11, 12). The reaction is shown in Scheme 2. Spraying with a dilute HCl solution makes the message disappear. Providing students with the structure of cyanidin diglucoside and asking them to predict what changes will occur under mild basic conditions requires them to consider which hydrogen atom would be lost most easily and the relative contributions of the various resonance structures. This example illustrates a resonance structure in which a charge is neutralized. The exercise also provides students with practice drawing more challenging resonance structures. An ethanolic solution of retinol, vitamin A (Figure 2), is used as a standard yellow dye solution that does not change color under basic conditions. Vitamin A is also yellow but is much less acidic than 2-hydroxy-1,4-napthoquinone as the anion formed (when it loses an Hþ) is not resonance stabilized. The structure of vitamin A (given in the supporting material) indicates that its acidity is likely to be similar to that of ethanol. Vitamin A is not likely to react with Na2CO3 and NaOH but only with strong bases such as NaH. However, there is no change in color, even when the anion is formed; the electrons on the oxygen are not in conjugation with the double bonds and cannot be delocalized. Vitamin A is not effective in making visible, a message written with a Na2CO3 or NaOH solution. Materials 0.1 M Na2CO3 (0.1 g in 10 mL of distilled H2O) 0.1 M NaOH (0.04 g in 10 mL of distilled H2O) 0.1 M HCl (10 mL) 0.040 M (0.50 g in 72 mL ethanol) 2-hydroxy-1,4-naphthoquinone 0.040 M (0.50 g in 44 mL ethanol) vitamin A, store in a refrigerator 4 g of henna 2 g of ground turmeric 3 red roses

r 2009 American Chemical Society and Journal of Chemical Education, Inc.

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4 Whatman #4 24.0 cm diameter filter papers 2 cotton swabs 4 spray bottles disposable gloves scissors 4 tea bags

Preparation of Extract from Henna Empty the tea from two tea bags. Place 4.0 g of henna into one bag, fold at the open end, and staple. Place the bag in the remaining empty tea bag, fold, and staple. Place 50 mL of ethanol in a 100 mL beaker, cover, and heat to just below the boiling point. Turn off the heat. Immerse the henna in the hot ethanol. Cover the beaker and wait 30 min. Filter the liquid and place it in the spray bottle. Preparation of Extract from Turmeric Empty the tea from two tea bags. Place 2.0 g of ground turmeric into one bag, fold at the open end, and staple. Place the bag in the remaining empty tea bag, fold, and staple. Place 50 mL of ethanol in a 100 mL beaker, cover, and heat to just below the boiling point. Turn off the heat. Immerse the turmeric in the hot ethanol. Cover the beaker and wait for 30 min. Filter the liquid and place it in a spray bottle. Preparation of Extract from Rose Petals Remove the petals from three roses. Place 75 mL of ethanol in a 200 mL beaker, cover, and heat to just below the boiling point. Turn off the heat. Immerse the rose petals in the hot ethanol. Cover the beaker and wait for 30 min. Filter the liquid and place it in a spray bottle.

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Vol. 87 No. 1 January 2010

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In the Classroom

Preparation of Filter Paper with Invisible Message Dip a cotton swab in a 0.1 M solution of NaOH and use it to write the word “NaOH” on each of a pair of Whatman #4 filter papers. Use a scissors to make a cut of 1 cm at the edge of each of the filter papers with NaOH. The cut is used to identify the paper. Repeat the procedure on four additional filter papers using a fresh cotton swab and 0.1 M Na2CO3 to write “Na2CO3”. Make two 1 cm cuts at the edges of each of these papers as a form of identification. Allow the filter papers to dry until the marks become invisible. This preparation of the filter paper should be done at least 15 min before the demonstration to facilitate evaporation of the aqueous solvent. Hazards Wear safety goggles and gloves when preparing the dye extracts and dye solutions, writing the messages, and spraying the developing solutions. These steps should be done in a fume hood. In addition, spray the filter papers with developing solutions very lightly and point away from yourself and students. The HCl and NaOH solutions are corrosive. Also, both 2-hydroxy-1,4naphthoquinone and vitamin A are irritants and can be harmful if they come in contact with the skin, are inhaled, or are swallowed. Ethanol is flammable and should be kept away from heat sources. In addition, the ethanolic extracts and solution of dye should be properly disposed of in designated organic waste bottles. Presentation and Discussion At the beginning of class, hold up the filter paper and invite students to look for any written messages. Students are usually puzzled and pay rapt attention at this stage. Invite one of the students to look closely for any written words and to share his or her observation with the class. Select the filter paper with the single cut and spray with the solution of 2-hydroxy-1,4-naphthoquinone. Students are usually surprised when they notice that from the blank paper, the word “NaOH” shows up in bright orange. Provide the students with the structure of 2-hydroxy-1,4naphthoquinone and ask them to explain the reason for the appearance of the message. Students usually are not able to explain the color change. This is a good opportunity to introduce the concept of relative acidity of alcohols. Point out that ethanol, C2H5OH has a pKa of around 16; it does not react with solutions of NaOH because the resulting conjugate acid would be H2O, which has a pKa of 15.73 (13). Remind students that an acid reacts with a base to form a weaker conjugate acid. Discuss the stabilization (of the alkoxide anion) by inductive effects using as an example CF3CH2OH, which has a pKa of 12.43, and explain how the net electron-withdrawing effects of the three fluoride atoms results in increased acidity (13). Point out that these alcohols do not react with weak bases and react only to a limited extent with the stronger metal hydroxide bases. Discuss the effect of resonance stabilization of the alkoxide anion using as an example, phenol, C6H5OH, pKa = 9.87 (13). The fact that phenol is acidic enough to react with NaOH is rationalized by the delocalization of the anion of the phenoxide. Have students draw the different resonance structures for the phenoxide ion. Next, use p-nitrophenol, p-NO2C6H5OH, (pKa = 7.15) and p-aminophenol, p-NH2C6H5OH (pKa = 10.46), as examples to 38


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show where both resonance and inductive effects can combine to affect the acidity of alcohols (13). The electron-withdrawing -NO2 group increases the acidity of the phenolic -OH while the electron-donating -NH2 group decreases it. Revisit the reason for the message displayed by 2-hydroxy-1,4-naphthoquinone (Scheme 1). Ask students to draw the structure of the anion formed and its resonance structures. At this stage, they are usually able to draw the structure and can also explain the color change observed. Hold up one of the filter papers with the two cuts to the class. Ask students whether the message (written with Na2CO3) would also become visible when sprayed with the solution of 2-hydroxy-1,4-naphthoquinone and the reason(s) for their answers. Many students are likely to indicate that the message will not be visible because Na2CO3 is not a strong enough base. They are usually under the impression that a -COOH functionality but not an -OH reacts with Na2CO3. Spray the paper with the dye solution; the message becomes visible (Figure 1). Point out that the results are consistent with the reported pKa values of 4.70 for 2-hydroxy-1,4-naphthoquinone and 6.37 and 10.31, respectively, for H2CO3 and HCO3-, the conjugate acids of CO32- (14, 15). Alternatively, the message can be displayed by spraying with an ethanolic extract of henna. The message is displayed but has a greenish-orange color. If this developing solution is used, inform students that henna is crushed leaves and the green tinge is likely due to the presence of chlorophyll extracted along with the 2-hydroxy-1, 4-naphthoquinone. Ask students to predict what would happen to the message if the filter papers are now sprayed with 0.1 M HCl. Many students are likely to indicate that the message disappears due to reformation of 2-hydroxy-1,4-naphthoquinone. Hold up one of the papers with the visible message and spray with the dilute HCl. The message becomes blurred and indecipherable and then disappears. Students are usually pleased that their prediction is validated. Provide the students with the structure of vitamin A and ask whether it can be used to develop a message written with Na2CO3. Without closely examining the structure of vitamin A, a number of students are likely to say that the message will show up. Spray the vitamin A solution onto the filter paper marked with Na2CO3. The message does not show up. Ask students whether the message written with NaOH will become visible if sprayed in a similar manner. Some of the students will likely point out that the anion formed is not resonance stabilized and, as such, the molecule is not acidic enough to react with NaOH. Spray the filter paper marked with the NaOH solution; no message appears. Explain that NaOH does not react with vitamin A, but that if a stronger base such as NaH is used, a reaction would occur as vitamin A likely has a pKa near to that of ethanol. Point out that the loss of Hþ from vitamin A is not expected to lead to a color change as the anion formed is not in conjugation with the double bonds and the charge is not delocalized. Continue by providing students with the structure of curcumin and explain that it exists predominantly as the enol form (12). Ask whether an invisible message written with Na2CO3 will show up if sprayed with a curcumin-containing extract from turmeric. Many students are likely to indicate that it would. Ask them to explain the reason for their answers. In discussing their responses, have them identify the most acidic


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In the Classroom

hydrogen atom and write resonance structures for the anion formed from loss of that hydrogen. Ask students to compare the acidity the phenolic -OHs in curcumin with that of phenol. As the ortho -OCH3 group on the aromatic ring of curcumin is electron donating, it destabilizes the anion formed (from loss of Hþ from the phenolic -OH). As such, the -OH on the aromatic ring of curcumin would be expected to be less acidic than that in phenol. This serves to reinforce the concept that both resonance and inductive effects contribute to the stability of anions and determine the relative acidities of functional groups. Spray with the extract from turmeric onto one of the filter paper on which an invisible message was written with Na2CO3. Students are usually pleased to see that a message is displayed in red while the rest of the paper is dyed yellow. Explain that the observations are consistent with the reported pKa value of 8.70 for curcumin corresponding to loss of its most acidic hydrogen atom (14). Ask students what would happen to the message if it is sprayed with HCl. Invariably, they are usually able to predict that the message will disappear. Spray the HCl onto the paper; the message does disappear. Provide students with the structure of cyanidin diglucoside and ask whether a message written with Na2CO3 will be displayed if sprayed with a solution of the compound extracted from roses. Many students are likely to indicate that it would. Have them identify the most acidic proton and write resonance structures for the anion formed. This requires them to consider which hydrogen atom would be lost most easily and the relative contributions of the various resonance structures. The exercise also provides students with practice drawing more challenging resonance structures. Spray the rose extract onto the paper; a message is displayed in blue. The results are consistent for the reported pKa value of 5.4 for cyanidin corresponding to loss of its most acidic hydrogen atom (12). In conclusion, hold the filter paper up high and spray with HCl. The message disappears. Instructors so inclined may take a bow. Conclusions This demonstration can be quickly, easily, and safely prepared. We have found the demonstration to be effective in attracting students' attention and in engaging them in lively discussions of resonance stabilization, keto-enol tautomerization, and relative acidities. The demonstration had some unintended positive consequences. Students were curious as to why compounds have different colors and as to the applications of pH-dependent reversible structural changes of molecules. These opportunities were used to reinforce additional chemical concepts and make students aware of some of the more fascinat-


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ing current and potential applications of organic chemistry in their lives. Acknowledgment Gratitude is expressed to students who took Organic Chemistry I with me and participated in the demonstration. In addition, gratitude is expressed to Ed Vitz and the checker for their helpful suggestions, many of which were incorporated into the demonstration. Special thanks to Kirran Tiwari, who took the colorful pictures of the messages developed by the dye solutions. Literature Cited 1. Helmenstine, A. M. About.com:Chemistry. http://chemistry. about.com/cs/howtos/ht/invisibleink3.htm 2. Kanda, N.; Asano, T.; Itoh, T.; Onoda, M. J. Chem. Educ. 1995, 72, 1131. 3. Breedlove, C. H. J. Chem. Educ. 1995, 72, 540. 4. Epp, D. N. J. Chem. Educ. 1993, 70, 326. 5. Fortman, J. J.; Stubbs, K. M. J. Chem. Educ. 1992, 69, 66. 6. Mebane, R. C.; Rybolt, T. R. J. Chem. Educ. 1985, 62, 285. 7. Forster, M. J. Chem. Educ. 1978, 55, 107. 8. Editorial Staff. J. Chem. Educ. 1997, 74, 1176A. 9. Curtright, R.; Rynearson, J. A.; Markwell, J. J. Chem. Educ. 1996, 73, 306. 10. Anderson, A. M.; Mitchell, M. S.; Mohan, R. J. Chem. Educ. 2000, 77, 359. 11. Senese, F. General Chemistry Online. http://antoine.frostburg.edu/ chem/senese/101/features/water2wine.shtml 12. Cortell, J. M. Influence of Vine Vigor and Shading in Pignot noir (Vitis vivifera L.) on the Concentration and Composition of Phenolic Compounds in Grapes and Wine. Ph.D. Dissertation Thesis, Oregon State University, Corvallis, OR, 2006; p 3. http://ir. library.oregonstate.edu/dspace/bitstream/1957/3142/1/JCthesisfinal3%5B1%5D.pdf. 13. McMurry, J. Organic Chemistry, 7th Ed.; Thompson, Brooks/ Cole: Belmont, CA, 2008; pp 602-606. 14. Borsari, M.; Ferrari, E.; Romano, G.; Saladini, M. Inorg. Chim. Acta 2002, 328, 61–68. 15. Brown, T. L.; LeMay, H. E.; Bursten, B. E. Chemistry: The Central Science, 8th Ed.; Prentice Hall: Upper Saddle River, NJ, 2000; p 620.

Supporting Information Available pH-dependent structural changes of curcumin and vitamin A; color pictures of messages displayed by extracts from henna, turmeric, and roses; table of pKa values of compounds used and cited in demonstration. This material is available via the Internet at http://pubs.acs.org.


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Developing an Invisible Message about Relative Acidities of Alcohols

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