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A Discovery-Learning 2,4-Dinitrophenylhydrazone Experiment Bruno M. Vittimberga and Ben Ruekberg* Department of Chemistry, University of Rhode Island, Kingston, RI 02881; *
[email protected] Formation of 2,4-dinitrophenylhydrazone derivatives of carbonyl compounds has long been a common exercise in organic chemistry laboratories. Recently, there has been an emphasis on discovery learning in laboratory experiments. Between 2000 and 2004, 61 items published in this Journal used the keyword “discovery learning”; for instance, “A Discovery Approach to Three Organic Laboratory Techniques: Extraction, Recrystallization, and Distillation” (1). A discovery aspect can be added to standard 2,4-dinitrophenylhydrazone exercises by asking students to discern the influence of properties such as structure on the color of the 2,4-dinitrophenylhydrazone formed. Rather than simply preparing derivatives of several carbonyl compounds, as in some texts (2), this enhanced exercise encourages students to deduce the influence of some structural characteristics from their results, reinforcing their familiarity with some important concepts. Basic the factors for the student to consider are molar mass, aldehyde–ketone, cyclic–acyclic, and conjugated–nonconjugated (of the carbonyl to the carbon–carbon double bond system). Students can accomplish this study using a limited number of readily available carbonyl compounds. They should complete a table similar to that shown below (Table 1). They should also know to expect and distinguish a spectrum of colors ranging from yellow to red. A greater number of properties may be examined if the range of compounds is increased. This provides a more challenging experience for more advanced students. A larger selection of compounds will permit a greater number of properties to be examined. A recommended group of carbonyl compounds is acetone, benzaldehyde, carvone, cinnamaldehyde, citral, cyclohexanone, dihydrocarvone, furfural, hydrocinnamaldehyde (3-phenylpropionaldehyde), β-ionone, nonyl aldehyde, and propiophenone. All are liquids. None is more toxic than 2,4-dinitrophenylhydrazine (2,4-DNP).1 None is excessively expensive. This selection permits the addition of several properties for investigation: density, index of refraction, and boiling point. While the students can be asked to decide on the properties in Table 1, the additional properties are probably best provided by the instructor. The larger compound list also affords an opportunity for com-
paring related compounds, for instance cinnamaldehyde, hydrocinnamaldehyde, and propiophenone. For this revision, some modification of the 2,4-dinitrophenylhydrazone procedure currently in use is desirable. For instance, one procedure calls for using a clean, dry, labeled test tube, adding 2 drops of the carbonyl compound to 1 mL of 2,4-DNP reagent, mixing, allowing the mixture to stand 15 min, and confirming precipitate formation (2d). However due to the red–orange color of the 2,4-DNP reagent, the color of the precipitate may be hard to discern. Thus after the reaction, decant the solution from the precipitate or remove it with a Pasteur pipet; rinse the precipitate with distilled water until the supernatant liquid is colorless.2 Because avoiding contamination of the carbonyl compounds is vital to the success of this experiment, it is strongly recommended that drop-dispensing bottles with tethered caps be used3 and that the screw-cap be secured with Parafilm M or other means of sealing. The purpose of this sealing is to ensure that the bottles cannot be opened without leaving a visible indication that the purity of the contents may have been compromised, in which case the bottle can be replaced. The use of these bottles also facilitates minimization of student exposure to these materials. The option of sharing of class data may be considered and, if chosen, all of the derivatives of the same compound should be placed in the same test tube rack bearing a label identifying the compound. With several examples of each derivative, outlying results can then be dealt with appropriately and the racks can then be arranged in order of the color from most yellow to most red. A further feature that may increase student interest is opportunity for computer analysis of the data. This is accomplished, using a spread sheet or word processor, by copying the chart as a table with an additional column in which are entered numbers reflecting the ranking of the derivatives’ colors from most yellow to most red. A number sort on this column should reveal which property best correlates with, or is the most accurate predictor of, the color of the 2,4dinitrophenylhydrazone derivative. Since a subsequent sorting will reflect the order of the previous sorting, as when one
Table 1. Blank Student Data Table for the 2,4-Dinitrophenylhydrazone Experiment Carbonyl Compound
Molar Mass/(g/mol)
Aldehyde or Ketone
Conjugated or Nonconjugated
Cyclic or Acyclic
Color
Acetone Cyclohexanone Benzaldehyde Heptaldehyde Acetophenone NOTE: Color refers to the color of 2,4-dinitrophenylhydrazone formed.
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In the Laboratory
sorts by first name and then last name to get a list of names in alphabetical order, a numerical sorting of another property before sorting by color value would reveal the influence, if any, of that property. Hazards 2,4-Dinitrophenylhydrazine reagent is prepared from solid 2,4-dinitrophenylhydrazine, sulfuric acid, ethanol, and water (3). 2,4-Dinitrophenylhydrazine is a poisonous, flammable solid that can explode if dry. Sulfuric acid is corrosive, carcinogenic, and poisonous and its mist is harmful to tooth enamel and damaging to the respiratory tract. Ethanol is flammable and may irritate skin, eyes, and respiratory tract. Furfural is flammable and toxic: it may be fatal if swallowed, inhaled, or absorbed through skin. Benzaldehyde may cause an allergic reaction. The other aldehydes and ketones listed here are flammable and potentially irritating to skin, eyes, and respiratory and digestive tracts. Gloves, goggles, and lab coat should be worn. The work should be performed in an area with adequate ventilation. W
Supplemental Material
CAS registry numbers are available in this issue of JCE Online.
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Notes 1. The most toxic, furfural, may be eliminated if desired. 2. Colorless water over the precipitate can provide an indication that the color of the precipitate is unaltered by leftover reagent. 3. Drop-dispensing bottles with tethered caps were purchased from Nalgene Labware, catalog number 2414-0015; http:// www.nalgenelabware.com/ (accessed Aug 2006). Others may work equally well.
Literature Cited 1. Horowitz, G. J. Chem. Educ. 2003, 80, 1039–1041. 2. (a) Campbell, B. N.; Ali, M. M. Organic Chemistry Experiments; Brooks/Cole Publishing Company: Pacific Grove, CA, 1994; p 508. (b) Ouellette, R. J.; Manchester, J. H. Experiments in General, Organic, and Biological Chemistry, 4th ed.; Prentice Hall: Upper Saddle River, NJ, 1997; pp 217–219. (c) Bettelheim, F. A.; Landesberg, J. M. Laboratory Experiments for Organic and Biochemistry, 4th ed.; Browhichoks/ Cole Publishing Company: Florence, KY, 2001; p 79. (d) Linstromberg, W. W.; Baumgarten, H. E. Organic Experiments, 6th ed.; Houghton Mifflin: Boston, MA, 2002; p 162. 3. Ruekberg, B.; Rossoni, E. J. Chem. Educ. 2005, 82, 1310.
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