In the Classroom edited by
Ed Vitz Kutztown University Kutztown, PA 19530
Dissolving Carboxylic Acids and Primary Amines on the Overhead Projector Sally D. Solomon* and Susan A. Rutkowsky Department of Chemistry, Drexel University, Philadelphia, Pennsylvania 19104
[email protected] Carboxylic acids (or primary amines) that are only slightly soluble in water are dissolved by the addition of aqueous NaOH (or HCl). The solubilities of selected acids and amines are listed in Tables 1 and 2 (1). Hexanoic acid and hexylamine are used in this colorful demonstration that is suitable for introducing properties of carboxylic acids and amines. Only simple glassware, a standard overhead projector, and small quantities of chemicals are required (2). Demonstration Overview A small volume of a carboxylic acid (or amine) along with a universal indicator to follow the pH change (3) is added to water placed in a transparent container on the stage of an overhead projector. Upon gradual addition of sodium hydroxide (or hydrochloric acid) solutions, there is movement in the mixture, which becomes increasingly rapid just before the entire organic compound dissolves. Chemicals and Glassware • • • • • • •
Hexanoic acid Hexylamine Universal indicator 6 M NaOH 6 M HCl Petri dish, 10 cm Disposable pipets
Hazards Although the amounts of chemicals used in this overhead projector demonstration are small, the demonstrator should wear suitable gloves and protective clothing, as well as safety glasses. Hexylamine and hexanoic acid are destructive to mucous membranes, can cause burns, and are harmful by inhalation, upon contact with skin, and if swallowed. The inorganic solutions, 6 M NaOH and 6 M HCl, are corrosive and can cause burns to the eyes and to the skin.
The Petri dishes are placed on a blank transparency where equations describing the reactions can be written as the demonstration is presented (Figure 1). The volume of NaOH or HCl needed is easily estimated for either acid or amine using their densities and molar masses (Tables 1 and 2). For instance, 1 mL of hexanoic acid
A 10 cm Petri dish is half filled with tap water (about 30 mL) to leave sufficient room for the NaOH or HCl solutions to be added. Using a disposable pipet, a 1-2 mL sample of acid or amine is added to the water in the dish. The easiest way to measure the volume is by first using water to calibrate a plastic or glass Pasteur pipet. Hexanoic Acid A 1-2 mL sample of hexanoic acid is added to 30 mL of water in a Petri dish. Upon addition of 8-10 drops of universal indicator, the aqueous mixture turns red because the acid is slightly soluble. Globules of colorless hexanoic acid can be seen on the red background. As 6 M NaOH is gradually added, the globules start to become smaller and smaller along with increasingly rapid motion in the mixture until dissolving is complete and a uniform
Journal of Chemical Education
Hexylamine A volume of 1-2 mL of hexylamine added to 30 mL of water forms globules visible upon a violet background once the universal indicator is added. In this case, 8 drops of indicator is sufficient, otherwise the mixture becomes too dark. The addition of 2-5 mL of 6 M HCl dissolves the amine, again with rapid motion near the end of the process. The uniform mixture appears red from adding excess acid. Students may also be able to see the white vapor of amine hydrochloride forming above the Petri dish.
Discussion
Procedure
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blue-violet color can be seen because of the excess NaOH. The volume of NaOH(aq) needed should be between 2 and 5 mL.
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Table 1. Properties of Carboxylic Acids Sol in H2O/ Molar Mass/ Density/ [g/(100 mL H2O)] (g mol-1) (g mL-1) CAS Number
Acid
Butanoic
miscible
88
0.96
107-92-6
Hexanoic
1
116
0.93
142-62-1
Octanoic
0.07
144
0.91
124-07-2
Table 2. Properties of Primary Amines Sol in H2O/ Molar Mass/ Density/ [g/(100 mL H2O)] (g mol-1) (g mL-1) CAS Number
Amine
Butylamine
miscible
73
0.74
109-73-9
Hexylamine
1
101
0.77
111-26-2
Octylamine
0.02
129
0.78
111-86-4
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Vol. 87 No. 4 April 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed800122y Published on Web 03/09/2010
In the Classroom
Figure 1. Reactions that can be written on the blank transparency near the Petri dishes.
has a mass of approximately 1 g or about 0.008 mol, thus, requiring 0.008 mol of 6 M NaOH, which is about 2 mL. Practical Applications The acid-base properties of carboxylic acids and amines provide a convenient method to separate them from organic
r 2010 American Chemical Society and Division of Chemical Education, Inc.
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reaction mixtures. For instance, a water-insoluble acid product can be extracted from an organic solvent using NaOH solution. After the organic layer is removed, dilute HCl is added to the aqueous layer containing the basic acid salt to produce the free acid. Benchtop demonstrations based on this principle show the extraction of an acidic substance from chloroform and benzene (4, 5). Likewise, a water-insoluble amine dissolved in an organic solvent can be extracted by acidifying the reaction mixture then treating the aqueous layer with NaOH to form the free base amine product. Amines used as therapeutic drugs are often administered in the form of their water-soluble salts. For example, the tertiary amine, procaine (Novocain) is a local anesthetic, which is injected in the form of its hydrochloride salt, procaine hydrochloride. Literature Cited 1. 2. 3. 4. 5.
pubs.acs.org/jchemeduc
Ralston, A. W. J. Am. Oil Chem. Soc. 1940, 17 (4), 89–91. Kolb, D. J. Chem. Educ. 1987, 64, 348–351. Solomon, S.; High, N. J. Chem. Educ. 1987, 64, 964–965. Kelly, T. R. J. Chem. Educ. 1993, 70, 848–849. Silversmith, E. F. J. Chem. Educ. 1972, 49, A694.
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