In the Laboratory edited by
The Microscale Laboratory
Arden P. Zipp SUNY-Cortland Cortland, NY 13045
Organic Chemistry in Capillaries Bharat G. Mahamulkar Department of Chemistry, Fergusson College, Pune 411 004, India Dilip D. Dhavale and Shriniwas L. Kelkar* Department of Chemistry, University of Pune, Pune 411 007, India; *
[email protected] Capillary tubes have traditionally been used in chemistry laboratories for recording melting points and spotting in thinlayer chromatography. They are also used in organic microscale experiments for determining boiling points (1). We are now utilizing capillaries in an innovative way, as reaction vessels and for the qualitative detection of functional groups. To the best of our knowledge, even recently published books still recommend use of test tubes for doing the reactions (2, 3). The low cost of capillary tubes, coupled with low reagent consumption, makes this technique attractive for use in any undergraduate or postgraduate laboratory. Capillaries closed on one end have been used for melting point and boiling point determination. The experiments described in this communication use capillaries open at both ends. Several experiments can be carried out using capillaries in combination with plastic Pasteur pipets. Beral-type Pasteur pipets made from plastic material could be used as a replacement for table reagent bottles for storing solutions of reagents during the laboratory periods. Unless squeezed, the long drawn capillary at the tip of these pipets does not allow any stored solution to come out even when the pipet is inverted or rested horizontally on the table. This reduces the possibility of chemical spills. Reagents such as aqueous KMnO4, Br2 in water (or CCl4 or acetic acid), 2,4dinitrophenylhydrazine, and acid or alkali solutions can be stored in these and conveniently used. Being unbreakable, plastic Pasteur pipets contribute to general laboratory safety. In addition, most of the stored reagents do not age or get spoiled for days, since the reagent solution at the tip indirectly seals the pipet. The pipet can still be sealed separately by heating with minimal flame and reopened when required by cutting off the sealed portion.
and without touching the open ends, the capillary is rocked in a seesaw motion within about 15 to 20° of the horizontal plane (Fig. 2). The solid dissolves if it is soluble in the cold solvent. Most solvents, except the heavier ones like chloroform or carbon tetrachloride, would not drip out even if the capillaries were held vertically. To be most effective, solutions of NaNO2, FeCl3, FeSO4, sodium nitroprusside etc., need to be freshly prepared. Using this technique, fresh solutions of reagents can be made with microgram quantities. Since these are used immediately, wastage of excessive solutions can be avoided.
Capillary-to-Capillary Transfers If the tips of capillary A (containing the reagent) and empty capillary B (with slightly smaller bore than A) are brought in contact with each other, the solution gets transferred from A to B. It is necessary to hold the large-bore tube at a slight angle and the empty capillary almost horizontal (Fig. 3). The solutions originally prepared in capillaries as described above can thus be used as stock solutions and just
Figure 1. Test for solubility of organic solids. The tip of a capillary containing the solid (left) and the tip of a Pasteur pipet containing solvent (right) are brought close together so that a drop of liquid squeezed from the pipet enters the capillary.
Manipulations Capillaries and Pasteur pipets can be used concurrently in the following ways.
Testing the Solubility of Organic Substances in Cold Solvents An open-ended capillary is tapped into the solid sample so that the solid gathers at the tip. The tip of this capillary and the tip of a Pasteur pipet containing the solvent are brought close together. The pipet bulb is squeezed gently until a small drop of the solvent appears at the outside of the tip of the pipet. When the capillary tip touches the drop, the solvent is transferred into the capillary instantaneously owing to the capillary action (Fig. 1). Taking out more drops in the same way can increase the quantity of the solvent in the capillary. Holding it in the center
Figure 2. The capillary is rocked to see if the solid dissolves in the cold solvent.
Figure 3. Illustration of transfer of liquid from larger-bore capillary A to smaller-bore capillary B.
JChemEd.chem.wisc.edu • Vol. 77 No. 3 March 2000 • Journal of Chemical Education
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In the Laboratory
a small portion of them can be used in different capillaries for further reactions.
Mixing Reagents By using the transfer technique, two different solutions can be mixed in a single capillary. Care has to be taken to ensure that no air bubble is trapped between the two layers of chemicals. Miscible liquids mix well in the capillaries. Reactions Determining the Nature of Organic Substances Organic acids, phenols, and bases in capillaries can be dissolved in 10% NaHCO3, dilute NaOH, and HCl solutions, respectively. These can then be reprecipitated in the same capillary by adding a drop or two of concentrated HCl or NaOH solution through a Pasteur pipet as described above.
Lassaigne’s Test for Element Detection The extract from a sodium fusion test for organic substances is taken in a capillary. A drop of freshly prepared FeSO4 solution (see above) is added through a Pasteur pipet and then a drop of sulfuric acid gives the characteristic blue color if nitrogen is present in the original substance. The test for sulfur using sodium nitroprusside can be performed in the same manner. If one or both of these tests are positive, the extract is acidified with nitric acid and boiled. This solution is then taken inside a capillary and silver nitrate is added via Pasteur pipet. The silver halide precipitate is formed conspicuously in the capillary. The quantity of silver nitrate saved by this method is enormous. Reactions Involving Functional Groups
Phenols To a few crystals or a drop of phenol in the capillary, a drop of aqueous solution of FeCl3 is added from the Pasteur pipet. The typical violet, blue, or green color appears instantaneously. Aromatic Primary Amines A drop of distilled aromatic primary amine in a capillary is mixed with HCl (1:1) and rocked in a seesaw motion. Aqueous NaNO2 solution prepared in another capillary and solution of β-naphthol in NaOH from yet another capillary are transferred to this in that order. The orange/red dye appears instantaneously. Nitrous acid prepared in situ by this method can be added to secondary amines taken up in capillaries to get separation of yellow liquid or to tertiary amines to get a green solid, to confirm their presence. Alternatively these tests may be done on filter paper. If the substance and the reagents are spotted with capillaries at the same spot on the filter paper in the required sequence, the characteristic colors appear on the paper. 2,4-DNP Derivatives of Aldehydes and Ketones An aldehyde or ketone at the tip of a capillary tube is mixed with 2,4-DNP reagent from a Pasteur pipet. When the
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capillary is rocked, the derivative forms within a few seconds. Students are asked to prepare derivatives of various aromatic and aliphatic aldehydes and ketones. They then compare the colors of the derivatives formed, which are indicative of extent of conjugation and the relationship between conjugation and color. The derivative thus prepared can be transferred onto a filter paper by blowing through the open tip with the help of a plastic Pasteur pipet. It can then be dried and used for determination of melting point.
Test for Unsaturation The usual test of decolorization of bromine water or aqueous KMnO4 can be performed in the capillary. If the substance to be tested is taken in capillary A and the reagent is added via capillary B (Fig. 3), one can observe the decolorization as the liquid goes through the solid and rises in the capillary. The instant color change of the reagent can be compared simultaneously in two capillaries. Limitations It is obvious that no reaction that requires heating can be done in capillaries. However, if even 50% of the reactions regularly done in qualitative organic chemistry experiments are performed in this manner, a great saving of reagents, chemicals, solvents, and energy will be achieved. Conclusions The capillary technique described here provides a means of performing many of the qualitative tests carried out in the organic chemistry laboratory using the smallest possible amounts of chemicals, although some tests (e.g., ester hydrolysis or reduction of nitro groups) will still need to be done in test tubes. For those tests that can be done by the capillary method, the small quantities of reagents employed represent substantial savings. Acknowledgments SLK wishes to thank R. D. McKelvey and other members of the Chemistry Department of the University of Wisconsin– La Crosse for fruitful discussions and experiments in their laboratories during a tenure as visiting professor. SLK and DDD thank the University Grants Commission, New Delhi, for a grant to carry out and popularize the microscale experiments. Literature Cited 1. Williamson, K. L. In Microscale Organic Experiments; D. C. Heath: Lexington, MA, 1987; p 52. 2. Pavia, D. L.; Chapman, G. M.; Kriz, G. S.; Engel, R. G. In Introduction of Organic Laboratory Techniques—A Microscale Approach; Saunders: Philadelphia, 1995. 3. Mayo, D. W.; Pike, R. M.; Trumper, P. K. In Microscale Organic Laboratory, 3rd ed.; Wiley: New York, 1994.
Journal of Chemical Education • Vol. 77 No. 3 March 2000 • JChemEd.chem.wisc.edu
