LABORATORY EXPERIMENT pubs.acs.org/jchemeduc
Qualitative Organic Analysis: An Efficient, Safer, and Economical Approach to Preliminary Tests and Functional Group Analysis Sunita Dhingra* and Chetna Angrish Department of Chemistry, Miranda House, University of Delhi, Delhi, India
bS Supporting Information ABSTRACT: Qualitative organic analysis of an unknown compound is an integral part of the university chemistry laboratory curriculum. This type of training is essential as students learn to approach a problem systematically and to interpret the results logically. However, considerable quantities of waste are generated by using conventional methods of analysis. This waste is a complex mixture of compounds, difficult to segregate or reuse, and its commercial disposal incurs a huge expense. Many thousands of undergraduate and postgraduate students study chemistry each year, and, at a rough estimate, each student produces about 225 g/year of the waste while performing qualitative organic analysis. Perturbed by these facts, we have made a concerted effort to reduce the quantities of waste generated and to replace the commonly used procedure by spot tests. The reagents used in the spot tests are the same as in the conventional analysis and are relatively inexpensive and easily available, in contrast to complex and expensive reagents mentioned for the spot tests in the literature. In the present work, spot tests have been developed for the detection of nitrogen and sulfur, Baeyer’s test for unsaturation, and the functional groups present in carbohydrates, carboxylic acids, phenols, alcohols, esters, aldehydes, ketones, aromatic nitro compounds, aromatic amides, and primary, secondary, and tertiary aromatic amines. The modified scheme is simple, eco-friendly, and energy- and time-efficient. KEYWORDS: Second-Year Undergraduate, Graduate Education/Research, Laboratory Instruction, Organic Chemistry, HandsOn Learning/Manipulatives, Microscale Lab, Qualitative Analysis, Green Chemistry, Molecular Properties/Structure Table 1. Preliminary Examination in the Qualitative Organic Analysis
Q
ualitative analysis of an unknown organic compound is an integral part of the chemistry curriculum for the undergraduate and postgraduate students. An unknown organic compound is analyzed by performing preliminary tests, functional group detection, and finally by preparation of a suitable derivative.1,2 This type of training provides an opportunity to students to learn a systematic and logical approach to a problem in the laboratory and subsequently take up a challenging career in isolation, characterization, and synthesis of new compounds that are useful for mankind. Students can also correlate the analytical work performed in the laboratory with the functional group chemistry studied in the organic lecture classes. Conventionally, the preliminary tests and functional group detection are carried out by performing chemical tests in test tubes.1,2 In this process of testing, considerable quantities of waste are produced that are difficult to segregate and reuse. According to a rough calculation, each student produces about 225 g of waste per year during the qualitative organic analysis. In this university alone, nearly 4000 students perform the qualitative organic analysis and hence 225 g 4000 = 900,000 g or 900 kg per year of waste is produced. The waste generated is enormous considering the number of students studying chemistry all over the world: a fact that cannot be ignored. The other major concern is that the waste is a complex mixture of chemicals, the collection, segregation, or reuse of which is a challenge. The commercial disposal of the waste involves a huge expense. Concern about the environmental hazards caused by the chemistry laboratories of academic institutions has resulted in a concerted effort to reduce the waste and inculcate the environmental awareness in the Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
Preliminary Examination Test for nitrogen
a
Testa Ferrous sulfate
Test for sulfur
Sodium nitroprusside
Test for unsaturation
Baeyer’s test
Whatman filter paper strip.
minds of young scientists and encourage them to use green practices in the laboratory.
’ PRESENT WORK A survey of the literature shows that preliminary tests and the functional group detection have been carried out by the following schemes: macroscale analysis,1,2 microscale analysis,3,4 and spot tests.5 Macroscale analysis is conventionally used in most universities. The major drawback of this scheme is the generation of large quantities of chemical waste. It is also more hazardous than the other schemes because organic solvents are used for extraction in some of the tests, relatively larger quantities of corrosive chemicals are used, and heating during the tests is done using liquefied petroleum gas (LPG), a nonrenewable energy source. However, the main advantage of the scheme is that commonly available and inexpensive reagents are used for the analysis. Some efforts have been made to reduce the waste by carrying out the Published: March 11, 2011 649
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Table 2. Functional Group Tests in the Qualitative Organic Analysis Functional Group
Test
Carbohydrates
Molisch testa,c, Fehling’s testb, Barfoed’s testb
Carboxylic acids
Sodium bicarbonate testc, Fluorescein testa
Phenols
Ferric chloride testb, Phthalein testa, Liebermann’s nitroso testa, special tests for R-naphthola,c and β-naphtholb
Alcohols
Ceric ammonium nitrate testb
Carbonyl compounds
2,4-DNP testb, Fehling’s testb, Schiff’s testb
Esters
Hydroxamic acid testc
Primary Aromatic Amines
Azo dye testb
Secondary Aromatic Amines Tertiary Aromatic Amines
Reaction with nitrous acid followed by Liebermann’s nitroso testa Reaction with nitrous acid followed by basificationb
Aromatic Nitro compounds
Reduction followed by Azo dye testb Hoffmann bromamide reaction followed by Azo dye testa,b
Aromatic Amides a
Glass TLC plate or Merck plate. Whatman filter paper strip. c Grooved tile. b
analysis at microscale level; however, the efforts have been partial and not completely successful. With the spot tests described in the literature, small quantities of reagents are used but many of these are not easily available and are also expensive. In the present work, we developed spot tests for functional group analysis and some preliminary tests using the same reagents typically used in the macroscale analysis. This enabled us to replace the relatively expensive reagents reported in the literature spot tests by inexpensive and easily available ones, thereby making the scheme economical and eco-friendly (by cutting down the consumption and hence disposal of chemicals drastically), energy-efficient (by saving the energy required for heating and cooling), and almost hazard-free.
’ GENERAL PROCEDURE The spot tests are carried out by spotting a concentrated solution of the given compound in a suitable solvent and other reagents, one over the other, on one of the following surfaces: commercial Merck silica gel-coated plate, a glass plate coated with silica gel, or Whatman filter paper. Some tests such as the Molisch and hydroxamic acid test have been performed in a grooved tile by adding a drop each of the solution of the compound and other reagents in the sequence given in the Supporting Information. Most of the tests are carried out at room temperature; however, when heating is required, an oven maintained at ∼120 °C is used. Blank tests are performed to avoid erroneous observations and conclusions. The spot tests for preliminary examination and functional group analysis are summarized in Table 1 and Table 2, respectively. ’ RESULTS AND DISCUSSION The reagents used in the conventional macroscale analysis have been adopted for spot tests. However, some tests were not suitable for adoption as spot tests. Modifications have been made either in the procedure or in the reagents in such cases. For example, conventionally, the -CONH2 group in aromatic amides is detected by carrying out its hydrolysis with alkali and testing the ammonia evolved. In the present scheme, the test has been modified. Aromatic primary amide is first converted to the corresponding aromatic primary amine (Hoffmann bromamide reaction), which is then tested by azo dye test. Similarly, the aromatic nitro group is detected by its reduction to the primary aromatic amine followed by azo dye test instead of the MullikenBarker test. At times, students fail to get a positive hydroxamic acid
test for esters by the conventional method. The test has been carried out successfully in a grooved tile at room temperature. The color changes in fluorescein, phthalein, and Liebermann’s nitroso tests were not very sharp when performed on a Merck plate or a silicacoated glass plate. The spotting, however, was done on the same surface and the spotted portion was scratched and transferred into a test tube containing 1-2 drops of dilute sodium hydroxide solution in 1-2 mL of water producing the respective colors instantaneously. The present scheme works well for monofunctional and simple bifunctional compounds (except in aromatic compounds containing -NH2, with -NO2 or -CONH2). MullikenBarker, Tollens’, iodoform, Lucas, carbylamine tests and so forth could not be successfully converted to spot tests.
’ HAZARDS The hazardous chemicals are either not used or a minimum quantity is applied for the spot tests. The use of organic solvents and LPG is almost eliminated. Hence, the scheme is relatively hazard-free. The CAS numbers of the reagents used are included in the online Supporting Information. The MSDS sheets for the reagents should be consulted. ’ CONCLUSION The qualitative organic analytical work can be carried out in any chemistry laboratory as no special equipment, instrument, and reagents are required in the present spot test scheme. The procedures have been used by undergraduate students of our institute. The proposed spot test scheme of analysis gives consistent and reproducible results. The present work is a step toward the introduction of green practices to young and budding scientists. ’ ASSOCIATED CONTENT
bS
Supporting Information Student handout; instructor notes; CAS numbers of the reagents. This material is available via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 650
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’ ACKNOWLEDGMENT The present work has been carried out in the D S Kothari Centre for Research and Innovation in Science Education (Miranda House) funded by Department of Science and Technology, Government of India. We are thankful to the Principal, Miranda House, for the permission and encouragement to pursue the present study. We also thank our undergraduate students and the laboratory staff who provided help. ’ REFERENCES (1) Vogel, A. I. Vogel’s Textbook of Practical Organic Chemistry; Longman: Essex, U.K., 1989. (2) Ahluwalia, V. K.; Dhingra, S.; Comprehensive Practical Organic Chemistry: Qualitative Analysis; Universities Press Private Limited: Hyderabad, India, 2008. (3) Finley, K. T. J. Chem. Educ. 2005, 82, 240. (4) Mahamulkar, B. G.; Dhavale, D. D.; Kelkar, S. L. J. Chem. Educ. 2000, 77, 387–388. (5) Feigl, F. Spot Tests in Organic Analysis, 7th ed.; Elsevier Science: Amsterdam, 1966.
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