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A Problem Involving Organic QualitativeAnalysis D. J. Silvert Dalhousie University Halifax, NS, Canada B3H A13 Five different organic compounds (unknowns A, B, C, D, E) a r e t o be identified from t h e results of three simple qualitative tests on each unknown. T h e compounds in this s e t of unknowns must fit the following specifications: 1. A particular unknown may he an alcohol, an aldehyde, or a
ketone. 2. Each compound contains only one oxygen atom, plus same carbon and hvdroeen. " - . in the molecular formula. 3. The molecular weights of all five unknowns are different. The highest molecular weight is 74, and this is the only unknown that contains four carbon atoms in the molecular formula. 4. One of the unknowns is a secondary alcohol. 5. None of the unknowns has carhon-carbon multiple bonds or rines . in the structural formula. T e s t results are obtained from a v e r y modest i n p u t of time a n d material, b u t considerable time is usually required t o deduce t h e unique solution t o this problem. T h e specifications limit t h e number of possible compounds to 15, a n d a n average student appears t o treat the deductive sequence as a n interesting challenge.
Equipment' 1250-mL beaker 1ringstand 1wire gauze 1burner 1 10-mL graduated cylinder At least 7 15-X 12-mm test tubes 1test tuhe rack Test Reagents (preferably available in squeeze bottles) Diehromote reagent (14 mL). Preparation: 10 g of NazCrnO, is dissolved in 1L of H20. The solution is then acidified by careful addition of 40 mL conc. H2S01. Iodine reagent (7 mL). Preparation: 250 g KI is dissolved in 1L of H20; 125 g of 1%is then dissolved in this solution. CAUTION: Preparation of dichromate solutions should he carried out in wellventilated areas. Avoid inhaling the powder, which contains chromium in the +6 oxidation state. DNPH reagent (7mL). Preparation: 165mL conc. HCI is added to 840 mL H20; 5 g of 2,4-dinitrophenylhydrazineis then dissolved in this solution. 10%NaOHsolution (estimated maximum 40 m 9 . Knowns (preferably available in dropping bottles) n-propyl alcohol (9 drops) acetone (9 drops)
These two knowns will enahle the student to observe a positive and negative result for each test, however, one may also wish to include an aldehyde, a secondary alcohol, and a tertiary alcohol.
Unknowns (preferably available in dropplng bottles labelled A, B, etc.) A 37%formaldehyde solution (formalin) (9 drops) B methanol (9 drops) C isopropyl alcohol (9 drops) D acetone (9 drops) E tert-hutyl alcohol (9 drops) Procedure Caution: The nature of the reagents, in particular the iodine and NaOH solutions, clearly make use of safety goggles mandatory in the area where this experiment is performed. Carry out (as directed below) the diehromate test, iodoform test, and DNPH test on the two known compounds. When test results are obtained that agree with the predicted results for each of these compounds, then proceed to carry out the three tests on each of the unkhown compounds A, B, C, D, E. Record the results of all tests in a data table indicating the suhstance tested. the test reaeent. formation - . observation of ureci~itate . and/or color change and conclusion whether test gave a positive or a negative result in each case.
Dichromate Test Add 4 drops of the substance to he tested to about 2 mL of diehromate reagent in a test tuhe. Mix well. Heat the test tuhe and its contents for about 3 min in a beaker half-filled with gently boiling water. Several labelled test tubes may be heated simultaneously to reduce the total time required in this part of the experiment. A positive dichromate test is observed as a bluish green solution. A negative diehromate test shows the original orange color of the reagent.
DNPH Test No heatinginuolued. Place about 1 mL of DNPHresgent in a test tuhe. Add one drop of the substance to be tested. A positive DNPH test is observed as the formation of a precipitate. A negative DNPH test shows an unchanged yellow solution.
lodoform Test
' Amounts lstedare for oneoperator. (Ifstudents work as partners. then one pannership = one operator.) Use of the 1 rst four items can be staggered thro-gh more lhan one operator as continual use is not required in the procedure.
No heating inuolued. Caution: Handle reagents with care. Report spills or splashes immediately. Place about 1 mL of iodine reagent in a test tuhe. Add 4 drops of the substance to he tested. Mix well. Then add 10% NaOH solution drop by drop with continual
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Number 11
November 1987
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mixing until the dark color of the iodine reagent has been removed. A pale yellow color may remain. A positive iodoform test is observed as a cream-colored heavy suspension from which a yellow precipitate will settle. No evidence of a suspension or precipitate is observed in a negative test.
Pre-Lab Assignment Students must have the following knowledge or available information: 1. Specifications that must he met by the set of five unknowns.
2. Features that distinguish the structural formulae of primary, secondary, and tertiary alcohols. 3. General structural formulae of aldehydes and ketones. 4. Knowledge that for this particular set of unknowns the test results may be interpreted as follows: a. The dichromate test is positive far primary and secondary alcohols and for aldehydes. I t is negative for ketones and tertiary alcohols. b. The DNPH test is positive for aldehydes and ketones. I t is negative for alcohols. c. The iodoform test is positive if the molecule contains
(X may be H or an alkyl group). Assuming that such information is available, any of the following should he fair questions. Nos. 1and 2 below are obviously desirable as part of a pre-lab assignment. 1. Write structural formulae for acetone and n-prapyl alcohol (1propanol). 2. Predict the result (positive or negative) when eaeh of the three tests (diehromate, DNPH, iadoform) is performed on acetone and on n-propyl alcohol. 3. Write structural formulae far the following compounds that could fit the specifications ofthe unknowns: a. Aldehydes (5 possible) h. Ketones (2 possible) c. Primary alcohols (5 possible) d. Secondary alcohols (2 possible) e. A tertiary alcohol 4. Predict the result (positive or negative) when eaeh of the three tests (dichromate, DNPH, iodoform) is carried out on eaeh of the compounds included in your answers to question 3. A table containing your answers to question 4 will be a valuable aid when drawing conclusions from your experimental results.
Obviously the chemistry of the three tests and the structures and reactivities of the 15 possible compounds offer considerable scope for class discussion. Sample data tahle including results that will he obtained if the proper procedure is applied to uncontaminated unknowns:
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Journal of Chemical Education
Test Dichmmate
Compound
DNPH
lodoform
acetone +propyl alcohol unknown unknown unknown unknown unknown
A B C
D E
The conclusion must he reached that the only compounds that will fit the experimental results a n d the particular unknown set specifications are A = tert-hutyl alcohol; B = acetone; C = formaldehyde; D = isopropyl alcohol; E = methanol. A "best" or "necessary" logic sequence can hardly he stated here. One example of a fairly common student sequence includes: a. A must be a tertiary alcohol, and so all other unknowns have molecularwts.
