Students Discover Organic Chemistry: A Phenomena-Oriented and Inquiry-Based Network Concept (PIN-Concept)

Abstract

The Phenomena-oriented and Inquiry-based Network-Concept (PIN-Concept) is a curriculum for the training of interconnected thinking in the field of fundamental organic chemistry. It has been developed by Harsch and Heimann [1–21] for the chemical and didactical education of prospective teachers at universities, and for practicing chemistry teachers and their classes at grammar schools. The PIN-Concept turned out to be motivating and effective for teachers’ training and for chemistry classes at stage 10–11 (age 16–17). Good experience has also been gained with some simplified components from the PIN-Concept at stages 8–9, but this has not yet been investigated systematically.

Experimental Details

9.1.1 Test Reactions as Analytical Tools

9.1.1.1 Cerium Nitrate Test

• Reagent: 40 g cerium ammonium nitrate are dissolved in 100 ml nitric acid (c = 2 mol/l).

• Procedure: 1 ml cerium ammonium nitrate reagent is to be diluted with 2 ml water. Add 5 drops of the test substance and shake well. Let rest for 5 min and observe.

• Result: A color change from yellow to red (or orange) indicates molecules with alcohol groups.

• Note: Alcohol molecules make red colored complexes with hexanitratocerate ions.

• Diethyl acetal is hydrolyzed (acid catalysis) to ethanol and acetaldehyde; the alcohol makes the test positive.

Sometimes, the red color, indicating an alcohol group, disappears immediately or slowly, and the solution becomes colorless. This indicates that cerium ions Ce⁴⁺ have been reduced to Ce³⁺, and that the test substance has been oxidized.

For example, ethanol is oxidized to acetaldehyde (after heating), 2-propanol to acetone, lactic acid to acetaldehyde and carbon dioxide, citric acid to acetone and carbon dioxide. These reactions can be discovered by the students when they make the cerium nitrate test with these substances in a larger scale in a synthesis apparatus (Fig. 9.3) and investigate the distillates with the reactions.

9.1.1.2 Bromothymol Blue Test (BTB test)

• Reagent: 0.02 g bromothymol blue and 0.6 g sodium hydroxide have to be dissolved in 100 ml ethanol.

• Procedure: 1-ml test substance is to be added to 1-ml BTB reagent. Mix it well and observe.

• Result: A color change from blue to yellow/orange indicates molecules with carboxylic acid groups.

• Note: The test is also positive with inorganic acids. Therefore, the detection of carboxylic acid groups is only conclusive in the absence of inorganic acids.

• If the test substance is not soluble (e.g., stearic acid), heating is helpful for detecting the carboxylic acid groups.

9.1.1.3 Phenolphthalein Test (Rojahn test)

• Reagent 1: 0.1 g phenolphthalein is dissolved in 100 ml ethanol.

• Reagent 2: Sodium hydroxide solution (c = 3 mol/l).

• Procedure: 1 ml of the test substance and 3 drops phenolphthalein solution are added to 1 ml ethanol. Sodium hydroxide solution is then to be added drop wise (constant shaking) until a permanent pink color is observed which even resists vigorous shaking. (It is very important to only add as much sodium hydroxide solution as needed to change the indicator to pink.) The test tube with the solution is then to be put into a water bath (40°C). It needs to be taken out after every minute to shake it. The test can be stopped, when a discoloration is observed. Otherwise, the test tube has to be observed for a maximum of 10 min.

• Result: Discoloration of the pink phenolphthalein solution indicates molecules with ester groups.

• Note: Esters are hydrolyzed in the alkaline test solution, and the consumption of hydroxide ions is indicated by phenolphthalein.

For the detection of ester groups in fat molecules, propanol is needed as a solvent instead of ethanol.

Lactic acid gives a positive test result because of intermolecular esterification. This is a nice exception, which can be used as a problem for inquiry learning.

This test was introduced by Carl August Rojahn (1889–1938) who was a Professor for Pharmacy and Food Chemistry at the University of Halle (Germany).

9.1.1.4 Dinitrophenylhydrazine Test (DNPH Test)

• Reagent 1: A mixture of 186 ml water, 33 ml hydrochloric acid (37%), and 1 g 2,4-dinitrophenyl-hydrazine are to be stirred well for 15 min. Undissolved ingredients are then to be removed by filtration.

• Procedure: To 1 ml DNPH reagent in a test tube is to be added one drop of the test substance. A stopper is to be put on the test tube which is to be shaken for 15 s and observed for another 45 s.

• Result: A yellow or orange precipitate (sometimes only a milky turbidity) indicates molecules with aldehyde or ketone groups.

• Note: the carbonyl groups of aldehydes or ketones react with the amino group of the DNPH reagent and make an insoluble yellow or orange hydrazone.

  For the detection of carbonyl groups in sugar molecules, heating with a boiling water bath (5 min) is necessary.

  Diethyl acetal is hydrolyzed (acid catalysis) to the ethanol and acetaldehyde; the aldehyde makes the test positive.

9.1.1.5 Fehling Test

• Reagent 1: 7 g CuSO₄ ·H₂O are dissolved in 100 ml water.

• Reagent 2: 35 g sodium potassium tartrate and 10 g sodium hydroxide are dissolved in 100 ml water.

• Procedure: 1 ml reagent 1 and 1 ml reagent 2 are mixed. The mixture is deep blue or purple and clear. Then 1 ml test substance and a boiling chip are added. After shaking the sample is put into a boiling water bath. The test can be stopped when a precipitate or turbidity is observed. Otherwise it is observed for a maximum of 5 min.

• Result: A red or reddish brown precipitate of copper oxide Cu₂O at the bottom of the test tube (often a small amount) indicates molecules with an Acetyl group.

• Note: Aldehydes are oxidized by copper ions Cu²⁺ in alkaline solution to acid ions; Cu²⁺ ions are reduced to Cu¹⁺, and these ions form insoluble red copper oxide Cu₂O.

Not only glucose but also fructose gives a positive test result because of rapid isomerization in the alkaline solution, in contrast to saccharose.

Diethyl acetal gives a negative test result because acetals are not hydrolized in alkaline solutions. (Note that saccharose is also an acetal!)

This test reaction was discovered by Hermann Fehling (1812–1885) who was a Professor for Chemistry at the University of Stuttgart (Germany).

9.1.1.6 Copper Sulfate Test

• Reagent 1: Copper sulfate in water (c = 0.1 mol/l).

• Reagent 2: Sodium hydroxide in water (c = 3 mol/l).

• Procedure: 1 ml of the liquid test sample (or a small spatula amount of a solid sample) is dissolved in 1 ml copper sulfate solution. The mixture must be clear. Then 5 drops of sodium hydroxide solution are added drop by drop (continuous shaking).

• Result: A color change from light blue to deep blue or purple without a permanent precipitate (the solution must be clear after adding 5 drops of sodium hydroxide solution) indicates molecules with alcohol groups on adjacent carbon atoms (diol groups).

• Note: In the copper sulfate test, the test substance (diol) has the same function as the tartrate ions (they are diols) in the Fehling test: They form deep blue and stable chelate compounds with Cu²⁺ ions in alkaline solution.

9.1.1.7 Bromine Test

• Reagent: 10 drops of bromine are dissolved in 250 ml water.

• Procedure: 2 ml test substance (or 1 spatula) are dissolved or suspended in 2 ml bromine water. The solution is shaken well. (Close the test tube with a stopper and use gloves.)

• Result: Decolorization indicates molecules with double bonds (alkenes).

• Note: Decolorization occurs by addition of bromine molecules to the double bonds of the test molecules. (The addition products are colorless.)

  If bromine is only extracted from the water phase into the organic phase without decolorization (e.g., with alkanes or esters as test substances), the test is negative.

9.1.1.8 Iodoform Test (Lieben Test)

• Reagent 1: 12.7 g iodine and 25.4 g potassium iodide dissolved in 100 ml water.

• Reagent 2: Sodium hydroxide in water (c = 3 mol/l).

• Procedure: To 0.5 ml test substance are added 1 ml iodine solution (brown) and without delay sodium hydroxide solution in 1 ml steps, if the substance is still brown.

  After every 1 ml step, the test tube has to be shaken, until the solution becomes yellow and clear. Then the test tube is observed for 5 min.

• Result: A yellow or pale precipitate (often only a slight turbidity) indicates molecules with special functional groups, namely:

Type 1: Acetyl groups of aldehydes and ketones

Type 2: Special alcohol groups with the structure

Type 3: Special ester groups with the structure

• Note: Esters of type 3 are hydrolyzed in the alkaline test solution to alcohols of type 2, and these are oxidized by iodine to acetyl groups of type 1, which after iodination of the methyl group are cleaved to iodoform I₃CH and an acid ion in the alkaline solution.

This complex and valuable test reaction has been discovered by Adolf Lieben (1836–1914) who was a Professor for Chemistry at the Universities of Paris (France), Palermo, Turin (Italy), and Vienna (Austria).

9.1.1.9 Iron Chloride Test

• Reagent 1: 8 g iron chloride FeCl₃ ·6H₂O dissolved in 100 ml water.

• Reagent 2: Amyl alcohol (1-pentanol).

• Reagent 3: 0.1 g phenolphthalein dissolved in 100 ml ethanol.

• Reagent 4: Sodium hydroxide solution (c = 3 mol/l).

• Reagent 5: Hydrochloric acid (c = 0.5 mol/l).

• Procedure: To 10 drops of the test substance and 2 drops phenolphthalein, sodium hydroxide solution is to be added drop by drop until the phenolphthalein indicator just changes to pink. Then the solution is discolored without delay by adding one drop of hydrochloric acid (or a few, if necessary). If the solution warmed up, it has to be cooled down to room temperature.

  Now, 2 drops of iron chloride solution and 1.5 ml amyl alcohol are to be added. The stoppered test tube has to be shaken vigorously for 5 s. Wait until the two phases separate.

• Result: The test is positive if an orange color can be observed in the upper or lower phase. This indicates carboxylic acid molecules with short chains. With formic acid and acetic acid, the lower phase is orange; with propanoic acid and butanoic acid, the upper phase is orange.

• Note: The iron chloride test does not work directly with the carboxylic acids, but with their salts. The purpose of the first step of the procedure is therefore to neutralize the carboxylic acids. (This step is not necessary for the salts, of course.) The acid ions form orange complexes with the iron ions. The complexes with propanoate and butanoate prefer the unpolar amyl alcohol (upper phase), whereas the more polar complexes with formiate and acetate prefer the lower water phase.

  The iron chloride test is negative with mineral acids, in contrast to the BTB test. However, it is limited to short-chain carboxylic acids.

9.1.1.10 Dichromate Test

• Reagent: 2.35 g potassium dichromate are to be dissolved in 150 ml water. Then 8 ml concentrated sulfuric acid have to be added. After stirring, fill up with water to 200 ml.

  This reagent is poisonous. Use gloves. Pay special attention to solid dichromate, because it is carcinogen. Do not inhale dichromate dust.

• Procedure: To 2 ml dichromate reagent are added 4 drops test substance and a boiling chip. The sample is to be put into a boiling water bath for 5 min.

• Result: A color change from orange to green or brown indicates molecules which are oxidizable (e.g., primary and secondary alcohols, aldehydes, esters, and other compounds) under the test conditions. Alkanes, ketones, and mono carboxylic acids (exception: formic acid) are not oxidizable.

• Note: The dichromate test should be used only by experienced students, who are aware of the necessary safety conditions. Another possibility is to restrict this test to teachers’ demonstrations, or even omit it because it is not strictly necessary as an analytical tool. (Note that for synthesis the dichromate reagent can be substituted by potassium permanganate.)

9.1.2 Syntheses

9.1.2.1 Esterification

With the purified product (ca. 15 ml), only the Rojahn test is positive. Additionally, the iodoform test should be made. It is positive as it should be for ethyl acetate, but only a slight turbidity is observable.

• Note: In the same way, propyl propionate can be synthesized from propanol and propanoic acid. This time, the iodoform test is negative as it should be. The yield is ca. 20 ml.

• Procedure: In an Erlenmeyer flask (100 ml), 20 ml ethanol, 20 ml ethanoic acid, and 5 ml sulfuric acid (conc.) are mixed. Shake the mixture well. It warms up spontaneously. Let it stand for 5 min and pour it then into another Erlenmeyer flask (100 ml) containing 75 ml water. Immediately, an organic phase separates in the neck of the flask. It smells like an ester.

  Test the crude product by means of analytical tests (Rojahn test, cerium nitrate test, BTB test, iron chloride test).

  In order to purify the crude product, transfer it with a pipe into a flask (100 ml), add the double volume of sodium carbonate solution (c = 1 mol/l) and shake the unstoppered flask well, until the gas evolution (CO₂) is finished. Then stopper the flask and shake well. Let the phases separate.

  Investigate the upper phase (purified product) with the tests.

• Result: With the crude product, the four tests are all positive. From this it can be concluded that an ester has been produced which is still contaminated with alcohol and acids.

9.1.2.2 Ester Hydrolysis

• Procedure: In a flask (250 ml), put 60 ml sodium hydroxide solution (c = 3 mol/l) and 15 ml ethyl acetate. Stopper the flask and shake it vigorously for 2 min. Then pour the reaction mixture by means of a funnel into a distillation apparatus (Fig. 9.3) and distil with a boiling water bath for 15 min. (Cover the upper part of the apparatus with an aluminum foil.) Investigate both the distillate and the residue with the analytical tests. Note that the residue must be acidified with sulfuric acid (c = 1.5 mol/l) before making the tests. (Mix 5 ml residue with 4.5 ml sulfuric acid.)

• Result: Ethyl acetate is hydrolyzed in alkaline solution into ethanol (distillate) and acetate (residue). In the distillate, only the cerium nitrate test, the iodoform test, and the dichromate test are positive. This is the pattern of ethanol.

  In the residue, the iron chloride test is positive (lower phase orange) indicating the presence of ethanoic acid (or acetate). The BTB test is also positive, but it is not conclusive for carboxylic acids because the residue was acidified with sulfuric acid.

• Note: Propyl propanoate is more stable than ethyl acetate. It is hydrolyzed only very slowly. In this case, it is better to let the mixture stand for a week and then shake it again vigorously before distilling it. But even then, the ester is not completely hydrolyzed.

9.1.2.3 Oxidation

• Procedure: In a synthesis apparatus (Fig. 9.3), 12 drops of the oxidizable substance (ethanol, 1-propanol, 2-propanol, ethyl acetate, diethyl acetal) are mixed with 14 ml dichromate synthesis reagent (82.0 g K₂Cr₂O₇ dissolved in 12.5 ml water and 1.5 ml conc. H₂SO₄). Use a funnel to put the reagent into the apparatus and a magnetic stirrer. Heat the oil bath to ca. 120°C and distil at this temperature for 10 min. Cool the distillation receiver with ice water. A charcoal absorber is not necessary in this case. Investigate the colorless distillate with the tests.

• Result: In all cases, the reaction mixture changes its color immediately from orange to dark green or brown. This indicates that the substances are oxidized.

  With ethanol, ethyl acetate, and diethyl acetal as educts, the distillate gives a positive test only with the BTB test and with the iron chloride test (lower phase orange). This indicates that the educts have been completely converted into ethanoic acid.

  With 1-propanol, propanoic acid is found in the distillate. (In the iron chloride test, the upper phase is orange.)

  With 2-propanol as an educt, the distillate makes the DNPH test and the iodoform test positive; all other tests are negative. It can be concluded that 2-propanol has been oxidized to acetone.

• Note: The dichromate synthesis reagent has not the same concentration as the reagent for the analytical dichromate test. Pay attention to the safety recommendations (see 1.10).