Postlab 8

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Feb 20, 2024

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TEXAS TECH CHEM 3106-382 Experiment 8: Identification of Unknowns: Classification Test Purpose: Experiment 8 is the second of three experiments we will conduct to determine the complete structure an unknown compound. In experiment 7, we focused on solubility of the compound. Today, we aim to enact various classification tests to identify our unknown compounds. These compounds may contain functional groups such as amines, ketones/methyl ketones, aldehydes, alcohols, phenols, and carboxylic acids. Physical Properties Table: Chemicals MW g/mol density g/mL boiling point (°C) melting point (°C) CrO 3 99.99 2.7 250 196 2,4- dinitropheenylhydrazin e 198.14 ~1.7 378.6 198-202 Ethanol 46.07 0.789 78.37 -114.1 Cu(SO 4 ) 159.61 3.6 150 110 5% NaHCO 3 84.01 1.03 100 0 2.5% FeCl 3 162.2 1.82 280-285 37 Acetone 58.08 0.79 56 -95 (NH 4 ) 2 Ce(NO 3 ) 6 548.22 1.10 83 107-108 Safety: CrO 3 2,4-dinitropheenylhydrazine Ethanol Tollen’s reagent Jone’s reagent Cu(SO 4 )

5% NaHCO 3 2.5% FeCl 3 Acetone (NH 4 ) 2 Ce(NO 3 ) 6 Procedures: I. Testing for Aldehydes and Ketones: Both aldehydes and ketones contain carbonyl groups that will react actively with 2,4, dinitrophenylhydrazine to form a hydrazone species. If a carbonyl is present, a precipitate will form in a color that ranges from bright yellow to red. Unsaturated species will brighter than saturated ones. 2,4-DNP Test and Hydrazone Derivative (Ketones/Aldehydes) Reaction : Procedure: 1. Add 7-8 drops of 2,4-DNP to a well on a spot plate. 2. Add your unknown.

a. 1-2 liquid or 2-3 drop solid dissolved in ~0.5 mL ethanol. 3. Stir with mixing rod. 4. If a colored precipitate forms, collect this using a Hirsch funnel. 5. *note: only do this test if you know a carbonyl is present. You can determine this by analyzing the IR spectrum of the compound. If this test is done an amine, an acid base reaction may ensue. Tollen’s Reagent Test Reaction : Promotes oxidation of aldehydes to carboxylic salts. Procedure: 1. Add ~1 mL Tollen’s reagent to shell vial. 2. Add a small amount of unknown (e.g. drop of liquid) 3. Cap vial and shake. 4. If the solution begins to darken and a silver solid appears at the surface, then your unknown is an aldehyde. Iodoform Test for Methyl Ketones: This reagent contains iodine in a potassium idodide solution. Reaction: Procedure: 1. Water-soluble compounds: a. In a shell vial, dissolve 2 drops/0.015 g of sample into 1 mL water. b. Add 1 mL 10% NaOH. c. Add 1 mL KI 3 solution. d. Shake vigorously and heat to 50-60° C. 2. Water-insoluble compounds: a. Same as above except use 1 mL of dioxane in place of water. 3. If CHI 3 precipitate forms, a methyl ketone is present. Jones Oxidation Test: Primary and secondary alcohols as well as aldehydes will react to this test. Reaction:

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Procedure: 1. Using a spot plate, add 1 drop of liquid unknown or 10 mg of solid unknown and dissolve with ~10 drops of acetone. 2. Add one drop of Jone’s reagent. 3. Chromium causes oxidation, a green chromium (III) salt will be produced. II. Testing for Alcohols Jones Oxidation Test: same as mentioned above. Sometimes the reaction produces a dark mixture and it is difficult to observe the green chromium (III) ions. Ceric Nitrate Test Procedure: 1. Place 5 drops of test reagent on a white spot plate. 2. Add 5 mg of solid or 1-2 drops of liquid unknown and look for a color change (brownish complex). 3. Amines and anilines may also produce a color change. III. Testing for Amines Copper Sulfate Test Procedure: 1. Add 7-10 drops of 10% Cu(SO 4 ) (aq) to a spot plate. 2. Add 10 mg solid or 1 drop liquid unknown. 3. Mis with stirring or pipet. 4. If a blue-green colored precipitate forms, then amines are present. Bromine Water Test Procedure: 1. Add 2 drops of liquid or 10 mg of solid to a small test-tube. 2. Add 1.5 mL of water to test tube. 3. Record pH. 4. Add bromine water solution until color persists. If there is a decolorization of bromine, followed by a white precipitate, then aromatic amines are present. If there is decolorization but no precipitate, then an alkyl amine may be present. III. Testing for Carboxylic Acids Procedure: 1. Add 1-2 unknown liquid or 10mg of unknown solid to a solution of 5% NaHCO 3 . If formation of gas is observed, then a carboxylic acid is present. When carboxylic acids are combined with a mild base, they produce carbon dioxide. IV. Testing for Phenols

Ferric Ion Test: Fe 3+ can form colored complexes (red, blue, purple, or green) when combined with phenols and some enols. Reaction: Procedure: 1. Add 2-3 drops of water to a white spot plate. 2. Add 1-2 drops of ethanol. 3. Add unknown and stir. a. Liquid—2 drops b. Solid—10mg 4. Add 1 drop of 2.5% FeCl 3. 5. Color change indicates presence of phenols or pyridine. If no change is seen after 1 minute of stirring, you may add a second drop of ferric chloride. Bromine Water Test: Use if you got a positive result for the Ferric Ion test, to determine if your unknown is a phenol. Reaction: Procedure: Refer to previous description. Results & Discussion: 1) In addition to solubility, we determined in experiment 7, we can include other physical properties by performing qualitative functional group tests. Checking melting points of derivatives. In some cases, you must create stable precipitation or product. Organic qualitative analysis involves four types of tests first measurements of physical properties, such as boiling point, melting point, density, and refractive index. Refractive index depends on the refraction of light. Every material has unique structure, and every structure can reflect unique waves of light. Refractive data therefore exposes distinctive properties of a structure, just like boiling point, melting point, and density. If you know can determine

these four physical properties, you can likely determine 80% of unknown. It is unlikely for any two compounds to share the same melting point, boiling point, density, and refractive index. 2) In experiment 7, we performed solubility tests which gave us a basic idea of the size, polarity, and pH properties of our unknown. As a result, for experiment 8 we will not have to perform all the functional group tests, but only the ones that would apply to our unknown based on the results of the solubility tests. 3) Classification tests can be performed to chemically alter an unknown so that a visible change is observed. A colorless, odorless compound may be transformed into something with recognizable features and identifiable properties such as a change in color, the formation of a precipitate, or gas formation. Gas formation is generally associated with an air mixture, H 2 , and CO 2 . 4) The formation of a solid derivative can be a critical step in identifying unknown. Some of these compounds share similar physical properties which makes them hard to distinguish as they yield similar results in these tests. However, some tests cause the formation of compound derivatives which make the compound distinctive. If your final product is not visible, you can make it visible via modifications. Classification Test Background: Classification tests are crucial in identifying an unknown because they are quick and reliable at exposing characteristic functional groups present, which narrows down the possible identities of the unknown. The potential functional groups present in our unknown include alkane, alkene, alkyl halide, alcohol, phenol, amine, aldehyde, ketone, and carboxylic acid. It may sound redundant to have phenol and alcohol as separate categories as they are both alcohols (phenol being a benzene alcohol), but they have distinct responses to these classification tests. For example, in the bromine water test, alcohols do not react, but phenols do. This is due to the reactive presence phenol’s aromatic ring. When phenol reacts with bromine water, it forms a white precipitate called 2,4,6-tribromophenol. Likewise, phenols also react with ferric ion tests while nonaromatic alcohols do not. A phenoxide ion is a good ligand for Fe (III)ion, which produces a characteristic green colored complex. Phenols also respond different from other alcohols in the Schiff’s reagent test. The Schiff’s test is generally used to detect the presence of aldehydes, but alcohols can be oxidized in the presence of SO 2 to aldehydes which then react with the reagent to give a pink color. Bromine in Cyclohexane : Bromine can easily enter an addition reaction with an alkene via the C=C. Bromine is colored (orange). If you react bromine with your unknown and product a colorless product, you know that your unknown includes an alkene. You may not know the total structure of your unknown, but you will know that it is in the alkene family. Therefore, this test is good if you are in the final stage and deciding between two molecules—one of which is an alkane and the other an alkene. Methods like NMR would not be able to do determine this difference. IR, however, may be used as a complementary technique to verify the presence of the C=C which shows up as a medium peak at 1650. Potassium Permanganate: Like the Bromine in Cyclohexane above, the reaction of an alkene with potassium permanganate also produces a color change. However, instead of the disappearance of color, you produce a brown color. Silver Nitrate : If you react your unknown with silver nitrate and produce shiny and silver precipitation, you have an alkyl halide such as methyl bromide, ethyl chloride, isopropyl fluoride and so on.

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Sodium Iodide: This test is similar to silver nitrate, so there is no need to perform both. If you suspect the presence of an alkyl halide, you can perform either the silver nitrate or sodium iodide reaction. In this test, if you see a strong and sudden precipitate, it means you have an alkyl halide. TCICA: It’s not a primary test. It is a lesser-known technique used to define primary and secondary alcohols. Iron (III) Chloride: Typically, Iron (III) Chloride reacted with phenols, produces a brightly color product. However, this test is not definite because not all phenols react with Iron (III) Chloride, and in some cases aldehydes and ketones will. Bromine in Water: This is an alternative test for phenols. Here, you will see a colorless product. 2,4-Dinitrophenylhydrazine & Schiff Test: These tests are used to identify the presence of aldehyde. However, the Schiff Test is very sensitive and does not always provide reliable results. 2,4-DNP tests for the presence of both aldehydes and ketones which will both form a solid yellow, orange, or red precipitate in the presence of 2,4-DNP.