«Identifying an Unknown Compound by Solubility, Functional Group Tests and Spectral Analysis This handout is a supplement to Signature Lab Series ANAL 0727 ...»
Identifying an Unknown Compound by Solubility, Functional
Group Tests and Spectral Analysis
This handout is a supplement to Signature Lab Series ANAL 0727 and contains material adapted from
Signature Lab Series ANAL 0727 and 0728, Cengage Learning. This lab contains material copywritten by
Cengage Learning and has been reproduced only to adapt the lab to experimental needs. This
supplement should not be used without the purchased lab manual, which contains the above
Purpose Of The Experiment:
Identifying an unknown organic compound through a three-step process involving selective solubility tests, selective functional group tests and spectral analysis.
You should be familiar with techniques for weighing, measuring by volume, and mixing in a test tube, as well as analysis of H NMR and C NMR spectra.
Organic qualitative analysis is an exercise in spectroscopy. Nuclear magnetic resonance spectroscopy and infrared spectroscopy are the major spectroscopic techniques used by organic chemists. However, much insight can be gained from using simple qualitative tests to determine the identity of unknowns. Structures of unknown compounds can be determined by comparing physical properties, performing functional group tests, and checking melting points of derivatives against those of known compounds reported in the literature. Solubility properties and chemical reactivity become apparent during these qualitative tests.
Spectroscopy has been discussed extensively in the lecture portion of this course.
Organic qualitative analysis involves four types of tests.
1. Measurement of physical properties includes determining refractive index, boiling points, melting points, and density.
2. Solubility tests can suggest the size and polarity of an unknown compound and the presence of basic or acidic functional groups. A compound’s solubility in aqueous acid or base involves ionization of the compound and, therefore, a chemical reaction. The salts produced are water-soluble.
3. Chemical tests transform an unknown into a different compound with an accompanying change in appearance. These tests are often called classification tests because they identify the possible functional groups present.
4. Formation of a solid derivative is a critical step in identifying an unknown. Many compounds have similar physical properties and give similar results in qualitative tests. However, an unknown can undergo reaction to form another compound called a derivative. The melting point of the purified derivative allows identification of the unknown.
In this lab we will focus on using Solubility Tests, Chemical Tests and Spectra Analysis to identify two unknown compounds.
In this experiment, you will combine both spectroscopy and qualitative tests to identify an unknown organic compound. For this experiment, the possible categories of the unknown are alkane, alkene, alkyl halide, alcohol, phenol, amine, aldehyde, ketone, and carboxylic acid. Each compound will contain only a single type of these functional groups. Each of these functional groups has a unique combination of solubility and reactivity that allows it to be distinguished from the others.
In Part A of the experiment (Week 1), you will use solubility tests to characterize your unknown compound. By comparing the solubility of your unknown in several aqueous solutions (described below), you will be able to limit the possible functional groups on your compound. A flow chart, Figure 2, will help guide you in this effort. In some cases, these tests will be sufficient to identify the functional group(s) of your unknown substance.
In Part B of the lab (Week 2), you will conduct a series of experiments to distinguish between the remaining functional group possibilities to uniquely identify the functional group on your unknown compound. You should note that in most cases, with proper planning and utilization of the information gained in Part 1, only a few functional group tests will be required to uniquely identify the functional group(s) contained within your molecule. You will not need to run all of the chemical tests on each sample. You job is to decide which are needed for each unknown in order to determine its functional group(s). If you are not thoughtful in this process, and try to run all of the tests, you will likely run out of material (see below).
Each group will be given approximately 1 gram of two different unknown samples. You must carefully think about what tests you wish to conduct as not to waste your sample. If you carelessly run too many tests and exhaust your material, you can obtain an additional sample of your unknown from your TA. Additional sample will cost you 10% of your grade for this lab per additional sample required.
Take-home message: plan your experiments carefully.
For both Part A and Part B, several known compounds will also be available so you can compare your results from your unknown to both known positive and negative tests. Figure 1 lists the known compounds that will be available to you. Tables 1 and 2 outline known substrates for each of the tests.
Figure 1. List of Known Compounds Available For Use As Positive and Negative Standards
In Part C of the lab (take home), once you have correctly identified the functional group present in your unknown compounds, your TA will provide you with the H NMR and C NMR spectra for your compounds, as well as the compound’s molecular formula. From this data, and the results of your experiments above, you will then assign the structure of the unknown and label the spectral data.
PART A (WEEK 1) - SOLUBILITY TESTS
Organic compounds follow three interdependent rules of solubility:
1. small organic molecules are more soluble in water than are large organic molecules;
2. polar organic molecules, especially those capable of hydrogen bonding, are more soluble in water than are nonpolar molecules; and
3. compounds in their ionic forms are more soluble in water than their neutral forms.
For example, benzoic acid is not soluble in water, yet it is soluble in sodium hydroxide solution and in sodium hydrogen carbonate solution because these bases react with benzoic acid to form the water-soluble benzoate ion. The solubility of carboxylic acids and amines is so characteristic that solubility tests alone differentiate these functional groups from all the others in this experiment.
The solubility flowchart shown in Figure 2 provides the scheme for this experiment. The first test to perform on all unknowns is water solubility.
Water Small, polar organic compounds such as alcohols, aldehydes, ketones, amines, carboxylic acids, and a few phenols are soluble in water. Water-soluble compounds are tested with pH paper to see if they are acidic or basic. A pH of 4 or lower indicates a carboxylic acid. A pH of 8 or higher indicates an amine.
Water-soluble compounds are tested with 5% sodium hydrogen carbonate (NaHCO3) to determine whether or not they are carboxylic acids. Carboxylic acids react with NaHCO3 to produce carbon dioxide bubbles, as shown below in Equation 3.
Large alcohols, aldehydes, ketones, amines, carboxylic acids, and phenols are not soluble in water. Alkanes, alkyl halides, and alkenes are not soluble in water, regardless of their size. These waterinsoluble compounds are tested for their solubility in the following reagents.
5% Sodium Hydroxide Water-insoluble compounds are first tested with 5% sodium hydroxide (NaOH). Sodium hydroxide is a strong base that ionizes strong or weak (Figure 2 Solubility flowchart) acids. Thus, both carboxylic acids and phenols are converted to salts and dissolve in aqueous solution. Non-acidic compounds will not dissolve. The reactions of carboxylic acids and phenols are shown in Equations 1 and 2, respectively.
5% Sodium Hydrogen Carbonate Water-insoluble compounds that are soluble in 5% NaOH are then tested with 5% sodium hydrogen carbonate (NaHCO3). Strongly acidic compounds such as carboxylic acids react with NaHCO3 to form water-soluble salts, as shown in Equation 3. The reaction also produces bubbles of carbon dioxide (CO2).
This test is commonly misinterpreted because CO2 bubbles are tiny. Careful observation is essential.
Phenols are less acidic than carboxylic acids and do not react with NaHCO3 to form water-soluble salts. As a result, phenols are insoluble in 5% NaCHO3.
5% Hydrochloric Acid Water-insoluble compounds that are insoluble in 5% NaOH are tested with 5% hydrochloric acid (HCl). If a compound is soluble in 5% HCl, it is an amine. Amines are organic bases that react with HCl to form water-soluble amine salts, as shown in Equation 4.
Concentrated Sulfuric Acid Water-insoluble compounds that are insoluble in 5% HCl are tested with concentrated sulfuric acid (H2SO4). Virtually all organic compounds containing alkene functional groups or oxygen or nitrogen atoms are soluble in concentrated H2SO4. These functional groups typically react with H2SO4 to form new compounds. Only alkanes, alkyl halides, and some aromatic compounds are insoluble in H2SO4.
Table 1 Known Positive And Known Negative Test Compounds For Solubility Tests
Perform the water solubility test on the known positive, known negative, and unknown • Perform subsequent solubility tests • If the solubility tests point to a carboxylic acid or amine, the classification is complete • If the solubility tests suggest any other functional groups, you will preform classification tests • during week 2 appropriate to those groups until the unknown is narrowed to only one functional group
CAUTION—Wear departmentally approved safety goggles at all times while in the chemistry laboratory. Always use caution in the laboratory. Many chemicals are potentially harmful. Follow safety precautions given for all reagents used in this experiment. Prevent contact with your eyes, skin, and clothing. Avoid ingesting any of the reagents. Perform all tests in duplicate using an unknown, a known positive, and a known negative. Mix well to make certain that liquid samples are not floating in the meniscus. Allow several minutes for compounds to dissolve. Be patient and observe closely.
Conduct the solubility tests following the pattern shown in Figure 1 above. Verify your solubility test results with your laboratory instructor before performing the classifications tests in Part 2. Use clean test tubes for each test.
1. Performing the Water Solubility Test CAUTION—Unknowns may be flammable, toxic, corrosive, or irritating. Keep away from flames or other heat sources.
Add 1 drops of a liquid sample or about 25 mg of a solid sample to 0.5 mL of distilled or deionized water in a test tube. Tap the tube with your finger to mix or stir gently with a glass stirring rod. Record the sample as soluble or insoluble.
If the unknown is water-soluble, test the solution with pH paper. Also test the pH of water as a control.
A solution at pH 4 of lower suggests a carboxylic acid. A solution at pH 8 or higher suggests an amine.
2. Performing the 5% Sodium Hydroxide Solubility Test CAUTION—Sodium hydroxide (NaOH) and hydrochloric acid (HCl) are toxic and corrosive. If your compound is water-soluble, proceed to Part 3.
For water-insoluble compounds, add 1 drops of a liquid sample or about 25 mg of a solid sample to 0.5 mL of 5% NaOH in a test tube. Tap the tube with your finger to mix or stir gently with a glass stirring rod.
Record the sample as soluble or insoluble.
To verify that a compound has dissolved, add 5% HCl to the NaOH mixture until the solution is acidic to pH paper. Look for a precipitate, indicating that the water-soluble salt has converted back into the water-insoluble compound.
Solubility in NaOH indicates either the carboxylic acid or phenol.
3. Performing the 5% Sodium Hydrogen Carbonate Solubility Test a. For Water-Soluble Compounds Put 1 drops of liquid sample or about 25 mg of solid sample in a dry test tube. Add 0.5 mL of 5% sodium hydrogen carbonate (NaHCO3). Do not stir. Watch for bubbles at the interface of the phases. Then tap the tube with your finger to mix or stir gently with a glass stirring rod. Record the sample as soluble or insoluble.
Generation of bubbles and solubility indicates a carboxylic acid. Solubility without generation of bubbles indicates a low molar mass alcohol, aldehyde, ketone, or amine. Conduct classification tests to determine which functional group is present.
CAUTION—Diethyl ether (ether) is highly flammable and toxic. Keep away from flames or other heat sources. Use a fume hood. If no bubbles were observed, put 1 drop of liquid sample or about 25 mg of solid sample in a dry test tube. Using a fume hood, add about 0.5 mL of ether. Then immediately add 0.5 mL of 5% NaHCO3.
Observe whether or not bubbles are generated at the ether-water interface.
Generation of bubbles indicate a carboxylic acid.
b. For Water-Insoluble Compounds
Put 1 drop of liquid sample or about 25 mg of solid sample in a dry test tube. Add 0.5 mL of 5% sodium hydrogen carbonate (NaHCO3). Do not stir. Watch for bubbles at the interface of the phases. Then tap the tube with your finger to mix or stir gently with a glass stirring rod. Record the sample as soluble or insoluble.
Generation of bubbles or solubility indicates a carboxylic acid.
If the compound is not soluble in NaHCO3 but is soluble in NaOH, it is likely a phenol. Confirm the presence of phenol with a phenol classification test.
4. Performing the 5% Hydrochloric Acid Solubility Test CAUTION—Hydrochloric acid (HCl) is toxic and corrosive.
For compounds insoluble in water and insoluble in 5% NaOH, add 1 drop of a liquid sample or about 25 mg of a solid sample to 0.5 mL of 5% HCl in a test tube. Tap the tube with your finger to mix or stir gently with a glass stirring rod. Record the sample as soluble or insoluble.
If the compound is soluble in 5% HCl, it is most likely an amine.
5. Performing Concentrated Sulfuric Acid Solubility Test CAUTION—Concentrated sulfuric acid (H2SO4) is toxic and oxidizing. Use a fume hood when working with H2SO4. If the compound is insoluble in 5% HCl and 5% NaOH, add 1 drop of a liquid sample or about 25 mg of a solid sample to 0.5 mL of concentrated sulfuric acid (H2SO4) in a dry test tube. Tap the tube with your finger to mix or stir gently with a glass stirring rod. Do not use a metal spatula.
Record the sample as soluble or insoluble. Interpret a color change or a precipitate as soluble.
If the compound is soluble in H2SO4, the sample is an alkene, an alcohol, an aldehyde, or a ketone. Conduct classification tests for each compound type.
If the compound is insoluble in H2SO4, the sample is an alkane or an alkyl halide. Conduct classification tests for alkyl halides.
If alkyl halide tests are negative, the compound is an alkane.
Based upon the positive and negative results from the above experiments you should now be able to narrow the possibilities for the functional group(s) present in your unknown sample. You should now carefully decide which experiments are needed during Part B (Week 2) to distinguish those possibilities.
PART B (WEEK 2) - CLASSIFICATION TESTS Solubility tests (Week 1) alone can indicate whether an unknown compound in this experiment is a carboxylic acid, a phenol, or an amine. The other functional groups must be identified or verified by classification tests.