Experimental Procedures: Recrystallization

Purpose

To explore the techniques for recrystallizing solids.Safety Alert1 Wear safety glasses or goggles while performing the experiments.2 Organic compounds are much more rapidly absorbed through the skin when

they are in solution, particularly in water-soluble solvents such as acetone and ethanol. For this reason, do not rinse organic materials off your skin with solvents such as acetone; wash your hands thoroughly with soap and warm water instead.

3 Do not use a burner in these procedures unless instructed to do so. Most solvents used for recrystallization are flammable (Table 3.1).

4 When using a hot plate, do not set it at its highest value. A moderate setting will prevent overheating and the resultant bumping and splashing of materials from the flask. Do not employ hot plates for heating volatile or flammable solvents; rather, use a steam bath.

5 Do not inhale solvent vapors. If a hood is not available to you, clamp an inverted funnel just above the Erlenmeyer flask in which you will be heating solvents. Attach this funnel to a source of vacuum by means of rubber tubing (Fig. 2.71b).

6 Never add decolorizing carbon to a boiling solution; doing so may cause the solution to boil out of the flask. Add the carbon only when the temperature of the solvent is below the boiling temperature. This same precaution applies when using a filter-aid to assist in the removal of the carbon during the hot filtration step.

Solvent Selection

ProcedurePreparationRefer to the online resources to answer Pre-Lab Exercises, access videos, and read the MSDSs for the chemicals used or produced in this procedure. Read or review Sections 2.5, 2.9, and 3.2, and review the solvent properties listed in Table 3.1. Although different criteria are used for defining solubility, plan to use the following definitions in this experiment: (a) soluble— mg of solute will dissolve in mL of solvent; (b) slightly soluble—some but not all of the mg of solute will dissolve in mL of solvent; (c) insoluble—none of the solute appears to dissolve in mL of solvent. Be certain to record all your observations regarding solubilities in your notebook!ApparatusTest tubes

, hot-water or steam bath.ProtocolFor known compounds, place about mg (a microspatula-tip full) of the finely crushed solid in a test tube and add about mL of water using a calibrated Pasteur pipet. Stir the mixture with a glass rod or microspatula to determine whether the solid is soluble in water at room temperature. If the solid is not completely soluble at room temperature, warm the test tube in the hot-water or steam bath, and stir or swirl its contents to determine whether the solid is soluble in hot water.If any of your solutes are soluble in the hot solvent but only slightly soluble or insoluble at room temperature, allow the hot solution to cool slowly to room temperature and compare the quantity, size, color, and form of the resulting crystals with the original solid material.Repeat the solubility test for the solutes using ethanol and then petroleum ether (bp , torr). After completing these additional tests, record which of the three solvents you consider best suited for recrystallization of each of the solutes.Compounds 1–4 contain a variety of functional groups that impart differing solubility properties to the molecules and are possible substrates on which to practice the technique of determining solubilities. Other known compounds may be assigned by your instructor.

For unknown compounds, a systematic approach is important for determining their

solubility. First, select the solvents from Table 3.1 to be used in the tests. It should not be necessary to test all the solvents, but you should consider trying those that are denoted with the symbol ‡ in the table. Your instructor may also suggest solvents to evaluate.After selecting the solvents, obtain enough clean, dry test tubes so that there is one for each solvent to be tested. Place about mg (a microspatula-tip full) of the finely crushed unknown in each test tube and add about mL of a solvent to a tube containing the solid. Stir each mixture and determine the solubility of the unknown in each solvent at room temperature. Use the definitions of soluble, slightly soluble, or insoluble given earlier.If the unknown is insoluble in a particular solvent, warm the test tube in the hot-water or steam bath. Stir or swirl the contents of the tube and note whether the unknown is soluble in hot solvent. If the solid is soluble in the hot solvent but only slightly soluble or insoluble at room temperature, allow the hot solution to cool to room temperature slowly. If crystals form in the cool solution, compare their quantity, size, color, and form with the original solid material and with those obtained from other solvents.It is a good idea to test the solubility of a solute in a variety of solvents. Even though nice crystals may form in the first solvent you try, another one might prove better if it provides either better recovery or higher-quality crystals. To assist in determining the best solvent to use in recrystallizing an unknown, you should construct a table containing the solubility data you gather by the systematic approach described above.If these solubility tests produce no clear choice for the solvent, mixed solvents might be considered. Review the discussion presented earlier in this section for the procedure for using a mixture of two solvents. Before trying any combinations of solvent pairs, take about mL of each pure solvent being considered and mix them to ensure that they are miscible in one another. If they are not, that particular combination cannot be used.Recrystallizing Impure Solids

Miniscale ProceduresPreparationRefer to the online resources to answer Pre-Lab Exercises, access videos, and read the MSDSs for the chemicals used or produced in this procedure. Read or review Sections 2.6, 2.7, 2.9, 2.11, 2.17, and 2.18.1. Benzoic Acid

ApparatusTwo -mL Erlenmeyer flasks, graduated cylinder, ice-water bath apparatus for magnetic stirring, vacuum filtration, and flameless heating.DissolutionPlace g of impure benzoic acid in an Erlenmeyer flask equipped for magnetic stirring or with boiling stones. Measure mL of water into the graduated cylinder and add a -mL portion of it to the flask. Heat the mixture to a gentle boil and continue adding water in -mL portions until no more solid appears to dissolve in the boiling solution. Record the total volume of water used; no more than mL should be required. Caution: Because the sample may be contaminated with insoluble material, pay close attention to whether additional solid is dissolving as you add more solvent; if it is not, stop adding solvent.DecolorationPure benzoic acid is colorless, so a colored solution indicates that treatment with decolorizing carbon (Sec. 2.18) is necessary. Caution: Do not add decolorizing carbon to a boiling solution! Allow the solution to cool slightly, add a microspatula-tip full of carbon, and reheat to boiling for a few minutes. To aid in removing the finely divided carbon by filtration, allow the solution to cool slightly, add about g of a filter-aid (Sec. 2.17), and reheat.Hot Filtration and CrystallizationIf there are insoluble impurities or decolorizing carbon in the solution, perform a hot filtration (Sec. 2.17) using a -mL Erlenmeyer flask to receive the filtrate (Fig. 2.52). Rinse the empty flask with about mL of hot water and filter this rinse into the original filtrate. If the filtrate remains colored, repeat the treatment with decolorizing carbon. Cover the opening of the flask with a piece of filter paper, an inverted beaker, or loose-fitting cork to exclude airborne impurities from the solution, and allow the filtrate to stand undisturbed until it has cooled to room temperature and no more crystals form.★ To complete the crystallization, place the flask in an ice-water bath for at least min.Isolation and DryingCollect the crystals on a Büchner or Hirsch funnel by vacuum filtration (Fig. 2.54) and wash the filter cake with two small portions of cold water. Press the crystals as dry as possible on the funnel with a clean cork or spatula. Spread the benzoic acid on a watchglass, protecting it from airborne contaminants with a piece of filter paper, and air-dry it at room temperature or in an oven. Be certain that the temperature of the oven is below the melting point of the product!AnalysisDetermine the melting points of the crude and recrystallized benzoic acid, the weight of the latter material, and calculate your percent recovery using Equation 3.1.

(3.1)

2. AcetanilideApparatusTwo -mL Erlenmeyer flasks, graduated cylinder, ice-water bath apparatus for magnetic stirring, vacuum filtration, and flameless heating.DissolutionPlace g of impure acetanilide in an Erlenmeyer flask equipped for magnetic stirring or with boiling stones. Measure mL of water into the graduated cylinder and add a -mL portion of it to the flask. Heat the mixture to a gentle boil.A layer of oil should form when the stated amount of water is added. (If you have not done so already, review the discussion of Crystallization in this section, focusing on how to crystallize compounds that form oils.) This layer consists of a solution of water in acetanilide. More water must be added to effect complete solution of the acetanilide in water. However, even if a homogeneous solution is produced at the boiling point of the mixture, an oil may separate from it as cooling begins. The formation of this second liquid phase is known to occur only under specific conditions: The acetanilide-water mixture must have a composition that is between and in acetanilide and be at a temperature above . Because the solubility of acetanilide in water at temperatures near exceeds , a homogeneous solution formed by using the minimum quantity of water meets these criteria. Such a solution will yield an oil on cooling to about ; solid begins to form below this temperature.Continue adding mL of water in -mL portions to the boiling solution until the oil has completely dissolved. Caution: Because the sample may be contaminated with insoluble material, pay close attention to whether additional solid is dissolving as you add more solvent; if it is not, stop adding solvent. Once the acetanilide has just dissolved, add an additional mL of water to prevent formation of oil during the crystallization step. If oil forms at this time, reheat the solution and add a little more water. Record the total volume of water used.Continue the procedure by following the directions for Decoloration, Hot Filtration and Crystallization, and Isolation and Drying given for benzoic acid in Part 1.AnalysisDetermine the melting points of the crude and recrystallized acetanilide, the weight of the latter material, and calculate your percent recovery using Equation 3.1.3. NaphthaleneApparatusTwo -mL Erlenmeyer flasks, graduated cylinder, ice-water bath apparatus for magnetic stirring, vacuum filtration, and flameless heating.DissolutionNaphthalene may be conveniently recrystallized from methanol, ethanol, or 2-propanol. Because these solvents are somewhat toxic and/or flammable, proper

precautions should be taken. The sequence of steps up through the hot filtration should be performed in a hood if possible. Alternatively, if instructed to do so, position an inverted funnel connected to a vacuum source above the mouth of the flask being used for recrystallization (Fig. 2.71b).Place g of impure naphthalene in an Erlenmeyer flask equipped for magnetic stirring or with boiling stones and dissolve it in the minimum amount of boiling alcohol. Caution: Because the sample may be contaminated with insoluble material, pay close attention to whether additional solid is dissolving as you add more solvent; if it is not, stop adding solvent. Then add mL of additional solvent to ensure that premature crystallization will not occur during subsequent transfers. Record the total volume of solvent used.Continue the procedure by following the directions for Decoloration, Hot Filtration and Crystallization, and Isolation and Drying given for benzoic acid in Part 1; however, use the solvent in which you dissolved the naphthalene rather than water.AnalysisDetermine the melting points of the crude and recrystallized naphthalene, the weight of the latter material, and calculate your percent recovery using Equation 3.1.4. Unknown CompoundApparatusTwo -mL Erlenmeyer flasks, graduated cylinder, ice-water bath apparatus for magnetic stirring, vacuum filtration, and flameless heating.DissolutionAccurately weigh about g of the unknown compound and transfer it to an Erlenmeyer flask equipped for magnetic stirring or with boiling stones. Measure about mL of the solvent you have selected on the basis of solubility tests into a graduated cylinder and add mL of it to the flask. Bring the mixture to a gentle boil using flameless heating unless water is the solvent, add a -mL portion of the solvent, and again boil the solution. Continue adding -mL portions of solvent, one portion at a time, until the solid has completely dissolved. Bring the solution to boiling after adding each portion of solvent. Caution: Because the sample may be contaminated with insoluble material, pay close attention to whether additional solid is dissolving as you add more solvent; if it is not, stop adding solvent. Record the total volume of solvent that is added.Continue the procedure by following the directions for Decoloration, Hot Filtration and Crystallization, and Isolation and Drying given for benzoic acid in Part 1.AnalysisDetermine the melting points of the crude and recrystallized unknown, the weight of the latter material, and calculate your percent recovery using Equation 3.1.5. Mixed-Solvent Crystallization

ApparatusTwo -mL Erlenmeyer flasks, graduated cylinder or calibrated Pasteur pipet, ice-water bath apparatus for magnetic stirring, vacuum filtration, and flameless heating.DissolutionPlace g of impure benzoic acid or acetanilide in an Erlenmeyer flask equipped for magnetic stirring or with boiling stones. Add mL of ethanol and heat the mixture to a gentle boil. If necessary, continue adding the solvent in -mL portions until no more solid appears to dissolve in the boiling solution. Caution: Because the sample may be contaminated with insoluble material, pay close attention to whether additional solid is dissolving as you add more solvent; if it is not, stop adding solvent. Record the total volume of ethanol used.DecolorationIf the solution is colored, treat it with decolorizing carbon (Sec. 2.18) and a filter-aid according to the procedure given for benzoic acid in Part 1.Hot Filtration and CrystallizationIf there are insoluble impurities or decolorizing carbon in the solution, perform a hot filtration (Sec. 2.17) using a -mL Erlenmeyer flask to receive the filtrate (Fig. 2.52). Rinse the empty flask with about mL of hot ethanol and filter this rinse into the original filtrate. If the filtrate remains colored, repeat the treatment with decolorizing carbon.Reheat the decolorized solution to boiling and add water dropwise from a Pasteur pipet until the boiling solution remains cloudy or precipitate appears; this may require adding several milliliters of water. Then add a few drops of ethanol to produce a clear solution at the boiling point. Remove the flask from the heating source, and follow the same directions as given for benzoic acid in Part 1 to complete both this stage of the procedure and Isolation and Drying.AnalysisDetermine the melting points of the crude and recrystallized product, the weight of the latter material, and calculate your percent recovery using Equation 3.1.

Microscale ProceduresPreparationRefer to the online resources to answer Pre-Lab Exercises, access videos, and read the MSDSs for the chemicals used or produced in this procedure. Read or review Sections 2.5, 2.6, 2.7, 2.9, 2.17, and 3.3. Plan to determine the melting points of both the purified and the crude solids as a way to show the benefits of recrystallization.1. Benzoic AcidApparatusA

test tube, a calibrated Pasteur pipet and Pasteur filtering and filter-tip pipets, ice-water bath apparatus for Craig tube filtration and flameless heating.DissolutionPlace of benzoic acid in the test tube. Add mL of water to the tube and heat the mixture to a gentle boil. To aid in dissolution, stir the mixture vigorously with a microspatula while heating; stirring also prevents bumping and possible loss of material from the test tube. If needed, add more solvent in - to -mL increments to dissolve any remaining solid. Bring the mixture to boiling and continue stirring after each addition. Once all the solid has dissolved, add an additional - to -mL portion of solvent to ensure that the solute remains in solution during transfer of the hot solution to a Craig tube. Caution: Because the sample may be contaminated with insoluble material, pay close attention to whether additional solid is dissolving as you add more solvent. If it is not, stop adding solvent. Record the approximate total volume of solvent used.DecolorationIf the solution is colored and the recrystallized product is known or suspected to be colorless, treatment with decolorizing carbon (Sec. 2.18) is necessary. Caution: Do not add decolorizing carbon to a boiling solution! Cool the solution slightly, add half of a microspatula-tip full of powdered carbon or, preferably, a pellet or two of decolorizing carbon, and reheat to boiling for a few minutes.Hot Filtration and CrystallizationPreheat a Pasteur filtering pipet by pulling hot solvent into the pipet. Then transfer the hot solution into this pipet with a Pasteur pipet or filter-tip pipet that has also been preheated with solvent, using the tared Craig tube as the receiver for the filtrate (Fig. 2.53). If decolorizing carbon or other insoluble matter appears in the Craig tube, pass the solution through the filtering pipet a second and, if necessary, a third time. Concentrate the clear solution in the Craig tube to the point of saturation by heating it to boiling. Rather than using a boiling stone to prevent superheating and possible bumping, continually stir the solution with a microspatula while heating. The saturation point will be signaled by the appearance of cloudiness in the solution and/or the formation of crystals on the microspatula at the air-liquid interface. When this occurs, add solvent dropwise at the boiling point until the cloudiness is discharged. Then remove the tube from the heating source, cap the tube with a loose plug of cotton to exclude airborne impurities, and allow the solution to cool to room temperature.★ If necessary, induce crystallization by gently scratching the surface of the tube at the air-liquid interface or by adding seed crystals, if these are available. To complete crystallization, cool the tube in an ice-water bath for at least min .Isolation and DryingAffix a wire holder to the Craig tube and, using the wire as a hanger, invert the apparatus in a centrifuge tube (Fig. 2.55). Remove the solvent by centrifugation, carefully disassemble the Craig tube, and scrape any crystalline product clinging

to its upper section into the lower part. Protect the product from airborne contaminants by plugging the opening of the tube with cotton or by covering it with a piece of filter paper held in place with a rubber band. Air-dry the crystals to constant weight either at room temperature or in an oven. Be certain that the temperature of the oven is below the melting point of the product!AnalysisDetermine the melting points of the crude and recrystallized benzoic acid, the weight of the latter material, and calculate your percent recovery using Equation 3.1.2. AcetanilideApparatusA test tube, a calibrated Pasteur pipet and Pasteur filtering and filter-tip pipets, ice-water bath apparatus for Craig tube filtration and flameless heating.DissolutionPlace mg of acetanilide in the test tube. Add mL of water to the tube and heat the mixture to a gentle boil. To aid in dissolution, stir the mixture vigorously with a microspatula while heating; stirring also prevents bumping and possible loss of material from the test tube. A layer of oil should form when the stated amount of water is added. An explanation for this phenomenon is found under the Dissolution stage of Part 2 of the Miniscale Procedures. Continue adding water in -mL portions to the boiling solution until the oil has completely dissolved. Caution: Because the sample may be contaminated with insoluble material, pay close attention to whether additional solid is dissolving as you add more solvent; if it is not, stop adding solvent.Once the acetanilide has just dissolved, add an additional mL of water to prevent formation of oil during the crystallization step. If oil forms at this time, reheat the solution and add a little more water. Record the approximate total volume of water used.Continue the procedure by following the directions for Decoloration, Hot Filtration and Crystallization, and Isolation and Drying given for benzoic acid in Part 1.AnalysisDetermine the melting points of the crude and recrystallized acetanilide, the weight of the latter material, and calculate your percent recovery using Equation 3.1.3. NaphthaleneApparatusA

test tube, a calibrated Pasteur pipet and Pasteur filtering and filter-tip pipets, ice-water bath apparatus for Craig tube filtration and flameless heating.DissolutionNaphthalene may be conveniently recrystallized from methanol, ethanol, or 2-propanol. Because these solvents are somewhat toxic and/or flammable, proper precautions should be taken. If instructed to do so, position an inverted funnel connected to a vacuum source above the mouth of the test tube being used for recrystallization (Fig. 2.71b) to minimize release of vapors into the laboratory.Place mg of naphthalene in the test tube and dissolve it in a minimum amount of boiling solvent. Caution: Because the sample may be contaminated with insoluble material, pay close attention to whether additional solid is dissolving as you add more solvent; if it is not, stop adding solvent. Then add mL of additional solvent to ensure that premature crystallization will not occur during subsequent transfers. Record the total volume of solvent used.Continue the procedure by following the directions for Decoloration, Hot Filtration and Crystallization, and Isolation and Drying given for benzoic acid in Part 1; however, rather than water, use the solvent in which you dissolved the naphthalene.AnalysisDetermine the melting points of the crude and recrystallized naphthalene, the weight of the latter material, and calculate your percent recovery using Equation 3.1.4. Unknown CompoundApparatusA test tube, a calibrated Pasteur pipet and Pasteur filtering and filter-tip pipets, ice-water bath apparatus for Craig tube filtration and flameless heating.DissolutionIf instructed to do so, position an inverted funnel connected to a vacuum source above the mouth of the test tube being used for recrystallization (Fig. 2.71b) to minimize release of vapors into the laboratory. Place mg of the unknown in the test tube and dissolve it in a minimum amount of boiling solvent you selected on the basis of solubility tests. Caution: Because the sample may be contaminated with insoluble material, pay close attention to whether additional solid is dissolving as you add more solvent; if it is not, stop adding solvent. Then add mL of additional solvent to ensure that premature crystallization will not occur during subsequent transfers. Record the total volume of solvent used.Continue the procedure by following the directions for Decoloration, Hot Filtration and Crystallization, and Isolation and Drying given for benzoic acid in Part 1; however, if you are not using water, use the solvent in which you dissolved the unknown.

AnalysisDetermine the melting points of the crude and recrystallized unknown, the weight of the latter material, and calculate your percent recovery using Equation 3.1.5. Mixed-Solvent CrystallizationApparatusA test tube, a calibrated Pasteur pipet and Pasteur filtering and filter-tip pipets, ice-water bath apparatus for Craig tube filtration and flameless heating.DissolutionPlace mg of benzoic acid or acetanilide in the test tube. Add mL of ethanol and heat the mixture to a gentle boil. If necessary, continue to add ethanol dropwise by Pasteur pipet until a homogeneous solution is obtained. Caution: Because the sample may be contaminated with insoluble material, pay close attention to whether additional solid is dissolving as you add more solvent; if it is not, stop adding solvent. Once all the solid has dissolved, add an additional - to -mL portion of solvent to ensure that the solute remains in solution during transfer of the hot solution to a Craig tube and, if necessary, decolorize the solution according to the directions in Part 1 for benzoic acid.Hot Filtration and CrystallizationPreheat a Pasteur filtering pipet by pulling hot solvent into the pipet. Then transfer the hot solution into this pipet with a Pasteur pipet or filter-tip pipet that has also been preheated with solvent, using the tared Craig tube as the receiver for the filtrate (Fig. 2.53). If decolorizing carbon or other insoluble matter appears in the Craig tube, pass the solution through the filtering pipet a second and, if necessary, a third time.Reheat the decolorized solution to boiling and add water dropwise from a Pasteur pipet until the boiling solution remains cloudy or precipitate forms. Rather than using a boiling stone to prevent superheating and possible bumping, continually stir the solution with a microspatula while heating. Then add a drop or two of ethanol to restore homogeneity. Remove the tube from the heating source, cap the tube with a loose plug of cotton to exclude airborne impurities, and allow the solution to cool to room temperature.★ Follow the same directions as given for benzoic acid in Part 1 to complete both this stage of the procedure and Isolation and Drying.AnalysisDetermine the melting points of the crude and recrystallized product, the weight of the latter material, and calculate your percent recovery using Equation 3.1.Discovery ExperimentFormation of PolymorphsDevelop an experimental procedure for forming polymorphs (see the Historical

Highlight Polymorphism, which is available online) of trans-cinnamic acid or derivatives thereof by referring to the following publications: Bernstein, H. I.; Quimby, W. C., J. Am. Chem. Soc. 1943, 65, 1845–1848; Cohen, M. D.; Schmidt, G. M. J.; Sonntag, F. I., J. Chem. Soc. 1964, 2000–2013. Consult with your instructor before undertaking your proposed procedure.Wrapping It UpFlush any aqueous filtrates or solutions down the drain. With the advice of your instructor, do the same with the filtrates derived from use of alcohols, acetone, or other water-soluble solvents. Use the appropriate containers for the filtrates containing halogenated solvents or hydrocarbon solvents. Put filter papers in the container for nontoxic waste, unless instructed to do otherwise.

Experimental Procedures: Melting Points

PurposeTo determine melting points using the capillary-tube method.

Safety Alert7 Wear safety glasses or goggles and suitable protective gloves while performing

the experiments.8 If a burner is used in this experiment, be sure that no flammable solvents are

nearby. Keep the rubber tubing leading to the burner away from the flame. Turn off the burner when it is not being used.

9 Some kinds of melting-point apparatus, such as the Thiele tube, use mineral or silicone oils as the heat transfer medium. These oils may not be heated safely if they are contaminated with even a few drops of water. Heating these oils above may produce splattering of hot oil as the water turns to steam. Fire can also result if spattered oil comes in contact with open flames. Examine your Thiele tube for evidence of water droplets in the oil. If there are any, either change the oil or exchange tubes. Give the contaminated tube to your instructor.

10 Mineral oil is a mixture of high-boiling hydrocarbons and should not be heated above because of the possibility of spontaneous ignition, particularly when a burner is used for heating. Some silicone oils may be heated to about without danger (Sec. 2.9).

11 Be careful to avoid contact of chemicals with your skin. Clean up any spilled chemicals immediately with a brush or paper towel.

12 If you use a Thiele tube, handle it carefully when you are finished, because

the tube cools slowly. To avoid burns, take care when removing it from its support.

Calibration of Thermometer

ProcedurePreparationRefer to the online resources to answer Pre-Lab Exercises, access videos, and read the MSDSs for the chemicals used or produced in this procedure. Read or review Sections 2.7 and 2.9.ApparatusCapillary melting-point tubes, packing tube, melting-point apparatus.ProtocolCarefully determine the capillary melting points of a series of standard substances. A list of suitable standards is provided in Table 3.2. The temperatures given in Table 3.2 correspond to the upper limit of the melting-point range for pure samples of these standards.Table 3.2Standards for Thermometer Calibration

Compound Melting Point

Ice water 0

3-Phenylpropanoic acid 48.6

Acetamide 82.3

Acetanilide 114

Benzamide 133

Salicylic acid 159

4-Chloroacetanilide 179

3,5-Dinitrobenzoic acid 205

Plot the corrections in your notebook as deviations from zero versus the temperature over the range encompassed by the thermometer. This allows you to tell, for example, that at about the thermometer gives readings that are too low, or that at the readings are about too high. These values should then be applied to

correct all temperature measurements taken. The corrections you obtain are valid only for the thermometer used in the calibration. If you break it, not only are you likely to be charged for a replacement, but you must repeat the calibration.Determining Capillary-Tube Melting Points

ProcedurePreparationRefer to the online resources to answer Pre-Lab Exercises, access videos, and read the MSDSs for the chemicals used or produced in this procedure. Read or review Sections 2.7 and 2.9. Plan to prepare two melting-point tubes if you are determining the melting points of unknown compounds.ApparatusCapillary melting-point tubes, packing tube, melting-point apparatus.1. Pure CompoundProtocolSelect one or more compounds from a list of available compounds of known melting point and determine the melting-point ranges. Repeat as necessary until you obtain accurate results and are confident with the technique.2. Unknown CompoundProtocolAccurately determine the melting range of an unknown pure compound supplied by your instructor.3. Mixed Melting PointsSample PreparationPrepare a sample for mixed melting-point determination by introducing of a second substance as an impurity into a solid whose melting point was determined in Part 1. Intimately mix the two components by grinding them together with a small mortar and pestle. Alternatively, use a small, clean watch glass and glass stirring rod or metal spatula to mix the components. Be careful not to apply too much pressure to the glass rod, however, because it is more fragile than a pestle and may break. Do not perform this operation on a piece of filter paper, because fibers from the paper may contaminate the sample.ProtocolAccurately determine the melting range of the sample and compare it to that of the major component of the mixture to study the effect of impurities on the melting range of a previously pure compound.

Discovery ExperimentMelting-Point DepressionObtain a pair of solids whose melting points are within of one another and determine whether they indeed depress one another’s melting points. Some possible examples are 3-furoic acid and benzoic acid, 1-naphthylacetic acid and 2,5-dimethylbenzoic acid, trans-cinnamic acid and 2-furoic acid, 2-methoxybenzoic acid and malic acid, or dimethyl fumarate, 9-fluorenone, and methyl 4-bromobenzoate. Consult with your instructor regarding these and other possibilities.Wrapping It UpReturn any unused samples to your instructor or dispose of them in the appropriate container for nonhazardous organic solids. Discard the used capillary tubes in a container for broken glass; do not leave them in the area of the melting-point apparatus or throw them in wastepaper baskets.Who Else Has My Compound?

Discovery ExperimentScience is a collaborative enterprise, and teams of scientists often work together to make discoveries. It is also common for groups of researchers from different parts of the world to collaborate by sharing experimental data and exchanging samples. The groups must then establish that the samples with which they are working are identical.How do you know if two compounds are the same? Among organic chemicals, there are many colorless (white) and colored solids and liquids, so a visual inspection is usually insufficient to determine identity. Rather, we must rely upon comparison of measured physical properties. In this experiment, you will be working with your classmates to discover which of you have the same compound and what that compound is. Through solving this puzzle, you will learn some basic techniques for analyzing and identifying organic compounds.The first part of the puzzle involves measuring physical properties of your compound and comparing them with data obtained by other students. The second part is proving that all the samples with the same or similar properties are indeed identical. Under the same conditions, compounds having comparable properties may be the same, but compounds having different properties are clearly not. Completing the puzzle requires that the compound be identified. In this regard, single measurements by themselves are insufficient for identifying a compound. For example, just knowing that a compound is a clear liquid at room temperature

and has a melting point of rules out many possibilities, but a variety of compounds melt at . Knowing that the compound melts at , boils at , has a density of g/mL, and is soluble in methanol but not in hexane gives us greater confidence that the substance is water. Accordingly, as you collect more data, your confidence in matching compounds that may be the same will increase as will your confidence in establishing the identity of the compound.You will be given a numbered sample of a colorless pure solid as your piece of the puzzle. The first goal in solving the puzzle is finding who else in the laboratoryhas the same compound as you. To accomplish this task, you will determine some properties of your sample, such as its melting point (Section 3.3), its solubility in two solvents (water and aqueous sodium hydroxide), and its on a thin-layer chromatography (TLC) plate (Section 6.2). You will then share your results by recording the data in a central location to find the other students who have a compound whose properties are the same as or similar to yours. Strategies for making unambiguous comparisons that will ultimately prove the identity of different samples are provided below.Your laboratory notebook should contain all the data you obtained, as well as a brief compilation of the data reported by the other students whose data are comparable to yours. These labmates and you will have a second goal of determining exactly which compound your group has. To accomplish this objective, the group will be given a list of possible comparison substances to use for making a final identification of the compound. These substances will also be available for actually making the comparisons. As before, suggestions will be provided that allow you to prove the identity of your sample and the comparison compound.

Experimental Procedures

PurposeTo demonstrate the equivalence of different samples and prove their structure.

Safety Alert13 Wear safety glasses or goggles and suitable protective gloves while

performing the experiments.14 The solvents used for TLC analysis are highly flammable, so do not handle

these liquids near open flames.15 The M solution of sodium hydroxide is caustic. Do not allow it to come into

contact with your skin. If this should happen, thoroughly rinse the affected area with water.

PreparationRefer to the online resources to answer Pre-Lab Exercises, access videos, and read the MSDSs for the chemicals used or produced in this procedure. Review Sections 3.2, 3.3, and 6.2.Determining Melting Points

ApparatusApparatus for determining melting points.AnalysisAccurately determine the melting point of your unknown solid.★

Testing Solubility

ApparatusTwo test tubes, calibrated Pasteur pipet, microspatula.DissolutionPlace approximately mL of water in the first test tube. Add about a microspatula-tip full of the unknown solid to the test tube and gently stir the mixture with the spatula. Place mL of M aqueous sodium hydroxide in the second test tube. Add about a microspatula-tip full of the unknown solid to this test tube and gently stir the mixture with the spatula. For both tests, record your observations in your laboratory notebook.★

Thin-Layer Chromatography

ApparatusWide-mouth bottles with screw-top caps, silica gel TLC plates, capillary pipet, test tube, microspatula.Setting UpDissolve a microspatula-tip full of your compound in approximately mL of acetone and stir the mixture to effect dissolution. Using a capillary pipet, spot the solution on each of three TLC plates.AnalysisDevelop one plate in ethyl acetate, one plate in hexane, and one plate in a mixture of ethyl acetate:hexane (by volume). Visualize your plates under UV light, calculate the values, and record the results in your laboratory notebook and where other students can see them.★

Comparing Data and Determining the Structure of the Unknown

ApparatusApparatus for determining melting points, several test tubes, calibrated Pasteur pipets, microspatula, wide-mouth bottle with screw-top cap, silica gel TLC plates, capillary pipet.Group AnalysisWorking with members of your group, obtain mixed melting points for each combination of samples in the group to prove that they are the same compound.★

Using the solvent system you determined to be the best for your compound, perform a TLC analysis by spotting the TLC plate with two different samples and a third “co-spot” (Section 6.2) that has both samples spotted at the same location. This technique minimizes the possibility that differing values for the two single spots result from variations in where the spots were applied to the plate or to the

concentration of the sample being spotted, as discussed in Section 6.2.The result of the TLC analysis should reinforce the conclusion you reach on the identity of samples based on the results of measuring mixed melting point.Using the melting point and TLC data your group has obtained and comparing it to the known compounds provided by your instructor, decide which compound you are likely to have and obtain an authentic sample of it. Demonstrate the identity of your sample and the known compound by performing appropriate mixed melting points, solubility tests, and TLC analysis.Wrapping It UpPut the organic solvents from TLC analysis in the appropriate container for nonhalogenated organic liquids. Dispose of the aqueous solutions from the solubility tests in the appropriate aqueous waste container. Discard TLC plates in the appropriate solid waste container and the capillary pipets in the broken glass container.