Melting Point Determination Lab Melting Point Determination Lab.doc

Name ___________________________ Date ________________ Period ____

Introduction: How Are Melting Points Used? The melting point (mp) of a pure compound is defined as that temperature at which the solid and liquid phase of the compound are in equilibrium at a certain pressure, usually 1 atmosphere. Most crystalline organic compounds have characteristic melting points that are sufficiently low (30-300° C) to be conveniently determined with simple equipment. Organic chemists routinely use melting points (1) to get an indication of the purity of crystalline compounds and (2) to help in identification of such compounds. Pure crystalline compounds usually have a sharp melting point. That is, the melting-point range (the difference between the temperature at which the sample begins to melt and the temperature at which the sample is completely melted) is small (narrow), usually within 1°. Impurities, even when present in small amounts, usually depress the melting point and broaden the melting-point range. A wide meltingpoint range (more than 5° C) usually indicates that the substance is impure; a narrow melting point range (0.5-2° C) usually indicates that the substance is fairly pure. However, there are some exceptions to both of these generalizations. If two samples have identical structures, their mixture melting point is not depressed and the melting-point range is not broadened. Small differences in melting point (on the order of 2-3° C) may also result from variations in technique, in thermometer accuracy, and in the amount of experience possessed by the person doing the melting-point determination. General Technique for Melting-Point Determination: To determine the melting point of a crystalline substance, a small amount of the finely powdered material is placed into a thin-walled capillary tube that is sealed at one end. The capillary tube is inserted into a melting point apparatus in which the temperature can be measured when heated. The first trial with the sample is to determine an approximate melting point of the sample. This will be done with heating at a rate of 10° per minute. To obtain an accurate melting point (A new sample will be set up just like the first), it is necessary to heat SLOWLY, proceeding through the melting point at no faster than 1° per minute. Two temperatures are recorded: the temperature at which the substance begins to liquefy and the temperature at which it becomes completely liquefied. The observed melting-point range is the interval between these two temperatures. The observed melting-point range can be influenced not only by the purity of the material but also by the size of the crystals, the amount of material, the density of its packing in the tube, and the rate of heating. A finite time is required to transfer heat from a hot liquid through the walls of the capillary tube and throughout the mass of the sample. When the heating fluid is heated too quickly, its temperature rises several degrees during the time required for melting to occur. This can result in an observed range that is higher than the true one. When the temperature of the heating fluid approaches the melting point of the sample, it is essential for good results to raise the temperature slowly and at a uniform rate, about 1-2° C/min. The sample should be small, finely powdered, and packed tightly in a thin-walled capillary tube of small diameter. The column of solid in the capillary tube should be just high enough to be seen clearly during melting (about 1-2 mm). The behavior of a material upon melting should be observed and recorded carefully in the notebook.

Objectives of the Experiment: In this experiment you will determine the melting points of two pure solids. You will then prepare a mixture of two substances and determine its melting-point range. Finally, you will obtain a sample of an unknown from your instructor. After you determine its melting point, you will identify the substance by finding a mixture melting. The Melting-Point Apparatus and Tubes: Several types of apparatus (see Figure 1) are commonly used to determine melting points. You will be using a Thiele-Dennis tube (third from left in Figure 1) which is specially designed to create a continuous fluid flow (oil) so no stirring or shaking is required. Convection currents created by the heat and the shape of the Thiele-Dennis tube do the mixing for you.

Figure 1

Pre Lab Questions: 1. Why are melting points of substances determined? a. _________________________________________________________________________. b. _________________________________________________________________________. 2. A pure substance has a _________________________________________ melting point. 3. An impure substance added to a pure substance does two things … a. ______________________________________________________________________________. b. ______________________________________________________________________________. 4. To determine an accurate melting point on a sample the heating should be done at a rate of __________________________________________. 5. What are the objectives of this lab? a. _______________________________________________________________________________. b. _______________________________________________________________________________. c. _______________________________________________________________________________.

Equipment: 1. Thiele-Dennis tube 2. Clamp 3. Stand 4. Bunsen burner with tubing 5. Paraffin Oil (Mineral Oil) - Peanut Oil can be used, but it might turn color 6. Thermometer 7. 50 ml beaker 8. Spatula 9. Electronic balance 10. Urea 11. Trans-cinnamic acid 12. Unknowns: See Table 1 13. Capillary tubes (one end closed) 14. Metric ruler 15. Packing tube 16. Flint starter 17. Watchglass 18. Small rubber band

Procedure: Part I: Identification of the melting point of Urea: Pulverize 50-100 mg of urea (see the structure in Figure 3) by crushing it with a spatula against the walls of a small (50-mL) dry beaker. Transfer the powered urea to the center of a watchglass. Introduce a small sample of urea crystals into a melting-point tube by thrusting gently the open end of a capillary tube into the urea powder several times (see Figure 2). To work the plug of solid material down to the sealed end of the tube drop the capillary tube, open end up, in a packing tube (see Figure 2). Repeat the procedure until the tube contains a 1-2 mm column of densely packed powder at the bottom of the capillary tube.

Figure 2 Attach a Thiele-Dennis tube to a stand using a clamp. Fill a Thiele-Dennis tube with peanut oil (or corn oil) so that the curved portion of the Thiele-Dennis tube is filled with oil then add one more centimeter (cm) of oil. Place the closed end of the capillary tube with the sample next to the bulb of the thermometer. Secure the capillary tube to the thermometer using a small rubber band near the open end of the capillary tube. Attach a notched cork to the thermometer above the capillary tube. Immerse the thermometer (with the capillary tube attached) so that the rubber band is not touching the oil inside the Thiele-Dennis tube. Heat the Thiele-Dennis tube at 10° per minute temperature rise, to get a rough estimate of the melting point. Then, repeat the procedure from the beginning (starting with a new sample and capillary tube) to determine an accurate melting point. The temperature may be allowed to rise fairly rapidly to within 30° C below the compound’s expected melting point (determined in the rough estimate trial). However, during determination of the actual melting-point range, the temperature should not rise more rapidly than 1-2° C/min. Therefore, decrease the rate of heating when the temperature is about 25° C below the expected melting point. The melting point of urea is approximately 130° C. Record the melting-point range of urea in the data table. The beginning of the mp range is the temperature at which the first drop of liquid forms within the solid. The end of the mp range is the temperature at which the entire solid has become liquefied.

Part II: Identification of the melting point of Trans-cinnamic acid: In a similar way, determine and record the melting point of a sample of trans-cinnamic acid (see the structure in Figure 3). This compound also melts at approximately 130° C. Part III: Identification of the melting point of a Mixture: To demonstrate the effect of impurities on the melting point of a pure substance, determine the melting-point range of a 50-50 mixture by weight (use about 50 mg of each compound) of urea and transcinnamic acid.

Figure 3 Part IV: Identification of the melting point of an Unknown: Procedure: Obtain an unknown sample (of one of the substances listed in Table 1) from your instructor. Use one tube to determine the melting point carefully and accurately with slow heating rate of about 1-2° C/min. once the temperature reaches 85° C. Use the data in Table 1 to make a preliminary identification of your unknown. Then confirm its identity by the mixture melting-point technique: Mix about 50 mg of the unknown with an equal weight of the substance you suspect it is from the side shelf and determine the melting point. Repeat the procedure, if necessary, to determine with certainty the identity of the unknown. Record your results and conclusion in your lab notebook. If a sharp-melting unknown substance X is suspected of being identical with some known substance A, the two should have the same melting points. If A is reported to have a melting point rather different from that observed for X, the two substances may be identical (the small differences being due to variations in technique of determining the melting points). Whether they are indeed identical can often be deduced quickly if a sample of A is available, by determining a mixture melting point. A mixture of X and A should have the same melting point as that of either substance alone, provided the two substances are identical.

Waste Disposal: Discard your used melting-point tubes in the waste glass container provided by your instructor. Place extra solids from the mixture melting point in a solid organic waste container provided by the instructor.

Table 1: Melting Points of some organic compounds Compound m.p., ° C Glutaric acid 97-99 Acetanilide 113-114 Benzoic acid 121-122 Benzoin 132-133 Cholesterol 148-150 Salicylic acid 156-158

Data Table 1: Compound

Rough Trial (°C)

Observed Melting Point Range (°C) Begins to liquefy

Literature mp (°C)

Completely liquefied

Urea

Trans-cinnamic acid

50/50 Mixture

Unknown ____

Unknown ____

Unknown plus mixture

Table 2: Unknown Identity Unknown Identities

Observations

Post Lab Questions: 1. Describe the process of melting in general. ___________________________________________________________________________________ ___________________________________________________________________________________ __________________________________________________________________________________. 2. Why are the melting points for pure compounds different? ___________________________________________________________________________________ ___________________________________________________________________________________ __________________________________________________________________________________. 3. What happens during the fast melting point determination? ___________________________________________________________________________________ __________________________________________________________________________________. 4. What happens during the slow melting point determination? ___________________________________________________________________________________ __________________________________________________________________________________. 5. Which one was more accurate, closer to the actual or literature value? ______________________Why? ___________________________________________________________________________________ __________________________________________________________________________________. 6. Describe the mixed melting point experiment. Was the value for the mixed sample lower or higher than the two pure compounds? _______________Why? ___________________________________________________________________________________ __________________________________________________________________________________. 7. Describe the thought processes you went through in identifying your unknown. ___________________________________________________________________________________ ___________________________________________________________________________________ ___________________________________________________________________________________ ___________________________________________________________________________________ ___________________________________________________________________________________ __________________________________________________________________________________.

References: 1. 2.

http://bucknell.edu/~mmdev2/casteel/chem211/labs/melting_points.html http://instruct.uwo.ca/chemistry/213a/lab1.htm diagrams

3.

http://www.chemistry.mcmaster.ca/~chem2o6/labmanual/expt1/exp1b-i.html

Data Table 1: Other Options Compound

Rough Trial

Observed Melting Point Literature mp (°C) Range Begins to Completely liquify liquified

Observations

2-methoxybenzoic acid vanillin acetanilide acetophenetidin sulfanilamide sulfapyridine caffeine urea benzamide mixed urea/benzamide unknown known #1 known #2 mixed unknown #1 mixed unknown #2 Melting points can be used in the following way to help identify a compound. Say a sharp-melting, unknown substance X is suspected of being identical to some known substance A. If the two are identical, they should have the same melting point. Thus if A is reported in the chemical literature to have a melting point significantly different form that observed for X, we can be quite certain that X does not have the same structure as A. On the other hand, if A is reported to have a melting point with a few degrees of that observed for X, the two substances may be identical (the small difference being due to variations in technique or purity). To make certain that X and A are identical, one can determine the mixture melting point-that is, the melting point of a mixture of X and A (when a sample of A is available). If X and A are identical, the mixture should have the same melting point as X or A has alone. On the other hand, if X and A are not the same substance (even though they separately have the same melting point), then a mixture of the two usually has a lower melting point and a broader melting-point range than either substance alone. This is because each substance acts as an impurity in the other.