CHM138 Lab 7 Melting Points Discussion: Identifying an unknown compound requires a chemist to measure several physical properties1 and to observe a few chemical properties2 of the compound. Even though it is quite possible for two different compounds to have a few physical and chemical properties in common, it is much less likely that the two different compounds will have many identical physical and chemical properties3. Useful physical properties that are utilized by chemists in identifying an organic compound include the compound’s: color, odor, physical state, melting point (mp), boiling point (bp), density (d), refractive index (nD). infrared (IR) spectrum, ultraviolet (UV) spectrum and nuclear magnetic resonance (NMR) spectrum4. A physical constant is a numerical value measured at the time certain physical properties are observed. As long as the physical constants of compounds are measured under standard conditions (temperature, pressure, etc.), the values are invariant and are, therefore, useful in determining the identity of unknown substances. Chemists regard a table of physical properties and physical constants to be extremely helpful in identifying unknown compounds. There are a number of references that contain extensive tables of physical properties and physical constants of compounds. Two of the more commonly used references are the CRC Handbook of Chemistry and Physics, published by the Chemical Rubber Company, and The Merck Index, by Merck & Co., Inc. If the physical properties of an unknown substance are identical to the physical properties of a compound listed in the tables, the two compounds are probably the same. Thus, a colorless liquid with a mp of 5.5°C, a bp of 80.1°C, and nD = 1.5011 at 20°C is very likely to be benzene, although we might want to make a few more observations to be certain. It must be pointed out, however, that it is not possible to accurately predict the physical properties of newly synthesized and isolated compounds. Therefore, tables of physical properties are only useful for identifying previously discovered compounds. However, useful information as to the compound's identity and purity can often be obtained by the measurement of its melting point. Crystalline solids are composed of atoms, ions, or molecules in a highly ordered geometric pattern (the crystal lattice). These atoms, ions or molecules are held in their positions by electrostatic, dipole and/or London (van der Waals) forces. When a pure crystalline solid is heated, the atoms, ions, or molecules begin to vibrate more and more rapidly until at a definite temperature the thermal motion of the particles is great enough to overcome the forces of attraction. Then they enter a more random and mobile state, the liquid state. The melting point of a solid is defined as the temperature at which the solid and liquid phases of a substance are in equilibrium5. The freezing point of a liquid is the same temperature as the melting point of its solid. However, freezing points are rarely measured in practice because solidification may not occur at the correct temperature due to the phenomenon of supercooling6. Determination of the temperature at which the solid and its liquid phases of a substance are in equilibrium is tedious and time consuming; it is also quite difficult to do if there is only a small amount of sample available. Thus, in practice, most melting points are determined as capillary melting points, which can be done quickly with a small amount of sample. A capillary melting point is the temperature range over which a small amount of solid in a thin-walled capillary tube begins to visibly soften (first drop of liquid)7 and then completely liquefies. Melting points recorded in chemical references are capillary melting points unless otherwise stated. A solid is said to melt SHARPLY if the melting point range is 1-2°C. A pure solid will generally melt sharply because the forces of attraction between its particles are the same. However, the presence of a foreign particle interrupts its uniform structure and the attractive forces are weakened. An impure solid melts at a lower temperature and over a wider range than a pure solid. Thus, the melting point of a solid not only helps to identify the compound but also gives an indication of purity. Suppose that two unknown compounds A and B have identical melting points and appear to be identical. One can easily determine whether or not A and B are really the same compound by determining the melting point of a mixture of A and B. (The melting point of a mixture is called a mixture melting point). If A and B are the same substance, the mixture melting point will be same. However, if they are different, one compound will act as an impurity in the other and the mixture melting point will be lower and wider than the melting point of pure A or pure B. It should be noted, however, that there is one unique mixture of two compounds A and B that has a lower melting point than the mixture of any composition of the two. That particular mixture is called the eutectic mixture. The melting point of the eutectic mixture is called the eutectic point. A mixture whose composition corresponds exactly to its eutectic mixture will have a relatively sharp melting point. Thus, there is a possibility that an eutectic mixture could be mistaken for a pure compound. However, if a small amount of either A or B is added to the mixture (assuming they are both known), the melting point of the resulting mixture will be higher and more spread out than the melting point of the eutectic mixture. Some solids pass directly from the solid state to the gaseous state without first liquefying. The phenomenon is called sublimation. The temperature at which sublimation occurs is called the sublimation point. Other solids decompose rather than melt. The temperature at which a solid decomposes is called the decomposition point. While both sublimation points and decomposition points are useful helping to identify compounds, neither is helpful in establishing the purity of the compound. 1 Physical properties are those properties that can be observed or measured without changing the composition of the substance. 2 Chemical properties are those properties observed only as a substance is changed chemically into another substance. 3 An exception would occur if the two compounds were enantiomers. 4 Spectra (NMR, IR, UV, etc.) are graphs of absorption intensity vs. wavelength or frequency as electromagnetic radiation of the proper wavelength is passed through a sample. 5 A system is at equilibrium when two opposing processes (for example, melting and solidification) occur at the same rate. 6 Supercooling occurs when a liquid, cooled below its freezing point, does not solidify. 7 Some solids begin to "sweat" a few degrees below their true melting points. Other solids suddenly shrink Just before melting. Such shrinkage of a solid being heated is called sintering. Neither phenomenon is melting. Procedure: A. Preparing the Sample Place a "pea-sized" mound of one of the "known" or "standard" compounds on a clean piece of paper and grind it to a fine powder using a spatula. Use the spatula to push a small amount of the solid into the opening of a capillary tube. Tap the capillary tube gently on the tabletop to cause the solid to fall to the bottom. Repeat the process until you have accumulated a sample 1-2 mm high in the bottom of the tube. You should try to tap the sample tightly together to remove air pockets since heat will transfer more consistently through a tightly packed sample. B. Determining the Melting Point of two known samples Select two known samples with similar or identical melting points from the “List of Melting Points for Standard Compounds” given below, and prepare the capillary tubes for both of the samples. A Mel-Temp apparatus will be used for the melting point measurements. Allow the temperature to rise rapidly at first (a setting of 3 or 4). When the temperature is about 20°C below the expected melting points of the compounds, slow down the heating by turning the dial down so that temperature rises at a rate of no more than 2-3°C/min. just before, during and after the solid melts. For each compound record the name and the range of temperature over which a solid first shows a definite drop of liquid to the temperature at which the entire sample is liquid. C. Mixture Melting Point Take approximately equal amounts of the two compounds for which you already have the melting points in B; grind and mix them together thoroughly. Prepare a third capillary tube containing a sample of this mixture. Determine the melting point of this mixture. Be aware that some mixtures do not lower and spread the melting point very much; However, other mixtures have very low and very wide melting points. In other words, one must cool the Mel-Temp down to about room temperature before attempting to determine the melting point of the mixture. D. Determining the identity of an Unknown Obtain an unknown solid. Your unknown sample is one of the compounds listed below. Prepare two capillary tubes containing the unknown. The first sample is heated rapidly to determine the approximate melting point. Then let the Mel-Temp cool to at least 20°C below this approximate melting point and use the second tube to obtain an accurate melting point with a heating rate of no more than 3°C per minute. From the melting point you determined for your unknown, decide which of the listed compounds it might possibly be. There may be more than one reasonable possibility. Prepare a 50:50 mixture of your unknown and the compound that is the most likely of the possible choices. Determine the melting point of this mixture. If the melting point of the mixture is the same as that of the pure unknown, it is likely that your unknown and the compound with which you mixed it are the same. If the melting point of the mixture is lower and wider than that of the unknown, you should prepare a new mixture of your unknown with the second most likely choice of known substance. You should continue to experiment until you identify a substance that does not lower the melting point of your unknown. When you have thus confirmed the identity of your unknown, report your findings to your instructor. The data can be listed in a table as follows: Name of Exp. mp (°C) Lit. mp (°C) Compound Benzoic acid - 122.4 2-Naphthol - 121-123 Mixture of Benzoic acid and - 2-Naphthol Data Tables Standard Compounds Lit. mp. °C Name of Test Compounds Exp mp (oC) Lit. mp (oC) Resorcinol 109-111 Acetanilide 113-115 d,l-Mandelic Acid 119 Benzoic Acid 122.4 2-Naphthol 121-123 Urea 132.7 trans-Cinnamic Acid 133 Benzoin 134-136 Maleic Acid 138-139 Unknown # Anthranilic Acid 144-146 Cholesterol 148.5 Mixed melting points with: Adipic Acid 152 Citric Acid 153 Salicyllic Acid 157-159 Benzanilide 163 Itaconic Acid 162-164 Unknown #______ Identity___________________________ Lab Exercises 1. List four (4) physical properties of organic compounds that are often measured by organic chemists in attempting to identify a compound. 2. List two ways in which the melting point of a solid organic compound is useful to organic chemists. 3. Define the following terms. a. melting point b. eutectic mixture c. eutectic point d. sintering 4. What is the effect of a small amount of impurity on the melting point of an organic compound? 5. Why does a sample in the melting point capillary tube have to be packed tightly? 6. Why are sublimation points and decomposition points less useful to a chemist than a melting point? 7. How is it possible that a mixture could fool a chemist into believing that the mixture was a pure compound? 8. The freezing point of a substance has the same numerical value as its melting point, yet melting points are routinely measured whereas freezing points are not. Why? 9. Which of the following temperature is the most likely to be the correct melting point range of a mixture of urea (132-3°C) and trans-cinnamic acid (mp=132-3°C)?