Melting Point, Boiling Point, and Index of Refraction Melting points, boiling points, and index of refractions are easily measured physical properties of organic compounds useful in product characterization and purity determination. A. Melting Points Pure, crystalline solids have a characteristic melting point, which is expressed as the temperature range over which the solid melts to become a liquid. The transition between the solid and the liquid is so sharp for small samples of a pure substance that melting points can be measured to ±0.1 o C. Typically it is no more than ±1 o C. Melting points of pure compounds are recorded in handbooks, such as the Handbook of Chemistry and Physics (CRC) or the Merck Index. Alternatively, you can find this information on the Internet, for example at http://chemfinder.cambridgesoft.com/. Measurements of the melting point of a solid can also provide information about the purity of the substance. Pure crystalline solids have a sharp melting point. They melt in a very narrow range (melting range) of temperatures, whereas mixtures melt with a broad temperature range. Mixtures also tend to melt at temperatures below the melting points of the pure solids. Many solid substances prepared in the organic laboratory are initially impure. These impurities affect the melting point of a substance. In a sample that contains a mixture of two compounds, each component usually depresses the melting point of the other, giving an observed melting point range that is lower and broader than the melting point of either component. A melting point composition diagram for two hypothetical solids, A and B, is shown below, as a graph of temperature versus composition. The eutectic point is the lowest temperature of the mixture and is determined by the equilibrium composition at which A and B melt in constant ratio. A sample whose composition is exactly that of the eutectic point will exhibit a sharp melting point at the eutectic temperature. This means a eutectic mixture can be mistaken for a pure compound since both have a sharp melting point. Because it is difficult to heat solids to temperatures above their melting points, and because pure solids tend to melt over a very small temperature range, melting points are often used to help identify compounds.
We will use the Mel-Temp apparatus for measuring the melting point in our lab. The Mel-Temp apparatus uses closed-end capillary tubes. The sample is placed into a predesigned slot and its melting behavior observed through a magnifying glass. Keep in mind that we have 5 Mel-Temps for the labs. Schedule your lab experiment to minimize waiting time. I. Melting points are best determined using a finely divided powder. Grind the sample using a mortar and pestle to ensure homogeneity. Fill a capillary tube to a height of no more than 2-3 mm with the packed urea. The sample can be packed tightly by dropping the capillary tube through glass tubing on a table top or the floor. Put the tube into the Mel-temp apparatus closed end down. Make sure that you can see the sample through the magnifying glass. Set the voltage to zero and turn on the Mel-temp. Turn the voltage to 45 and observe both the sample and temperature reading as you heat. http://orgchem.colorado.edu/hndbksupport/meltingpt/mtset.html. (Never set the voltage at more than 70). Note (a) the temperature at which the column of urea first collapses or shows some liquid and (b) the temperature at which the sample is completely liquid. This is the melting range, which we call a melting point. Always report a melting range. The melting point is not accurate if the thermometer and the sample are not at the same temperature. For accuracy the sample should be heated through the melting range at a rate of 1 o C or less per minute. Turn off the apparatus and let it cool. If you did not get a good result for the melting point of urea, prepare a sample in a new capillary, and repeat the measurement. Capillaries cannot be reused. Put used capillaries in the glass waste container. II. Prepare a melting point diagram for a mixture of two compounds. We will use urea and cinnamic acid. Work in groups of two for this part. Record the melting point ranges of urea (from part I), pure cinnamic acid, a 1:1 urea:cinnamic acid mixture, a 4:1 urea:cinnamic acid mixture, and a 1:4 urea:cinnamic acid mixture Plot your data in a melting point composition diagram similar to the above diagram. Make an accurate diagram using graph paper and record melting point ranges. http://ull.chemistry.uakron.edu/organic_lab/melting_point/
III. You will be given a solid unknown. Your unknown is one of the following compounds: salicylic acid, benzoic acid, succinic acid, acetanilide, benzophenone, or napthalene. Samples of all of these compounds are available in the lab. In the procedure part of your Prelab explain how you plan to identify your unknown. Data and Observations Record the melting ranges obtained directly into your lab notebook. List any important observations you make while performing the experiment. For example, describe the appearance of a compound when it melts and any other visible changes occurring prior to, or during, the melting process, i.e. water vapor, gas bubbles, color changes, clarity of the liquid melt. Analysis Compare the literature melting points of all substances you have used and compare to the values you have determined experimentally. Comment on any discrepancies. Compare the melting point ranges of your pure urea and cinnamic acid with the mixture. Is it possible to estimate the eutectic point from your graph? Questions 1. How fast do you heat the sample in the Mel-temp when determining a melting point? 2. If you heat too fast, will your observed melting point be higher or lower than the true value? Explain. 3. What is meant by the term melting range? What happens at this range? 4. Why should you always use a new capillary tube with a sample of your compound when doing a second melting point determination? B. Boiling Points The boiling points of pure organic liquids are, like the melting points, characteristic physical properties. The process of determining the boiling point is more complex than that for the melting point. It requires more material, and because it is less affected by impurities, it is not a good indication of purity. Like the melting point, the boiling point of a liquid is affected by the forces that attract one moleucle to another-ionic attraction, dipole-dipole interaction, hydrogen bonding, and van der Waals forces. A very clean liquid in a very clean vessel will superheat and not boil when subjected to a temperature above its boiling point. If boiling does occur under these conditions, it occurs with explosive violence. To avoid this problem boiling stones or a boiling stick is always added to liquids before heating them to boiling. 1. Practice your boiling point determination techniques, by placing about 0.3 ml of one of the liquids provided and a boiling stone in a reaction tube. Fit a distillation head (connecting adapter) on top of the reaction tube to ensure that the system is open to the atmosphere. REMEMBER TO NEVER HEAT A CLOSED SYSTEM! Using a thermometer adapter clamp a thermometer so that the bulb is just above the liquid, and then heat the liquid with a sand bath or a water bath (depending on the boiling point of your liquid, above 100 o C a sand bath, below 100 o C a water bath). Heating is regulated so that the boiling liquid refluxes (condenses the drips down) about 3 cm up the thermometer bulb in order to heat the mercury thoroughly. The boiling point is the highest temperature recorded by the thermometer and maintained over about a 1-min
time interval. True boiling is indicated by drops dripping from the thermometer and a constant temperature recorded on the thermometer. If the temperature is not constant, then you are probably not observing true boiling. 2. You will be given a liquid unknown. Your unknown is one of the following liquids: toluene, ethanol, cyclohexane, 1-butanol, or distilled water. All of the liquids will be available in the lab. In the procedure part of your lab, explain how you plan to identify your unknown using the boiling point and refractive index. Observation Record all your data directly in your lab notebook and compare experimental results with literature values. Comment on any discrepancies. Analysis Explain how you identified your liquid unknown using the boiling point and index of refraction. Comment on the technique of boiling point determination and refluxing and the use of the refractometer. Questions 1. What are the consequences of heating a closed system? 2. Which would you expect to have a higher boiling point, ethanol or dimethyl ether? Explain. C. Refractive Indices The refractive index is a physical constant that, like the boiling point, can be used to characterize liquids. It is the ratio of the velocity of light in air to the velocity of light in the liquid. The angle of refraction is a function of temperature and the wavelength of light. Because the velocity of light in air is always greater than that through a liquid, the refractive index is a number greater than 1; for example, hexane n 20 D 1.3751. The superscript 20 indicates that the measurement was made at 20 o C, and the subscript D refers to the yellow D-line of a sodium vapor lamp, light with a wavelength of 589 nm. The measurement is made with a refractometer using a few drops of liquid. Compensation is made within the instrument for the fact that white light and not sodium vapor light is used, but a temperature correction must be applied to the observed reading by adding 0.00045 for each degree above 20 o C. n 20 D = n t D + 0.00045(t 20 o C)
The refractive index can be determined to 1 part in 10,000, but because the value is quite sensitive to impurities, there is not always very good agreement with the literature with regard to the last figure. To master the technique of using the refractometer, measure the refractive indices of several known, pure liquids before measuring an unknown. For more information on the Abbe-3L refractometer (the instrument we have in the lab) and some video clips demonstrating its us, please go to http://web.uccs.edu/bgaddis/chem337/expts/nd/nd.htm Two or three drops of the sample are placed on the open prism using a polyethylene pipette (to avoid scratching the prism face). The prism is closed, and the light is turned on and positioned for maximum brightness as seen through the eyepiece. If the refractometer is set to a nearly correct value, then a partially gray image will be seen. Turn the knob so that the line separating the dark and light areas is at the crosshairs. Sometimes the line separating the dark and light areas is fuzzy and colored. Turn the chromatic adjustment until the demarcation line is sharp and colorless. Then read the refractive index by pressing the button down to light up the scale in the field of vision. Read the temperature on the thermometer attached to the refractometer, and make the appropriate temperature correction to the observed index of refraction. Write up a procedure for using the value of the index of refraction in addition to the boiling point determination, to identify your unknown.