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Home , Melting point

Points

Background The melting point of a is a reproducible, easily measured property of organic compounds. Melting points are one of the primary ways to characterize and identify products in organic class. The melting point can also tell you about the compound’s purity since both lower and broaden the melting range of a compound. As you learned in general chemistry, impurities elevate the point of substances and depress the point (a.k.a. melting point). Melting “point” is a bit of a misnomer because all compounds melt over a range. Pure samples generally melt over a narrow (2 degree) range. Impure compounds generally melt over larger ranges. Melting points can be used to confirm the identity of an unknown compound and to assess the purity of a known compound.

Preparing a Melting Point Sample When taking a melting point, your sample should be a fine, dry powder. If your are not finely powdered, crush a small amount with a mortar and pestle before filling the melting point tube. If your sample is wet from , wait until the next class for it to dry. If your sample looks oily it may be due to high boiling impurities that can only be removed through recrystallization.

To fill a melting point tube, tap the open end of the tube into your powdered sample to pick up a few grains. Flip the tube and gently tap the closed end on the bench to knock the material to the bottom of the tube. Long narrow tubes are also available to pack the crystals down. The tubes allow you to drop the melting point tube from a height without breaking it. No more than 2 millimeters of densely packed crystals are needed to take a melting point. Loosely packed samples and large samples to uneven heating.

Calibrating Your The thermometer you are assigned for this class may read too low or too high by varying degrees at various so it must be calibrated. Known samples of high purity are used to calibrate the . To calibrate, compare the high in your experimental range to the high temperature in the literature melting point range (often only the high temperature reported for literature values). The difference between the two high numbers should be added to (for thermometers that read low) or subtracted from (for thermometers that read high) each value in melting ranges that you take in the future. Your thermometer may be off by different amounts in different temperature ranges. Use whichever calibration is closest to the temperature your solid melts at.

Using the Mel-Temp The melting point machines can be set to at different rates (see graphs on page 2). It is important to heat at a slow rate (2 degrees per minute) near the melting point to get an accurate reading. If you have no idea where your sample will melt, make up two sample tubes. Heat the first tube at a fast rate to get a rough idea of the melting point. Allow the machine to cool to 10 degrees below your estimate, insert the second sample and heat at a slow rate until melted. Making two samples and using this procedure much faster than making one sample and heating at 2 degrees per minute until the sample melts, particularly if it is a high melting sample. Note that the machine does not need to be completely cooled between measurements; it only needs to be cooled lower than the first melting point. Additionally, each machine holds up to three samples – please utilize all slots if you are doing a lab that requires more than one melting point. Finally, Mel- Temp barrels get very hot – be careful not to burn yourself.

The melting point (range) begins when the first grain just begins to liquefy and ends when the sample is completely . Even very pure samples melt over at least a 2 degree range. The melting points quoted in the literature usually only print the highest temperature. You will always quote both temperatures – always record melting points as ranges.

How to Take a Mixed Melting Point One way to confirm the identity of an unknown is to mix it with the compound you believe it to be and take a melting point. If your unknown and the known compound are identical, the melting point of the should not change. If your unknown is not identical to the known compound, it will act as an and the melting point range will be lower and broader.

To take a mixed melting point mix a small amount of each compound (~1:1 ratio is recommended) and thoroughly crush them together with a mortar and pestle, then proceed with the melting point as usual.

A note on thermometers: The thin of the thermometer bulb will break if cooled or heated too rapidly. Do not put a thermometer in a hot mel-temp apparatus. Do not put a hot thermometer in cold .

Procedure

Part I: Calibration Your thermometer will be calibrated with pure samples of , and . Since you know the literature melting points of the three calibration , a quick melting point determination is not necessary. Measure the melting point range of each compound at least twice. Group members should take turns at the measurements. Heat at a rate of 2 degrees per minute near the three melting points. Between the melting points, please heat faster! Repeat until you get consistent ranges. Record all melting ranges in your notebook and compare the highest number in each recorded range to the high literature value.

Part 2: Identification of an Unknown You will be given a sample of an unknown to identify through mixed melting points. The sample will have one of three identities: , benzoin or . All three compounds melt at a similar temperature. Make the following : 1) your unknown mixed with urea; 2) your unknown mixed with benzoin; 3) your unknown mixed with cinnamic acid. Record the melting point range of the three samples (utilize the three slots in the mel-temp). Use this information to determine the identity of your unknown.

Chemicals: benzophenone, naphthalene, benzoic acid, urea, benzoin, cinnamic acid

Waste: melting point tubes and excess solids should be put in the melting point waste jar. Contaminated weigh-boats go in the plexiglass bin. No glass should ever go in the regular trash.