Lab 01: Melting Point of an Unknown Compound
Reading
Pavia et al., Introduction to Organic Laboratory Techniques: A Microscale Approach (3rd edition): Technique 6,
Sections 6.1 to 6.8 (pp. 576–584, 3rd edition). (Pavia 4th edition: Technique 9)
Pre-Lab Planning
The planning portion of the notebook should have the following sections: Date, Title, Introduction, HazMat (see below), Apparatus, and Procedure.
The Haz-Mat portion should be completed for the following three compounds: urea, benzoic acid, and
fluorene. It may be necessary to find MSDS information online for one or more of these compounds.
Procedure
You will first gain practice at taking and recording the melting point of a compound with a known
melting point. Obtain a sample of one of the known compounds as assigned by the instructor. Record the identity
of the known sample in the lab notebook, then look up and record the literature values of the melting points of the
known compound. Be sure to record visual observations of the sample. Once you are confident in recording MPs,
you can move on to your unknown, where you will first determine an accurate MP and then perform a mixed MP
with your unknown and a possible suspect compound.
Preparing a melting point sample:
Carefully crush the sample to a coarse powder using a test tube. One method for doing this is to place the
sample between two pieces of clean weighing paper and gently rolling a test tube over the sample. Another
method is to place the sample in a clean watch glass, and gently crush the sample with the bottom of a clean test
tube. Try to recover as much of the sample as possible after crushing.
Capillary tubes containing the known compounds should be prepared using the procedure described in
Section 6.5 (p. 580). Initially, one tube of each compound should be prepared; additional tubes should be
prepared as needed. Try not to overfill the capillary tubes; only 1–2 mm of sample in the bottom of the tube are
needed.
Setting up a Thiele Tube melting point apparatus:
Obtain an oil-filled Thiele tube from the center lab bench. Set up the melting point apparatus as illustrated in
figure 6.3 (p. 581) with the following important exceptions:
1. A two or three-pronged utility clamp should be used on the top of the Thiele tube to secure it to a ring stand.
2. The quantity of sample suggested in Figure 6-3 is about five times too much. Use only the amount suggested
in Section 6.5.
Do not position the burner directly below the sample. Instead, position the burner about two-thirds the way out
along the side arm as suggested in Figure 6.3. This will result in more uniform heating of the sample.
Determining the melting of a compound for which the melting point is already known:
Heat the melting point apparatus rapidly until the temperature is about 20 oC below the expected melting
temperature. There is a delay between the time that heat is added or removed and the time that the temperature
stops increasing—this is called an induction period. Avoid melting the sample during or after the rapid heating.
When the temp has stabilized approximately 10–20 oC below the expected melting temperature, start heating
gently to raise the temp at approximately 1–2 oC per minute. The melting point is determined by heating the oil in
the Thiele tube until the sample melts. Ideally, the temperature of the oil should be changing by only 1–2 oC at
the time of melting to measure an accurate melting point.
Reporting melting point data:
Melting points are always reported as temperature ranges. The lower number in the range is the
temperature at which the sample first starts to melt. The upper number is the temperature at which the sample has
completely melted. Be careful of impurities in the sample (for example, dust or broken glass) which will never
melt; waiting for them to melt could cause the melting point to be reported as much higher than its actual value.
Chem 221
Lab 01: Melting Point
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The melting behavior should also be recorded. The melting behavior includes observations regarding
shrinkage, sweating, discoloration, and decomposition. These are explained in more detail in section 6.8.
As a general rule, all instruments should be read as precisely as both the instrument and the method allow.
Thus, although the thermometer can be read to the nearest 0.1°, it may not be possible to obtain this precision,
particularly if the rate of heating is rapid. Even at a low rate of heating, it may not be apparent precisely when the
sample starts to melt. If it is apparent, the thermometer should be read to the nearest 0.1 °C; if it is not possible,
then indicate so in your lab notebook.
The samples should be replaced with fresh tubes after each melting point determination, because
frequently chemical changes occur at high temperatures. These could introduce impurities that distort the melting
point.
Obtain an unknown sample from the instructor after determining accurate melting points of your known
sample. Immediately record the number of the unknown sample directly in the lab notebook. Then, record visual
observations of the sample. It may be useful to examine the unknown under a magnifying glass or the dissecting
microscope.
Determining the melting point of an unknown sample:
Ideally, the temperature of the oil should be changing by only 1–2 oC at the time of melting to measure an
accurate melting point. If an unknown sample were heated from room temperature at this rate, it could take hours
to measure the melting point; this would be a very inefficient use of lab time. As a consequence, organic chemists
have developed the following method for shortening the amount of time needed to determine an accurate melting
point of an unknown compound.
This efficient method involves measuring the melting point at least two times. The first time, an
approximate melting point is determined by raising the temperature at a rate of about 10 oC per minute. Care
should be taken not to raise the temperature too quickly, or the melting point will be too inaccurate. After the
approximate melting point has been determined, the temperature of the apparatus is lowered to about 10 oC below
the approximate melting point. A new sample is inserted in the apparatus, and the temperature is raised at a rate
of 1–2 oC per minute. This permits a very accurate measurement that will only take a few minutes.
Follow the procedures above to prepare two capillary tubes of the unknown sample. Determine an
inaccurate melting point first by rapid heating (about 10 oC per minute), then lower the temperature, replace the
first sample with the second capillary tube, and determine an accurate melting point. If necessary, perform
additional melting point determinations with fresh melting point tubes until obtaining a precise range.
When an accurate melting point range has been determined, make a conclusion regarding the purity of the
unknown. Melting-point ranges greater than 2 °C usually indicate an impure compound, although there are
exceptions.
Mixed Melting Point:
Examine the list of possible compounds found on the center lab-bench. The melting points on this list are
literature values, and usually only the lowest temperature of the melting-point range is reported in the literature.
Most of the possible compounds come in pairs with similar melting point values. Indicate in your lab book which
compounds the unknown might be.
Obtain a pure sample of one of the possible compounds from the chemical dispensing area. Use this
sample to check the identity of the unknown using the Mixed melting Point Method as described in Section 6.4
(p. 579). Carefully record the melting behavior of the mixture along with the temperatures for the range.
Based on the results of the mixed melting point determination, determine whether the unknown and the
selected sample compound are the same.
After finishing the experimental section, place any used capillary tubes and any excess unknown in the
designated waste containers in the hood. Allow the Thiele tube to cool, and then return it to the box on the center
bench. Return any clamps to the front of the room. Clean the area around your lab bench, including wiping up
any spills with a moist paper towel.
After writing up the conclusions section in your lab notebook, answer the following questions on the next
page. Fold and neatly tape this page in your lab book after the conclusions.
Chem 221
Lab 01: Melting Point
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Chem 221: Additional CRC Handbook Exercise (Fold and tape this in your notebook)
The preceding exercise was designed to illustrate the technique of melting point determinations as well as the
method of mixed melting points. However, the exercise is somewhat artificial because very few compounds listed
on the table of compounds could correspond to the observed melting point of the given unknown. A more
realistic exercise would involve the use of the more extensive melting point tables found in handbooks such as
Lange's or the CRC Handbook. To illustrate the use of the CRC Handbook in particular, perform the following
steps and answer the questions regarding the melting point index.
1. Obtain a copy of the CRC 55th Edition or earlier. Indicate the edition used:
2. Look in the alphabetical INDEX at the end of the handbook under "Index of melting points for organic
compounds". What section and page number is given for this entry?
3. Turn to the section and page number cited above. At the top of this page you will find information regarding
the values in the index. In the case where a melting point range is known for a compound, how will the
compound be listed?
4. If we wanted to make the melting-point exercise less artificial and more realistic, you would have been
directed to determine the melting point range of your unknown and then refer to the CRC Handbook melting
point index to determine the potential compounds that your unknown might be. Write down the accurate
melting point range you determined for your unknown:
Assuming that your observed melting point range is accurate to the nearest 1 °C, under what temperature in
the index would you look to find the list of potential compounds?
5. Turn the entry corresponding to the above listed temperature. How many compounds are listed corresponding
to this temperature?
6. Your answers to questions 4 and 5 were based upon the assumption that your observed melting point range
was accurate to the nearest 1 °C. However, this is not a very good assumption. Much more likely, the
observed melting point range is accurate to ±1 °C. Less likely, but possible, would be an error has high as
±2 °C. List two reasons why the observed m.p. range may not be highly accurate.
7. Lange's Handbook reflects the likely uncertainties in melting point determinations. The entries in Lange's
index of melting points are grouped together in temperature-intervals of about 4 °C. Obtain a copy of
Lange's. What edition is it?
Find the m.p. index in Lange's. On what section and page does the index begin?
Using the lower temperature of your observed m.p. range and assuming an error of ±2 °C, under what
temperature-interval(s) would your unknown be found?
The large number of compounds listed under these intervals should convince you that more information
besides the m.p. is needed to easily identify a compound. Fortunately, we will usually have additional
information about the compound, such as the functional groups present, solubility in various solvents, and
chemical properties. The combination of such information will usually narrow down our search to just a few
compounds. The mixed melting point method will then allow (nearly) positive identification.