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Unformatted text preview: Melting Point
The melting point of a compound is the temperature at which liquid and solid
phases coexist in equilibrium under atmospheric pressure. The melting point is recorded
as a melting-point range. The first number of the range is the point when the crystals
start to melt and the second number is the point at which the solid is completely melted to
liquid. For example, benzhydrol has a melting point range of 65-67 °C, that means the
compound melted over a 2-degree range.
The melting point not only can be used in organic chemistry to identify an
unknown but it can also be used to determine the purity of a compound. There are two
ways to determine the purity of a compound using melting points. First, the purer the
compound, the narrower the melting point range. A melting point range is generally
accepted to have a range of 3 °C. Second, impurities also cause the melting point to be
lower than that of a pure compound (melting point depression).
Preparation: Mayo pg 52-56; View the "Melting Point Determination" DVD in the
Vasche Library prior to this experiment.
Purpose: Identify an unknown substance by first determining the melting point of the
unknown, and then using mixed melting points to confirm the identity of the unknown.
Procedure: Melting point of an unknown compound.
Melting point determination is a simple, but critical skill for every organic
chemistry student to learn. Therefore, this experiment will be performed individually.
Obtain an unknown from the instructor and make sure to sign the “Unknown Sign-Up
Sheet” in the appropriate spot. Record your unknown number in your notebook!
Determine its melting point range (see below). The unknown will be one of the
substances in the table below. It is advisable that you should run duplicate samples that
show a consistent melting point range. After your unknown melting point range has been
determined, mix a few mg of your unknown with known samples of comparable melting
point ranges and use this information to identify the unknown. You need to mix your
sample with at least two different known compounds.
Use the Mel-temp apparatus to measure the melting point of your unknown and
mixed substrates (Figure 1).
Figure 1. Mel-temp apparatus
(b) Mel-temp capillary tubes Melting Point 1 First, obtain a capillary tube from the reagent bench or from your own drawer.
Load the sample into the capillary tube by dipping the open-ended side of the tube into a
bed of material and introduce about 1 mg of the sample. Gently tap the closed ended side
of the tube on the bench top. The sample will slowly move down the tube. Be patient
with this process since the capillary tube will break if you tap too hard. Insert the
tube into one of the sample slot of the Melt-Temp with the closed-end down (Figure 2). (a)
Figure 2. Closeup view
(a) slot for sample placement
(b) view through eyepiece Each Mel-temp has three wells for capillary tubes, that means more than one
melting point can be measured at once; share with other lab mates. Ideally, if you know
the expected melting point range you can rapidly raise the temperature to about 20-30 °C
below the expected range. At this point, the temperature should only be raised at about
2 °C/min. For this lab since you don't really know when your unknown will melt, you
will set your Melt-Temp at about 30% - 50% maximum, which will raise the temperature
at about 3-8 °C/min (Figure 3). Observe the sample through the eye-piece (Figure 2b). (a)
Figure 3. Closeup of power dial
(a) power dial, 0 = min, 10 = max
(b) on/off switch Once you know the melting point of your unknown you can make an educated
guess from the known list of what your unknown might be. Consequently, you will make
three melting point samples:
1. your unknown, again, by itself as the reference compound
2. your unknown mixed with the suspected compound
3. your unknown with another compound that has a similar melting point to your
These three samples should be run at the same time to avoid melting point errors caused
by using a different Mel-temp apparatus. If you run the samples at different heat settings,
or with different Mel-temp apparatuses, the results may vary. Melting Point 2 Possible unknowns Compound
p -nitrophenylacetic acid
benzanillide Melting Point MP Range (°C)
164-166 3 CHEM 3012 - Spring 2009
Organic Chemistry Laboratory I Name ______________________________ Melting Point – Report Form
unknown # _______________
Part I: Melting Points unknown - trial 1: _____________ °C
unknown - trial 2: _____________ °C Suspect Compounds suspect 1: ____________________
suspect 2: ____________________ Part II: Melting Points unknown: _____________ °C
unknown + suspect 1: _____________ °C
unknown + suspect 2: _____________ °C Observations
appearance of unknown: appearance of suspect 1: appearance of suspect 2: Results
unknown identity: ____________________ Melting Point – Report Form 4 Discussion (Must be duplicated in notebook. Limit yourself to the available space.) Conclusion (Must be duplicated in notebook. Limit yourself to the available space.) Melting Point – Report Form 5 Questions (Answer in complete, full sentences. Limit yourself to the available space.)
1. Three test tubes, labeled A, B, and C, contain substances with approximately the same
melting points. How could you prove the test tubes contain three different
chemical compounds? 2. One of the most common causes of inaccurate melting points is too rapid heating of
the melting-point apparatus. Under these circumstances, how will the observed
melting point compare with the true melting point? Explain your answer. Melting Point – Report Form 6 3. Melting-point ranges are ordinarily determined in a glass capillary tube. This means
that the glass constitutes an impurity in the system. Why does the glass have no
noticeable effect on the melting-point ranges? 4. A student suspected that an unknown was undergoing a chemical change at its melting
point. Suggest a simple method for testing this hypothesis. Melting Point – Report Form 7 ...
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- Spring '08