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Chem 2123
Experiment 1
Melting Point Determination
Purpose:
To learn how to use the melting point instruments by determining the mp of pure compounds
and to determine identity of unknown solids using mixed melting points. (Chapter 14 in Mohrig) Also to
expose you to other means of identification, such as IR spectroscopy.
Background:
The melting point of a substance is a physical property that can be used for its identification. It is a measure
of the amount of heat energy that must be supplied to the particles of a solid to overcome crystal packing
forces and intermolecular forces of attraction (Van der Waals, dipole-dipole, and H-bonding) that confine
them to the solid state. The determination of melting points is particularly important to organic chemists,
since the solids we work with generally have low melting points (below 300°C), which can be conveniently
measured.
Melting points are also used as an indication of purity. Substances melt throughout a temperature range in
which both the solid and liquid phases of the substance coexist in a state of equilibrium and as such, melting
points are always reported as a range. Above that range, the substance exists only as a liquid, and below it
only the solid phase is present (no wetness observed). The size of this temperature range and its closeness
to the actual melting point are a measure of the purity of the substance…that is, impure samples of solids
have lower and broader temperature melting ranges. For example, if a pure sample of a compound melts
from 110 to 111.5°C, the addition of substantial amounts of another compound might result in a new melting
point range from 108-112 or even 85 to 100ºC.
A very pure substance will typically have a melting point range of 1 to 3 degrees. Larger ranges usually
indicate one of two things: 1. impurities in the sample or 2. poor technique in determination of the melting
point. The former can only be corrected by further purification, while the latter requires more practice and
patience. The most common technique flaw is heating the sample too fast near the melting point. If the rate
of melting lags behind the heating rate, you’ll end up with a broad range.
An identical or near identical temperature melting point range is not, in itself, proof of identity. There are
thousands of solid organic compounds that melt within any relatively short temperature range and overlaping
of melting points is inevitable. However, if an unknown solid sample is believed to be a certain known
compound, it is a relatively simple task to prove or disprove this belief by performing a “mixed” melting
point. The unknown and known compounds are mixed in relatively equal quantities. If the two samples are
the same compound, a 50:50 mixture will still be a pure sample and the melting point will be unchanged. If
they are different compounds, the 50:50 mixture will be very impure and the melting point range will be
lowered and broadened.
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Requirements:
Equipment:
Chemicals:
Melting point instruments, capillary tubes
Known Compounds
trans-Cinnamic acid
Urea
mp (°C)
132-133
132-133
Possible Unknowns
Benzophenone
Diphenylamine
Biphenyl
Stearic acid
Dibenzoylmethane
Napthalene
o-Toluic acid
m-Toluic acid
Benzoic acid
exo-Norborneol
trans-Cinnamic acid
Acetylsalicylic acid
Diphenylacetic acid
Adipic acid
Citric acid
Salicylic acid
Triphenylmethanol
d-Tartaric acid
p-Toluic acid
mp (°C)
47-49
52-54
68-70
68-70
75-79
79-81
104-105
111-113
122-123
124-126
133-135
134-136
146-149
151-153
152-154
158-160
161-163
171-174
177-180
General Procedure:
Fisher-Johns Apparatus
Place a coverslip on the heating block and add a few crystals, just enough to see through the magnifying
lens. Add a coverslip to the top and begin heating. If you don’t know the melting point of the solid, perform
a rough measurement by setting the controller to 60. Once you have an approximate melting point, repeat
the measurement with a setting around 20-30 once you are ~15 degrees from the melting point. If heating
becomes too sluggish, adjust the controller gradually.
Stuart 20 Apparatus
Fill a capillary tube with ~ 1-2 mm of sample and insert the tube in one of the holes on the side of the
instrument (there are two…make use of both). For unknown compounds or mixtures, perform a rough
measurement by adjusting the set temperature to 200 and pressing “start”. This will heat the sample to 200
°C at 20 degrees/minute. Once you have an approximate melting point, repeat the measurement by setting
the temperature 15 degrees below the rough melting point and pressing “start”. When the yellow “plateau”
light comes on, press “start” again to heat the sample at 2 degrees/minute.
Repeat measurements where necessary. RECORD ALL MELTING POINTS AS RANGES!!
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Required melting point determinations:
For this experiment you will work with a partner. Alternate between readings, so that you each get a chance
to use the instrument.
Part 1 Melting points and mixed melting points of known compounds.
Perform a melting point determination for each of the following
1) Pure urea
2) Pure cinnamic acid
3) 50 mol % urea/50 mol % cinnamic acid
Part 2 Determination of unknowns using melting point
Obtain an unknown assignment from the TA and make note of the letter you are given.
Perform a mp determination for your unknown, repeat and compare with the list of possible substances
given above.
Perform at least one mixed melting point to confirm the identity of the unknown or eliminate other possible
compounds.
Part 3 Determination of unknowns using IR
Use the IR instrument to get a spectrum of your unknown. Compare the spectrum of your unknown to the
spectrums produced by the known compounds to help determine the identity of your unknown.
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