CHM 137 General Chemistry II Lab
Exploring Melting Points of Organic Compounds
Adapted from “Melting Point Determination: Purity and Identity of Crystalline Organic
Compounds,” Hart, Harold; Craine, Leslie; Hart, David; Organic Chemistry, A Short Course, 10th
Ed., Houghton Mifflin Co., Boston, MA, 1999.
Pre-Lab:
None
Objective:
In this experiment you will determine the melting points of two different
pure solids that have approximately the same melting point range. You
will then prepare a mixture of the two substances and determine its
melting point range. Finally, you will obtain a sample of an unknown
from your instructor. After you determine its melting point, you will
identify the substance by finding a mixed melting point.
Background: Most solid organic compounds have characteristic melting points that are
low enough (below 300°C) we can conveniently observe them with simple
equipment. Organic chemists routinely use melting points (a) to get an
indication of the purity of crystalline compounds and (b) to help identify
such compounds.
Pure crystalline compounds usually have a sharp melting point range.
This means that the difference between the temperature at which the
sample begins to melt and the temperature at which the sample is
completely melted is small (narrow). Impurities in a sample, even when
present in small amounts, typically lower the melting point and widen
the melting point range. A wide melting point range (more than 5°C)
usually indicates that the substance is impure; a narrow melting point
range (0.5-2°C) usually indicates that the substance is fairly pure.
However, there are some exceptions to both of these generalizations.
Small differences in melting point (on the order of 2-3°C) may also result
from variations in technique, thermometer accuracy, and the experience
of the person doing the melting point determination.
Because each compound has a characteristic melting point, we can use
melting points to help identify unknown compounds. As an example, say
a sharp-melting, unknown substance X is suspected of being identical to
some known substance A. If the two are identical, they should have the
same melting point. Thus if A is reported in the chemical literature to
have a melting point significantly different from that observed for X, we
can be quite certain that X does not have the same structure as A. On
the other hand, if A is reported to have a melting point within a few
degrees of that observed for X, the two substances may be identical (the
small difference being due to variations in technique or purity, as
mentioned above).
To make certain that X and A are identical, we can determine perform a
“mixed melting point” experiment – in this case we observe the melting
point of a mixture of X and A (assuming a sample of A is available). If X
and A are identical, the mixture should have the same melting point as X
or A alone. On the other hand, if X and A are not the same substance
(even though they separately have the same melting point), then a
mixture of the two usually has a lower melting point and a broader
melting point range than either substance alone. This is because each
substance acts as an impurity in the other.
To determine the melting point of a crystalline substance, we introduce a
small amount of the finely powdered material into a thin-walled capillary
tube that is sealed at one end. The capillary tube is inserted into a
melting point apparatus and heated. An image of a properly filled
capillary tube and one type of melting point apparatus are shown in
Figure 1. Once set-up, two temperatures are recorded: the temperature
at which the substance begins to liquefy (say, for example, 75 °C) and the
temperature at which it becomes completely liquefied (say, for example,
77 °C). The observed melting point range is the interval between these
two temperatures (i.e. 75 – 77 °C, not 2 °C). The melting point is a
characteristic property of a pure chemical substance.
While the melting point range is
strongly affected by the purity of the
material, other factors can influence it
as well, including the size of the
crystals, the amount of material, the
density of its packing in the tube, and
the rate of heating. A finite time is
required to transfer heat from a hot
liquid bath or metal block through the
walls of the capillary tube and
throughout the mass of the sample.
When the bath or block is heated too
quickly, its temperature rises several
degrees during the time required for
melting to occur. This can result in an
observed range that is higher than the
true one.
When the temperature of the bath or
block approaches the melting point of
the sample, it is essential for good
Figure 1. Filled capillary tube (left, shown
results to raise the temperature slowly
approx. full size) and a Mel-Temp. unit
and at a uniform rate, usually about
2°C/min. The sample should be small,
finely powdered, and packed tightly in a thin-walled capillary tube of
small diameter. The column of solid in the capillary tube should be just
high enough to be seen clearly during melting (about 1-2 mm).
The behavior of a material upon melting should be observed and
recorded carefully. Write, for example, "Melts sharply at 89.0-89.5 °C" or
"mp 131-133 °C, with decomposition" or "Discolors at 65 °C; melts slowly
at 67-69 °C."
Procedure:
1. Obtain three clean, dry watch glasses and five melting point tubes.
2. Weigh out a small sample (approximately 50 mg – that’s 0.050 g!) of urea
onto one of the watch glasses. If needed, crush a small amount of the
sample first in a mortar and pestle before weighing it.
3. Fill a melting point tube with a sample of urea by thrusting the open end
into the powder several times. To work the plug of solid material down to
the sealed end of the tube, vigorously tap the sealed end on the table, or
lightly draw a file across the tube held loosely in the hand, or drop the
melting point tube, closed end down, through a length glass tubing onto
the bench top. Repeat the procedure until the tube contains a 1-2mm
column of densely packed powder at the bottom.
4. Insert the tube into the melting point apparatus and start heating. The
temperature may be allowed to rise fairly rapidly to within 15-20 °C
below the compound's expected melting point (approx. 130 °C for urea).
However, during determination of the actual melting point range, the
temperature should not rise more rapidly than 2 °C/min. Therefore,
decrease the rate of heating when the temperature is about 15 °C below
the expected melting point. Record the melting point range of urea in
your lab notebook.
5. Repeat steps 2 – 4 to determine and record the melting point range of a
sample of trans-cinnamic acid. This compound also melts at
approximately 130 °C.
6. To demonstrate the effect of impurities on the melting point of a pure
substance, determine the melting point range of a roughly 50-50 mixture
by weight of urea and cinnamic acid. To do so, thoroughly mix the
remaining amounts of urea and cinnamic acid in one of your beakers,
then fill a melting point tube and record the melting point range as you
have done above.
Identification of an Unknown
When the melting point of an unknown substance is to be determined,
you can save time if you first obtain its approximate melting point using
a rapid heating rate – say, 15-20 °C/min. Then allow the apparatus to
cool to 15-20°C below the approximate melting point. Use a second
sample of the substance to determine the melting point accurately, with a
slow heating rate of about 1-2°C/min.
7. Obtain an unknown sample (of one of the substances listed in Table 1)
from your instructor. Weigh out approximately 50 mg of the sample onto
a watch glass (crush in mortar and pestle, if necessary).
8. Fill two melting point tubes with samples of the unknown. Use one tube
to determine the approximate melting point, and use the second tube to
determine the melting point more precisely, as described above. Record
the results in your lab notebook.
9. Use the data in Table 1 to make a preliminary identification of your
unknown. Then confirm its identity by the mixed melting point
technique: Mix the remainder of your unknown (now a bit under 50 mg)
with an approximately equal weight of the substance you suspect it to be
from the samples available on the supply cart. Use good chemical
hygiene – DO NOT CONTAMINATE THE REFERENCE SAMPLES!
Record your results and conclusion on the report sheet. Remember, if
your unknown sample and reference do not give a sharp melting point
range, they are not the same compound – try another reference in this
case.
10. Discard your used melting point tubes in the waste glass container
provided in the waste hood. Place extra solids from the mixture melting
point in a solid organic waste container provided there, as well.
Table 1. Melting points of some organic compounds.
Compound
mp (°C)
Compound
mp (°C)
Biphenyl
70-71
Benzoin
132-133
vanillin
81-82
trans-cinnamic acid
132-133
Phenanthrene
100-101
urea
132-133
Benzoic acid
121-122
salicylic acid
156-158
2-Naphthol
121-122
sulfanilamide
165-166
Lab Report: For this lab you should complete the report sheet (available in the usual
location) by supplying your data and answering the discussion questions
on the report sheet. Print out a copy of your report (or email it to your
instructor, if they are willing to accept it in that format).
If you worked with a partner, you should each produce and hand in
separate reports; your partner’s name should be included in your report.