Experiment 1
Melting Point Determination and Identity of
Crystalline Organic Compounds
General Principles
Most crystalline organic compounds have characteristic melting points that are sufficiently low
(50-3000C) to be conveniently determined 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. That is, the melting point range
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 even when present in small
amounts, usually lower the melting point and broaden the melting-point range. A wide melting
point range (more than 50C) usually indicates that the substance is impure; a narrow melting-point
range (0.5-2 0C) usually indicates that the substance is fairly pure. However, there are some
exceptions to both of these generalisations. Small differences in melting point (on the order of
2-30C) may also result from variations in technique, in thermometer accuracy, and in the amount
of experience possessed by the person doing the melting point determination. The melting point is
therefore a valuable criterion of purity for an organic compound.
Melting points can be used in the following way to help identify a compound. 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. If A is reported in the chemical literature to
have a melting point significantly different from that observed for compound 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).
To make certain that X and A are identical, one can determine the mixture melting point that is,
the melting point of a mixture of X and A. If X and A are identical, the mixture should have the
same melting point as X or A has 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
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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 summarise, if a crystalline substance is pure, its melting point is likely to be narrow. If two
samples have identical structures, their mixture melting point is not depressed and the melting
point range is not broadened.
General Technique for Melting Point Determination
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. Two temperatures are recorded: the
temperature at which the substance begins to liquefy and the temperature at which it becomes
completely liquefied. The observed melting-point range is the interval between these two
temperatures. The melting point is a characteristic property of a pure chemical substance.
The observed melting point range can be influenced not only by the purity of the material but also
by 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 results to raise the temperature slowly and at a uniform rate, usually about
1-2 0C/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 behaviour of a material upon melting should be observed and recorded carefully. Recording
for example; "melts sharply at 89.0-89.50C" or "melts at 131-133 0C, with decomposition" or
"discolours at 85 0C; melts slowly at 67-69 0C."
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
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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 mixture melting point. Your instructor will show directions for the proper use of the
melting point apparatus available in your laboratory.
Chemicals and Equipment
Urea, cinnamic acid, benzoic acid, 2-naphthol, benzoin, capillary tube, melting-point apparatus,
50-70 cm length narrow glass tubing.
Melting Point Determination of Some Organic Compounds
Procedure
The capillary tube is filled as follows. About 30 mg of the dry substance (for example urea) is
placed on a glass slide or upon a fragment of clean porous porcelain plate and finely powdered
with a clean metal or glass spatula, and then formed into a small mound. The open end of the
capillary tube is pushed into the powder, 'backing' the latter, if necessary, with a spatula. The solid
is then shaken down the tube by tapping the closed end on the bench or dropping the melting
point tube, through a 50-70 cm length of a narrow glass tubing onto the bench top (protect the
melting point tube with a paper sheet or tissues). The procedure is repeated until the length of
densely packed material is 1-2 mm, and the outside of the tube is finally wiped clean.
Insert the tube into the melting point apparatus and start heating. The temperature may be allowed
to rise fairly rapidly to within 10-150C below the compound's expected melting point. However,
during determination of the actual melting point range, the temperature should not rise more
rapidly than l-20C/min. Therefore, decrease the rate of heating when the temperature is about
100C below the expected melting point. The melting point of urea is approximately 1300C.
Record the melting point range of urea on the report sheet and the behaviour of a material upon
melting.
In a similar way, determine and record the melting point of a sample of trans-cinnamic acid. This
compound also melts at approximately 1300C.
To demonstrate the effect of impurities on the melting point of a pure substance, determine the
melting-point range of a 50-50 mixture by weight (use about 25 mg of each compound) of urea
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and cinnamic acid. If time permits, repeat the procedure with 75-25 and 25-75 mixtures.
(Alternatively, your instructor may assign different ratios to different students in the class, and
each student can use both his or her results and those of others.) Using midpoints of the meltingpoint ranges, plot the data on the report sheet.
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 0C/min. Then allow
the apparatus or bath to cool to 15-200C 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 2 0C/minute.
Procedure
Obtain an unknown sample (of one of the substances listed in Table 1.1) from your instructor. Fill
2 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. Use the data in Table 1.1 to make a preliminary identification of your unknown. Then
confirm its identity by the mixture melting-point technique: Mix about 30 mg of the unknown
with an equal weight of the known substance you suspect and determine the melting point of the
mixture. Repeat the procedure, if necessary, to determine with certainty the identity of the
unknown. Record your results and your conclusion on the report sheet.
Table 1.1. Melting Points of Some Organic Compounds.
Compound
Melting Point, 0C
Benzoic Acid
121-122
2-Naphthol
121-122
Benzoin
132-133
trans-Cinnamic Acid
132-133
Urea
132-133
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Questions
1. Define the term melting point range
2. For what purposes are melting points routinely used?
3. Three test tubes contain white crystalline organic solids A, B, and C, each of which melts at
140-142 0C. A 50-50 mixture of A and B melts at 122-130 0C. A mixture of 50-50 of A and C
melts at 140-142 0C. In what range would a 50-50 mixture of B and C probably melt? What
can you say about the identities of A, B, and C?
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