ORGANIC I LABORATORY
KELLY
Melting Points
READ
Rodig pp 17-23, Exp’t 2A and C
I . Introduction
The melting point (or freezing point) of a pure compound may be defined
as that temperature at which the solid and liquid phases of the compound are in
equilibrium at some particular pressure, usually taken as 1 atm, or 760 mm Hg.
T˚ = mp
Solid Q
Liquid Q
The definition implies that when the solid Q is in heterogeneous equilibrium with
liquid Q at some temperature T 0, the melting point, a slight increase in
temperature will result in the complete conversion of solid Q to the liquid phase.
Conversely, a slight decrease in temperature will result in the complete
conversion of liquid Q to solid Q.
Because the volume change on going from the solid phase to the liquid phase is
usually very small, a change in pressure has very little effect on the melting
point. For example, an increase in pressure from 1 to 2 atm lowers the melting
point of ice by only 0.00240, an amount too small to be of significance in routine
organic laboratory work.
Phase Behavior
Some understanding of the physical nature of the melting process is essential
before an explanation can be given of the use of melting points as a method of
identification and a criterion for purity. It is at the melting point of a pure
crystalline material that sufficient thermal energy has been introduced to begin
breaking the crystal lattice. For many (but not all) pure compounds the range of
temperature change, or melting range, over which this breakdown occurs will be
from 0.5 to 20. The melting range, therefore, is sometimes taken as
circumstantial evidence of purity.
The anticipated behavior of a mixture of compounds can be conveniently
explored using a temperature-composition phase diagram such as the one
illustrated below for hydroquinone and camphor.
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The left and right coordinates of the plot represent pure camphor and
hydroquinone with melting points of 178 and 130.80, respectively. The region of
the graph above and in between curves AB and BC represents a liquid mixture of
hydroquinone and camphor, while that area of the graph to the right of BC is
pure solid hydroquinone in contact with a liquid-phase mixture of camphor and
hydroquinone. The area to the left of AR is pure solid camphor in contact with a
liquid-phase mixture of camphor and hydroquinone. A mixture of the solid
compounds exists below point B. Line AR represents an equilibrium between
pure solid camphor and the liquid mixture. For example, at a temperature of
1090, two equilibria are possible in this binary mixture. Either pure solid
camphor will be in equilibrium with a liquid mixture of 80 mole % camphor and
20 mole % hydroquinone or pure solid hydroquinone will be in
equilibrium with a liquid mixture of 70 mole % hydroquinone and
29.5 mole % camphor.
It is often more instructive to determine the fate of a liquid mixture of a certain
composition as it is cooled. A mixture that contain 20 mole % hydroquinone will
remain entirely liquid until it is cooled to 109˚, at which time pure camphor
crystallizes from the solution. A further drop in temperature causes more
camphor to crystallize out and the remaining liquid mixture becomes richer in
hydroquinone. By proceeding to the right along line AB we reach temperature
of 21˚ which corresponds to a liquid with a composition of 31 mole %
hydroquinone. This point on the curve (point B on the phase diagram) represents
the eutectic temperature and a composition known as. the
eutectic
composition. This is the lowest temperature at which a liquid mixture can exist
in equilibrium with either or both of the two solid components If the temperature
is lowered below 2l˚, solid material will continue to crystallize at the point of
eutectic composition until all of the liquid mixture has solidified. In a similar
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manner we cold have started with a liquid mixture rich in hydroquinone, cooled
it until more hydroquinone began to crystallize, and followed line BC to the left
until we reached the same eutectic temperature, point B.
If we reverse the foregoing considerations, the significance or the eutectic
temperature become. more apparent because this is the temperature at which any
solid mixture begins to melt, and melting may continue over a very wide range.
To use the example illustrate by the figure below, suppose we heat a solid
camphor-hydroquinone mixture that contains 80 mole % hydroquinone. Melting
begins (as it will for any mixture or hydroquinone and camphor) at 21˚ and
continues until all material at the eutectic composition is liquid. The remaining
solid is pure hydroquinone which will gradually melt as the temperature is raised
from B to X . When a temperature of 118.8˚ is reached, all of the solid has
melted and the overall melting range has been 97.8˚. In actual practice, a solid
mixture that contains only a few percent of a second component may exhibit a
mp range of only a few degrees, because the experimental technique usually used
does not allow detection of the minute amount of melting that begins at the
eutectic temperature. Material having the eutectic composition will, of course,
always exhibit a sharp melt mg point, which could be interpreted incorrectly as a
property indicative of a pure compound.
Mixed Melting Points
Subject to the considerations discussed above, melting points can often be used
to establish the identity of two compounds. As demonstrated in the discussion of
phase behavior, the melting point of a mixture varies from that of either pure
compound. Therefore, if the melting point of a mixture is not
different, the two compounds forming the mixture are likely to be
identical. Providing the identity of one compound is known, that of the other is
evident.
It is preferable to carry out the melting-point determination on the mixture
and at least one of the pure compounds simultaneously in the same apparatus to
ensure that the experimental conditions for the two determinations are identical.
Decomposition Points
Many compounds decompose at or near their melting points, and although
decomposition temperatures are often highly reproducible and useful in
themselves, decomposition can complicate the determination of a melting point.
Decomposition may be evidenced by a significant change in the color of the solid
or melting with gas evolution. If decomposition occurs before melting, the
decomposition products will depress the melting point.
The extent of
decomposition often depends on the rate of heating, and decomposition may be
avoided by setting the temperature of the heater to just below the mp of the solid
and introduce the sample and quickly obtain the mp before any appreciable
decomposition has had a chance to occur.
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II. Procedure
A. Obtain a melting point unknown from the lab assistant. This unknown will
be one of the compounds listed in Table 2.2 on page 22 of your lab
manual.
B. Always fill melting point capillaries to a depth of no more than 1/8 inch
from the bottom. Make sure the crystals are finely ground and well
packed in the capillary.
C. DO not use the apparatus until you have been instructed in its use..
D. Sign up for a period during the week during which you will
take your melting points. Before you leave the lab today, you should
have prepared and labeled two melting capillaries each for your unknown,
two for the recrystallized acetanilide, one each for the two standards.
E. Before using the melting point apparatus, make sure it has cooled down
below 60°C.
F. Using the procedure in part 'A' on page 24, obtain an approximate melting
point of the unknown and of your acetanilide from your crystallization
experiment. Pure acetanilide melts at 114°C.
G. Determine a more accurate melting point of each of the following in a
second melting run (repeat procedure in part 'A', page 24) with capillaries
of the following samples all in place.
1. pure unknown
2. recrystallized acetanilide
3. Two mixtures of your unknown with standard compounds that have a
similar melting point ( i.e. within plus or minus 5˚C).
H. Record all your data on the attached Report Sheet.
II. Report
A. Fill out the Report Sheet attached and turn in.
B. STUDY QUESTIONS FOR EXAM
1. Answer the following questions: Page 26; 2, 3; Page 27; 1, 3, 4, 5
2. Know the procedure for taking a melting point.
3. Know how to prove the identity of an unknown using the mixture
melting point technique.
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MELTING POINT REPORT SHEET
Name_________________________
Unknown number __________
Fill in the melting points you obtain, below:
First Melting run (10°/min.)
Pure unknown
Softening point
____________
Melting point
____________
Recrystallized acetanilide
Softening point
____________
Melting point
____________
1st Possible Compound________________________
Softening point
Melting point
____________
___________
2nd Possible Compound_______________________________
Softening point
____________
Melting point
____________
Second Melting run (1-2°/min in vicinity of melting points)
Pure unknown
Softening point
____________
Melting point
____________
Mixture mp #1
Softening point
____________
Melting point
____________
Mixture mp #2
Softening point
____________
Melting point
____________
Recrystallized Acetanilide
Softening point
____________
Melting point
____________
My unknown compound was ________________________________________
My conclusion is based on the following reasons: (use back side of sheet)
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