1 Lab 01: Melting Points
Reading
Pavia et al., Introduction to Organic Laboratory Techniques: A Microscale Approach. Read the technique
section “Physical Constants of Solids: The Melting Point.”
Introduction
In this experiment you will become familiar with the Thiele tube and the melting point apparatus for
taking melting points of organic compounds. You will first practice taking melting points of a compound
with a known melting point using the Thiele tube. After you are confident of your technique using the
Thiele tube, you will obtain a quick-and-dirty melting point of an assigned unkown compound. A more
accurate melting point of your unknown will then be obtained using the melting point apparatus (MPA).
A mixed melting point using the MPA will then enable you to determine the identity of your unknown.
After the identity of your unknown has been determined you will work in groups of three to determine the
melting point of a mixture of urea and trans-cinnaminc acid. The class data will be compiled and you
will construct a melting point composition diagram.
I.
Melting Point of an Known and an Unknown using a Thiele tube
Reporting melting point data:
Melting points are always reported as temperature ranges. The lower number in the range is the
temperature at which the sample first starts to melt. The upper number is the temperature at which the
sample has completely melted. Be careful of impurities in the sample (for example, dust or broken glass)
which will never melt; waiting for them to melt could cause the melting point to be reported as much
higher than its actual value.
The melting behavior should also be recorded. The melting behavior includes observations
regarding shrinkage, sweating, discoloration, and decomposition.
The samples should be replaced with fresh tubes after each melting point determination, because
frequently chemical changes occur at high temperatures. These could introduce impurities that distort the
melting point.
Tips on MP Determination of a Known (for Part-I (a)):
Heat the melting point apparatus rapidly until the temperature is about 20 oC below the expected
melting temperature. There is a delay between the time that heat is added or removed and the time that
the temperature stops increasing—this is called an induction period. Avoid melting the sample during or
after the rapid heating.
When the temperature has stabilized approximately 10–20 oC below the expected melting
temperature, you should start heating gently to raise the temp at approximately 1–2 oC per minute. The
melting point is determined by heating the oil in the Thiele tube until the sample melts. Ideally, the
temperature of the oil should be changing by only 1–2 oC at the time of melting to measure an accurate
melting point.
Tips on MP Determination of an Unknown (for Part-I (b)):
When we do not know the MP of a compound, we must take at least two runs. The first time, an
approximate melting point is determined by raising the temperature at a rate of about 10 oC per minute.
Care should be taken not to raise the temperature too quickly, or the melting point will be too inaccurate.
After the approximate melting point has been determined, the temperature of the apparatus is lowered to
about 10 oC below the approximate melting point. A new sample is inserted in the apparatus, and the
temperature is raised at a rate of 1–2 oC per minute. This permits a very accurate measurement that will
only take a few minutes.
2 Setting up
p a Thiele Tu
ube melting point apparatu
us:
Obtain an oill-filled Thielee tube from thhe center lab
bench. Set up thhe melting poiint apparatus as illustrated in
heree. Be sure to nnote the folloowing:
1. A two or threee-pronged uttility clamp should be usedd on
the top of the Thhiele tube to seecure it to a rring stand.
2. Position the bburner about two-thirds thhe way out aloong
the side arm. Thiis will result iin more unifoorm heating of the
sam
mple.
3. Be certain too pay careful aattention to your thermometer.
Do not let it fall; it will break releasing meercury into thee
room
m.
a. Proceedure for thee MP of a Kn
nown:
Obtain
O
a comp
pound with a known
k
meltin
ng point. Youu can choose aany compounnd that is availlable
from the list
l in the lab. Crush the saample to a po
owder using a test tube by pplacing the saample on a cleean
watch glaass, gently cru
ushing the sam
mple with the bottom of a cclean test tubee. Prepare a ccapillary melting
point tubee as demonstrrated by your professor. Seecure the capiillary tube agaainst the therm
mometer usinng a
cut piece of latex tubin
ng. Place the thermometer
t
inside
i
the Thiiele tube and determine thhe MP range oof the
compound
d using a micro-burner to control
c
the raate of heating.. Be sure to nnote the MP bbehavior.
MP
M Range
MP
M Behaviorr
3 b. Procedure for the Determination of the Melting Point of an Unknown Sample:
Obtain an unknown compound from your instructor and be sure to record the unknown number.
Crush the sample to a powder as before. Prepare one capillary melting point tube, secure the capillary
tube against the thermometer, place the thermometer inside the Thiele tube, and determine the MP of the
compound. You will take on fast MP using the Thiele tube and a more accurate slow MP using the
melting point apparatus in the next section.
Unknown
Number
Observation of
Solid
Appearance
MP Range
MP Behavior
II.
Melting point of an Unknown using a Melting Point Apparatus (MPA)
Procedure:
Prepare a capillary melting point tube of your unknown. You only need to prepare one since you
already found an approximate MP using the Thiele tube. Determine the MP of the compound using the
melting point apparatus. Your instructor will demonstrate how to use the MPA.
MP Range
MP Behavior
4 III.
Identification of an Unknown using Mixed MP using MPA
Examine the list of possible compounds found in the lab. The melting points on this list are literature
values, and usually only the lowest temperature of the melting-point range is reported in the literature.
Most of the possible compounds come in pairs with similar melting point values.
Suspect
Compound #1
Suspect
Compound #2
Procedure
Of the compounds that you have narrowed your choices to, only one will be available for
performing a mixed MP. Obtain a small amount of the available pure sample and place it on a watch
glass. Mix an equal amount of your unknown with the pure sample. Use this sample to check the identity
of the unknown using the mixed melting point method. Your instructor will demonstrate the proper
technique for this part of the lab.
Based on the results of the mixed melting point determination, determine the identity of your
unknown.
Unknown #_____________________is ________________________
Briefly describe how you determined the identity of your unknown.
5 IV.
Construction of a Melting Point Composition Diagram
A series of mixtures of urea and trans-cinnamic acid have been prepared and you will use these to
make your own melting-point/composition diagram. You will be assigned to a group of three and each
member of your group will prepare a melting point capillary tube and take a melting point (for a total of
three MP measurements). Average the lower range and the higher range to get an average MP range for
your assigned composition.
Mole Fraction
Urea
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
Mole Fraction
trans-cinnamic
acid
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
When you finish, report your data your instructor. Once all of the data has been collected, make a melting
point composition diagram. See if you can locate (approximately) the eutectic point.
MP 1
MP 2
MP 3
6 Melting Point Composition Diagram of trans-Cinnamic Acid and Urea