Download Melting Point, Boiling Point, and Index of Refraction

Melting Point, Boiling Point, and Index of Refraction
Melting points, boiling points, and index of refractions are easily measured physical
properties of organic compounds useful in product characterization and purity
determination.
A. Melting Points
Pure, crystalline solids have a characteristic melting point, which is expressed as the
temperature range over which the solid melts to become a liquid. The transition
between the solid and the liquid is so sharp for small samples of a pure substance that
melting points can be measured to ±0.1oC. Typically it is no more than ±1oC. Melting
points of pure compounds are recorded in handbooks, such as the Handbook of
Chemistry and Physics (CRC) or the Merck Index. Alternatively, you can find this
information on the Internet, for example at http://chemfinder.cambridgesoft.com/.
Measurements of the melting point of a solid can also provide information about the
purity of the substance. Pure crystalline solids have a sharp melting point. They melt
in a very narrow range (melting range) of temperatures, whereas mixtures melt with a
broad temperature range. Mixtures also tend to melt at temperatures below the melting
points of the pure solids.
Many solid substances prepared in the organic laboratory are initially impure. These
impurities affect the melting point of a substance. In a sample that contains a mixture
of two compounds, each component usually depresses the melting point of the other,
giving an observed melting point range that is lower and broader than the melting point
of either component. A melting point composition diagram for two hypothetical solids,
A and B, is shown below, as a graph of temperature versus composition.
The eutectic point is the lowest temperature of the mixture and is determined by the
equilibrium composition at which A and B melt in constant ratio. A sample whose
composition is exactly that of the eutectic point will exhibit a sharp melting point at the
eutectic temperature. This means a eutectic mixture can be mistaken for a pure
compound since both have a sharp melting point.
Because it is difficult to heat solids to temperatures above their melting points, and
because pure solids tend to melt over a very small temperature range, melting points
are often used to help identify compounds.
We will use the Mel-Temp apparatus for measuring the melting point in our lab. The
Mel-Temp apparatus uses closed-end capillary tubes. The sample is placed into a predesigned slot and its melting behavior observed through a magnifying glass. Keep in
mind that we have 5 Mel-Temps for the labs. Schedule your lab experiment to
minimize waiting time.
Procedure
I. Melting points are best determined using a finely divided powder. Grind the sample
using a mortar and pestle to ensure homogeneity. Fill a capillary tube to a height of no
more than 2-3 mm with the packed urea. The sample can be packed tightly by
dropping the capillary tube through glass tubing on a table top or the floor. Put the tube
into the Mel-temp apparatus closed end down. Make sure that you can see the sample
through the magnifying glass. Set the voltage to zero and turn on the Mel-temp. Turn
the voltage to 45 and observe both the sample and temperature reading as you heat.
http://orgchem.colorado.edu/hndbksupport/meltingpt/mtset.html. (Never set the voltage
at more than 70). Note (a) the temperature at which the column of urea first collapses
or shows some liquid and (b) the temperature at which the sample is completely liquid.
This is the melting range, which we call a melting point. Always report a melting range.
The melting point is not accurate if the thermometer and the sample are not at the
same temperature. For accuracy the sample should be heated through the melting
range at a rate of 1 oC or less per minute. Turn off the apparatus and let it cool. If you
did not get a good result for the melting point of urea, prepare a sample in a new
capillary, and repeat the measurement. Capillaries cannot be reused. Put used
capillaries in the glass waste container.
II. Prepare a melting point diagram for a mixture of two compounds. We will use urea
and cinnamic acid. Work in groups of two for this part. Record the melting point
ranges of urea (from part I), pure cinnamic acid, a 1:1 urea:cinnamic acid mixture, a 4:1
urea:cinnamic acid mixture, and a 1:4 urea:cinnamic acid mixture Plot your data in a
melting point composition diagram similar to the above diagram. Make an accurate
diagram using graph paper and record melting point ranges.
http://ull.chemistry.uakron.edu/organic_lab/melting_point/
III. You will be given a solid unknown. Your unknown is one of the following
compounds: salicylic acid, benzoic acid, succinic acid, acetanilide, benzophenone, or
napthalene. Samples of all of these compounds are available in the lab. In the
procedure part of your Prelab explain how you plan to identify your unknown.
Data and Observations
Record the melting ranges obtained directly into your lab notebook. List any important
observations you make while performing the experiment. For example, describe the
appearance of a compound when it melts and any other visible changes occurring prior
to, or during, the melting process, i.e. water vapor, gas bubbles, color changes, clarity
of the liquid melt.
Analysis
Compare the literature melting points of all substances you have used and compare to
the values you have determined experimentally. Comment on any discrepancies.
Compare the melting point ranges of your pure urea and cinnamic acid with the
mixture. Is it possible to estimate the eutectic point from your graph?
Questions
1. How fast do you heat the sample in the Mel-temp when determining a melting point?
2. If you heat too fast, will your observed melting point be higher or lower than the true
value? Explain.
3. What is meant by the term melting range? What happens at this range?
4. Why should you always use a new capillary tube with a sample of your compound
when doing a second melting point determination?
B. Boiling Points
The boiling points of pure organic liquids are, like the melting points, characteristic
physical properties. The process of determining the boiling point is more complex than
that for the melting point. It requires more material, and because it is less affected by
impurities, it is not a good indication of purity. Like the melting point, the boiling point of
a liquid is affected by the forces that attract one moleucle to another-ionic attraction,
dipole-dipole interaction, hydrogen bonding, and van der Waals forces. A very clean
liquid in a very clean vessel will superheat and not boil when subjected to a
temperature above its boiling point. If boiling does occur under these conditions, it
occurs with explosive violence. To avoid this problem boiling stones or a
boiling stick is always added to liquids before heating them to boiling.
Procedure
1. Practice your boiling point determination techniques, by placing about 0.3 mL of
one of the liquids provided and a boiling stone in a reaction tube. Fit a distillation head
(connecting adapter) on top of the reaction tube to ensure that the system is open to
the atmosphere. REMEMBER TO NEVER HEAT A CLOSED SYSTEM! Using a
thermometer adapter clamp a thermometer so that the bulb is just above the liquid, and
then heat the liquid with a sand bath or a water bath (depending on the boiling point of
your liquid, above 100 oC a sand bath, below 100 oC a water bath). Heating is
regulated so that the boiling liquid refluxes (condenses the drips down) about 3 cm up
the thermometer bulb in order to heat the mercury thoroughly. The boiling point is the
highest temperature recorded by the thermometer and maintained over about a 1-min
time interval. True boiling is indicated by drops dripping from the thermometer and a
constant temperature recorded on the thermometer. If the temperature is not constant,
then you are probably not observing true boiling.
2. You will be given a liquid unknown. Your unknown is one of the following liquids:
toluene, ethanol, cyclohexane, 1-butanol, or distilled water. All of the liquids will be
available in the lab. In the procedure part of your lab, explain how you plan to identify
your unknown using the boiling point and refractive index.
Observation
Record all your data directly in your lab notebook and compare experimental results
with literature values. Comment on any discrepancies.
Analysis
Explain how you identified your liquid unknown using the boiling point and index of
refraction. Comment on the technique of boiling point determination and refluxing and
the use of the refractometer.
Questions
1. What are the consequences of heating a closed system?
2. Which would you expect to have a higher boiling point, ethanol or dimethyl ether?
Explain.
C. Refractive Indices
The refractive index is a physical constant that, like the boiling point, can be
used to characterize liquids. It is the ratio of the velocity of light in air to the velocity of
light in the liquid. The angle of refraction is a function of temperature and the
wavelength of light. Because the velocity of light in air is always greater than that
through a liquid, the refractive index is a number greater than 1; for example, hexane
n20D 1.3751. The superscript 20 indicates that the measurement was made at 20 oC,
and the subscript D refers to the yellow D-line of a sodium vapor lamp, light with a
wavelength of 589 nm.
The measurement is made with a refractometer using a few drops of liquid.
Compensation is made within the instrument for the fact that white light and not sodium
vapor light is used, but a temperature correction must be applied to the observed
reading by adding 0.00045 for each degree above 20 oC.
n20D = ntD + 0.00045(t – 20oC)
The refractive index can be determined to 1 part in 10,000, but because the
value is quite sensitive to impurities, there is not always very good agreement with the
literature with regard to the last figure. To master the technique of using the
refractometer, measure the refractive indices of several known, pure liquids before
measuring an unknown.
For more information on the Abbe-3L refractometer (the instrument we have in
the lab) and some video clips demonstrating its us, please go to
http://web.uccs.edu/bgaddis/chem337/expts/nD/nD.htm
Procedure
Two or three drops of the sample are placed on the open prism using a
polyethylene pipette (to avoid scratching the prism face). The prism is closed, and the
light is turned on and positioned for maximum brightness as seen through the
eyepiece. If the refractometer is set to a nearly correct value, then a partially gray
image will be seen. Turn the knob so that the line separating the dark and light areas
is at the crosshairs. Sometimes the line separating the dark and light areas is fuzzy
and colored. Turn the chromatic adjustment until the demarcation line is sharp and
colorless. Then read the refractive index by pressing the button down to light up the
scale in the field of vision. Read the temperature on the thermometer attached to the
refractometer, and make the appropriate temperature correction to the observed index
of refraction.
Write up a procedure for using the value of the index of refraction in addition to
the boiling point determination, to identify your unknown.