CMU Stereochemical Relationship Between Borneol and Isoborneol Lab Report

Experiment 7Sublimation of Camphor
There are two main objectives in this experiment. The first objective is to demonstrate that
common household bleach can be used to oxidize a secondary alcohol to a ketone. The
oxidation is based on a procedure from the Journal of Chemical Education that describes
oxidation of a secondary alcohol, cyclohexanol, to a ketone, cyclohexanone. This is a very
environmentally friendly process since the by-products are water and NaCl.
The second objective is to learn the technique of sublimation. Overall, you will determine
if the secondary alcohol known as isoborneol will undergo oxidation to the ketone called
camphor and then purify the product using sublimation.
Sublimation is the process in which a solid passes directly into the gas phase without
passing through the liquid phase. Deposition is the phase transition in which gas transforms
into solid without passing through the liquid phase. The solid that forms in a sublimation
process is called the sublimate. Dry ice is probably the most familiar example of
sublimation. Solid CO2 passes directly into a dense white vapor that sinks to the floor and
dissipates. Another common example occurs with incandescent lightbulbs. A filament of
tungsten wire is heated by an electric current to temperatures close to 3000º C which causes
the wire to glow. Even though tungsten has the highest melting point of any metal
(3422°C), at these high temperatures, tungsten will sublime.
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The vaporization of tungsten is represented by the equilibrium expression:
According to Le Chatelier’s Principle, to generate bright light, the temperature of the
tungsten filament must be increased. However, as the temperature increases, the tungsten
metal vaporizes at a faster rate. When hot tungsten gas strikes the much cooler wall of the
light bulb, the vapor condenses back to a solid. Since tungsten is a gray metal, this is an
obvious problem for two reasons: The metal coats the inside of the bulb so less light is
transmitted, and the tungsten filament gets thinner and eventually breaks.
Just as water exerts a vapor pressure sufficient to cause evaporation well below its boiling
point, there are a number of relatively high-melting organic compounds that exert an
appreciable vapor pressure and will sublime at moderate temperatures. That is why we can
smell organic solids like naphthalene, camphor and isoborneol. At room temperature, these
solids produce vapor concentrations sufficient to stimulate odor receptors. How humans
detect and recognize a multitude of odors is a complex process not fully understood, but
the basic premise is that molecule/receptor interactions depend on gaseous substances
entrained in the air that passes over the olfactory receptors.
Any solid-vapor transition is often called sublimation, but the behavior of camphor and
other odoriferous compounds is really an evaporation. Technically, a sublimation point is
defined similarly to a melting point or boiling point. Sublimation is defined as the point at
which the vapor pressure of a solid equals the atmospheric pressure. Likewise, boiling point
is defined as the point at which the vapor pressure of a liquid equals atmospheric pressure.
Every substance has a non-zero vapor pressure at a given temperature which is why liquids
and solids both evaporate at room temperature; of course evaporation of solids is usually
insignificant.
For evaporation of liquids then, the greater the vapor pressure, the lower the boiling point.
Compare the vapor pressure of water (24 mmHg/25 oC) with sweet-smelling, volatile
liquids like ethyl acetate (95 mmHg/25 oC) and diethyl ether (520 mmHg/25 oC). None of
these liquids are boiling at room temperature because none have a vapor pressure that
equals 1 atm at that temperature, yet each of these liquids still vaporize to an extent.
Because vapor pressures are dependent on intermolecular forces, polar compounds often
have high boiling points.
For solids, vapor pressures are much lower: camphor (0.065 mmHg/25 oC), isoborneol
(0.0035 mmHg/25 oC) and naphthalene (0.084 mmHg/25 oC). These low vapor pressures
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are sufficient to produce detectable levels of odors, even though none of them are melting
at room temperature. Solid carbon dioxide is an anomaly; its vapor pressure at room
temperature is over 60 atm. What used to be considered a college prank, sealing dry ice
pellets in a plastic soda bottle, is now, rightfully so, treated as an explosive act of terrorism.
In principle, sublimation can be used to separate two solids that have different vapor
pressures, just like distillation is used to separate liquids that have different boiling points.
If the difference in vapor pressures of two solids is big enough, it may be possible to
separate the solid with the higher vapor pressure from the other solid with a lower vapor
pressure. For example, ammonium chloride can be separated from calcium carbonate
(chalk dust) by sublimation. At 300°C, the vapor pressure of ammonium chloride is 250
mmHg whereas the vapor pressure of calcium carbonate at that temperature is negligible.
In contrast, separation of a mixture of camphor and isoborneol by sublimation does not
occur since they both have appreciable vapor pressures. But if camphor contains isoborneol
as an impurity, the purity of camphor can be estimated by the melting point of the
sublimate. This is because camphor has an unusually large freezing-point depression
constant. The sublimate can be regarded as a solid solution with camphor as the solvent
and isoborneol as the solute. The equation for freezing/melting point depression can be
used to calculate the concentration of isoborneol in the sublimate:
T = Kf X m
T = mp depression (difference between literature mp and experimental mp)
Kf = freezing-point depression constant for camphor = 40°C kg/mol
m = molal concentration of isoborneol (mol isoborneol/kg camphor)
Once molality (m) is determined, the mass percent of isoborneol and camphor can be
calculated.
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Sublimation Apparatus
Sublimation is carried out by heating the sublimand, the solid before it has sublimed, and
collecting the sublimate on a cool surface. In the figure above, a mixture of NH4Cl and
CaCO3 is the sublimand while an inverted funnel closed with a plug of cotton or stopper is
the cool surface on which the sublimate will deposit. Two nested beakers is another simple
yet effective system. A small layer of sublimand is added to the outer beaker, and the
deposition (inner) beaker is fill with cold water or ice chips. The outer beaker is heated and
crystals of sublimate collect on the bottom of the inner beaker. A variation of this apparatus
uses an Erlenmeyer flask for the inner surface and, if needed, cold water can be run through
the system. Sublimation under vacuum is also a common operation.
Various heat sources can be used, such as water baths for compounds that sublime at low
temperatures or a hot plate/sand bath when higher temperatures are required. For the brave
at heart, waving a cold yellow flame from a Bunsen burner on the bottom glass surface
provides a well-controlled method of heating, but one must use caution so that the
sublimand does not melt.
Sublimation requires a trial-and-error approach, perhaps trying more than one apparatus. It
is recommended to work with a small sample first until the appropriate system is identified.
For best results, the sample should be a finely divided solid that is completely dry. If the
solid is contains traces of water, the sublimate may get wet and pasty as water droplets
condense with the solid. If the inner condensing beaker or test tube is filled with ice or ice
water, droplets of water may start to condense inside the apparatus, especially on humid
days. Often this is a matter of timing. One should add the ice or cold water just as heating
begins. This avoids prolonged cooling of the inner surface, which promotes condensation.
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Procedure
Suppose you follow this procedure and obtain the results given below:
Add 25.0 mmol of isoborneol to a 125-mL Erlenmeyer flask. Carefully add 2.0 mL of
glacial acetic acid followed by 5.0 mL of hypochlorite solution (5.25% Clorox or other
hypochlorite laundry bleach). Over a period of 10 minutes, add another 40 mL of
hypochlorite solution to the flask in small portions. Stir well, and monitor the temperature
of the solution during the addition taking care that the stir bar does not break the
thermometer tip. Control the rate of addition so that the temperature does not go above
50°C. Keep an ice water bath nearby to cool the reaction mixture if needed.
After the addition is complete, seal the flask with parafilm, and stir at room temperature
for 30 minutes. To ensure that excess hypochlorite is available to complete the reaction,
test the reaction mixture periodically, about every 7 minutes. Take a drop of solution, and
spot a test strip of starch-iodide paper. An intense blue/black color is a positive test that
confirms the presence of hypochlorite.
If any test is negative add more hypochlorite (1-2 mL) until a positive test results.
When the reaction period is finished, take one final test. If the test is positive, add saturated
sodium bisulfite solution dropwise to destroy excess hypochlorite
.
Cool the reaction mixture to 5°C and, after a few minutes, collect the solid by vacuum
filtration. Wash the solid on the filter paper with two portions of ice-cold water. Dry the
product at room temperature (not in the oven). Weigh the product, and set aside a small
portion for a melting point.
Purify the product by sublimation. Weigh the sublimate, and measure its melting point.
Your mp should be reported as a range, but use the higher temperature of the range when
calculating the freezing-point depression.
Results:
Starting with 3.875g of isoborneol (154.2g/mol; mp = 212°C), 1.225g of product was
obtained from the filter paper after air-drying. The product was purified by sublimation
and the mp of the sublimate was 161-168 ºC. The literature mp for camphor (152.2 g/mol)
is 179 ºC.
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For your report, submit to Canvas your answers to questions 1-8.
1. Using the mass from the filter paper, calculate the percent yield of camphor. (Assume
for this calculation only that the product is pure, even though it is not pure.)
2. Using the highest temperature from the melting point range of the product, calculate the
mass percentages of isoborneol and camphor in the sublimate.
3. This experiment started with a strong-smelling white solid and produced a
strong-smelling white solid. What evidence supports the fact that these are two different
compounds?
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4. Perspective drawings of isoborneol and camphor give a better representation of their
bicyclic structures. Determine the configuration (R or S) for each asymmetric center in both
structures. Write your designation next to the asymmetric carbon center.
5. What is the maximum number of stereoisomers possible for isoborneol? For camphor?
6. Draw the enantiomer for both structures above and label each stereocenter as R or S.
7. What is the stereochemical relationship between borneol and isoborneol; geometric
isomers, constitutional isomers, enantiomers, diastereomers, meso or regioisomers?
8. What would be the product if borneol was oxidized using the same procedure? Draw its
structure.
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