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ORGANIC CHEMISTRY LABORATORY EXPERIMENTS
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EXPERIMENTSYNTHESIS OF ASPIRIN FROM SALICYLIC ACID
Aspirin is one of the oldest and most common drugs in use today. It is both an analgesic (pain
killer) and antipyretic (reduces fever). One method of preparation is to react salicylic acid (1 )
with acetic anhydride (2) and a trace amount of acid (equation 1).
The chemical name for aspirin is acetylsalicylic acid (3)
PROCEDURE
Place 3.00 g of salicylic acid in a 125 ml Erlenmeyer flask.
Cautiously add 6 ml of acetic anhydride and then 5 drops of concentrated H2SO4.
Mix the reagents and heat the flask in a beaker of water warmed to 80-90°C, for 10 minutes.
Remove the Erlenmeyer flask and allow it to cool to room temperature. Add 40 ml of H2O
and let the sample crystallize in an ice-water bath.*
Filter and wash the crystals with cold water.
Allow them to air dry overnight and weigh the product. What is the percent yield? One
drawback to this synthetic procedure is that there is the possibility of some left over salicylic
acid. To test for unreacted salicylic acid, add a few drops of 1% ferric chloride solution to a
tube containing a few mg of salicylic acid dissolved in water. What do you observe? Do the
same for a few mg of your sample dissolved in water. Is there any salicylic acid?
MELTING POINTS AND SUBLIMATION
MELTING POINT
The temperature range at which a crystalline solid changes into a liquid is defined as
the melting point. To obtain the melting point of a compound, a small sample is slowly
heated. The sample is carefully observed (usually through a small tube) and the
temperature at which liquid is first observed is noted. When all of the solid has liquified,
this temperature is noted as well. In most instances a sample will melt over a small
range of temperature. Thus the temperature at which the liquid is first observed and the
solid is totally liquified is referred to as the melting point range. Most pure samples
melt over a very small (<1°) temperature range while some samples may melt over a
couple of degrees. In general, samples that melt over a broad range (>5°) probably
have soluble impurities which depress the melting point. Consequently, the melting
point range of a compound can be an indication of purity.
SUBLIMATION
Sublimation is a process by which a compound goes from a solid to a gas without going
through a liquid phase. Most of you have observed this process when you have seen
“dry ice” (CO2(s)) or “freeze dried” a substance. Many organic compounds “sublime” at
readily accessible temperatures and pressures which gives us a route to a simple and
quick purification.
,PROCEDURE
(Since we only have a limited number of melting point apparatuses, some of you should
do the sublimation first and melting point second.)
1) Melting point
A) Obtain a small sample of cinnamic acid or urea and obtain its melting point range.
Repeat the process with another sample. Compare the melting point you recorded to
the melting point in the literature.
B) Take a “mixed melting point” of one of the cinnamic acid / urea mixtures provided.
What do you observe?
2) Sublimation
Obtain a 50 mg sample of salicylic acid and place it into the side arm Erlenmeyer flask
from your microscale kit. Assemble the apparatus as depicted on page 212 of Zubrick or
shown in the lab demo. Fill the centrifuge tube with ice. Heat the flask gently on a
heating mantel. You should observe the solid evaporating into “whiffs” of gas and
condensing as a solid on the cold surface of the centrifuge tube. (This is often referred
to as a “cold finger”)
Carefully disassemble the apparatus so as not to dislodge any solid on the cold finger.
Scrape the solid off the cold finger and weigh it. Calculate the % recovery.
Compound Purification: Recrystallization
Purification of compounds that are either synthesized in the lab or that have been
isolated from sources in nature is a very important part of organic chemistry. A variety
of methods may be used including distillation, sublimation, extraction, different kinds of
chromatography and recrystallization. The basic process of recrystallization involves
dissolving the substance in a solvent to remove insoluble impurities then letting the
desired compound crystallize.
Products obtained from an organic reaction are seldom pure when isolated
directly from the reaction mixture. If the product is solid, it may be purified by
recrystallization from a suitable solvent. A good recrystallization solvent should dissolve
a moderate quantity of the substance to be purified at elevated temperatures but only a
small quantity of the substance at lower temperature. It should dissolve impurities
readily at low temperatures or not at all. Finally, the solvent should be readily removed
from the purified product. This usually means that it has a relatively low boiling point.
A chemist can consult the literature for information regarding recrystallizing solvents for
a particular substance, or if that information is not available, test several solvents. A
small amount of the substance to be recrystallized is placed in several test tubes and a
small amount of a different solvent is added to each. Solubility is then noted both at cold
and elevated temperatures. The quality and quantity of crystals obtained when the
solution is cooled are also noted. To get a good yield of purified material, the minimum
amount of hot solvent to dissolve all the impure material is used. In practice 3-5% more
solvent than necessary is used so the solution is not saturated. If the impure compound
contains traces of colored material that are not native to the compound, they may be
removed by adding a small amount of decolorizing charcoal to the hot solution, quickly
filtering it and allowing it to crystallize. Usually crystallization spontaneously occurs
upon cooling the solution. If it does not, crystallization may be induced by cooling the
solution in an ice bath, scratching the vessel wall with a glass stirring rod or by adding a
single crystal of pure material (a seed crystal). The crystals are then isolated using
vacuum filtration. The collected crystals are then washed with ice cold solvent to further
remove impurities.
, Procedure
Solubility Tests
Place about 10 mg of anthracene into each of 4 reaction tubes or micro test tubes.
Weigh out the 10 mg quantity until you are familiar with the appearance (size) of
approximately 10 mg of sample. Once familiar with 10 mg as a small pile on the end of
your spatula, you may estimate the amount and not weigh it out. Add 0.25 mL of ethanol
to tube 1 and observe the mixture. Repeat with water (tube 2), toluene (tube 3), and
ligroin (tube 4). The sample is considered dissolved when the solution is clear with no
cloudiness or solid apparent. A solution of dissolved solute may have color; it is still
considered dissolved if no solid is apparent. If you observe any solid on the bottom of
the tube, floating on the top of the solvent, or dispersed in the solvent (cloudy), the
sample is considered not to have dissolved.
If the samples dissolve in a solvent at room temperature, you do not need to heat
the sample in the next step. If the sample readily dissolves in ethanol at room
temperature, add one or two drops of water to see if cloudiness (precipitate) forms.
Continue adding one or two drops of water and checking for precipitation until you have
added 10 drops total.
For those samples which did not dissolve in a solvent at room temperature, gently
heat the mixture on the steam bath and observe if the sample dissolves in hot solvent.
Pay particular attention to the ligroin mixtures. Ligroin has a low boiling point and can
be easily boiled away completely. If you boil away a solvent, simply add the 0.25 mL
again and continue.
Repeat the experiment with [4-amino-1-naphthalenesulfonic acid, sodium salt]
and then again with benzoic acid.
Record your observations in your notebook. Did the sample dissolve in a solvent
at room temperature? Did the sample dissolve in hot solvent? Did the sample precipitate
(crystallize) upon cooling of the solvent? Recording this data in table form works best.
Microscale Recrystallization of acetylsalicylic acid from water.
Calculate the required minimum volume of hot water to dissolve 60 mg of
acetylsalicylic acid. The solubility of acetylsalicylic acid in 25° C water is 1.0g/300 mL.
The solubility of acetylsalicylic acid in 37° C water is 1.0g/100 mL. Aspirin will
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hydrolyze in boiling water, so do not heat the recrystallization solution very long. Heat
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until you just see thermal gradients (wavy heat lines) or bubbles begin to form. Use the
aspirin solubility at 37° C for your hot solubility calculation and reduce the solvent
volume by 1/3 to 1/2 (we are guessing at the higher solubility of aspirin in 100° C
water).
Place the 60 mg of acetylsalicylic acid in a reaction tube and add the required
minimum volume of water calculated above. Add a boiling stick and begin gently
heating on the sand bath. As the solvent begins to boil (see above), add water dropwise
until the sample just dissolves. Add 1 more drop of water. Record the total volume of
water needed to dissolve the sample (21 drops = 1 mL) Remove the solution from the
heat and place in the test tube holder to cool to room temperature undisturbed.
If crystals have not formed upon cooling, scratch the side of the reaction tube with
a glass stir rod. If crystals still do not form, ask your instructor for help on the next steps
to take.
Once crystals have formed, place the reaction tube in an ice bath and allow crystallization
to complete. Once the reaction tube is ice cold and no further crystallization is occurring,
proceed with the following isolation steps.
Place the tip of your Pasteur pipette firmly on the bottom of the reaction tube.
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