Fall Organic Chemistry Experiment #1
Glassware Identification and Keeping a Notebook
Recrystallization and Melting Point
Part 1 (Introduction to the Organic Chemistry Lab)
Suggested Reading:
"The Student's Lab Companion: Laboratory Techniques for Organic Chemistry”, by John W.
Lehman
Introduction
p. 1-9 Laboratory Safety,
p. 11-19 A. Basic Operations,
pgs. 21-38: OP-1 (Cleaning and Drying Glassware) OP-2 (Using Specialized
Glassware) OP-3 (Using Glass Rods and Tubing) OP-4 (Weighing) OP-5
(Measuring Volume) OP-6 (Making Transfers)
Appendices I-V
Procedure:
You should record all laboratory procedures as well as observations and calculations in your
laboratory notebook!
(1) Preparing for laboratory work. In order to prepare for work in the organic chemistry
laboratory, you must compile a list of all reagents used along with their physical properties and
safety issues. So, complete each of the following exercises according to the directives:
a. Give this experiment a and record today’s date
b. Write out the Purpose/Hypothesis of today’s experiment
c. Acetone – find its melting point, boiling point, flash point, and synonyms according
to information from a filed MSDS in S303 – according to its flashpoint and boiling
point, is acetone considered a highly flammable (Class 1A) material?
d. Methylene chloride – find all health effects, first-aid measures, and fire-fighting
measures for this compound listed in an on-line MSDS resource
e. Diethyl ether – find all synonyms, exposure limits, and personal protective equipment
for this compound from an on-line MSDS resource – according to its PEL or TLV
values, can we use methylene chloride out on the bench top or should we use it in a
fume hood?
f. Hexanes – find health effects and first-aid measures pertaining to this compound
g. Chloroform – find its density in Appendix VI of the Lehman text
h. Benzoic acid – find solubility data for this compound in various solvents by looking
in the Merck Index
i. Acetanilide – convert from 0.335 mmol to grams (HINT: you’ll need to look up a
piece of information first)
j. Toluene – find safety data ratings (health, flammability, reactivity, and contact),
handling, storage, and stability/reactivity information for this compound via an ICSC
(on-line via NIOSH)
k. Ethylene glycol – convert from 1.65 mmol to mL (HINT: you’ll need to look up a
piece of information first)
l. Write out (using structures) the reaction between Acetic acid and Ethanol using
Hydrochloric acid (as a catalyst) to yield Ethyl acetate and Water
(2) Operations.
a. Walk through the laboratory and locate where glassware and equipment is routinely
stored. It may be helpful to use the glassware/equipment inventory sheet posted on
the organic website.
b. Locate the items in Figure A2 (page 24) from the lab drawers and assemble them
according to the diagram. Show to your instructor for evaluation.
c. Using appropriate glassware and volumetric devices, measure out the following
volumes of water and then transfer according to the directive shown. Demonstrate to
your instructor for evaluation.
1. 22 mL to a 250 mL round-bottom flask
2. 6 mL to a 25 mL Erlenmeyer flask
3. 1.5 mL to a 25 mL round-bottom flask fitted with a rubber septum
4. 500 µL to a 5 mL long-neck microscale round-bottom flask
d. Perform Part 2 below (recrystallization and melting point). Be sure to save your
product in a properly labeled vial. It is vital that you keep an accurate record of this
experiment in your laboratory notebook.
(3) Clean-up.
a. Dispose of any remaining organic solids and liquids in the appropriate waste
container
b. Dispose of broken or disposable glassware in appropriate containers
c. Clean-up all equipment and glassware according to the directives in Operation 1 of
the Lehman text (p. 21-22) – be sure to dispose of any acetone washings in the
appropriate waste container
d. Place all clean glassware in its appropriate drawer
e. Clean your work area/bench/fume hood
f. Write up lab summary statement/conclusion
g. Have laboratory partner sign your lab notebook
h. Check-in with instructor before leaving.
Part 2 (Recrystallization and Melting Point)
Suggested Reading:
"The Student's Lab Companion: Laboratory Techniques for Organic Chemistry”, by John W.
Lehman
OP-7 (Heating using hotplates and hot water baths), p. 42-44
OP-8 (Cooling), p. 52-53
OP-12 (Gravity Filtration), p. 61-64
OP-13 (Vacuum Filtration), p. 65-68
OP-14 (Centrifugation), p. 68-70
OP-25 (Recrystallization), p. 130-145
Introduction
Crystallization is the most important tool for the purification of solid organic compounds.
In some cases, the crystals can be grown from a molten state. However, the removal of impurities
becomes very difficult for crystals grown in this fashion. Therefore, recrystallization techniques
involve dissolving the solid material in the minimum amount of hot solvent and enabling the
solution to become saturated with respect to the solute, which then crystallizes (precipitates) from
the solution. The goal is for a perfectly regular array of crystallizes to form in which foreign
materials (impurities) are excluded. The ideal situation involves the complete precipitation of
pure crystalline material (of a single substance) while the impurities remained dissolved in
solution. In order to achieve this scenario, the impurities must be in a sufficiently low
concentration relative to the desired substance. Once crystallized, the solid can be collected by
vacuum filtration, washed with cold solvent, and dried (usually air-dried). The crystallization
process generally involves the following seven steps.
1.
Choosing the crystallization solvent
2.
Dissolving the solute.
3.
Decolorizing the solution.
4.
Removing suspended (undissolved) solids.
5.
Crystallizing the solute.
6.
Collecting and washing the crystals.
7.
Drying the product.
(1) Choosing the crystallization solvent.
The general rule of thumb to follow here is “like dissolves like”. If our solute is
relatively nonpolar, then we will likely choose a solvent like hexane or petroleum ether.
If our solvent is polar, then we will choose something more toward methanol or acetone.
At any rate, the best solvent will dissolve all of the solute when the solution is hot and
none of the solute when cold. In addition, the solvent should either not dissolve
impurities at all or dissolve them very well (so they remain in solution at all times). The
table shown at the end of this experiment has been included to help in your choice of
solvent. The solvents are listed in decreasing order of polarity.
(2) Dissolving the solute.
When dissolving the solute, we want to be aware of a few details. The usual procedure
involves adding the crystals to some beaker, flask, etc. and slowly (if not dropwise)
adding the solvent with stirring. We want to be sure that we are also heating our solution
over a hot plate or steam bath. We add just enough boiling solvent to get the crystals to
dissolve (the MINIMUM amount). We will also like to avoid superheating our solution
(a solution that is heated above the boiling point without boiling). These types of
solution may all of a sudden boil violently and explosively. To avoid this, we generally
insert a wooden applicator stick directly into the solution to promote even boiling.
Boiling chips can also work and both devices provide an excellent surface for nucleation
(the initial crystals that come out of solution). If you have added too much solvent, you
may have to boil it off to a sufficient volume to allow crystallization.
(3) Decolorizing the solution.
To remove colored impurities from the solution, we often add a bit of activated carbon
(charcoal) to which impurities can adsorb onto the surface. Pelletized Norit is the most
common form and can be easily removed by gravity filtration.
(4) Removing suspended solids.
Any material that does not dissolve can be readily removed by a HOT gravity filtration or
by decantation. Vacuum filtration is not a good option because it cools the solution in the
process. The two primary gravity filtration options are: (a) filtration through a cottonplugged Pasteur pipette; and (b) gravity filtration through a glass funnel fitted with fluted
filter paper. A diagram of each apparatus can be found in the suggested reading.
(5) Crystallizing the solute.
This step is the most time consuming and often the most frustrating. However, keep in
mind a few key points and you should be able to readily crystallize your solute. The most
important thing is to be sure that you do not have too much of your hot solvent. If this is
the case, then boil some of it off until you reach the saturation point. Once you are at this
point, then you will want to SLOWLY cool your solution to room temperature on the
benchtop. If crystallization does not begin (after a reasonable amount of time), then you
can employ a few tricks to aid in the process. Some of the tricks include: (a) adding a
seed crystal; (b) scratching the inside of the container with a glass stirring rod; or (c)
further cooling the solution in an ice bath. Always remember to be patient. We want to
do this step slowly to insure the formation of large, pure crystals.
(6) Collecting and washing the crystals.
Once crystallization is complete, the crystals can be separated from the ice-cold mother
liquor (the filtrate) by vacuum filtration. We will also want to remove excess impurities
the may be dissolved in the solvent by washing with pure solvent. Keep in mind that we
will want to wash with COLD solvent so that we do not redissolve any of the crystals.
(7) Drying the product.
Normally we let the suction run for a few minutes to pass as much of the solvent through
to the filtrate as possible. Afterwards, we let the crystals air-dry for a sufficient period
(24 hours) before performing further experiments or analyses (e.g. melting point).
Be aware that you may run into problems. If such is the case and you are unable to acquire
crystalline material, do not give up. The process is really more art than it is science. You need to
be patient and seek out help if you get stuck or things do not work out as expected.
Techniques:
1.
Crystallization
2.
Vacuum Filtration
3.
Melting Points
Procedure:
Obtain a sample of a solid unknown and record the unknown number and mass in your
laboratory notebook. Split the material into five "equal" portions (weighed). Place each of the
five solid fractions into five different 10 x 75 mm test tubes. Label the tubes according to the
type of solvent used to dissolve each sample (you may choose from water, methanol, hexanes,
toluene, diethyl ether, ethyl acetate, ethanol, and acetone). I would suggest that you select a wide
variety of solvents based upon their polarities. Attempt to dissolve each of the solid samples
using approximately 2 mL of the appropriate solvent. Record your results (soluble or insoluble).
If the solid is soluble, discard the contents into the appropriate waste container in fume hood #2.
For the insoluble ones, add a boiling chip and heat each to the boiling point in a boiling-water
bath (boil 35-50 mL of water in a 150 mL beaker) or sand bath. Again record your results
(soluble or insoluble). If the solid remains insoluble, discard the contents into the appropriate
waste container in fume hood #2.
Select one of the remaining solutions, perform a HOT gravity filtration to remove undissolved
impurities. You may want to use a small funnel fitted with filter paper and a 50 mL Erlenmeyer
flask to collect the filtrate. Allow these solutions to cool to room temperature and observe
whether crystallization occurs or not. If crystals do not appear, try to induce it to occur. Collect
the crystals by vacuum filtration on a Hirsch funnel and wash the crystals with 1-2 mL of the
COLD recrystallization solvent. Allow the crystals to air dry until later in the week and then
obtain its mass and melting point (See Lehman OP-30, page 186-191 -- you'll be determining
melting point by the Mel-Temp method). Determine the identity of your unknown according to
the table provided in Appendix VI (pgs. 290-300) of the Lehman text.