Experiment 1: Recrystallization and Identification of an Unknown Solid

Experiment 1: Recrystallization and Identification of an
Unknown Solid
I. Recrystallization
Procedure:
You will be given an unknown solid compound to purify by recrystallization using water as
the recrystallizing solvent. The unknown will either be benzamide (mp 128-130°C) or 2ethoxybenzamide (mp 132-134°C) and may contain a water-soluble organic impurity and a
black carbon insoluble impurity.
O
O
NH2
NH2
OCH2CH3
benzamide
2-ethoxybenzamide
Pertinent information about the chemicals you are using:
Reagents
MW (g/mol)
benzamide
2ethoxybenzamide
water
MP (ºC)
BP (ºC)
Density (g/mL)
100
1
~130-131
~130-131
18
0
Throughout this experiment, clean all glassware and equipment before using it. To clean
it, wash with soap and water, rinse with water, and shake as much water as possible
from it. If soap and water does not completely clean it (i.e. there is still some visible
residue in it), wash with a little acetone then with water. The residue is likely organic
material that the acetone will remove. This cleaning technique can be used in this
experiment because water is being used as the solvent. A trace of water in the
glassware will do no harm.
Perform the following steps:
Dissolving the Solid:
1. Place a clean 250-mL Erlenmeyer flask containing approximately 100-mL water and
several boiling stones on a hot plate set to about 4; this will be used as your source
of hot water when dissolving your compound for the experiment. If necessary,
increase the hot plate setting a little until the water is boiling.
Do we have to dry all the glassware when we clean it?
Because we are going to put water into it, this flask does not have to be dry. A
few more drops of water, more or less, won’t make any difference. In this and other
experimental procedures in our organic chemistry courses, the following distinction should
be noted regarding glassware and other apparatus. “Clean” means to wash with soap and
water and then with plain water, followed by a gentle shaking to remove as much water as
possible. “Clean, dry” means to clean as mentioned above in the previous sentence,
followed by rinsing with a couple “squirts” of acetone to remove the water, followed by
drying on the laboratory dryer.
2. Record the Identification Number of your unknown compound in your notebook.
Weigh 1-2 grams of the unknown compound and place it in a 125-mL Erlenmeyer flask
containing several boiling stones. Record the weight of your compound.
How accurately do I have to make the measurements of weight and volume in all
experiments?
If a volume is specified, such as “5 mL”, this may be measured with a graduated
cylinder – that is, the volume is somewhat approximate. On the other hand, if you were
asked to add 5.0 mL, you have to measure it more accurately and the use of a syringe or
other calibrated device would be required.
The weight of a compound that is required (i.e. 1-2 grams) means you must weigh it
and get as close to the specified amount as possible. A piece of weighing paper, or a
weighing boat, is placed on the balance and then the balance is zeroed. The paper is then
removed and some compound is placed on the paper and the paper is then placed on the
balance again to see how much has been added. This process is repeated until the correct
weight has been reached. In most cases, we try to avoid weighing compounds directly on
the balance. Students very often “miss” the weighing paper and get organic compounds on
the balance pan, which degrades the balance itself. It’s less messy to not weigh on the
balance directly, especially when you are new to using balances.
When weighing liquids, sometimes it is unavoidable – a weight is taken directly on
the balance as the liquid is slowly dripped into the weighing vessel. Care must be taken
not to let the chemicals get on the balance!
Last comment – You must always know how much of each component you are using.
Record your values. This is necessary so that work can be duplicated if desired and so you
can calculate the theoretical yield of your product in a reaction.
3. Using a 2 mL Pasteur pipette, add about 10-mL of boiling hot water to the flask and
place the flask containing the sample on the hot plate. Using a Pasteur pipette, add
hot water in small portions until the unknown solid just dissolves; use a minimum of
hot water. Remember, the unknown contains traces of black carbon that will NOT
dissolve. Add only enough hot water to dissolve the whiter crystalline solid. Then
add 2-3 milliliters extra to prevent premature crystallization during the hot gravity
filtration.
The water isn’t boiling but it is warm – can I get started now?
Recrystallization involves the use of hot solvents, in this case water is the solvent.
The hotter the solvent, the more soluble your compound will be in the solvent. So, the
hotter the solvent, the less solvent you need to dissolve the compound. You must have a
saturated solution in order to get back a maximum amount of crystals from your process.
The more solvent, the less saturated. Keep the water boiling hot the entire time and leave
the Erlenmeyer flasks on the hot plate. Some students tend to remove them and set them
on the bench top, not realizing that they are now cooling off!
The extra water added at the end, to prevent premature crystallization, is
important for the hot gravity filtration. You will be using a piece of filter paper – a DRY
piece of filter paper. If you pour a saturated solution through the dry filter paper, the
filter paper will absorb some of your water and your solution will become super-saturated!
This could lead to premature crystal formation in your funnel of your filtration.
Filter the Insoluble Impurities using the hot, gravity filtration technique.
1. Place your plastic powder funnel in a clean 125 mL Erlenmeyer flask and add an
appropriate-sized piece of “fluted” filter paper. Clamp this flask to the bench top,
to prevent it from tipping.
Why gravity filtration with fluted filter paper?
Gravity filtration using filter paper is required to remove the finely divided charcoal
impurity in your solution. Other types of filtration (such as vacuum) would result in the
product being contaminated with the charcoal. Fluted filter paper (as opposed to the
“cone”) is commonly used because it provides more surface area through which the solvent
can flow, allowing the filtration to occur quicker.
2. Pre-wet the filter paper with a couple milliliters of hot water just prior to pouring
hot solution through the filter paper and pour as quickly as possible, allowing it to
drain into the Erlenmeyer flask.
I’m filtering a solution with water in it – why do I need to wet the filter paper?
Pre-wetting the filter paper will help prevent premature crystallization from occurring. It
will help keep the saturation level of your solution uniform, as the filter paper won’t be
removing any of your solvent.
Why isn’t the funnel/filter paper being heated prior to gravity filtration?
Most hot gravity filtrations require this to be done to prevent premature crystallization
of the product in the funnel. In this case, however, we are using a plastic wide-mouth
funnel, which will not cool the solution too quickly.
Crystallizing and Isolating the Compound:
1. Allow the filtrate to cool somewhat by occasionally swirling the flask at room
temperature and then place the flask in a 400 mL beaker of ice and water, called an
“ice-water bath” until the solution is cold and crystallization is complete. While the
flask is cooling in the ice-water bath, prepare a clean filter flask and a clean Büchner
funnel fitted with the appropriately sized filter paper for use in the next step.
Attach the vacuum hose (heavy walled tubing attached to the vacuum control) and
clamp the flask to ensure it does not fall over.
What if I don’t see any crystals form?
Sometimes the crystals have a difficult time forming a lattice. Adding a “seed crystal”, a
crystal of the compound you want to grow in the solution is one option – once the first
crystal appears in the solution, others will line up with it to form a lattice. Since we don’t
know what the compound is, this isn’t an option for us. Alternatively, we can “scratch” the
solution with a clean glass sitrring rod. To do so, hold the glass rod firmly and stir the
solution, moving the glass rod around the bottom surface of the flask. It should feel like
you are scraping the surface of the glass flask. This process tends to help assist the
crystals in aligning themselves so they can form a lattice.
Why clean the filter flask?
Filter flasks should always be cleaned before using in the event that product gets into the
filtrate. If this happens, the contents of the filter flask can be re-filtered without
contaminating the product.
How cold is cold?
To maximize the amount of crystals to be recovered, the colder the better. Solid
compounds are less soluble in colder solvents. Unfortunately, it takes a long time to cool
to ice temperature (0C). A good rule of thumb is to cool the solution to 5-10C. You won’t
get much more product by waiting longer.
2. When the solution is cold and crystallization is complete, turn on the vacuum to the
filter flask, “seat” the filter paper on the Buchner funnel by pouring a couple mL of
water on it and press the funnel down on the filter flask. Swirl the contents of the
crystallizing flask to suspend the crystals, and then pour the mixture as quickly as
possible onto the filter funnel.
Why vacuum filtration?
Vacuum filtration is routinely used to remove a solid compound from a solvent. On the
other hand, hot gravity filtration is used to remove traces of inert impurities from a
solution Filter flasks should always be cleaned before using in the event that product gets
into the filtrate. If this happens, the contents of the filter flask can be refiltered
without contaminating the product.
3. Rinse the product remaining in the crystallization flask with a couple mL of ice-cold
water from the ice bath and pour it onto the product on the Buchner funnel. Solid
stuck on the sides of the flask may be loosened with a clean, glass stirring rod.
Why rinse the flask?
This will get as much solid as we can and will help to maximize the recovery. The solvent
(water) must be cold to avoid redissolving the product.
4. Allow the material to dry somewhat by pulling air through the product on the
Buchner funnel for a few minutes. In the meantime, wash all glassware and clean
your work area. Place the top of the Büchner funnel containing the solid in your
drawer to dry until the next lab period. Do not cover the funnel or the beakerleave open to the air to facilitate the drying process.
Waste disposal and Clean Up:
a. Filtrate from the filter flask – wash down the drain
b. Filter paper – place in trash can
c. Clean and put all glassware and equipment in proper locker. Wash and dry your work
area.
End Day One, Lab 1.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
During the next lab period, transfer the crystals from the funnel to a clean, dry preweighed sample vial and reweigh. Determine the weight of the pure solid in grams (g)
and the percent recovery, which is defined as:
Percent Recovery =
weight of pure sample (in g) x 100
weight of impure sample (in g)
Determine the melting point of the solid and identify the unknown using the mixed
melting point technique, as described in part (b) below.
II.
Melting Points
Read Zubrick, Chapter 12 for background information on the technique of melting point
analysis
Since the melting points of the two compounds, benzamide and 2-ethoxybenzamide, are
so close to each other in value, a simple melting point analysis will not be sufficient in
discriminating which unknown you have. Remember that even a small amount of impurity
can cause a drop in temperature and confuse your results.
You will do four melting point analyses. Your first analysis will be a quick, fast run
(sometimes called a “ballpark run”) that will be taken intentionally too fast to get an
estimate of the melting point temperature. Your second analysis will be a slower, more
accurate analysis to tell you purity and your last two analyses will be to determine the
identify of the unknown.
Procedure:
A. Determine the melting point of your purified compound:
1. Your crystallized unknown alone – one fast run, and one or two slow runs (see
instructor). Load two capillary tubes with small amount (1-2 mm of material only) and
pack as shown in prelab lecture. Set-up and run with the parameters shown below on
the Digimelt:
http://pages.towson.edu/jdiscord/WWW/331_LabAssignments/DigitalMelTempDire
ctions.pdf for a “How To” on using the Digimelt.
Fast Run Parameters for Digimelt apparatus:
START TEMP: 60ºC, RAMP RATE: 20ºC/min, STOP TEMP: 140ºC.
Slow Run Parameters for Digimelt apparatus:
START TEMP: (Begin 10ºC below where your Fast Run began), RAMP RATE:
2ºC/min, STOP TEMP: 140ºC.
Your Slow Run will give you information on the PURITY of your compound. Should you be
asked to discuss your results, do not include any discussion from the Fast Run as it is
just an inaccurate estimate.
B. Determine the identity of your unknown compound
Perform two runs, simultaneously, of mixture- or mixed-melting points. A porcelain tray
has been provided for you. In one of the three wells, you will mix a small portion of your
unknown compound with an equally small portion of benzamide. In another of the three
wells, you will mix a small portion of your unknown compound with an equally small portion
of 2-ethoxybenzamide. Be sure not to cross contaminate. Clean your spatula between
uses in the compounds’ vials. Use the end of your glass, stirring rod to mix the mixtures
together thoroughly – a mixed melting point analysis only works if you are truly taking
the melting point of a mixture! Use one end of the stirring rod for one mixture and
then flip it over and grind up the other mixture with the other end. Again – do not
contaminate one mixture with the other. Just as you did for your other runs, load one
capillary tube with the smallest amount you can still see of one mixture and then repeat
for a second capillary tube and the second mixture. You may wish to LABEL the tubes so
you don’t get them confused. Pack the capillary tubes correctly and run these analyses
simultaneously, using the same parameters for the Digimelt as you used for your slow
run.
2. Your unknown mixed with benzamide – one run
3. Your unknown mixed with 2-ethoxybenzamide – one run
Your instructor will demonstrate how to use the Digimelt melting point apparatus. On
the basis of your mixed melting point data, identify your unknown compound.