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Q: Is the composition
of Spartacetin given?
A: No - we are using an unknown proportion of starch, aspirin and unknown
analgesic component. In addition we have 0.25 ml of tetraglyme (tetraethyleneglycol
dimethyl ether) as a soluble impurity.
Q: Why can't I find
Spartacetin in the Merck Index?
A: Because it is a mythical analgesic mixture, not an actual compound.
Phenacetin and acetanilide, on the other hand, are actual compounds and
are the candidates for the unknown substance in this experiment.
Q: Do we need to
have Amounts and Ratios of Moles in the table for the prelab.
A: No. This is an investigative experiment, so it's just a table of physical
properties.
Q: Will we write
up the two parts of the experiment separately?
A: No. It will be combined as one report.
Q: Will we turn in
the starch?
A: No. After it has dried overnight, weigh it and the discard it. We will
be turning in the purified unknown and aspirin with the report.
Q: During extraction
with aqueous sodium hydroxide, is it necessary to remove the dichloromethane
layer form the funnel between the first and second extraction?
A: Yes, because we are extracting from the lower layer into
the top layer. The lower layer (dichloromethane in this case) is drained,
then the first sodium hydroxide portion is poured out the top of the funnel
into a receiving flask. The dichloromethane is returned to the funnel
and the process is repeated with a fresh portion of sodium hydroxide (to
be combined with the first sodium hydroxide extract).
Q: After extraction,
which layer do I work up first?
A: The aqueous layer, since aspirin will hydrolyze in aqueous base or
acid. Aspirin must be isolated on the same day as the extraction. The
work on evaporating the dichloromethane from the organic layer if you
have time, or do the evaporation next period. (To store the dichloromethane
solution in a sidearm flask, use a pipet bulb to seal the sidearm and
a cork on the neck of the flask (not a rubber stopper!).
Q: How can I scrape
all of the crude unknown out of the sidearm flask?
A: You can't. It will help if you check out a (smaller) 125 mL sidearm
flask for the evaporation. If you were to measure it, you would find the
weight of the residue left in the flask is smaller than it appears.
Q: Does the crude
unknown have to be dried in a desiccator before the recrystallization?
A: No, if you have time to proceed with the recrystallization on the day
you evaporate, go ahead. (Be aware that part of the weight of crude unknown
you measure is water.)
Q: How do I know
how much water to use in the recrystallization?
A: From the weight of the crude unknown, calculate the amounts of hot
water needed to dissolve it if the unknown is acetanilide or if it is
phenacetin. Since we will use the steam bath for heat, our temperature
will be less than 100 °C and these estimated will be a bit low. Begin
by assuming your unknown is the more soluble compound. Add the water in
portions, and swirl on the steam cone for about a minute between portions,
so you can see how much solid is dissolving. This will allow you to prepare
a nearly saturated hot solution, while at the same time giving a clue
as to the identity of the unknown (by its solubility).
Remember that you want to avoid adding too much water. We'll wash with
just a small portion of boiling water (have some distilled water boiling
on a heating mantle in a 100 mL round-bottom flask for this purpose -
use boiling chips).
Q: Does the water
for recrystallization have to be preheated?
A: No. Add portions of room temperature water, it will heat quickly enough
on the steam bath.
Q: I added way too
much water, and no crystals come out upon cooling. What can I do?
A: Evaporating down the water is not practical - it would take too long.
Instead, try adding sodium chloride to "salt out" the unknown
compound. This increases the ionic strength of water and helps force the
compound out of solution. Add about 20g/100 mL and stir well to dissolve
all the salt. Then scratch the inside of the flask with a glass rod and
chill in an ice bath. Good luck!
Q: How many melting
points are we doing?
A: Five. One of the crude unknown, two of the purified unknown and one
mixed melting point each with phenacetin and with acetanilide (We are
setting aside three samples of the crude unknown so we will have spare
samples).
Q: Can I use the
same sample to measure the melting point twice?
A: Use a fresh sample each time.
Q: Does the purified
unknown have to be dried in a desiccator overnight before the mp is taken?
A: Before the mixed-melting points, yes. But air-drying on the Buchner
funnel followed by a half hour in the desiccator is enough drying for
the samples of pure unknown. This is because both of our possible compounds
melt well above 100 °C and the sample will fully dry as we heat it
slowly.
©2001,2002 Daniel A. Straus
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