CHEM 233 Lecture Notes - Lecture 10: Potassium Carbonate, Paracetamol

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Published on 7 Dec 2015
School
University of Illinois
Department
Chemistry
Course
CHEM 233
Professor
Williamson Ether Synthesis:
Preparation of Phenacetin from Acetaminophen
Methods and Background:
Figure 1: Mechanism of Diels Alder Reaction
The objective of this lab is to prepare the compound of Phenacetin from Acetaminophen through
Williamson Ether Synthesis in the lab. Phenacetin is essentially prepared while iodoethane is in
the presence of base. The hydrogen which is phenolic has enough acidity to become
deprotonated from the potassium carbonate and allow reaction to occur. The product is then
purified by recrystallizing and is distinguished with TLC, melting point analysis, and IR
spectroscopy. What is expected from this lab is to monitor the reaction with TLC after
performing the experiment, learn how to utilize the rotary evaporator to remove solvent from the
solid product, learn to purify via recrystallization, and eventually calculate percent yield of the
products received from performing synthesis reactions. The hazards that had to be recognized
was that 2-Butanone, ethyl acetate is quite flammable, iodoethane and methyl chloride are both
toxic whereas iodoethane is quite corrosive, and sodium hydroxide could cause burns. Because
of these, it was important to wear goggles and gloves at all times and to clean beakers or other
glassware appropriately.
The Williamson Ether Synthesis was commonly used for ethers since the approach could apply
to the basis of alcohols and alkyl halides. Two steps occur in which deprotonation of the alcohol
by a strong base forms an alkoxide ion. Then, SN2 occurs in which the alkoxide becomes the
nucleophile and the alkyl halide becomes the electrophile. Referring to the first step of the
mechanism, there is an acid-base reaction in which the alcohol serves as the acid in this case
even though it’s not very acidic. So, a strong base is important so that deprotonation occurs. If
the base isn’t strong enough, then the alcohol would not serve as a nucleophile for the second
step of the process. Even phenols would serve as a better acid in which a weak bases could be
used rather for the deprotonation occurrence. In regards to the second step of the mechanism,
SN2 reaction happens from which a particular alkyl halide is prominent to which the nucleophile
can attack the electrophilic carbon simultaneously with the carbon-halide bond being broken. So,
methyl and primary alkyl halide react faster than a secondary one. There’s also the factor in
which substitution and elimination compete in their occurrences. Since alkoxides serve as good
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bases, they can also be used in elimination reactions which makes the alkyl halides become
alkenes. The more sterically hindered the alkyl halide is though, elimination would be favored
over substitution reaction which is something to consider. So, in this way, the Williamson ether
synthesis has its limitations in the case of secondary alkyl halides along with tertiary alkyl
halides not even yielding ether products. Factors that allow SN2 to occur when Williamson ether
synthesis is conducted is that the reaction rate is dependent on carbon-oxygen bond formation
such that the nucleophile is strong, the carbon-hydrogen bond breaks such that a particular
leaving group allows for this, and the nucleophile will always be an alkoxide with the exception
of choosing a particular electrophile to carry out this process. Weaker bases can take the role as
leaving groups as well as alkyl iodides over alkyl bromides or chlorides. Also, the kind of solvent
used in the reaction can affect the reaction such that polar, aprotic solvents rather than protic
solvents can result in a more rapid reaction. The solvents usually used in the case of Williamson
reaction is either dimethyl sulfoxide, dimethyl formamide, acetonitirile, or acetone.
Experimental Procedure:
Williamson Ether Synthesis Procedure:
The reflux apparatus was put together in the same matter as done in the 8th lab for the
preparation of alkenes. When the reflux apparatus is put together, without the round bottom flask
yet, four Tylenol tablets weighing 325 mg in total were crushed with a mortar and pestle. 2.5 g of
potassium carbonate, 15mL of 2-butanone, and one boiling stone was added to the
acetaminophen all in the round-bottom flask. 1mL of iodoethane was added to the mixture in the
round-bottom flask; this addition took place in the fume hood. The round-bottom flask was
placed unto the reflux apparatus in which reflux was done for one hour. After an hour, the
reaction mixture was cooled below its boiling point and the solids were placed unto the vacuum
filter where they would be washed a couple ties with 5mL of ethyl acetate each. We made two
TLC plates (a back-up in case) which had three lanes: one lane of pure acetaminophen, one of
pure acetaminophen with the reaction mixture (called co-spot), and the third lane was just the
reaction mixture. The TLC plate was eluted with 4:1 ethyl acetate:methylene chloride that were
filled in beakers. After the lanes proceeded before the top of the line, the TLC plates were
removed, the plates were examined under UV light and circle spots were observed and measured
out, so that we measured the distance as well as to calculate the R factor. Afterwards, the filtrate
was placed into a separatory funnel and extracted with 20mL of 5% NaOH (aq) and then 20mL
of water respectively. The part that became cloudy was the organic layer which we transferred to
an empty Ernlemeyer flask and dried it using the sodium sulfate, adding it until the solid wasn’t
clumping anymore. The flask was swirled as we added the solid sodium sulfate and the
phenacetin solution became clear. Afterwards, the dried phenacetin solution transferred to a
50mL round-bottom flask and the solvent was removed using the rotary evaporator which the TA
operated. This allowed for the recrystallization of the solid to occur with the use of hot ethanol.
The littlest amount of hot ethanol was used to dissolve the solid while keeping the ethanol with
the solid consistently hot. When the phenacetin all dissolved, the solution was removed from
heat and cooled to room temperature before being placed in an ice bath. The final product was
dried and attached to a vacuum. The amount obtained was 0.49 g. Afterwards, melting point
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Document Summary

The objective of this lab is to prepare the compound of phenacetin from acetaminophen through. Phenacetin is essentially prepared while iodoethane is in the presence of base. The hydrogen which is phenolic has enough acidity to become deprotonated from the potassium carbonate and allow reaction to occur. The product is then purified by recrystallizing and is distinguished with tlc, melting point analysis, and ir spectroscopy. The hazards that had to be recognized was that 2-butanone, ethyl acetate is quite flammable, iodoethane and methyl chloride are both toxic whereas iodoethane is quite corrosive, and sodium hydroxide could cause burns. Because of these, it was important to wear goggles and gloves at all times and to clean beakers or other glassware appropriately. The williamson ether synthesis was commonly used for ethers since the approach could apply to the basis of alcohols and alkyl halides. Two steps occur in which deprotonation of the alcohol by a strong base forms an alkoxide ion.

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