Post on 28-Jul-2020
Aspirin lab report
TA: Dr. Dehghan
3/17/2016
Erik Maradiaga
Introduction:
Aspirin is an important drug in that it has helped many individuals who are
experiencing pain. Aspirin, or acetylsalicylic acid, is derived from salicylic acid that
was used by the Native American. However, salicylic acid is problematic in that it
causes stomach irritation. As a result, acetylsalicylic acid was synthesized in order
to overcome this problem in the late 1800s.
The process of converting salicylic acid to acetylsalicylic undergoes a process
call esterification. In other words, esterification is the transformation of alcohols
into esters (Shan, 2014). Salicylic acid contains an alcohol and carboxylic acid
functional group and is converted to acetylsalicylic by converting the alcohol group
into an ester functional group. This reaction is an equilibrium process and is shown
by the double arrow in the chemical reaction. As a result, Le Chatelier principle can
be applied to this process. Le Chatelier principle states that when stress is added to
an equilibrium process, then the system will react in a way in that the effect of the
stress will be minimized (Last, 1997). For instance, if you have too much
concentration of the reactants, then the chemical reaction will shift and make more
products in order to keep it at equilibrium. Thus, the chemical reaction will favor
either the products or the reactants depending on what is being added to the
reaction. In this case, if there is more acetic anhydride being added to this reaction,
then the chemical reaction will shift toward the products and make more
acetylsalicylic acid. Likewise, if there is more acetic acid being added, then the
chemical reaction will shift and make more salicylic acid.
However, there are many other ways in order to catalyze an esterification
reaction. As an illustration, an aldehyde can be converted to an ester by using N-‐
heterocyclic carbine (NHC). This mechanism differ in that carbine undergoes a
nucleophilic addition to an aldehyde and forms an alcohol. Next, an oxidant oxidized
the alcohol to form an acyl carbine, which is known as Breslow intermediate. Last,
nucleophilic substitution by alcohol occurs in order to get an ester (Shan, 2014).
This is different from the aspirin reaction since an aldehyde is the reactant instead
of an alcohol.
The purpose of this experiment is to synthesize acetylsalicylic from salicylic
acid in order to see the process of esterification. Phosphoric acid, a strong acid, was
used in order to speed up the reaction since it is a catalyst, and this reaction is slow
without the catalyst. Likewise, the reaction is also heated up to 70-‐80 degrees
Celsius in order to speed up the reaction and help it reach equilibrium faster. Also,
acetic anhydride was used as the reagent rather than acetic acid since one of the
products of the reaction is acetic acid, so this reaction would not be as effective
because it is an equilibrium process. On the other hand, recrystallization and
purification was also done in order to compare the compounds in a TLC plate. Water
was added in order to recrystallize the product but also to eliminate some of the
excess acetic acid from the reaction. In order to get the crude product to
recrystallize, the product was added to hot water/ ethanol mixture. For instance,
since the crude product still has some acetic acid present in the solution, the hot
water/ ethanol mixture will react with the acetic acid in order to purify the aspirin
more. Overall, the three main products that was analyze was salicylic acid, crude
acetylsalicylic acid, and the recrystallized acetylsalicylic acid.
Results:
Figure 1:
Figure 2: Reaction mechanism for salicylic acid to acetylsalicylic acid
Figure 2 shows the mechanism of reaction for the conversion of salicylic acid to acetylsalicylic acid. It also shows how phosphoric acid is used in the first step as a catalyst and donates a proton to acetic anhydride. It also shows the intermediate in this reaction and how the electrons are moving throughout each step. Figure 3: TLC Plate
Figure 2 shows the results after TLC. In the bottom, it compares salicylic acid to the crude aspirin and recrystallized aspirin. The solvent traveled 4.1 cm from the spots. Also, salicylic acid traveled about 2.0 cm away from the spot, and the crude aspirin and recrystallized aspirin traveled about 2.4 cm and 2.5 cm respectively. Table 1: Calculated masses of synthesized products
Mass of Salicylic Acid 2.002 g
Mass of Aspirin and Filter Paper 3.513 g
Mass of Filter Paper .110 g
Mass of Aspirin Synthesized 1.693 g
Table 1 shows the masses that were weighed throughout the experiment. The calculated mass of the aspirin that was synthesized was about 3.403 grams while 2.002 grams of salicylic acid was used. Table 2: Calculated Rf Values Salicylic Acid .49
Crude Aspirin .59
Recrystallized Aspirin .61
Table 2 shows the calculated Rf values for each compound after TLC was performed. Salicylic acid has a Rf value of .49 and crude aspirin has an Rf value of .59. Similarly, Recrystallized aspirin has an Rf value of 61. Based on these results, it can be illustrated how crude aspirin and recrystallized aspirin have close Rf values. Equations: Rf= Distance Spot traveled/ Distance Solvent Traveled Recrystallized Aspirin: 2.5/4.1 Rf= .61 Limiting reagent: Mass= density*volume Density of acetic anhydride: 1.08 g/mL Volume of acetic anhydride= 5 mL Molar mass of salicylic acid= 132.12 g/mol Molar mass of acetic anhydride= 102.09 g/mol Mass of acetic anhydride= 1.08 g/mL * 5 mL= 5.4 g acetic anhydride 5.4 g acetic anhydride/ 102.09 g/mol = .05289 mol acetic anhydride 2.002 g salicylic acid/ 132.12 g/mol= .01449 mol salicylic acid Percent Yield: Mass of aspirin= 180 g/mol (Actual Yield/ Theoretical Yield) * 100% 1.693 g/ 2.611 g* 100%= 64% Discussion:
Figure 2 shows the mechanism in detail as to how salicylic acid is converted
to acetylsalicylic. First, the acetic anhydride is protonated and gets a proton from
the phosphoric acid. As a result, the oxygen that received the hydrogen has a +1
formal charge. Afterward, the alcohol on salicylic acid acts as a nucleophile and
attacks the carbonyl carbon on acetic anhydride because there is a partial positive
charge on the carbon due to the electronegativity of oxygen. Therefore, the bond
that is formed between these two compounds form an intermediate where there is a
+1 formal charge on the alcohol of the salicylic acid. However, an acetate anion
deprotonates the alcohol that is part of the salicylic acid and also protonates the
alcohol that is part of the acetic anhydride. That is, the electrons that is part of the
hydrogen bond breaks and forms a double bond with the carbon that is bonded to it.
Since carbon is not stable when it has 5 bonds, the oxygen that has a +1 formal
charge receives the electrons and leaves in order to form acetic acid. Overall, the end
result of this reactions forms acetylsalicylic acid and acetic acid.
Table 2 shows the Rf values after TLC was performed. It shows how the Rf
values for salicylic acid, crude aspirin, and recrystallized aspirin is .49, .59, and .61
respectively. This matches the experiment’s expectation because it shows how the
reaction was successful. The mobile phase is appropriate since one compound is
more polar than the other, thus will have an effect in the separation of compounds
from the stationary phase. That is, since acetylsalicylic is less polar than salicylic
acid because it does not contain the hydrogen bonding from the alcohol group, it
will traveled farther in the TLC plate. Salicylic acid is polar and will react with the
TLC plate since polar likes polar, causing it not to travel farther. However, figure 3
also show that the purification may have not been successful due to the presence of
salicylic acid. In other words, salicylic acid is still present because this reaction is
reversible and the temperature of the reaction cannot be controlled. As a result, the
TLC plate still shows some salicylic acid present after purification was done.
One experimental error that may have affected the results is that the
ethanol/water was boiling when the crude aspirin was added to the warm bath. As a
result, this may have affected the recrystallization process of the crude aspirin and
might have an effect in the percent yield. One way to improve this part of the
procedure is to indicate a temperature that the warm water should be at. Another
experimental error that occurred was that temperature could have been controlled
better for this experiment. In other words, if temperature was controlled, then there
should not have been salicylic acid present in the recrystallized aspirin TLC plate. As
a result, the TLC plate could be misleading and show that the recrystallization
process was successful when it really was not. Overall, temperature could have been
better monitor throughout the whole experiment since this is an equilibrium
process.
References: Last, Arthur. A Colorful Demonstration of Le Chatelier’s Principle. Journal of College Science Teaching. 1997. 27.2: 143-‐145. Shan, Tang. Direct Oxidative Esterification of Alcohols. Dalton Transactions. 2014. 43: 13460-‐13470.