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Lab Report Marco Huerta

Partner: Sami Mrestani Chem 12B – Organic Chemistry




Objective: To prepare a local anesthetic, Benzocaine, by the direct estherification of p-aminobenzoic acid with ethanol.

Abstract: Over the years many new local anesthetics have been synthesized and tested. Though many have not come into general use, the search for the perfect local anesthetic is still under way. An interesting fact about these compounds is that they all share common feature (see general structure below). At one end of the molecule is an aromatic ring. At the other is a secondary or tertiary amine. These two ends are separated by a central chain of atoms usually one to four units long. The aromatic part is typically and ester of an aromatic acid which is important to the bodily detoxification of these compounds. In our experiment, we created a topical anesthetic, Benzocaine, by the direct estherification of p-aminobenzoic acid. This was accomplished by reaction with absolute ethanol catalyzed by concentrated sulfuric acid and later neutralized with a 10% sodium carbonate solution. Neutralization raised the pH of the overall reaction system to about 8 creating a precipitate of Benzocaine. Recrystallization of the crude product was performed to increase purity. Our experiment produced a 0.0705g crude sample of Benzocaine with a melting point of 86.55 – 88oC. Recrystallization produced a 0.0411g pure sample with a melting point of 89.5 – 90.9oC. Pure Benzocaine melts at 90oC. Benzocaine is an odorless, white, rhombohedric crystal with low water solubility. It is sensitive to light exposure and to temperatures above 30° C. As a drug, it has a low potency and low systemic toxicity. It is also a possible sulfonamide antagonist.

Reaction: H+

p-Aminobenzoic acid ethanol Benzocaine

Since the reaction is in equilibrium, excess water would cause the reaction to favor the reactants and thus not produce the desired product. Our intent is to create the ethyl esther functional group.

Results: Benzocaine Product
Crude Purified
Mass .0705g .0411g
M.P. 86.55-88oC 89.5-90.9oC
Melting Point % error
Crude = 90o-88o/90 x 100 = 2.2% error
Purified = 90.9o-90o/90 x 100 = 1.0% error

Theoretical yield

0.131g p-aminobenzoic acid x 1mol/137.14g = 9.55x10-4 mol p-aminobenzoic acid *
1.20 mL ethanol x .789g/mL x 1mol/46.07g = 2.055x10-2 mol ethanol *-limiting reagent

9.55x10-4 mol p-aminobenzoic acid x 1mol/1mol x 165.19g/1mol = .1578g Benzocaine

Percent Yield

.0411g*/.1578g Benzocaine x 100 = 26.05 %* Recrystallized Benzocaine

Percent Loss

.0705-.0411/.0705 x 100 = 41.7 % loss due to recrystallization*

*= reported values are much lower than actual yields because m.p. and IR analysis were done prior to weighing final product
Major IR peaks

NH2 peaks = ~ 3300-3400cm-1
Strong Aromatic C-H bonds = ~ 3000cm-1
Strong Carbonyl signal = ~ 1700cm-1

Questions 1. What is the structure of the precipitate that forms after the sulfuric acid has been added? *

2. When 10% sodium carbonate solution is added, a gas evolves. What is the gas? Give a balanced equation for this reaction. * Carbon dioxide * 2 NH3 + Ph-CO2Et + CO3-2 -- 2 H2N-Ph-CO2Et + H2O + CO2

3. Explain why benzocaine precipitates during the neutralization. * During neutralization the acidic proton is removed making the benzocaine insoluble in the solvent we used to dissolved the reactants. The solvent system resulting from neutralization is more polar since water was added to it. The product ends up precipitating out of solution.

I do not feel that there was an absolute need for recrystallization. We reported a 2.2% error from melting point analysis and an 81.89% match from IR comparison. There was not a noticeable gain in purity after recrystallization. Alternatively, we experienced a significant loss in yield during the recrystallization process. Our only gain was an increased melting point for our final product. After purification, we reported a 1.0% m.p. error but a 0.44% decrease in IR comparison. Overall, we obtained a close to pure product. Even a sample test performed on our hand was given to be effective. A numbing sensation was felt for about 30 sec. Once the product was produced a melting point was measured to identify the product and to test for purity. If the melting was not the literature melting point, which was obtained from the MSDS sheet for benzocaine, of 90°C then the product was not pure and would have to be recrystallized. The melting point for the product produced from the esterification was 88°C which confirmed the identity and that the product was close to pure and a recrystallization was not really necessary.
From the IR and the melting point it can be concluded that benzocaine was successfully synthesized with a percent yield of 26.05% and rather pure. The reason for the discrepancy in the melting point might be that there is still some reactant in the product or other impurities. Some possible reasons for the low percent yield might be that not all of the product was retrieved from the filter, some solvent was lost during the refluxing, and some of the reaction was lost when it foamed over the edge of the flask during the addition of the sodium bicarbonate.

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