The local anesthetic, benzocaine, was synthesized via the esterification of p-aminobenzoic acid with ethanol. The percent yield of crude product was determined to be 21% and the melting point was recorded at 86. 2°C ± 0. 2°C, with a 6. 3% error from 92°C, the literature melting point of pure benzocaine. The crude product was then recrystallized to improve the purity of benzocaine and 57. 4% was recovered. The new melting point range was measured at 89. 1°C ± 0. 3°C, which has a 3. 15% error.
The infrared spectrum of the recrystallized product was measured to further verify that the synthesized product was benzocaine. Introduction The discovery of benzocaine as a local anesthetic came out of necessity to find a replacement for other anesthetic compounds with high toxicity levels such as cocaine and similar synthetic drugs. Cocaine has been used for its pain relief and stimulant effects for centuries, specifically by the Amerindian population in the Peruvian Andes, in the form of chewing the coca leaf (Erythroxylon coca) (Pavia et al, 283).
The pure crystalline tropane alkaloid and active component of the coca leaves, cocaine, was isolated in 1862, and was used as an anesthetic in surgical and dental procedures in the 1880’s (Pavia et al, 284).
However, it was soon realized that the use of cocaine was not safe because the lethal dose was very close to the treatment dose and because of the toxic effects on the central nervous system, including addiction (McMaster University).
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As a result, scientists began to make substitute synthetic compounds similar in structure to cocaine, which consists of an aromatic residue, an intermediate chain, and a basic tertiary amino group, shown in figure 1. Figure 1: Structure of Cocaine (ChemWiki) All of the synthetic drugs that derived from the structure of cocaine had similar functional groups including an aromatic ring at one end, which is typically an ester of an aromatic acid, a basic tertiary amino group at the ther end (which increases the compound’s solubility in the injection solvent), and a central chain of atoms one to four units in length that connects the two ends (Pavia et al, 284).
Benzocaine does not possess the tertiary amino group and thus is not used for injection, but only as a topical anesthetic. To synthesize an aromatic ether involves the esterification of a benzoic acid in the presence of acid. The benzoic acid is not reactive enough to undergo nucleophilic addition so a strong acid is required to protonate the carbonyl oxygen, which gives it a positive charge, thus making the molecule more reactive.
The tetrahedral intermediate then loses a water molecule to yield the ester product for an overall substitution of a hydroxide group (-OH) by an alkyl group (-OR) (McMurry, 796).
The general mechanism for esterification is shown in figure 2. Figure 2: Mechanism of Esterification Reaction. 1. Protonation of carbonyl & N? attack 2. Formation of good leaving group 3. Loss of water and another deprotonation forms the ester (Organic Chemistry Help) In this experiment, Ethyl p-aminobenzoate, or benzocaine, was synthesized by the esterification reaction mechanism of p-aminobenzoic acid and ethanol in the presence of sulfuric acid.
The general reaction is shown in figure 3. Figure 3: Esterification of p-aminobenzoic Acid to Synthesize Benzocaine (ChemWiki) Experimental An analytic balance was used to measure 0. 1212g of p-aminobenzoic acid. The p-aminobenzoic was transferred to a 3mL conical vial along with 1. 2mL of absolute ethanol, and a magnetic spin vane was added to dissolve the solid. Next, 1. 0mL of concentrated sulfuric acid was added drop-wise to the vial while the solution was still being mixed by the spin vein, and a white precipitate formed in the vial.
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The mixture was then refluxed; a water cooled condenser was attached to the vial and the mixture was allowed to come to a gentle boil at 105°C with constant stirring by the spin vein. After 70 minutes of reflux, the vial mixture was allowed to cool to room temperature and the contents were transferred via Pasteur pipette into a beaker with 3mL of water. Next, 1mL plus an additional 10 drops of 10% sodium carbonate was added drop-wise to the beaker until the solution reached a pH of 8. The precipitate formed (crude benzocaine product) was collected via vacuum filtration and washed with water during the transfer into the Hirsch funnel.
The product was allowed to dry for one week after which the mass and melting point of the crystals were measured. The crude product was then recrystallized in a Craig tube over a warm water bath (60-70°C) by adding methanol drop-wise until the solid completely dissolved. Eight drops of hot waster were then added to reform the precipitate, followed by subsequent addition of 15 methanol drops to re-dissolve the precipitate. The solution was then chilled in an ice bath and “seeded” with a spatula to induce crystallization. The recrystallized product was then collected via gravity filtration using an air vacuum to accelerate the process.
An analytical balance was used to determine the mass of the purified product. The crystals were collected in two capillary tubes and a MelTemp device was used to measure the new melting point. A sample of crystals was run through the IR spectrometer to obtain the infrared spectra for the purified benzocaine product. The data and calculations sheets are attached to the report. Results and Discussion Ethyl p-aminobenzoate, or benzocaine, was synthesized via the esterification reaction mechanism of p-aminobenzoic acid and ethanol in the presence of sulfuric acid. The mass of the crude benzocaine product was determined to be 0. 31g for a 21. 2% yield. The yield was very low but can be accounted for by the loss of crude product in the second week. The crystals were mistakenly first transferred into a conical vial before they were transferred into a Craig tube. The crude product was very fine and adhered to the walls of the conical vial so that not all of it was recovered. In addition, there was minimal loss of product during the various mixture transfers from container to container throughout the lab procedure. Yet another possibility for such low yield remains that the reaction did not proceed to completion.
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Acetic Anhydride and p-Aminophenol were heated in a vial attached to an air condenser to synthesize crude acetaminophen, resulting in 0.097 grams (47.48% yield). The crude acetaminophen was then recrystallized in a solvent of water and methanol over heat resulting in 0.082 grams (39.61% yield) of pure acetaminophen. Melting points of both crude and pure acetaminophen were taken, and found to be ...
During the neutralization process, sodium carbonate was added until the pH was 8, however, it is likely that the pH was actually slightly less than that because a 100% color match of pH paper was not achieved, and addition of sodium carbonate may have been prematurely stopped. The melting point range for the crude benzocaine product was measured at 86. 2°C ± 0. 2°C. There is a 6. 3% error from the literature melting point of pure benzocaine which has been established at 92°C. The lower melting point implies that there were impurities present in the crude product.
The crude product was then recrystallized in order to increase the purity of benzocaine and in the process only 0. 0178g or 57. 4% of the product was recovered. The new melting point range was determined to have a range of 89. 1°C ± 0. 3°C, which only has a 3. 15% error based on the literature melting point of pure benzocaine. This lower percent error indicates that the recrystallization of crude benzocaine did have purifying effects on the product. Ultimately, the purifying effects outweigh the product loss during recrystallization because in this case, the product benzocaine, a topical anesthetic, is most effective in its pure state.
The goal for this synthesis is to obtain pure benzocaine, thus one recrystallization step or more are an important part of the experimental procedure. In order to further characterize the synthesized benzocaine, an infrared spectrum was taken from a sample of the recrystallized product. The structure of benzocaine, shown in figure 4, has an amino group, an aromatic component, and an ester component. Figure 4: Structure of Ethyl p-aminobenzoate, or benzocaine (ChemWiki) As can be seen in the IR spectrum in figure 5, the functional groups are all present, indicting a successful synthesis of benzocaine.
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The amino group peaks between 3200-35000cm-1, the strong aromatic C-H bonds show absorbance at approximately 3000cm-1, and the ester absorbs near 1700cm-1. A sample of the purified benzocaine was tested for its effectiveness as an anesthetic on the skin. A small amount was rubbed on the skin (top of hand).
Next, several tests were performed including: addition of a drop of hot water to the treated area and to a non-treated area, addition of a drop of cold water to both areas, and slight poking with a pen on both the treated and untreated areas.
It was determined that the benzocaine produced did have anesthetic properties because the sensations were less intense on the treated skin than on the untreated skin. The effects lasted for approximately five minutes. In conclusion, benzocaine was successfully synthesized, despite the low yield, in the esterification reaction of p-aminobenzoic acid and ethanol. The recrystallization procedure yielded more pure and more desirable benzocaine product, which can be seen by the higher melting range and further characterized by the IR spectrum.
