Download Christopher Smith

Christopher Smith
Chem 216 Sec 111
Discussion: Ex. #5
In Experiment Five, ethyl p-aminobenzoate (Benzocaine) was prepared by esterification
of p-aminobenzoic acid in ethanol using sulfuric acid as a catalyst for the reaction.
During the initial preparation of ethyl p-aminobenzoate, the sulfuric acid was added drop
wise to a solution of p-aminobenzoic acid in ethanol. Upon slowly adding sulfuric acid, a large
amount of white precipitate was formed. The pKa of p-aminobenzoic acid is 4.88 while the pKa
of sulfuric acid is -9. Therefore, the p-aminobenzoic acid, which exists is its conjugate base
form, was easily protonated to form the white precipitate which was the hydrogen sulfate salt of
ethyl p-aminobenzoic acid.
After precipitating the hydrogen sulfate salt, the solid was placed into a large excess of
ethanol and refluxed. Refluxing was used to heat the reaction but also to prevent the loss of
solution to evaporation. The ethanol solution containing the precipitate was heated to a boil on a
heater and the vapors were condensed back to liquid form in a water-cooled column above the
solution. The liquid then trickles back down into the solution and was boiled again. CaCl2 was
used at the top of the water jacket as a dehydrating agent. By absorbing water from the product
of the transesterification reaction, the CaCl2 drove the reaction forward due to Le Chatelier’s
principle. By decreasing the amount of water, the ethanol and the hydrogen sulfate salt of paminobenzoic acid were forced to react further and produce more product. But in addition to
forming more water, more of the hydrogen sulfate salt of ethyl p-aminobenzoate was formed as
well.
Thus, during the reflux, the reaction was accelerated due to the heat and Le Chatelier’s
principle, and transesterification between ethanol and the hydrogen sulfate salt of ethyl paminobenzoic acid progressed over the span of 75 minutes. After the reflux was finished, the
product, the hydrogen sulfate salt of ethyl p-aminobenzoate, was ready to be reacted with 10%
sodium carbonate solution. Sodium carbonate is a base which reacted with the hydrogen sulfate
anion attached to the ethyl p-aminobenzoate as in addition to the amino group which was still in
its NH3+ form. The base was added drop wise until the pH was above 8. Approximately 140
drops of sodium carbonate solution was needed to completely deprotonate the hydrogen sulfate
anion as well as the amino group.
The precipitate formed (ethyl p-aminobenzoate) after the deprotonation reaction had a
yield of 240mg. Having begun with 240mg of ethyl p-aminobenzoic acid, the percent yield was
83%. The melting point of the precipitate was 740C. The true melting point of benzocaine is
920C, so there were impurities in the solid and recrystallization was necessary. An IR spectrum
of the solid before recrystallization does show the important bonds in benzocaine though. Two
medium intensity peaks at 3423.0 and 3343.9 indicate the presence of NH bonds. A short peak
at 3222.9 represents the eleven C-H bonds. A C=O bond is present due to a strong absorption
peak at 1684.6. The normal range for a C=O bond in an ester is 1750-1735, but due to the
resonance effects of the nitrogen atom in benzocaine, the C=O bond has single bonded properties
and thus a lower wavelength due to less energy needed to stretch the bond.
There was insufficient time at the end of the experiment to complete recrystallization of
the impure benzocaine solid. Therefore, no melting point test could be conducted to verify the
actual formation of benzocaine. But the IR spectrum taken before recrystallization clearly
indicates that benzocaine was formed during the transesterification reaction in addition to a few
small impurities.