B1: Esterfication: The synthesis of Aspirin
Reaction equation & mechanism
97.994 g/mol
+ +
138.121 g/mol 102.09 g/mol
180.158 g/mol 60.052 g/mol
The esterification
of aspirin is a SN2 reaction, see appendix fig. 1 for the whole mechanism
Yield
The theoretical yield with 36 mmol of salicylic acid and an excess of acetic anhydride would be 6.49
grams of aspirin (see appendix calculation 1).
5.28
After the first crystallization the amount was 5.28 grams ∙ 100 %=81.4 %
6.49
2.23
After recrystallization the amount was 2.23 grams ∙ 100 %=34,4 %
6.49
Procedure
First off all, 4.96 grams of salicylic acid, 10 mL of acetic anhydride and 2 mL of phosphoric acid were
used. The temperature was measured to show the completion of the reaction. Having left the mixture
for approximately 10 minutes, the reaction was heated on a stove for 5 minutes on 50 °C to complete
the reaction. Afterwards, ice cold water was added to start the formation of crystals. When no more
crystal appeared the mixture was filtered through a glass filter. The residue was collected and had been
spread out on a glass plate to enhance drying. The crystals were recrystalized with use of ethanol as
the solvent. After complete dissolving, the mixture was transferred into a glass filter, it should have
been cooled down first, and the crystals were once again dried on a glass plate. This final product is
aspirin with little to no contamination.
Assignment
The melting point of the crystalized aspirin had a longer melting trajectory than the
recrystalized one (see appendix table 1). The recrystalized one was also closer to the actual
melting point from aspirin, 135 °C. This means that the recrystalized sample had fewer
contaminations.
The infrared (IR) spectra of the recrystalized sample (appendix figure 2) showed an extra peak at
around 1700 cm-1 which the salicylic acid (appendix figure 3) didn’t have. This peak is a C=O stretch
of an ester which is characteristic for aspirin and not for salicylic acid. The IR spectra also showed a
97% similarity to commercial aspirin, on the computer in the measuring room.
, APPENDIX
Figure 1. Mechanism of the esterfication of salicylic acid to aspirin.
Calculation 1
36 mmol of salicylic acid was used with an excess of acetic anhydride, the mol ratio between
salicylic acid and aspirin is 1:1. So at the end of the reaction there will be 36 mmol of aspirin
as well. The molar mass of aspirin is 180.158 g/mol. This means that 36 mmol of aspirin
weighs 0.036 ∙ 180.158=6.49 g
Table 1. Melting point determination
Crystalized aspirin Recrystalized aspirin
Started melting 103 133
(°C)
Completely melted 133 136
(°C)
Reaction equation & mechanism
97.994 g/mol
+ +
138.121 g/mol 102.09 g/mol
180.158 g/mol 60.052 g/mol
The esterification
of aspirin is a SN2 reaction, see appendix fig. 1 for the whole mechanism
Yield
The theoretical yield with 36 mmol of salicylic acid and an excess of acetic anhydride would be 6.49
grams of aspirin (see appendix calculation 1).
5.28
After the first crystallization the amount was 5.28 grams ∙ 100 %=81.4 %
6.49
2.23
After recrystallization the amount was 2.23 grams ∙ 100 %=34,4 %
6.49
Procedure
First off all, 4.96 grams of salicylic acid, 10 mL of acetic anhydride and 2 mL of phosphoric acid were
used. The temperature was measured to show the completion of the reaction. Having left the mixture
for approximately 10 minutes, the reaction was heated on a stove for 5 minutes on 50 °C to complete
the reaction. Afterwards, ice cold water was added to start the formation of crystals. When no more
crystal appeared the mixture was filtered through a glass filter. The residue was collected and had been
spread out on a glass plate to enhance drying. The crystals were recrystalized with use of ethanol as
the solvent. After complete dissolving, the mixture was transferred into a glass filter, it should have
been cooled down first, and the crystals were once again dried on a glass plate. This final product is
aspirin with little to no contamination.
Assignment
The melting point of the crystalized aspirin had a longer melting trajectory than the
recrystalized one (see appendix table 1). The recrystalized one was also closer to the actual
melting point from aspirin, 135 °C. This means that the recrystalized sample had fewer
contaminations.
The infrared (IR) spectra of the recrystalized sample (appendix figure 2) showed an extra peak at
around 1700 cm-1 which the salicylic acid (appendix figure 3) didn’t have. This peak is a C=O stretch
of an ester which is characteristic for aspirin and not for salicylic acid. The IR spectra also showed a
97% similarity to commercial aspirin, on the computer in the measuring room.
, APPENDIX
Figure 1. Mechanism of the esterfication of salicylic acid to aspirin.
Calculation 1
36 mmol of salicylic acid was used with an excess of acetic anhydride, the mol ratio between
salicylic acid and aspirin is 1:1. So at the end of the reaction there will be 36 mmol of aspirin
as well. The molar mass of aspirin is 180.158 g/mol. This means that 36 mmol of aspirin
weighs 0.036 ∙ 180.158=6.49 g
Table 1. Melting point determination
Crystalized aspirin Recrystalized aspirin
Started melting 103 133
(°C)
Completely melted 133 136
(°C)
