Preparation and Purification of Acetylsalicylic Acid

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Preparation and purification of acetylsalicylic acid

Results and Data treatment
(A) Preparation of aspirin
i) Details about the reactants
Reaction of the acetylation of salicylic acid is following

From the balanced reaction above, it can be seen that the stoichiometry between salicylic acid and acetic anhydride is 1: 1. In this experiment, 21.7mmol of salicylic acid was used to react 6.0mL of acetic anhydride and salicylic acid was limiting reagent. The expected amount of salicylic acid used:

21.7/1000*138.12=2.99g=3.0g
The expected amount of acetic anhydride:
21.7/1000*102.09=2.215g=2.2g
Reactants| Formula/MWt| Min. Mole Ratio| Expected amount used| Expected vol./density used| Actual amount used| Actual vol./density used| Actual mole| Actual mole ratio| Salicylic acid| C9H8O4138.12gmol-1| 1| 3.0g| -| 2.97g| -| 21.5mmol| 1| Acetic anhydride| (CH3CO)2O102.09gmol-1| 1| 2.2g| 2.06mL/1.08gmL| 6.48g| 6.00mL/1.08gmL| 63.5mmol| 2.95| Phosphoric acid| H3PO4| catalyst| -| -| 5 drops| -| -| -|

ii) Yield of crude product and purified product
Preparation of aspirin| Purification of aspirin|
Mass of filter paper| 1.30g| Mass of filter paper| 1.17g| Mass of filter paper+ crude product| 6.13g| Mass of filter paper+ purified aspirin| 4.36g| Mass of filter paper+ residue| 1.33g| Mass of purified aspirin| 4.36-1.17=3.2g| Mass of crude product| 6.13-1.33=4.8g| | |

iii) Theoretical yield of aspirin
2.97/138.12*180.16=3.9g

iv) Percentage yield of the product
3.19/3.9*100%=82%

(B) Characterisation of aspirin
I. Melting Point
The melting point of the purified product is 125.4-128.3oC which is lower than the authentic values.

II. Infrared Spectrum
The IR spectrum of the purified product is below.

According the IR spectrum table cited from internet, few peaks standing for different bonding are found. Wavenumber cm-1| Appearance| Inference|
2871.95| broad| VOH carboxylic acid|
2698.57| medium| VC-H aldehyde |
1753.44| sharp| VC=O ester|
1689.84| strong| VC=O carboxylic acid|
1605.61| sharp| VC=C aromatic ring|

III. Nuclear Magnetic Resonance
The NMR full spectrum and aromatic spectrum of the purified product are below.

Full spectrum

Aromatic spectrum

Chemical shift /ppm| Multiplicity| Integral ratio| Hydrogen representing| 2.351| singlet| 3| HA|
8.117| doublet| 1| HB|
7.358| triplet| 1| HC|
7.624| triplet| 1| HD|
7.141| doublet| 1| HE|
7.260| singlet| -| HF|

Discussion
From the result, the percentage yield of the product is 82% which is very high but this kind of result is not normal in this experiment. Such a high percentage yield may be due to the presence of water in the sample during the crystallisation. It is possible that the water in the sample is not completely dry in the suction filtration. And also the humidity on the experiment operation day is higher than usual and this may cause the existence of water in the crystal.

The melting point measured for the purified product is between 125.4-128.3oC, however, it is a little lower than the literature value which should be between 128-137oC. It may contain some impurities which disturb the distribution of molecules in the pure aspirin and hence affect the interactions between the aspirin molecules.

Let see the structure of Aspirin, there are a carboxylic acid group, an aromatic ring and an ester group.
Structure of Aspirin
From the IR spectrum of the purified product obtained, it is found out that there is a broad peak at 2871.95 cm-1 and 1689.94 cm-1 of wavenumbers. They belong to the stretch of the –OH bond and –C=O bond in carboxylic acid group respectively. There is a sharp peak at 1605.61 cm-1 which stands for the stretch of –C=C bond in aromatic ring. Also, peak at 1753.44 cm-1 is found in the spectrum and it comes from the stretch of –C=O bond of ester. The...
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