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 presence of these peaks shows the existence of carboxylic acid, aromatic ring and ester group in the crystal, and hence it can proof the crystal being tested is aspirin. Besides, a peak of 2698. 57 cm-1 is shown in the spectrum, it indicates that the crystal also has aldehyde which is not expected to be exist.
It is possible there is impurity that has aldehyde group in the extracted crystal. Let see the detailed structure of aspirin, the hydrogen in aspirin is labelled as follows Singlet peak of 2. 351ppm of chemical shift accounts for the existence of methyl group at ester group since the protons in this methyl group cannot see any other protons within three bonds. Also, its integral ratio is 3 which mean it has three indistinguishable protons comparing to others proton groups. It is the similar in the proton in carboxylic acid, singlet but with a 1 of integral ratio.
The protons in aromatic ring should be around 7 ppm but they have different splitting due to their different environment. For HC and HD, they both have triplet splitting because they both have two neighbouring protons. HD has a larger chemical shift than HC. For HB and HF, they only have one neighbouring proton, their respective peak can only be split into doublet. HB has a larger chemical shift than HF as HB is much near the electronegative oxygen. In this experiment, 5 drops of phosphoric acid was added into the mixture of reactant because it acts as a catalyst for the acetylation of salicylic acid.
From the mechanism of the reaction of salicylic acid posted above, it can be seen that the carbon of the carboxylic acid group in acetic anhydride is protonated by hydrogen ion first and it comes from phosphoric acid. At the end of the mechanism, a hydrogen ion is released back so only a little amount of acid is used. Panadol is also a pain killer as well as aspirin. The key component of panadol which relieve pains is paracetamol. It can be prepared industrially by three main organic steps. First, phenol, the reactant of panadol, is nitrified by sodium nitrate, and p-nitrophenol and o-nitrophenol are derived.
Second, the wanted p-nitrophenol produced from the first step is reduced by NaBH4 to p-aminophenol. At last, p-aminophenol is acetylated with acetic anhydride to paracetamol. Overdose of Panadol would cause liver failure and affect its functions. The paracetamol is metabolised in liver. There are three pathways for paracetamol to be hydrolysed into non-toxic substance. One of the pathways is N-hydroxylation and rearrangement, then is GSH conjugation. The intermediate, NAPQI, in this pathway is toxic substance which can have toxic reactions with nucleic acids and proteins.
It may affect our body bio-reactions and even DNA molecule. Conclusion The percentage yield is 82% which is not expected, it may contain some water in the crystal. After analysing the melting point of the product which is lower than the authentic value, it can be ensured that the product obtained impurities and it may be water. High humidity and not enough drying process may cause error here. Also, the main functional groups in aspirin are found in IR spectrum and the structure of the aspirin can be deduced from the HNMR spectrum.