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To familiarize themselves with the preparation of some simple organic compound and to purify the compound by Recrystallisation. This experiment also enables students to conduct the synthesis of aspirin, reinforce the skills of Recrystallisation and also the technique of melting point determination. By the end of the experiment, one should have acquired the skills to use Recrystallisation to purify solids.
This experiment is mainly broken into 3 main procedures. The Preparation of Aspirin, the Recrystallisation of Aspirin and the Melting Point Determination of Aspirin.
At the end of the experiment, the result gotten should be a result of 40 - 60 % yield and a melting point that is around the theoretical melting point of aspirin, which is 135Â°C - 136Â°C.
Organic Synthesis is a branch of chemical synthesis which concentrates on the construction of organic compounds through organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has developed into one of the most important branches of organic chemistry. In this experiment, the product that was synthesized was acetylsalicylic acid, also known as aspirin.
Aspirin can be made by reacting salicylic acid with acetic anhydride.
Figure 1 Reaction of Acetic salicylic acid with acetic anhydride
Salicylic acid reacts with acetic anhydride when an acid is present as a catalyst (In this experiment, sulfuric acid, H2sO4 was used as the catalyst.). Once the aspirin is synthesized, it can be collected by suction filtration while the more water soluble unreacted starting materials are washed away in the aqueous solvent. At this point the product is already isolated. To purify the product further, there is a need to recrystallise the crude aspirin.
Brief History of Aspirin
Aspirin is a very important medicine and has been around since 1900 when Felix Hoffman and Friedrich Bayer and Company invented it. Born on January 21, of 1868 in Germany Felix Hoffman studied chemistry and pharmacy at the University of Munich and obtained his Doctoral degree there in 1894.
(In view of the first and second step of the general procedure]
Preparation of Aspirin
The synthesis of Aspirin, during the first step of the procedure, is classified as an esterification procedure.
Simply put, esterification is a reaction between a carboxylic acid and an alcohol which is catalyzed by a mineral acid which is, in this experiment, sulfuric acid. In general, the mechanism regarding esterification follows as what is shown through the following image.
Figure 2 Mechanism of Esterification
(Image gotten from a report regarding the synthesis of Aspirin)
As from the report found at < http://www.drcarman.info/kem220lb/01lab220.pdf>, the proton from the acid attacks the carboxyl oxygen which in turn "pushes" the two electrons in one of the bonds "down", i.e., it delocalizes the electrons and "spreads them out" between the two oxygen atoms. The delocalized electrons then, in the presence of the alcohol, rearrange in such a manner as to create a temporary bond between the two reactants. Note that the alcoholic oxygen atom has a temporary positive charge to it in the intermediate in step #2.
The proton (Step #3) from the alcohol attacks the oxygen in the original -OH portion of the acid forming a positively charged oxygen atom and a sort of water molecule still attached to the intermediate. The electrons "holding" the "water" to the intermediate "flip down", releasing this water, leaving the delocalized intermediate in the end of Step 3. In Step #4, the proton added in Step 1 leaves and the electrons left behind flip down "closing" the double bond on the oxygen atom and leaving.
The starting reactants for this experiment are salicylic acid and acetic anhydride, which are as follow.
Figure 3 Salicylic Acid and Acetic Anhydride
(Image gotten from a report regarding the synthesis of Aspirin)
Recrystallisation of Aspirin
After the aspirin is prepared, the crude product is relatively impure and needs to be purified by Recrystallisation. The solvent that is used for this is ethanol and water.
The report from < http://www.chem.utoronto.ca/coursenotes/CHM249/Recrystallization.pdf>, in general, Recrystallisation is a procedure for purifying compounds. The process of recrystallisation involves dissolution of the solid in an appropriate solvent at an elevated temperature and the subsequent re-formation of the crystals upon cooling, so that any impurities remain in the solution.
Almost all solids are more soluble in a hot than in a cold solvent, and solution crystallization takes advantage of this fact. Thus, if a solid is first dissolved in an amount of hot solvent insufficient to dissolve it when cold, crystals should form when the hot solution is allowed to cool. The extent of precipitation of the solid depends on the difference in its solubility in the particular solvent at temperatures between the extremes used. The upper extreme is determined by the boiling point of the solvent, whereas the lower limit is usually dictated by experimental convenience. For example, an ice-water bath is often used to cool the solution to 0Â°C.The solid should be recovered with greater efficiency at these temperatures, provided the solvent itself does not freeze.
General Outline of Recrystallisation
Rapid and convenient way of purifying a solid organic compound
The material to be purified is dissolved in the hot appropriate solvent
As the solvent cools, the solution become saturated with respect to the substance, which then crystallize
Impurities remain in the solution
The selection of the solvent for the recrystallisation process is perhaps the most critical part of it since the correct solvent must be selected to form a product of high purity and in good recovery or yield.
The website <http://www.erowid.org/archive/rhodium/chemistry/equipment/recrystallization.html> provides the following details on the factors of the solvent :
The compound should be very soluble at the boiling point of the solvent and only sparingly soluble in the solvent at room temperature. This difference in solubility at hot versus cold temperatures is essential for the recrystallisation process. If the compound is insoluble in the chosen solvent at high temperatures, then it will not dissolve. If the compound is very soluble in the solvent at room temperature, then getting the compound to crystallize in pure form from solution is difficult. For example, water is an excellent solvent for the recrystallisation of benzoic acid. At 10Â°C only 2.1 g of benzoic acid dissolves in 1 liter of water, while at 95 Â°C the solubility is 68 g/L.
The unwanted impurities should be either very soluble in the solvent at room temperature or insoluble in the hot solvent. This way, after the impure solid is dissolved in the hot solvent, any undissolved impurities can be removed by filtration. After the solution cools and the desired compound crystallizes out, any remaining soluble impurities will remain dissolved in the solvent.
The solvent should not react with the compound being purified. The desired compound may be lost during recrystallisation if the solvent reacts with the compound.
The solvent should be volatile enough to be easily removed from the solvent after the compound has crystallized. This allows for easy and rapid drying of the solid compound after it has been isolated from the solution.
Ethanol and Water
As stated earlier, the solvent used in this specific experiment was Ethanol and Water. The reason as to why Ethanol and Water was used was in short, that the solvent that was used must be of sufficient density to dissolve the acetic acid and Ethanol and Water fits the criteria.
Firstly we have to prepare aspirin. 2.4g of salicylic acid is weighed in a dry 100ml conical flask. The actual weight was recorded.
6ml of acetic anhydride was added to the salicylic acid in the flask. This whole step must be done in the fume hood.
3-4 drops of concentrated sulfuric acids added to the mixture and it is swirled to mix the solution evenly.
The mixture is heated in water bath for 10 to 15 minutes in order to complete the reaction.
The flask is removed from the water bath. 1ml of distilled water was added from a dropper to decompose excess acetic anhydride when the mixture is still hot.
An additional 40ml of cold water is added. The mixture was stirred and rubbed with a stirring rod to induce crystallization.
The crude product was collected by suction filtration and it was washed with a little cold water.
Next, we have to prepare recrystallisation of aspirin.
The crude product aspirin prepared is relatively impure and may be purified by recrystallisation. A
Solvent suitable for this recrystallisation process is a mixture of ethanol and water.
The crude product was dissolved in approximately 5ml of ethanol in a 100ml conical flask. The solution will be warmed if necessary.
The solution was added with approximately 30ml of hot distilled water.
The mixture is warmed until the solid dissolves completely in the solution.
The solution is cooled.
A clean and dry glass is weighed together with a filter paper. The weight was recorded.
The recrystallised product was obtained by suction filtration using your weighed filter paper.
The crystals were transferred onto the weighed watch glass. The crystals filter paper and watch glass was to b dried in the oven for 15-20 minutes.
The crystals were placed in a desiccator for 5-10 minutes.
The dried crystals were weighed together with the filter paper and watch glass. The weight was recorded. The weight of dried aspirin was weighed.
The expected yield of aspirin was calculated from the amount of salicylic acid that was used. The percentage yield is calculated.
The melting point of aspirin is determined. The aspirin waste was discarded in the waste in the 'Aspirin Waste' container.
Results and Calculation
Mass of salicylic acid (a) = 2.40 g
Mass of filter paper & watch glass (b) = 34.29 g
Mass of dried, recrystallised aspirin, filter paper & watch glass Â© = 35.86g
Mass of dried, recrystallised aspirin (d) = (c) - (b)
= 35.86g - 34.29g
Number of moles of salicylic acid used (e) = 2.40/138 = 0.0174 mol
(Mol. Wt of salicylic acid = 138)
Expected number of moles of aspirin (f) = 0.0174 mol
Expected mass of aspirin (g) = 3.132g
(Mol. Wt = 180)
Percent yield = (d)/ (g) x 100%
= 1.57/3.132 x 100%
= 50.13% yield
Temperature Range 135.4 - 136.8 Â°C
White and needle-like shaped.
The main point of the experiment was to recrystallise aspirin that was prepared so as to purify the compound. The aspirin was prepared through the synthesis of salicylic acid and acetic anhydride in the presence of a catalyst before crystallizing it.
The balanced chemical equation for the above reaction is
C4H6O3 + C4H6O3 â†’ C9H8O4 + C2H4O2
which can also be written as
C6H4OHCOOH + (CH3CO)2O â†’ C9H8O4 + CH3COOH
which is salicylic acid + acetic anhydride > Aspirin + acetic acid
The results gotten from the experiment was a yield of 50.13%. Based on the amount of salicylic acid that was used in the first place, which was 2.40g, the amount of aspirin I should have expected to get should be 3.132g. But only 1.57g of aspirin was gotten by the end of the experiment. This could have some effects towards the purity of the final product as it was not as expected. But by determining the melting point of the dried, recrystallised acid, which was 135.4Â°C - 136.8Â°C, it is shown that it is very close to the expected melting temperature of pure aspirin, which lies between 135Â°C - 136Â°C. This tells me that the sample that I had gotten at the end of the experiment was quite pure and thus, close to the expected result instead.
While this may seem to be a 'mistake' in the calculations, but the percent yield plays no part in the calculation of the purity of the product. The yield is just the amount of product you obtain from a reaction. This meant that from the experiment, I only achieved 50.13% of what had been expected from it and that the yield only plays a huge role when you're trying to, let's say, making paint, to achieve a lower cost rate.
Also, since it was determined that the aspirin gotten at the end was quite pure, we can say that from the 1.57g of the aspirin, it is very likely that majority of the product is pure. But further analysis would have to be done to prove the exact purity of it. There could be problems that might have affected the product of course, such as, when dissolving the initial amount of salicylic acid in the solution of acetic anhydride and concentrated sulfuric acid, it did not completely dissolve into the solution, even when it was being heated. This could have a slight impact on the results of the overall yield of aspirin because it was possible that not all of the salicylic acid was synthesized.
The crude aspirin was recrystallised in the end as to improve the purity of the substance. While well formed crystals are expected to be pure because each molecule or ion must fit perfectly into the lattice as it leaves the solution. Impurities would normally not fit as well in the lattice, and thus remain in solution preferentially. Hence, molecular recognition is the principle of purification in crystallization. However, there are instances when impurities incorporate into the lattice, hence, decreasing the level of purity of the final crystal product. Also, in some cases, the solvent may incorporate into the lattice forming a solvate. In addition, the solvent may be 'trapped' (in liquid state) within the crystal formed, and this phenomenon is known as inclusion. (This part was extracted from the Wikipedia page involving crystallization)
Also, during the procedures, one should remember to avoid skin contact while handling acetic anhydride and sulfuric acid. This is as acetic anhydride is a dangerous chemical. The vapors will irritate the eyes and
nose. The liquid can cause burns on the skin. Use the chemicals in your hood area and if you spill
any, wash with large amounts of water. Sulfuric acid is another dangerous chemical. It likes a diet
of cotton clothing but will eat other materials, including skin. If you spill any (even a drop) clean it
up immediately using lots of water.
The experiment very much went as expected as the aspirin gotten at the end of the product was considered pure as its temperate range falls almost within the intended target. But not all 1.57g of the product is pure as the temperature falls slightly off the mark. The percent yield could also have been better but as 50.13% was within the target set, the experiment is successful.