National University of Singapore Essay
National University of Singapore
The appearance of the yellow crystals obtained tallies with the reference physical appearance of Dibenzalacetone1. The appearance of the white powder obtained also tallies with the reference physical appearance of o-Chlorobenzoic acid2. The yields of the compounds are relatively low at 34.2% and 29.5%. This may be due to some possible sources of error and limitations which will be discussed in the next section. Limitations and Sources of Error
Loss of compounds was an important factor that caused the yield of the purified compounds to be reduced. This may have occurred first during the extraction phase where extraction may not be complete. During the filtration and vacuum filtration processes, there was also a high level of difficulty in retrieving all the wanted products due to the products forming on the edges of the filter paper or sticking to the sides of the apparatus. Although the yield can be improved by increasing the number of extractions, the increase will not be significant enough (explained under exercise question 1) to overcome the loss of the compounds due to frequent transferring of the products.
The usage of the separatory funnel to separate the 2 organic and aqueous layers limited the accuracy of the experiment due to potential contamination. There was difficulty in determining the exact boundaries between the organic and aqueous layers accurately even under precautions such as conducting the separation at slow speed and at eye level.
Contamination of the purified products will cause the melting point determination to deviate from actual results and identifying the compounds incorrectly. The solubility test conducted to determine the recrystallization solvent might be another possible source of error. The amount of compound to be added into 2mL of the solvent was estimated rather than accurately weighed. This may have caused errors in determining the solubility of the compounds in the solvents and hence, led to incorrect use of the proper recrystallization technique for the compounds. Furthermore, cloudy mixtures obtained upon mixing the compound and the solvent might cause the solubility of the compounds to be inaccurately determined. Conclusion
A mixture of 2 organic compounds was separated using recrystallization and purified. Melting Point Determination was the method used to identify the 2 organic compounds and cross-checking the experimental results obtained with a list of possible organic compounds showed that the Neutral Compound was Dibenzalacetone and the Acidic Compound was o-Chlorobenzoic acid. Verifying the identified compounds via their appearances with references further validated the experimental results. References
1. Royal Society of Chemistry, Chemspider, Search and Share Chemistry http://www.chemspider.com/Chemical-Structure.86113.html, Retrieved 14 September 2013 2. Royal Society of Chemistry, Chemspider, Search and Share Chemistry http://www.chemspider.com/Chemical-Structure.8071.html, Retrieved 14 September 2013 Exercise Questions
a) D = [X]o / [X]w
When D = 8, [A]o = 8 [A]w
Since mass can be expressed as a product of volume and concentration as shown in Equation 1 below, by letting Vo be the volume of organic layer, Vw the
volume of aqueous layer with m being the total initial mass of A, Equation 2 can be obtained. ———- Equation 1
———- Equation 2
Using Equation 2 and subbing in known values, we can obtain
———- Equation 3
Solving, we get [A]w = 1.11 X 10-2 g/mL
b) Using Equation 2,
For 1st Extraction,
———- Equation 4
Solving, we get [A]w = 2.00 X 10-2 g/mL
For 2nd Extraction,
———- Equation 5
Solving, we get [A]w = 4.00 X 10-3 g/mL
c) Repeating the above method as shown in (b) for multiple extractions, we can obtain For 4th Extraction using 20 mL of dichloromethane,
[A]w = 1.23 x 10-3 g/mL
For 8th Extraction using 10 mL of dichloromethane,
[A]w = 3.91 X 10-4 g/mL
d) Based on calculation above, while the total amount of dichloromethane used in (a), (b) and (c) remained constant at 80 mL, the values of [A]w obtained decreases with increasing number of extractions done with smaller amounts of dichloromethane used during each extraction. Multi-extraction allows more of the solute to dissolve in the solvent. This leads to a higher amount of solute extracted.
e) To access if excessive extraction (8 times) is necessary, the yield difference of solute extracted between doing 4 and 8 times extraction must be considered.
Mass of solute not extracted by 4-times Extraction = 1.23 X 10-3 X 80
= 0.0984 g
Mass of solute not extracted by 8-times Extraction = 3.91 X 10-4 X 80
= 0.0313 g
Percentage yield of solute using 4-times Extraction =
Percentage yield of solute using 8-times Extraction =
The percentage yield increases by 0.8% which is not very significant as calculated above. This does not justify the cost and time needed to carry out excessive extraction since the yield difference is small. Question 2
c) A better solvent for recrystallization of B will give a higher yield of B crystals. Based on the calculation in (a) and (b), water gives a higher yield of crystals (8.54 g at 25°C and 9.81 g at 0°C ) as compared to ethanol (7.38 g at 25°C and 8.21 g at 0°C). Hence, water is a better solvent for the recrystallization of B.
d) The crystals should be washed with cold solvent (0°C). The solubility of compounds increases with increasing temperature. Washing the crystals with cold solvent ensures that the crystals do not dissolve back into the solvent. Washing the crystals with warm solvent will cause the yield of crystals to diminish.