This lab uses a variety of tools to create nitrophenol (electrophilic aromatic substitution). Here, phenol is nitrated using a moderate process that uses diluted nitric acid. The method created includes choosing the reaction conditions to match the reactivity of the starting components; using TLC, recrystallization, and melting point for the product's characterisation. The reaction produces a combination of p-nitrophenol and o-nitrophenol as well as minute amounts of 2,4,6-trinitrophenol and 2,4-dinitrophenol. Subsequently, the mixture of the reaction is submitted to the separation of column chromatography (purification) to help in obtaining the p-nitrophenol and the o-nitrophenol. At this time, the TLC is used to monitor the process of purification. Furthermore, the obtained nitrophenols are analyzed by the use of NMR and IR spectroscopy.
Introduction
By definition, an electrophilic aromatic substitution refers to an organic reaction in which an electrophile replaces an atom (usually hydrogen) that is attached to the aromatic system (Coombes 2006). Some of the most essential substitutions of electrophilic aromatic include the alkylating and the acylation Friedel-Crafts reaction, the aromatic halogenation, aromatic nitrogen, and the aromatic sulfonation (Lederer 1982). Concerning the mechanism of the reaction, the electrophile A is attacked by the aromatic ring. As a result, a positively charged cyclohexadienyl is formed in the process (Mahdavi and Heidari 2017). Several examples of such carbocation have already been characterized (Morlock 2015). However, under the normal conditions of operations, this carbocation suffer the loss of the proton from the group of methylene (Zhang and Wang 2012).
Ordinarily, the substituents already attached to the ring of benzene affects both the electrophilic aromatic substitution and the regioselectivity (Ramsteiner 1987). Regarding the regioselectivity, the substitution is promoted by some groups at the para or ortho positions. Remember, the substitution is favored by other groups at the meta position. Such groups are referred to as the meta directing or the ortho-para directing respectively (Roth 1993). Moreover, some of the groups play a significant role in increasing the reaction rate (activating). Conversely, other groups aim at decreasing the rate of reaction (deactivating). The use of the resonance structure can also help in explaining the patterns of regioselectivity. Also, both the inductive effect and the resonances can be useful in explaining the influence on kinetic (Sahiner and Ozay 2012).
Nitration
Nitration is one of the most common methods involve in bonding the aromatic system to nitrogen (Vasiliades and Sahawneh 1982). In this process, a wide variety of the conditions of nitration are available for the nitrating compounds of both relatively low and very high reactivity towards electrophiles. In total, there are about four common methods of bonding the aromatic system to the nitrogen. The first method involves a mixture of the concentrated sulfuric and the nitric acids (Office 2015). Also, there is the method of fuming the nitric acid in the solution that contains acetyl nitrite (acetic anhydride) (Kolla 1994). Other methods include mixing the concentrated nitric acid by the use of various quantities of water. The concentrated acid can also be mixed with the glacial acetic acid (Knox 1978).
Experimental Procedure
All the equipment/ apparatuses were assembled in the right place. Afterward, phenol (4.7 grams, 0.05 mole) was weighed into the weighing boat before carefully transferring it to a 250 ml conical flask. After adding toluene (15 ml), the solution was cooled in an ice bath to below 100C. Subsequently, an aliquot of concentrated nitric acid (4 ml) was added dropwise by the use of a plastic pipette. This process was done while maintaining the temperature between 50C – 100C. At this point, the flask was swirled for ensuring thorough mixing of its contents. After completing this addition (which took about 5 minutes), the flask was kept in the ice bath for the next five minutes. Henceforth, the flask was allowed to stand at room temperature for twenty minutes. In this position, the reaction mixture was being swirled periodically.
Additionally, the solution was chilled in an ice bath with approximately 00C. At this time, the crystals of the product began precipitating out. Next, the solid under the suction filtration was collected by the use of the Buchner funnel and flask. These solid substances were washed using the small quantity of cold toluene (approximately 5 ml). The combined washing and filtrate were then kept for later analysis using thin layer chromatography (TLC). This product was again transferred to the pre-weighed container that was labeled ready for drying. The yield of the product was then recorded after drying.
Regarding the analysis of the thin layer chromatography, the small tip of the spatula (about 2 mg) of the products was placed into a separate, test tubes or small samples. These products included the ortho-nitrophenol, para-nitrophenol, a few drops of the filtrate, and the 2,4-dinitrophenol. At this time, about 0.2 ml of the 50:50 (v/v) mixture of the solvent of petrol/ toluene was added to each of the sample tubes. Each of the samples was then spotted onto the TLC plate’s baseline such that each spot is about 1.5 cm above the plate’s bottom and approximately 1.5 cm distant to its neighboring spot. After applying all the five samples, the TLC plate was placed into the tank and covered using an aluminum foil lid. The plate was then allowed to develop until the solvent front traveled to about 1 cm of the top of the plate. After removing the plate form the tank, a pencil mark was made where the solvent front reached.
Results/ Discussion
Various spectra of chromatography (i.e., A, B, C, D, and E) dissolves and pull across the piece of paper upon reaching the optimum time needed for effecting the reduction under the conditions of reactions used. Because the components of the mixture travel across the paper with different speeds, its colors separate based on their attraction to the solubility of the solvent or paper.
In this lab, the mass of the product = mass of the container with the product – the mass of the container without the product = 7.91 grams – 7.23 grams = 0.68 grams.
Therefore, the yield of the product = (7.23 grams/ 7.91 grams) x 100 = 91.4%.
Clearly, the product is not pure because it separates into various colors. Some of the material’s composition in the filtrate include the para-nitrate, the azobenzene, ortho-nitrate, and the benzophenone. In question one, “a” is the o-nitro toluene, “b” is the bromobenzoic acid, and “c” is the glycolic acid.
The method involved in converting phenol into OCH2CH3 consists of the addition of the zinc dust to the solution (Kochi 1974). Also, the addition of the sodium sulfate or the sodium carbonate can be used in converting the phenol into other products such as OHNH2.
Conclusion
The lab provided an experimental possibility of separating colors by the method of TLC. Also, it achieved its goal of preparing nitrophenol (electrophilic aromatic substitution) using various equipment. The colors spread out different on the chromatography paper. These colors had the different rate of solubility. Even though it was possible to achieve the primary objective of the lab, there were some errors. However, such errors can easily be reduced by being more careful.
References
Coombes, R. (2006). Electrophilic Aromatic Substitution. ChemInform, 37(22).
Dong, X. (2013). The Current Trends in Organic Chemistry. Organic Chemistry: Current Research, 03(01).
Iwan, A. (2013). Current Trends in Organic Chemistry Existing in Organic Solar Cells. Organic Chemistry: Current Research, 03(01).
Knox, J. (1978). Liquid chromatography detectors (Journal of Chromatography Library, Vol. II). Journal of Chromatography A, 150(2), p.569.
Kochi, J. (1974). Electrophilic aromatic substitution: Electron transfer routes in side chain substitution. Tetrahedron Letters, 15(49-50), pp.4305-4308.
Kolla, P. (1994). Gas chromatography, liquid chromatography, and ion chromatography adapted to the trace analysis of explosives. Journal of Chromatography A, 674(1-2), pp.309-318.
Lederer, M. (1982). Ion chromatography. Journal of Chromatography A, 245(3), p.385.
Mahdavi, H. and Heidari, A. (2017). Chelated palladium nanoparticles on the surface of plasma-treated polyethersulfone membrane for an efficient catalytic reduction of p-nitrophenol. Polymers for Advanced Technologies.
Morlock, G. (2015). Miniaturized planar chromatography using office peripherals — Office chromatography. Journal of Chromatography A, 1382, pp.87-96.
Office, C. (2015). Acknowledgement to Reviewers of Chromatography in 2014. Chromatography, 2(1), pp.19-19.
Ramsteiner, K. (1987). On-line liquid chromatography-gas chromatography in residue analysis. Journal of Chromatography A, 393(1), pp.123-131.
Roth, M. (1993). Thermodynamic pitfalls in chromatography revisited: Supercritical fluid chromatography. Journal of Chromatography A, 641(2), pp.329-335.
Sahiner, N. and Ozay, O. (2012). Enhanced Catalytic Activity in the Reduction of 4-Nitrophenol and 2-Nitrophenol by p(AMPS)-Cu (0) Hydrogel Composite Materials. Current Nanoscience, 8(3), pp.367-374.
Vasiliades, J. and Sahawneh, T. (1982). Midazolam determination by gas chromatography, liquid chromatography and gas chromatography—mass spectrometry. Journal of Chromatography B: Biomedical Sciences and Applications, 228, pp.195-203.
Zhang, J. and Wang, H. (2012). Mass Spectrometry: Insightful Window for Organic Chemistry. Organic Chemistry: Current Research, 1(4).
