Separating Binary Mixture of Naphthalene and Biphenyl
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Napthalene and biphenyl mixture is a binary mixture of substances where the substances are very similar to each other as can be observed from their structures. Due to their similarities in physical properties their separation cannot be accomplished by simple crystallisation solvent extraction. Therefore, in such case, there must be a chemically modification of one of the components to render it significally different from the other constituents. Then separation of the derivative from the mixture; and finally the derivative is transformed back from the mixture.
When several benzene rings are fused together to give more extended pi systems, the molecules are called polycyclic benzenoid or polycyclic aromatic hydrocarbons (PAHa). In these structures two or more benzene rings share two or more carbon atoms.
The fusion of one benzene ring to another results in a compound called naphthalene. Further fusion can occur in a linear manner to give anthracene, tetracene, pentcacene, a series called the acenes. Angular fusion (annulation) results in phenanthrene, which can be further annulated to a variety of benzenoid polycycles.
In contrast to benzene, which is a liquid, naphthalene is a colourless crystaline material. It is best known as a moth repellent and insecticide. The spectral properties of naphthalene strongly suggest that is shares benzeneâ€™s delocalised electronic structure and thermodynamic stability. Basically when observing spectra and peaks, it is concluded that the electrons in napthalene are delocalised more extensively than in benzene. Therefore the added four pi electrons enter into efficient overlap with those of the attached benzene ring. Hence, several resonance forms can be drawn:
Resonance Forms of Naphthalene
Naphthalene is an example of a polycyclic aromatic molecule where resonance forms are used to describe the reactivity of these molecules. Resonance is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis formula. Is is noted that a molecule or ion with such delocalized electrons is represented by several contributing structures, called resonance structures or canonical forms.
Basically the aromatic character of naphthalene is proven in this reactivity. Naphthalene undergoes electrophilic substitution rather than addition. For example, treatment with bromine, even in the absence of a catalyst, results in smooth conversion into 1-bromonaphthalene. The mild conditions required for this process prove that naphthalene is activated with respect to electrophilic aromatic substitution. It is noted that these reaction can occur in the absence of a catalyst. However if benzene instead of naphthalene, an aluminium chloride catalyst is required for the reaction to occur. Similarly, whereas both benzene and naphthalene can be alkylated using Friedel-Crafts reactions, naphthalene can also be alkylated by reaction with alkenes or alcohols, with phosphoric acid as the catalyst.
Regarding electrophiles, it is noted that these attack substituted naphthalenes regioselectively. This basically refers to the preference of one direction of chemical bond making or breaking over all other possible directions. It is noted that an activating group usually directs the incoming electrophile to the same ring anda deactivating group directs it away. This ring carrying the substituent is the most affected.
It is important to also mention the picric acid apart from the naphthalene. This is because this acid was useful for the formation of the derivative, solid naphthalene picrate. Picric acid is actually the chemical compound called 2,4,6-trinitrophenol (TNP). This yellow crystalline solid is one of the most acidic phenols. It is an explosive, like other highly nitrated compounds such as TNT. Picric acid is called acid because of the unusually high acidity of its hydroxyl group (pKa 0.38) which is increased beyond that of acetic acid (pKa 4.7) and even hydrogen fluoride (pKa 3.2) by the electron-withdrawing effect of the three nitro groups. It is noted that this property was in part responsible for its replacement by TNT in military uses.
Moving on to the derivative formed, one should first say that a picrate is actually a salt or an ester of picric acid. However, it is important to say that it could also be an additional compound which picric acid forms with many aromatic hydrocarbons, as in this case in this experiment, where the aromatic hydrocarbon is naphthalene. Hence, additional compounds are also called picrates, even though they are not a salt of picric acid.
Discussing the results obtained in this experiment it is noted that no data of the results obtained could be recorded for the biphenyl. This is because the biphenyl did not crystallise. For recrystallisation the solubility of the substance is required to be high in the hot solvent but low when it is cooled, so that the desired product will crystallise whilst impurities remain dissolved. However, this did not happen for the biphenyl. After cooling in ice-cold water for several minutes, it did not crystallise. A reason for this could be due to the solubility of the biphenyl, which is found to be very soluble at room temperature and so most of the biphenyl remained dissolved. Hence poor recovery is obtained such that the biphenyl was discarded. Hence no vacuum filtration was carried out since no crystals were formed.
One should say that in order to increase the yield, a second miscible solvent could be added. This would alter the solvent polarity sufficiently to decrease the solubility of biphenyl. As the second solvent diffuses into the mixture, the biphenyl should crystallise out.
Sources of error:
The naphthalene crystals to be collected on the Hirsch funnel simply passed through the filter paper into the Buchner flask during vacuum filtration. This might occurred due to the crystals not being formed completely or due to the crystals which might dissolved a bit in the methnaol (solvent) during the process and so passed through, even though a minimum amount of solvent was used. The process was then repeated so as to collect as much product as possible, and the yield claculated was quite reasonable.
The solid naphthalene picrate crystals formed and collected on a watch glass were not completely dry when they were weighed and the melting point temperature range was determined. This was due to lack of time available. Hence the mass obtained might varied a bit and was slightly greater due to some solvent still present. This was noted during the melting point determination process where the powdered sample was very difficult to be tapped down to the bottom of the melting tube. It just kept being blocked in the middle within the tube, because it was not completely dry.
It was concluded that the naphthalene (50 % w/w) and biphenyl mixture was separated and the separation involved the formation of a derivative of naphthalene, a solid napthalene picrate, which was isolated from biphenyl. It was also concluded that at the end of the day, it was the individual components which were required to be obtained individually, i.e. the purified biphenyl and the purified naphthalene which was reconverted from picrate. Hence since this was achieved the experiment was quite successful since the aimwas reached.
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