Naphthalene and Biphenyl Separation Experiment

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.

It is noted that although the napthalene and biphenyl are two extremely similar chemicals, the naphthalene is a Lewis base that reacts with picric acid to form a picrate. However, biphenyl does not. It is also noted that the separation is made possible because solid naphthalene picrate has different solubility properties from naphthalene and biphenyl.

Apparatus

Spatula, glass rod, electronic balance, ice-cold water, distilled water, Bunsen flame, capillary tubes, melting point apparatus, thermometre, Hirsch funnel and suitably sized filter papers, Buchner flask, filter funnel, stemless funnel, small beaker, pipette, boiling tubes, 100 mL and 250 mL conical flasks, fluted filter papers, watch glass.

2.3 Procedure

Part a) Forming the naphthalene derivative (naphthalene picrate) and isolating it from biphenyl

A beaker was prepared, half-filled with water, and placed on a steam bath in the fume cupboard. This was done in order to prepare a water bath.

A clean boiling tube was placed in a small beaker, which were put on an electronic balance and the TARE was used to eliminate the weight of the glass ware.

1.001 g of naphthalene (50 % w/w) and biphenyl mixture were weighed using an electronic balance. These were placed the boiling tube directly using a spatula.

2 mL of methanol were added to the mixture in the boiling tube. The tube was then placed in the water bath and heated on the steam bath until the mixture was completely dissolved. Once it dissolved the white colour of the solid mixture disappeared as it dissolved and the solution obtained was colourless.

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In another boiling tube 1.3 g of picric acid were weighed and 10 mL methanol were added to the boiling tube. The volume was measured using a measuring cylinder. The picric acid was yellowish in colour with the solid powder at the bottom of the container.

Then the boiling tube was warmed in the water bath until a yellow solution was obtained.

A few drops of the picric acid solution were put aside using a pipette. The pipette was placed in a boiling tube on the rack and left there to be used later.

The picric acid solution was added slowly to the mixture of naphthalene and biphenyl solution. Then the boiling tube was warmed for 1 minute on the steam bath. Once the yellow solution became deeper and stronger and crytals were just about to start forming the boiling tube was removed and allowed to cool on the bench. Then after cooling it was placed in ice-cold water. This resulted in the formation of solid naphthalene picrate.

Vacuum filtration was carried out using a Hirsch funnel, fitted with a filter paper of a suitable size, and corked to a Buchner flask. The rubber tubing was attached to the arm of the flask and the vacuum source was turned on.

The crystals formed were then collected in the Hirsch funnel. They were washed with methanol, of which the minimum of volume was used.

The crystals collected were transferred onto a watch glass using a spatula, and left to dry on the bench. The mass of the naphthalene picrate crystals was then found by weighing it. This means that the mass found included the weight of the watch glass too. Hence the mass of the watch glass alone was then taken after the experiment and a subtraction was done in order to obtain the mass of the crystals alone.

Then the melting point was determined. This was done by placing the middle of a capillary tube into a Bunsen flame. This melted into two halves due to heat. A melting point tube was then filled up to 2-4 mm by tapping gently the open end into the naphthalene picrate on the watch glass. Then the tube was inverted and tapping was done again so as to allow the sample to move downwards the closed end of the tube. Once this was done the tube was placed in the melting point apparatus which was switched on some time beforehand. The melting point temperature range was then recorded, by recording the temperature at which the product first started to melt and the temperature at which the product melted completely.

The presence of naphthalene in the picrate was checked by the addition of a few drops of fresh picric acid solution; (the few drops that were kept aside previously, with a pipette) It was noted that no picrate was formed. This means that the separation was complete.

Part b) Re-extracting biphenyl, purifying it and testing its purity

1 mL concentrated ammonia and 30 mL water were measured using a meauring cylinder. These were placed in the filtrate. This resulted in the precipitation of the biphenyl.

The biphenyl was collected by filtration using a filter funnel fitted with a fluted filter paper and a conical flask beneath. The filtrate was added slowly into the filter paper until filtration was complete.

The residue left on the filter paper was collected and transferred onto a watch glass using a spatula and left to dry a bit. Then it was dissolved in a minimum amount of hot water in a conical flask.

Hot gravity filtration was carried out using a stemless funnel with a fluted filter paper, and a 100 mL conical flask. Then the filtrate was allowed to cool.

After cooling it was placed in ice-cold water, and after some time the sides of the conical flask were scratched with a glass rod, to allow the formation of the crystals. However, no crystals were observed. Therefore the solution was discarded and so the mass and the melting point of the recrystallised biphenyl were not found. Hence no results were obtained.

Part c) Re-converting the picrate to naphthalene, purifying it and testing its purity

The picrate crystals were placed in a 100 mL conical flask, and were dissolved in a little hot methanol by warming on the steam bath.

Then 1 mL concentrated ammonia and 30 mL water were added to the conical flask. After warming for 1 minute the solution was allowed to cool to room temperature, and then placed in ice-cold water.

The naphthalene was filtered by hot gravity filtration and recrystallised from methanol by vacuum filtration.

Diagrams:

Hot Gravity Filtration Vacuum Filtration

Precautions:

It was ensured that all the apparatus was rinsed with distilled water so as to prevent contamination.

It was ensured that it was the picric acid solution that was added to the mixture solution of naphthalene and biphenyl, and not vice-versa, and the addition was does slowly and gently so as to prevent spilling of some of the solution, and also so as to allow time for the reaction to occur slowly, for the formation of the solid naphthalene picrate.

It was ensured that the required time was allowed for complete filtration to occur. This filtration was completed until no more drops of filtrate appeared to fall.

It was ensured that the Buchner flask was pressed downwards with the hands during the vacuum filtration process. This was done so as to ensure that full suction was present and so vacuum was created, so that filtration was done completely.

It was ensured that the rubber tubing was disconnected first and then the water aspirator turned off, so as to prevent sucking back and so contamination of the substance.

It was ensured that both the stemless funnel and the conical flask were warmed just a little before hot gravity filtration was carried out, by placing them in an oven for a short time. This was important so as to prevent premature crystallisation.

It was ensured that care was taken when the capillary tube was placed into the Bunsen flame in order to melt it into half. This was important so as to prevent and avoid injuries of the hands or fingers even though gloves were worn.

It was ensured that parallax errors were avoided as much as possible by looking normally to the scale of the thermometre when taking the reading of the melting point temperature range. Also, when measuring volume of solutions using a measuring cylinder.

3. Results and Calculations

3.1 Results

mass of napthalene (50 % w/w) and biphenyl mixture used: 1.001 g

mass of naphthalene picrate crystals: 1.958 g

melting point temperature range of solid naphthalene picrate: 129 – 136 ÌŠC

mass of separated biphenyl: /

melting point temperature range of biphenyl: /

mass of separated naphthalene: 0.347 g

melting point temperature range of naphthalene: 81 – 92 ÌŠC

3.2 Calculations

mass of napthalene picrate crystals watch glass : 30.391 g –

mass of watch glass only : 28.433 g

hence, mass of naphthalene picrate crystals : 1.958 g

Percentage Recovery for purified separated naphthalene:

% yield = actual mass X 100 %

theoretical mass

= 0.3470 g X 100 %

0.5005 g

% yield = 69.33 %

4. Discussion

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 benzene’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.

Naphthalene picrate

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.

5. Conclusion

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 aim was reached.

References:

McMaster University 1997/98: http://www.chemistry.mcmaster.ca/~chem2o6/labmanual/expt1/exp1b-i.html, retrieved 2010, 1st December.

Founder -Jimmy Wales: Wikipedia the free encyclopedia 2010:

http://en.wikipedia.org/wiki/Naphthalene, retrieved 1st December 2010.

The organic compounds database maintanied by Colby College:

http://www.colby.edu/chemistry/cmp/cmp.html