Aim: To use the extraction separation technique so as to separate a mixture of benzoic acid and 4-nitrotoluene provided. This is carried out so as to obtain the individual components of the mixture in a purified form by recrystallisation process and whose metling point is determined to prove the identity of the compound obtained.
Separation by extraction is a technique which allows the separation of compounds based on their difference in solubility in two immiscible solvents.
If a solute X is allowed to distribute itself bewteen two immiscible solvents, A and B, then the following equilibrium will be reached:
X (A) X (B)
where A is usually water, the aqueous phase and B is the water-immiscible organic solvent, such as diethyl ether as in this case.
The equilibrium constant for this process is known as the distribution coefficient or the partition coefficient, KD or BDA and is given by:
KD (X) = BDA (X) = the concentration of X in phase B = [X]B
the concentration of X in phase A [X]A
similarly, for solutes X and Y distributed into two immiscible solvents A and B we may define the following equilibria:
X (A) X(B)
Y (A) Y(B)
and for effective separation by extraction of X and Y using solvents A and B, we require that:
KD (X) >> KD (Y)
in which case most of X will find itself in solvent B and most of Y will find itself in solvent B, or,
KD (X) << KD (Y)
in which case most of X will find itself in solvent A and most of Y will find itself in solvent B.
Structural formula of 4-nitrotoluene
Structural formula of benzoic acidAll this is just a brief background simply showing that the aqueous layer, being water and the ether, which are both solvents and are referred to as A and B, are in equilibrium with each other. Then the benzoic acid / 4-nitrotoluene mixture to be separated by extraction, referred to as Y and X, are the solutes. These species are both insoluble in water and so do not like the polar environment. Hence this is why sodium bicarbonate solution is used. This is useful as to convert the benzoic acid to the water soluble sodium benzoate. This will then go into the aqueous layer rather than staying in the ether layer. Then concentrated hydrochloric acid is used to convert back to benzoic acid since the benzoic acid is required to be collected and be purified!
C6H5COOH(s) + NaHCO3 (l) C6H5COO-Na+ (aq) + H2O (l) + CO2 (g)
C6H5COO-Na (aq) + HCl (l) C6H5COOH (s) + NaCl (aq)
Adding on the above equations one could also mention the fact that according to the Bronsted – Lowry definition, an organic acid is a compound that reacts to donate a proton (H+) to a base such as OH- ion.
R-COOH (s) + OH- (aq) R-COO- (aq) + H2O (l)
Organic acid conjugate base
(insoluble in water) (soluble in water)
As can be seen from this reaction, the result of the loss of a proton causes the formation of the carboxylic acid’s conjugate base with a negative charge. This charge now allows the compound to dissolve in water. A carboxylic acid’s property of forming an ionic conjugate base is useful as it allows the separation of carboxylic acids from other compounds that do not have this chemical property.
In actual practice the acid and non – acid mixture is first dissolved in ether, an organic solvent that dissolves most organic compounds. It is noted that most compounds will dissolve in ether or in water, but not in both.
In order to conclude once the mixture is separated by extraction, recrystallisation processes are carried out in order to obtain the individual purified substances. Then the melting point is determined for each in order to confirm the purity of the substance obtained.
2.1 Chemicals used
Riedel de Haem
Sodium hygrogen carbonate
Riedel de Haem
a spatula, glass rod, electronic balance, Bunsen burner, capillary tubes, melting point apparatus, thermometre, separating funnel, stopper, measuring cylinder, 250 mL conical flasks, 100 mL conical flask, beaker, Buchner flask, Buchner funnel, cork, filter paper, fluted filter paper, stemless funnel, oven, steam bath (flameless oven) in the fume cupboard, ice-cold water, distilled water.
Part a) Separating the benzoic acid and 4-nitrotoluene mixture
2.001 g benzoic acid / 4-nitrotoluene mixture were weighed on an electronic balance and placed in a 250 mL conical flask by means of spatula.
20 mL of diethyl ether were measured using a measuring cylinder and added to the mixture in the conical flask.
The mixture was the transferred to a separating funnel and 20 mL diethyl sodium bicarbonate solution (5% by mass) were added to it. It was noted that this coverted the benzoic acid to the water soluble sodium benzoate.
The separating funnel was stoppered and shaken for a short while. Then it was inverted and the tap was opened so that pressure was released. It was then closed and shaking was continued, and the pressure was released frequently until further shaking developed little or no additional pressure.
Finally the layers were allowed to separate for some time and then the bottom layer was run off into a 100 mL conical flask.
Then 10 mL of water were added to the separating funnel and the aqueous layer was run off into the same conical flask.
Part b) Recovering benzoic acid and testing its purity
A few drops of concentrated hydrochloric acid were added to the aqueous phase until the pH was less than 2. The pH was checked by adding a drop with the glass rod onto pH papers provided and the colour obtained was checked and compared with the ones provided on paper for reference. The colour observed was orange.
The solution was allowed to stand until crystallisation of benzoic acid was complete. The beaker was placed in ice-cold water so that all the solid was allowed to form.
Vacuum filtration was carried out. The apparatus was set up and the solution was filtered under suction. The crude benzoic acid crystals were collected in the Buchner funnel on the filter paper. The crystals were washed twice with water and allowed to drain.
They were then recrystallised from water and the benzoic acid crystals collected were now purified and were transferred to a clean watch glass using a spatula. Then they were allowed to dry on the bench for some time.
The mass of the purified benzoic acid was found by weighing the watch glass and the product. Then when the experiment was over the watch glass was weighed empty and subtraction was done in order to obtain the mass of the purified benzoic acid.
The melting point was then determined. This was done by first placing a capillary tube, from its centre, in a Bunsen flame so that two melting point tubes were produced as the capillary tube melted into half. Then one tube was used to tap 2-4 mm of pure benzoic acid into the tube. This was then inverted and tapped against the bench so that the sample taken in was allowed to move to the bottom of the tube. The tube was then placed in a melting point apparatus and the melting point temperature range was found.
Then the percentage recovery was worked out.
Part c) Recovering 4-nitrotoluene and testing its purity
The ether solution was evaporated to dryness over a steam bath (flameless oven) in the fume cupboard.
It was noted that when no further loss of the solution, by evaporation, was present the conical flask was removed and was allowed to cool to room temperature. When cooled it was placed in ice-cold water until the formation of crystals occurred.
The crystals formed were collected and recrystallised from ethanol.
The mass of the pure 4-nitrobenzene was found using an electronic balance. The melting point was determined and the percentage recovery was worked out.
Separation of two immiscible liquids
(the lighter liquid being the ether layer and
the denser liquid being the aqueous layer, in this case)
It was ensured that all the apparatus was washed with distilled water so as to prevent contamination.
It was ensured that the tap of the separating funnel was tightly closed from the bottom before the mixture was transferred to it, otherwise this will run out and hence there would be loss of sample mixture prepared!
It was ensured that the separating funnel was shaken slowly and carefully and the stopper was pressed during the process so as to prevent it from detachment due to pressure.
It was ensured that the tap of the separating funnel was frequently opened once the funnel was shaken and inverted. This was important so that excess pressure was relieved preventing the funnel from breaking or collapsing.
It was ensured that the aqueous layer was run out very slowly once the interphase between the ether and the aqueous layer approached. This was important so as to be able to control the tap and close it quickly in time so as to prevent drops of ether from running out of the funnel with the aqueous layer.
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 solid product.
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, otherwise one could have injured the hands or fingers even though gloves were worn.
3. Results and Calculations:
mass of benzoic acid / 4-nitrotoluene mixture weighed = 2.001 g
For purified benozic acid:
mass obtained = 0.333 g
melting point temperature range obtained = 120 – 122 ÌŠC
percentage yield calculated = 33.3 %
For purified 4-nitrotoluene:
mass obtained = 0.268 g
melting point temperature range obtained = 51 – 52 ÌŠC
percentage yield calculated = 26.8 %
mass of purified benzoic acid & watch glass = 25.142 g –
mass of watch glass only = 24.809 g
mass of purified benzoic acid = 0.333 g
Percentage Recovery for purified benzoic acid:
% yield = actual mass X 100 %
= 0.333 g X 100 %
% yield = 33.3 %
mass of purified 4-nitrobenzene & watch glass = 28.702 g –
mass of watch glass only = 28.434 g
mass of purified 4-nitrotoluene = 0.2680 g
Percentage Recovery for purified 4-nitrobenzene:
% yield = actual mass X 100 %
= 0.268 g X 100 %
% yield = 26.8 %
Sources of error
Parallax errors were avoided as much as possible by looking normally to the scale of the thermometre when taking the melting point temperature range. Also when measuring volumes of reagents using measuring cylinders.
During the vacuum filtration for the recrystaiisation process, some of the product i.e. benzoic acid and in another suction , 4-nitrotoluene just went throught the filter paper and into the Buchner flask. Hence the purified product obtained on hte Buchner funnel was not the total product obtained by separation. Unfortunately, due to lack of time, the process could not be repeated again. Hence the results obtained varied and percentage yield calculated was lower.
The purified products obtained were not allowed to dry completely before their mass was taken. Hence this might slightly affected the results obtained since the mass could might have been slightly higher due to some solvent still present.
During the separation of the 4-nitrotoluene / benzoic acid mixture by means of extraction using the separating funnel, it was quite difficult to stop the run at the interphase between the ether and the aqueous layer. Hence a drop of the ether layer was run out into the conical flask by mistake and hence mixed with the aquoeous layer. This means that there was no complete separation of the two.
The objective of an extraction is to recover valuable soluble components from raw materials by first dissolving them in the liquid solvent, so that the components can be later separated and recovered from the liquid.
Is is found that extraction is applied to a variety of food products, for example the extraction of sugar from sugar-cane or sugar-beets, the extraction of oil from oil seeds, the extraction of coffee extract from coffee beans, the extraction of various other components such as proteins, vitamins, pigments, essential oils, flavoured compounds, amonsgts others. Hence, the extraction of these products allows their separation from many different materials.
Extraction process can be calssified based on combination of phases (solid, liquid, gas and supercritical fluid). The solid – liquid is a type of extraction that is useful for the isolation and purification of naturally occuring sources. On the other hand the liquid – liquid is a more common method depending on solubility propertiesof components.
Regarding solvents, it can be said that there is a variety of solvents that can be used. Solvents used in separation of a mixture by extraction may be organic solvents which are denser than water; for example dichloromethane, chloroform and carbon tetrachloride. On the other hand they may also be organic solvents which are less dense than water; for example diethyl ether, toluene, hexane. In this experiment ether was used and this is very frequently used because ether is very immiscible with water, it has very powerful solvent properties for non-polar solvents, it has very low boiling point (35 ÌŠC) hence it can be evaporated easily after the extraction. However special care must be taken when handling this solvent since it is extremely volatile and flammable. In particular, all naked flames in the immediate vicinity should be extinguished.
A solvent is very important in the extraction process. A solvent is actually a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature.
In this experiment liquid-liquid extraction was carried out. This is also known as solvent extraction and partitioning and this method was used in order to separate compounds based on their relative solubilities in two different immiscible liquids, which are water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid phase. Liquid-liquid extraction is a basic technique in chemical laboratories, where it is performed using a separatory funnel.
In other words, liquid-liquid extraction is the separation of a substance from a mixture by dissolving that substance in a suitable solvent. By this process a soluble compound is usually separated from an insoluble compound.
One should say that solvent extraction is used in nuclear reprocessing, ore processing, the production of fine organic compounds, the processing of perfumes, and other industries.
While solvent extraction is often done on a small scale by laboratory chemists using a separatory funnel, it is normally done on the industrial scale using machines that bring the two liquid phases into contact with each other. Just for the sake of mentioning them such machines include centrifugal contactors, thin layer extractors, spray columns, pulsed columns, and mixer-settlers.
This brings us to the importance of using a separating funnel by which an extraction separation process can be carried out. A separating funnel is a pear-shaped laboratory apparatus which is used to separate two immiscible liquids that are usually very difficult to separate, for instance ether and water. It is very useful because the layers of the two liquids could be seen very easily, with the naked eye, separated from the side of the funnel. The separating funnel usually has a short stem and is fitted with a ground glass interchangeable stopper. The size of the separating funnel must always be about twice the volume to be extracted. It is usually very useful and convenient to have the separating funnel mounted in a ring on a stand with a firm base.
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Discussing the results obtained it was noted that the percentage yield calculated was quite low. Hence much of the product was lost and a reference to this was mentioned as one of the sources of error, where some product was not taken into account since it passed through the filter paper into the Buchner flask and the recystallisation process was not repeated. This might occurred due to the filter paper used, not being of the appropriate size.
In order to conclude one could discuss a modification which could be done to the above protocol for the separation of a mixture of benzoic acid and 4-nitrophenol. As mentioned in the introduction and further on with it in the procedure, the steps carried in the laboratory during the experiment were different from the ones below.
Initially acid anhydride is added and the 4-nitrophenol becomes 4-nitrophenyl ethanoate which is an ester that is soluble in ether. Then sodium bicarbonate is added which is not alkaline enough in order to convert the ester into alcohol but is alkaline enough in order to convert the benzoic acid into benzoate, which is soluble in water. Then extraction is done and NaOH is added to the ester and 4-nitophenol is obtained again. Concentrated hydrochloric acid is then added to the benzoate until crystallisation of benzoic acid occurs. Hence the individual compenents are obtained and in a pure form once the recrystallisation process is carried out.
It as concluded that the benzoic acid / 4-nitrotoluene mixture was separated by means of extraction. Hence the individual components of the mixture were obtained separately and in a purified form by recrystallisation processes. The melting point of the purified crystals was also found, confirming the purity of the substances.
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