Recrystallisation As A Technique For Purification Biology Essay

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Recrystallisation is a means of purifying solids. If carried out correctly the final product will be both of a high yield as well as pure. Recrystallisation is essential because products from an organic reaction are seldom obtained in a pure state directly from the reaction mixture. Hence if the product is a solid it may be purified by recrystallisation from a suitable solvent. The process of crystallization depends on the fact that most organic compounds are more soluble in hot solvents than in cold, and that the impurities present will have different solubilities from the desired compounds. The ideal solvent for recrystallisation is therefore one in which the compound as a high solubility when it is hot and a low solubility when it is cold. Hence the experiment starts with choosing the ideal solvent to be used for the recrystallisation process. The best solvent is chosen from several others depending on the results obtained.

In this experiment filtration is introduced, which is a technique used either to remove impurities from an organic solution or to isolate an organic solid. The two types of filtration are done. These are gravity filtration and vacuum or suction filtration. Basically gravity filtration is done to remove solid impurities from an organic liquid. On the other hand vacuum filtration is done in order to collect a desired solid, for instance, the collection of crystals in a recrystallisation procedure.

One should also mention the last step of the experiment being the melting point determination. The melting point temperature range is important and useful since it confirms the purity of organic solid substance obtained. The melting point of a slightly impure compound will always differ from the melting point of the pure compound, of course.

The crystals obtained were not completley dry. Drying methods such as using a flameless oven set or a dessicator under vacuum could not be used due to lack of time. Hence extra mass was present due to some ice-cold pure water still present within the crystals. The fact that the crystals were not very well dried was a disadvantage when it came to tapping the melting tube over them and taking up some of the substance into the tube. This is because it was quite difficult especially when tapping the tube on the bench to allow the substance to move towards the bottom. The moisture in the substance made it stick and get blocked in the middle. Hence more time was taken to to prepare the tube!

It was difficult to know when to stop heating and stirring the solution before the filtration was done. This is because it was known that not all the substance would dissolve due to 5% naphthalene impurities present.

The substance's purity depends on for example the volumes of solvent used. The smaller the number but sufficient enough, the greater the purity of the substance obtained. This could affected the results because the volume of solvent to be used is not known. Hence one could exceed the volume used without knowing. Therefore care was taken to minimise this as much as possible.

The hot filtered solution in the conical flask was not left to cool down slowly to room temperature before immersing it in an ice-bath. This affected the results obtained because this caused the dissolved substance not to crystallise out completely. This occurred due to lack of time available during the experiment.

4. Discussion

The purification of a solid by recrystallisation depends upon the fact that different substances have different solubilities in different solvents. In the simplest case, the unwanted impurities are much more soluble than the desired compound. The impure sample is dissolved in the minimum volume of hot solvent to form a saturated solution, then as the solution cools slowly, crystals of the desired compound form and can be collected by filtration. The soluble impurities remain in cool solution and so pass through the filter paper with the solvent.

Regarding the choice of the solvent used, one must note that the choice is critical to the purification by recrystallisation, but there is no simple way to know which solvent will work best. However, some general principles of solubility will assist in this choice. Crystallisation depends primarily on solubility relationships. In most instances the solubility of a compound in any solvent increases markedly with temperature. This means that if a compound has adequate solubility in a hot solvent, it can often be recrystallised almost completely by cooling to a temperature at which the solution is supersaturated, i.e., the solubility limit is exceeded.

The solubility of crystalline organic compounds depends on the functional groups that are present and the polarity of the solvent to a very large extent. In this context the expression "like dissolves like" is a very useful principle. Compounds with groups such as -OH, -NH-, -CONH- and others are usually more soluble in solvents such as alcohols or water than in hydrocarbons.

Decreasing Polarity (approximate)

most polar: H2O water

RCOOH organic acids (acetic acid)

RCONH2 amides (N,N-dimethylformamide)

ROH alcohols (methanol, ethanol)

RNH2 amines (triethylamine, pyridine)

RCOR aldehydes, ketones (acetone)

RCOOR esters (ethyl acetate)

RX halides (CHCl3>CH2Cl2>CCl4)

ROR ethers (diethyl ether)

ArH aromatics (benzene, toluene)

least polar: RH alkanes (hexane, petroleum ether)

In contrast, non-polar molecules possessing very few or no polar groups will dissolve in hydrocarbons. Regardless of the type of compound, however, the more stable the crystal lattice (the higher the melting point), the less soluble the compound.

One essential characteristic of a useful solvent is that the desired compound must be considerably more soluble in the solvent when it is hot than when it is cold. To determine whether or not the solvent fulfils this requirement, a spatula of sample was placed into a small test tube, and a few drops of solvent were added, and, if the substance did not dissolve in the cold solvent (which would render that particular solvent unsuitable), the mixture was heated to 80 ÌŠC on a steam bath or a water bath. If the material did not go into solution at this point, more solvent was added, a little at a time with continued heating, until it did. This ensured that the minimum volume of hot solvent has been used to dissolve the sample. The solution was then cooled by placing the tube in a beaker of cold water to see whether the compound would crystallise out.

What was important to note was the possibility that the compound might dissolve only slowly in the boiling solvent, and also that insoluble impurities might be present in the sample.

In this experiment two types of filtrations were carried out. These are gravity filtration and vacuum or suction filtration, and these are the two most commonly used types of filtrations in organic chemistry laboratories. In order to define what filtration is, it could be said that it is a technique used either to remove impurities from an organic solution or to isolate an organic solid.

Gravity filtration is the method of choice to remove solid impurities from an organic liquid. The "impurity" can be a drying agent or an undesired side product or leftover reactant. Gravity filtration can be used to collect solid product, although generally vacuum filtration is used for this purpose because it is faster.

A filtration procedure called hot gravity filtration is used to separate insoluble impurities from a hot solution. Hot filtrations require fluted filter paper and careful attention to the procedure to keep the apparatus warm but covered so that solvent does not evaporate.

On the other hand vacuum filtration is used primarily to collect a desired solid, for instance, the collection of crystals in a recrystallisation procedure. Vacuum filtration uses either a Buchner or a Hirsch funnel. Vacuum filtration is faster than gravity filtration, because the solvent or solution and air are forced through the filter paper by the application of reduced pressure. The reduced pressure requires that they be carried out in special equipment. The equipment consists of a Buchner or Hirsch funnel, a heavy-walled, side arm filtering flask, a rubber adaptor or stopper to seal the funnel to the flask when under vacuum, and a vacuum source.

It is important to note that vacuum filtration is not used to filter a solid from a liquid if it is the liquid that is required and if the liquid is low boiling. Any solvent which boils at about 125 ÌŠC or lower will boil off under the reduced pressure in the vacuum flask.

Regarding part C of the procedure i.e the determination of the melting point temperature range of the pure substance obtained which was benzoic acid. Using the organic compounds database maintanied by Colby College and inserting the metling point range obtained in the experiment done, benzoic acid, of chemical formula C6H5COOH, was one of the listed substances. This means that the experiment was quite successful.

In order to include some information about this concept one must say that the melting range of a pure solid organic is the temperature range at which the solid is in equilibrium with its liquid. As heat is added to a solid, the solid eventually changes to a liquid. This occurs as molecules acquire enough energy to overcome the intermolecular forces previously binding them together in an orderly crystalline lattice. Melting does not occur immediately, because molecules must absorb the energy and then physically break the binding forces. Typically the outside of a crystal will melt faster than the inside, because it takes time for heat to penetrate.

The melting range of a compound is one of the characteristic properties of a pure solid. The melting range is defined as the span of temperature from the point at which the crystals first begin to liquefy to the point at which the entire sample is liquid.

One should note that the presence of impurity has two effects on a substance's melting range. These are melting range depression, where the lower end of the range drops and melting range broadening, where the range simply increases. A melting range of 5° or more indicates that a compound is impure.

One should note that the reason for this depression or broadening is because contaminants disrupt the consistency and organisation of the crystal lattice at the molecular level. It can be said that contaminants do not sort of "fit" correctly into what would be the normal pure lattice. Now, the disruption weakens the lattice, so that the lattice can be broken down more easily. The weakened structure melts more easily at reduced temperature, i.e. depression. One should note that disruption of the lattice makes it non-uniform. At the molecular level, the molecules closest to the impurities melt fastest. Further away from the impurities, the crystal lattice is relatively undisturbed and therefore melts at or nearer the normal temperature.

One should understand that only impurities, which are incorporated into the crystal structure at the molecular level, cause depression and broadening. At the chemical level, it is impossible to raise the melting point of an already pure substance. Its melting point can be depressed by contamination, but not raised! Therefore, most likely, the unknown sample is an impure version of the higher melting sample.

One should of course mention the fact that often contaminated solids are purified by recrystallisation, which the experiment was mainly all about. If the resulting melting range is unchanged, the original sample probably was pure to begin with. But if the resulting melting point gets higher, the original sample was, obviously, impure. When crystals are isolated by filtration from a solvent, it is important to allow complete drying or evaporation of the solvent in order to get a good melting range. Residual solvent functions as a contaminant and will depress or broaden the melting range for a crystal. When two chemicals are mixed, the resulting melting point is not the average of the two melting points. It is always depressed from the melting point of the major component in the mixture. Even when two chemicals with exactly the same melting point when pure are mixed, the resulting melting point is depressed.

In order to conclude the discussion onecould also mention additional steps in the procedure for a better result :

A watch glass could have been placed over the solution, (i.e over the conical flask neck), once all the solid product was dissolved, before moving on the hot gravity filtration. The watch glass was quite essential as this prevents evaporation of the solvent.

Some anti-bumping granules (also known as boiling chips which are small, irregualrly shaped pieces of material) could be added to the conical flask with the solid and solvent added. These allow smoother prolonged boiling without evaporation of the liquid. Boiling did not occur in this experiment, (the temperature was not exceeded more than 80 ÌŠC) However the solution was heated on hot plate for quite some time. And anti-bumping granules always help, in fact they are frequently employed in distillation and heating.

5. Conclusion

It was concluded that recrystallisation process was done and thus the product which is crystalline in its pure form was purified. The % yield was calculated and this was quite high. Also the melting point temperature range matched actual melting temperature of benzoic acid. Hence the substance obtained was pure and hence the experiment was quite successful.

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