Purification of Organic Compounds: Methods of Liquids Purification

Edited By Shivani Poonia | Updated on Oct 19, 2024 12:38 PM IST

The busy world of organic compounds is purified continuously, which is this fundamental activity that gives access to pure compounds for every form of useful application, be it in research or in fine chemicals for the production of drugs, subject to demonstration of feasibility and improved manipulability, then it calls for these stringent protocols for the separation and purification of organic compounds. This is a technique that cannot simply be box-contained within the closed walls of a laboratory setup.

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This Story also Contains

The Techniques of Purification
Sublimation and Crystallization
Distillation and Fractional Distillation
Differential Extraction and Chromatography
There are various pharmaceutical applications and benefits behind organic compound purification:
Some Solved Examples
Conclusion

Purification of Organic Compounds: Methods of Liquids Purification

It finds application in almost every type of industrial setting, be it in the area of health sciences or food processing, environmental sciences, and many more. Consider, for instance, a field that highly takes up this: pharmaceuticals. In this field, the effectiveness of a given drug counts highly on the level of purity the biologically active compound attains. Impurities tamper in a major way with the efficiency and safety profile of the drug and are thus tantamount to setting a precedent of offering health risks to patients. Equally, in the case of environmental science, at many times the purification of organic compounds may be needed to rid of pollutants in water and soil, which can harm ecosystems and hence human health.

The Techniques of Purification

Purification of organic compounds is the process of separating the compound under interest from impurities and unwanted by-products of the reaction. These enterprises are considered to be significant in qualitative analysis, synthetic purposes, and the use of compounds in their pure form. Among these properties that enable such decisions on the choice of the method of purification include the melting or boiling point, solubility, and polarity of the compound. Some general methods of purification of organic compounds include sublimation, crystallization, distillation, extraction, and chromatography.

Sublimation and Crystallization

This is a process by which a solid substance changes directly into the state of a gas, without passing through the liquid phase.

The technique can be best employed on the purification of substances that can sublime at middle to high melting points; examples of such substances are benzoic acid and naphthalene. On the other hand, crystallisation is the process where solid crystals can be formed from a solution or melt. It is one of the most used techniques daily in the pharmaceutical industry for the purification of active pharmaceutical ingredients and for the production of compounds obtained in a higher degree of purity.

Sublimation

It is the transition of a substance directly from the solid phase to the gas phase without passing through the intermediate liquid phase. This method of purification is used for those sublimable substances which are associated with non-volatile impurities. On heating the impure compound, volatile compound gets sublimed and its vapours are collected whereas, the non-volatile impurity remains as such. e.g., camphor, naphthalene, anthracene, benzoic acid, iodine, etc.

Crystallisation

It is the process of formation of solid crystals from solution, melt or by deposition directly from a gas phase. In this process a saturated solution of sparingly soluble compounds is prepared at high temperature and filtered.The clear solution thus obtained is set undisturbed to get cooled so that the pure solid organic substance is seperated out in the form of fine crystals which can be filtered out and dried. For example, Benzoic acid mixed with naphthalene can be seperated using hot water.

Note: Sometimes the process of crystallisation takes a long time. In such cases, crystallisation is initiated by adding a small crystal of some substance. This process is called seeding and the small crystal is known as seed which act as a nuclei for crystallisation.

Distillation and Fractional Distillation

This is fractional distillation, a process whereby the principle is allowed to set down on the difference between the boiling points of the constituents or installments of a mixture.

The technique majorly applies in the purification of liquid mixtures, such as ethanol and water. Fractional distillation is a further developed method of distillation that permits the separation of might-be mixtures into isolated components based on boiling point. This is mostly practicable in the production of fuels, especially petrol and diesel-related products. It is also helpful in separating air to its constituents of both oxygen and nitrogen gases.

Simple distillation

It is a procedure by which two liquids with different boiling points can be separated. Simple distillation can be used effectively to separate liquids that have some major degrees difference in their boiling points. As the liquid being distilled is heated, the vapours that form will be richest in the component of the mixture that boils at the lowest temperature. Purified compounds will boil, and thus turn into vapours, over a relatively small temperature range (2 or 3°C) by carefully watching the temperature in the distillation flask, it is possible to affect a reasonably good separation.

As distillation progresses, the concentration of the lowest boiling component will steadily decrease. Eventually, the temperature within the apparatus will begin to change; a pure compound is no longer being distilled.

Steps to be followed in the simple distillation process:

(i) Take a mixture(Acetone and water) in the distillation flask and fit it with the thermometer.
(ii) Arrange the apparatus as shown in the given figure.
(iii) Heat the mixture slowly keeping a close watch on a thermometer.
(iv) Since acetone has a lower boiling point, it starts to vaporise and condense in the condenser which is finally collected in the beaker.

Note: Simple distillation is effective only when separating a volatile liquid from a nonvolatile substance. If the liquids comprising the mixture that is being distilled have boiling points that are closer than 50 degrees to one another, the distillate collected will be richer in the more volatile compound but not to the degree necessary for the complete separation of the individual compounds.

Fractional distillation

It is used for the mixture of two liquids which differ in their boiling point by 10-15 K. It is a type of distillation which involves the separation of miscible liquids. The process involves repeated distillations and condensations and the mixture is usually separated into component parts. In this method, a fractionating column is used to increase the cooling surface area so that the ascending vapour phase becomes richer in more volatile components and the descending liquid phase becomes richer in less volatile components.

The basic principle of this type of distillation is that different liquids boil and evaporate at different temperatures. So when the mixture is heated, the substance with a lower boiling point starts to boil first and convert into vapours.

Fractional distillation is used for the separation of

(i) Acetone (b.pt. 329 K) from methyl alcohol (b.pt. 338 K)

(ii) Crude oil into various useful fractions such as gasoline, kerosene oil, lubricating oil, etc.

Steps to be followed in the fractional distillation process:

1. After setting up the apparatus, a mixture of two miscible liquids A and B is taken where A has more volatility than substance B.

2. The solution is added to the distilling flask while the fractionating column is connected at the tip of the flask. Heat is applied which increases the temperature slowly. The mixture then starts to boil and vapours start rising in the flask.

3. The vapours are from the volatile component A. The vapours then start moving through the fractionating column into the condenser where it is cooled down to form a liquid which is collected in the receiver.

Throughout the process, vaporization and condensation take place repeatedly until the two mixtures are separated completely.

Distillation under reduced pressure or vacuum distillation

It is a type of distillation process that is used for the purification of high-boiling liquids and liquids which decompose at or below their normal boiling point.

If the pressure on the surface of the liquid is reduced by a suction pump or vacuum pump, the liquid boils at a lower temperature without decomposition. e.g., glycerol (b.pt. 563 K) can be distilled at 453 K under 12 mm Hg pressure without decomposition.

Raw juice in sugar factories is generally concentrated by vacuum distillation.

Steam distillation

This process is used for the seperation and purification of organic compounds (solid or liquid) which are volatile in steam, immiscible with water, possess a high vapour pressure of about 10-15 mm Hg at 373 K and contain non-volatile impurities.

In this process, steam is passed through the organic mixture to be distilled so that the distilling mixture consists of steam and volatile organic compound, which follows that

Atmospheric pressure = vapour pressure of organic substance + vapour pressure of steam.

From the above relation, it can be interpreted that organic compounds distil below their normal boiling point without decomposition. For example,
(1) Aniline can be distilled at 371.5 K against its normal boiling point of 457 K.
(2) O-nitrophenol can be separated from p-nitrophenol since o-Nitrophenol is volatile in steam.

Differential Extraction and Chromatography

This is where a single-compound solubility technique of purification is applied in the mixture of solvents.

This technique is much more specifically used for the isolation of organic compounds from aqueous solutions or to purify organic solvents from impurities. Chromatography is based on the principle of differential partitioning of compounds between stationary and mobile phases. There are many types, the most common being column, TLC (thin layer), and HPLC (high-performance liquid chromatography).

Differential extraction

It means extraction with solvent. This method is based on the fact that organic substances are more soluble in organic solvents than in water. The organic substance is extracted from its aqueous solution by adopting the following process:

(1) The aqueous solution containing organic substance is shaken with a suitable organic solvent which dissolves the substance but is immiscible with water. Two layers are formed an organic layer and an aqueous layer.

(2) The solvent layer containing the organic substance (organic layer) is separated using a separating funnel. The impurities remain in the aqueous layer.

(3) The organic solvent is removed by distillation to obtain the organic substance.

Chromatography

It is the technique for the separation, purification, and testing of compounds. In this process, we apply the mixture to be separated on a stationary phase (solid or liquid) and a pure solvent such as water or any gas is allowed to move slowly over the stationary phase, carrying the components separately as per their solubility in the pure solvent.

There are four main types of chromatography:

1. Adsorption chromatography

2. Column chromatography

3. Thin layer chromatography

4. Partition chromatography

Adsorption chromatography

In the process of adsorption chromatography, different compounds are adsorbed on the adsorbent to different degrees based on the absorptivity of the component. Here also, a mobile phase is made to move over a stationary phase, thus carrying the components with higher absorptivity to a lower distance than those with lower absorptivity.

Column chromatography

It is the technique used to separate the components of a mixture using a column of suitable adsorbent packed in a glass tube, as shown in the figure below. The mixture is placed on the top of the column, and an appropriate eluant is made to flow down the column slowly. Depending upon the degree of adsorption of the components on the wall adsorbent column, the separation of the components takes place. The component with the highest absorptivity is retained at the top, while the other flows down to different heights accordingly.

Thin layer chromatography

In this process, the mixture of substances is separated into its components with the help of a glass plate coated with a very thin layer of adsorbent, such as silica gel and alumina, as shown in the figure below.

The plate used for this process is known as a chrome plate. The solution of the mixture to be separated is applied as a small spot at a distance of 2 cm above one end of the plate. The plate is then placed in a closed jar containing a fluid termed an eluant, which then rises up the plate carrying different components of the mixture to different heights.

Partition chromatography

In this process, a continuous differential partitioning of components of a mixture into a stationary phase and mobile phase takes place. Paper chromatography is a type of partition chromatography. A special quality paper known as chromatography paper is used. In this process, chromatography paper is used as a stationary phase which is suspended in a mixture of solvents that act as a mobile phase. Here, we put a spot at the base of the chromatographic paper with the mixture to be separated and as the solvent rises up this paper, the components are carried to different degrees depending upon their retention on the paper. The components are thus separated at different heights.

There are various pharmaceutical applications and benefits behind organic compound purification:

1. Pharmaceutical Industry:

In the pharmaceutical industry, purification methodologies play a pertinent role, not only in design but also at the manufacturing stage for the purpose of safety and efficacy of drugs. For example, antibiotic synthesis will have exact steps of purification to get rid of harmful impurities.

2. Chemical Industry:

While manufacturing chemicals that are used to make plastics, dyes, and perfumes, among many others, purification processes are of the essence to result in highly pure chemicals. The quality of such chemicals obtained will therefore determine the resultant features and safety attributes in produced products.

3. Environmental Remediation:

Efforts are facilitated to conduct analyses on removal methodologies for water and soils through the detoxification of such environments. For instance, adsorption and chromatography techniques may be used to extract pollutants in industrial wastewater.

4. Forensic Science:

Chromatographic identification and quantification techniques have found applications in forensic laboratories for drug, explosive, and general substance identification and quantitation since their respective crime against humanity. The applications are carried out to be used in legal investigations bound both to the public and the conventional scientist.

5. Food Industry:

Foodstuffs, high-quality amidated ingredients, and natural sweeteners and flavours are produced using purification processes in the manufacturing process. Consumers in the food industry are sensitive to such things as the purity of ingredients and end products.

In the same regard, in academic laboratories, several purification techniques are applied to have the students get to terms with the concepts of separation and the related practical skills in working with and analyzing organic compounds.

These kinds of experiences not only deepen students' levels of understanding of chemistry but also prepare students for further career opportunities in many branches of science.

Some Solved Examples

Example 1

Question: What technique is used for the separation of a mixture of calcium sulphate and camphor?

1)Sublimation

2) Chromatography

3) Distillation

4) Fractional DIstillation

Solution: Camphor is a sublimable substance, meaning it transitions directly from solid to gas without passing through a liquid phase, while calcium sulphate is not. Therefore, the technique used for the separation of these compounds is sublimation. On heating the mixture, camphor sublimates and its vapours are collected, leaving behind calcium sulphate. Hence, the answer is sublimation.

Example 2

Question: Consider the process where an impure compound is dissolved in a solvent and heated at a high temperature. Upon cooling the hot solution, pure compounds separate from the solution. What is this process called?

1) Sublimation

2) Crystallization

3) Distillation

4) None of the above

Solution: This process is known as crystallisation. The impure compound is dissolved in a solvent and heated. As the solution cools, the pure compound forms solid crystals which can then be filtered out and dried. Hence, the answer is crystallisation.

Example 3

Question:What is the basis of the crystallisation technique used in the purification of solid organic compounds?

1) Same solubility

2) difference in solubility

3) Solvent solubility

4) All of the above

Solution: The crystallisation technique is based on the difference in the solubilities of the compound and the impurities in a suitable solvent. The impure compound is dissolved in a solvent and heated. Upon cooling, the pure compound crystallises out of the solution, leaving the impurities dissolved. Hence, the correct answer is the difference in solubilities.

Example 4

Question: What is the principle behind the separation of two substances by fractional crystallisation?

1) Solubility

2) Molarity

3) Mole fraction

4) No of moles

Solution: Fractional crystallisation depends on the differences in solubility of the substances in a suitable solvent. When the solution is slowly cooled, the substance with lower solubility crystallises first. This allows for the separation of substances based on their different solubilities. Hence, the correct answer is solubility.

Conclusion

From the development of pharmaceuticals to environmental remediation, the techniques covered here help guarantee those products we are going to use in our everyday lives are of high quality, safe, and effective. The novel and revolutionary approaches in the broad spectrums of organic compound purification will further evolve in unison with research and technological advancement in the solving of challenging problems encountered by chemists and researchers across the globe.