A Short Note On Purification Of Organic Compounds

Purification of chemicals can be accomplished using a variety of approaches.

However, the technique chosen is determined by the type of the material (whether solid or liquid). It also relies on the pollutants that are present. These procedures are widely used to purify substances:

• Simple crystallisation
• Fractional crystallisation
• Sublimation
• Simple distillation
• Fractional distillation
• Distillation under reduced pressure
• Steam distillation
• Azeotropic distillation
• Chromatography

Filtration

Filtration is a technique for separating insoluble solid components of a mixture from soluble components in a solvent.

Using water as a solvent, it is used to separate a combination of naphthalene and urea. While urea dissolves in water, naphthalene does not. Naphthalene stays on the filter paper after filtering, but urea is extracted from the filtrate by evaporating water.

For instance, separating anthracene and benzoic acid requires dissolving the mixture in hot water and filtering the hot solution. Anthracene does not dissolve in hot water, although benzoic acid does. Anthracene remains as a residue on the filter paper after filtering, whereas benzoic acid crystallises from the filtrate after cooling.

The filtering process is slow and time-consuming. Filtration is done at decreased pressure with a Buchner funnel and a water suction pump in such instances.

Crystallisation

The concept is that the solubilities of the chemical and the contaminants in a solvent are different. A solvent is chosen in which the purified chemical is sparingly soluble, that is, sparingly soluble at lower temperatures but soluble at higher temperatures. The solution is heated to a saturated state, and the crystals of the compounds are filtered out after cooling.

Crystals of benzoic acid, for example, can be crystallised using water. In cold water, benzoic acid is very slightly soluble; in hot water, it is completely soluble.

Repeated crystallisation is used if the combination contains impurities with the same solubility as the chemical to be purified.

Fractional Crystallisation

This approach is used to separate and purify two or more compounds in the same solvent that have differing solubilities.

The mixture is dissolved in a solvent that has distinct solubilities for the two components of the mixture. The less soluble material crystallises out first when a hot saturated solution of this mixture is allowed to cool, while the more soluble substance stays in the solution. When the crystals of the second (i.e., more soluble) chemical are formed, they are removed from the mother liquor and concentrated once more before the hot solution is allowed to cool.

Fractional crystallisation is the process of separating various components of a mixture by multiple crystallizations.

Sublimation

It entails the direct conversion of a solid into a gaseous form without passing through a liquid stage in between, and vice versa when cooled. Sublimation is only possible for substances whose vapour pressures reach atmospheric pressure well before their melting points. Sublimable compounds are those that can be vaporised.

Sublimation is a highly helpful method for purifying solids that sublime when heated and are related with non-volatile contaminants.

An inverted funnel is put over the impure substance in a china dish covered with perforated filter paper. A smidgeon of cotton is stuffed inside the funnel’s stem. When the dish is heated in a sand bath, the volatile solid vapours rise up, pass through the perforations in the filter paper, and condense on the funnel’s cooled walls, leaving the nonvolatile impurities behind.

This procedure purifies camphor, naphthalene, anthracene, benzoic acid, iodine, and other compounds.

Simple Distillation

Distillation is the process of turning a liquid into vapours by heating it, then cooling it to condense the vapours.

The approach is typically employed for liquids with non-volatile contaminants that are sufficiently stable at their boiling points.

Simple organic liquids like benzene, ethanol, acetone, chloroform, carbon tetrachloride, toluene, and xylenes, for example, can be purified by simple distillation.

Fractional Distillation

If the boiling points of the two liquids in the combination are within 10 K of each other, the separation cannot be done using the basic distillation process mentioned above. This is why, at the boiling point of the mixture’s more volatile liquid, there will be enough vapours of the less volatile liquid as well. As a result, the mixture’s liquids will distil together, and separation will be impossible.

Fractional distillation, which includes successive distillations and condensations, can be used to separate such a liquid mixture into discrete components. A fractionating column is used in fractional distillation. A long glass tube with a broad bore is frequently filled with glass beads, tiny stones, porcelain rings, or coke, or blown into a number of spherical or pear-shaped bulbs. The fractionating column’s main aim is to enhance the cooling surface area and offer hurdles or impediments for ascending vapours and descending liquid.

Steam Distillation

this is a simple method for separating organic molecules from non-volatile organic or inorganic contaminants and purifying them. Only those chemicals that are volatile in steam, insoluble in water, have a vapour pressure of around 10-15 mm at 373 K, and include non-volatile impurities are suitable for this approach.

When the item to be purified boils over 373 K at 760 mm and decomposes at or below its boiling point, steam distillation is very useful.

Steam distillation allows high-boiling compounds to be distilled at moderate temperatures, avoiding breakdown.

Chromatography

Chromatography is a separation process for separating constituent particles of a mixture of substances, purifying chemicals, and determining the purity of organic molecules. A combination of substances is applied to a stationary phase (solid or liquid) with this approach. In the stationary phase, the gas mixture or pure solvent is permitted to move slowly. As a result, the mixture’s components begin to separate from one another.

There are two types of chromatography:

Adsorption Chromatography

It is based on the idea that the elements are absorbed to variable degrees on an adsorbent. Adsorbents such as silica gel or alumina are commonly employed. Different elements of the mixture are adsorbed at varied distances across the stationary phase when a mobile phase passes over it.

Adsorption chromatography is further divided into the following categories:

1. Column Chromatography: A mixture is separated in this step using a glass column filled with silica gel or alumina. At the top, the ingredient with the strongest affinity for the fixed phase is adsorbed, and so on. It is then recovered with the help of an eluent. After that, the solvent is evaporated to obtain the component.
2. Thin Layer Chromatography: A sheet of alumina (0.2 mm thick) is used to cover a tiny patch of the mixture, which is then stored in a suitable solvent. Due to capillary action, the solvent rises, and the components rise with it based on their differential adsorption, allowing them to be separated.

Partition Chromatography

The method of partition chromatography is defined as Component separation between two liquid phases, namely the original solvent and the solvent film utilised in the column.

Conclusion 

Among all methods for purification of organic compounds the most important is chromatography.

Chromatography is a vital biophysical method for qualitative and quantitative analysis that allows for the separation, identification, and purification of the components of a mixture. Size and shape, total charge, hydrophobic groups on the surface, and binding ability with the stationary phase are all factors that may be used to purify proteins. Ion exchange, surface adsorption, partition, and size exclusion are four separation approaches based on molecule properties and interaction type. Column, thin layer, and paper chromatography are examples of chromatography procedures that use a stationary bed. One of the most prevalent methods of protein purification is column chromatography.