Purification and Characterisation of Organic Compounds

In this chapter, we will discuss the different processes and techniques such as chromatography, distillation, sublimation during purifications of the organic compounds. Purification and characterisation of organic compounds are required once it has been produced. Purification techniques vary depending on the nature of the organic chemistry and the presence of contaminants. Sublimation converts the substance directly into vapour, allowing it to be separated and purified. A substance is refined by crystallisation, which involves cooling it and transforming it into a crystal form. Distillation is the process of heating a substance and converting it into vapour, which allows it to be separated. The chemicals are purified in chromatography with the help of stationary and mobile phases. 

Methods for Purification and characterisation of organic compounds 

The purification of the organic compound is based on the nature of the compound and the impurity present in it. The common techniques used for the purification are as follows:-

• Chromatography
• Sublimation
• Crystallisation
• Distillation
• Differential extraction

Chromatography (Modern method) 

It is the process that is used to separate mixtures from their compounds, purify compounds and also test the sterility of a compound. The name “process chromatography” is derived from the Greek word chroma, which means colour. Chromatography is a vital biophysical method for quantitative and qualitative analysis that allows for the identification, purification, and separation of the components of a mixture. Size and shape, total charge, hydrophobic groups on the surface, and binding capacity with the solid surface are all factors that can be used to purify proteins. Simply, we can say that,

• The component of the mixture gets gradually separated from one to another.
• Moving phase is known as the mobile phase.

So, based on the working principle, the chromatography process is categorised into various categories. The two methods of chromatography are as follows:-

Adsorption Chromatography

Adsorption Chromatography is based on the concept that solid substances known as adsorbents can keep molecules at their surface. The low, non-ionic forces of attraction of the van der Waals’ & hydrogen bonding, which only occur at specific adsorption beds, account for this holding force. These adsorbent beds can distinguish between organic molecules based on their binding strength. Differences in the compound flow rate finally lead to the separation of the organic mixture as the solvent is repeatedly passed over the adsorbent in the column. Silica, alumina, and carbon are the most often utilised adsorbents.

Partition Chromatography

Partition Chromatography is a type of chromatography technique used to separate different mixtures. In the 1940s,  this process was discovered by Richard Laurence Millington Synge and Archer Martin. 

The main difference between paper chromatography and TLC are as follows:

•  The theory of paper chromatography is partition-based, while the theory behind TLC is adsorption based.
• In TLC, glass is coated with silica gel, while in paper chromatography, the water present in the pores of the cellulose is in a stationary phase.
• Corrosive reagents can be used in TLC, while in paper chromatography, corrosive agents could damage the paper.

Sublimation

On heating, some solid substances change from solid to vapour state without passing through the liquid state; this process is called sublimation. It is used to separate sublimable compounds from non-sublimable impurities—for example, Iodine, Camphor, Naphthalene, Ammonium chloride etc.

Crystallisation 

Crystallisation is the process of atoms or molecules arranging right into a well-described, inflexible crystal lattice so that it will reduce their active state. The smallest entity of crystal lattice is known as a unit cell, which could be given atoms or molecules to develop a macroscopic crystal. During crystallisation, atoms and molecules bind collectively with well-described angles to shape a feature crystal form with clean surfaces and facets. Although crystallisation can arise in nature, crystallisation additionally has a huge commercial software as a separation and purification step within the pharmaceutical and chemical industries. 

Distillation

The distillation process is used to separate the following impurities:-

(i) Volatile liquid from non-volatile impurities

(ii) the liquids having enough variation in their boiling points (BP).

• A liquid having dissimilar BP  vaporises at different temperature
• The vapours are cooled and formed  liquids are collected separately

Examples of distillation are Chloroform (BP 334K) and aniline (BP 457K)

Benzene (BP 353K) and Chloroform (BP 334k)

Fractional Distillation

If the difference in boiling points of two liquids is not much, simple distillation cannot be used to separate them. The vapour of such liquid is formed within the same temperature range and is condensed simultaneously. In this technique, vapours of a liquid mixture are passed through a fractionating column before condensation.

 Conclusion 

In this chapter, we have studied that purification is required for organic chemicals taken from natural sources or manufactured in the lab. Purification is done in a variety of ways, depending on the compound’s composition and the impurities it contains. A compound’s purity can be determined by measuring its melting or boiling points, as well as using chromatographic and spectroscopic techniques.