Purification of organic compounds – II

We have already discussed organic compounds in some detail in another article. Here, we will discuss some important methods of purification of organic compounds since they contain some level of impurities.

Methods of purification of organic compounds

The general methods of purification are:

• Sublimation
• Crystallization
• Distillation
• Differential extraction
• Chromatography

We will explore purifying organic compounds by crystallisation and purification methods of organic compounds by sublimation.

Methods of purification of organic compounds by Crystallisation

Crystallization is the formation of a solid with highly organised atoms or molecules into  a crystal structure. Crystals can form via various processes such as solution precipitation, freezing, and, in rare cases, direct deposition from a gas. 

Temperature, air pressure, and, in the case of liquid crystals, evaporation time all impact the properties of the resulting crystal. 

The majority of minerals and organic molecules readily crystallise, and the resulting crystals are generally of high quality, with no visible flaws. Proteins, for example, reduce more difficulty than smaller biochemical particles. 

The strength of atomic forces (in the case of mineral substances), intermolecular forces (organic and biochemical substances), and intramolecular forces all have a strong influence on how easily molecules crystallise (biochemical substances).

Crystallisation is a chemical solid-liquid separation technique in which a solute is a mass transfer from a liquid solution to a pure solid crystalline phase. 

Crystallisation occurs in a crystallizer in chemical engineering. As a result, crystallisation is associated with precipitation, though the result is a crystal rather than amorphous or disordered matter.

Nucleation is the stage of crystallisation in which dispersed solute molecules or atoms in a solvent begin to gather into tiny clusters (raising solute concentration in a small region) that become stable under the current operating conditions. 

The nuclei are made up of these stable clusters. As a result, clusters must reach a critical size to become stable nuclei. Critical size is influenced by a number of factors (temperature, supersaturation, etc.). 

The atoms or molecules are arranged in a defined and periodic manner during the nucleation stage, representing the crystal structure – note that “crystal structure” is a special term that refers to the relative arrangement of the atoms or molecules, rather than the macroscopic properties of the crystal (size and shape). However, these are a result of the internal crystal structure.

Crystal growth refers to the subsequent size increase of nuclei that reach the critical cluster size. Crystal growth is a dynamic equilibrium process that precipitates and dissolves solute molecules or atoms back into the solution.

Methods of purification of organic compounds by sublimation

Sublimation is converting a solid to a gas without going through the liquid state. Sublimation is an endothermic process that occurs when a substance approaches its triple point or the lowest pressure to remain a liquid.

Deposition, also known as desublimation, is the reversal of sublimation, which occurs when a substance transitions from a gas to a solid without going through the liquid phase. Sublimation is defined as a solid-to-gas transition followed by a gas-to-solid transition (deposition).

Suppose the temperature is below the boiling point. In that case, evaporation from the surface occurs, and if the temperature is above the boiling point, boiling with bubble formation in the liquid’s core appears. If the temperature is above the boiling point, the transition from solid to gas occurs as sublimation from the surface.

Most chemical compounds and elements exist in three states at normal pressures and temperatures. In these situations, the presence of an intermediate liquid state is essential for the transition from solid to gaseous.

The pressure at issue is the partial pressure of the drug, not the system’s total pressure (for example, atmospheric). As a result, any solid with a perceptible vapour pressure (for example, water ice slightly below 0 °C) may generally sublimate in the air at a given temperature.

Sublimation is the physical transformation of a solid into a gas instead of chemical conversion. For example, solid ammonium chloride decomposes into hydrogen chloride and ammonia when heated. This is not sublimation but a chemical reaction.

Similarly, a chemical reaction with oxygen is used to convert paraffin wax candles to carbon dioxide and water vapour rather than sublimation.

Heat absorption provides enough energy for specific molecules to overcome their attractive connections with their neighbours and escape into the vapour phase, resulting in sublimation.

Because the process requires more energy, it is an endothermic shift. By summing the enthalpies of fusion and vaporisation, the enthalpy of sublimation (also known as the heat of sublimation) may be calculated.

A substance is heated under a vacuum in sublimation equipment. The solid volatilizes and condenses as a purified chemical on a cooled surface (cold finger) under this lowered pressure, leaving a non-volatile residue of impurities behind.

When the heating is turned off and the vacuum is released, the purified material may be collected from the cooling surface. A temperature gradient is applied to boost purification efficiency while separating various fractions. An evacuated glass tube is heated slowly and accurately in most applications.

Conclusion:

Some solids can move directly from the liquid to the vapour state, bypassing the liquid phase. Sublimation is one of the methods of purification of organic compounds that takes advantage of this property. In addition, it can differentiate between sublimabal and non-sublimable compounds. 

Crystallization’s fundamental concept in purification methods of organic compounds is that different compounds and impurities are soluble in various solvents. Therefore, it is decided to use a solvent in which the compound to be purified is sparingly soluble, that is, sparingly soluble at low temperatures but soluble at high temperatures. Filtration is used to remove the crystals of the compounds after heating the solution to achieve a saturated solution.