Purification of Organic Compounds - Practice Questions & MCQ

Sublimation 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 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.

Simple distillation 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 simple distillation process:    

(i) Take a mixture(Acetone and water) in the distillation flask 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 thermometer.
(iv) Since acetone has a lower boiling point, it starts to vaporise and condenses 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 complete separation of the individual compounds.

Fractional distillation 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 become richer in more volatile component and descending liquid phase become richer in less volatile component.

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 lower boiling point starts to boil first and convert into vapours.

Fractional distillation is used for 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 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 into 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 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 suction pump or vacuum pump, the liquid boils at 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 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 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 interpretated that organic compounds distil below its normal boiling point without decomposition. For example,
(1) Aniline can be distilled at 371.5 K against its normal boiling point 457 K.  
(2) o-Nitrophenol can be seperated from p-Nitrophenol, since o-Nitrophenol is volatile in steam.

Differential extraction also 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 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 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 that 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 flow 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 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 as 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.