Question

Motivational factor in purification of organic compound....

Answer 1

 

Types of Purification

A large number of methods are available for the purification of substances. The choice of method, however, depends upon the nature of substance (whether solid or liquid). It also depends on the type of impurities present in it. We commonly use these methods for purification of substances:
Simple crystallisation
Fractional crystallisation
Sublimation
Simple distillation
Fractional distillation
Distillation under reduced pressure
Steam distillation
Azeotropic distillation
Let us now study about these methods in brief for better understanding.

Simple Crystallisation
This is the most common method that we use to purify organic solids. For crystallisation, a suitable solvent is one

which dissolves more of the substance at a higher temperature than at room temperature

in which impurities are either insoluble or dissolve to an extent that they remain in solution (in the mother liquor) upon crystallisation

which is not highly inflammable and

which does not react chemically with the compound to be crystallized. The most commonly-used solvents for crystallisation are water, alcohol, ether, chloroform, carbon- tetrachloride, acetone, benzene, petroleum ether etc.

Fractional Crystallisation
It is the process of separation of different components of a mixture by repeated crystallisations. In the first step, we dissolve the mixture in a solvent in which the two components have different solubilities. When we cool a hot saturated solution of this mixture, the less soluble component crystallises out first while the more soluble substance remains in solution.
The mother liquor left after crystallisation of the less soluble component is again concentrated and then we allow it to cool. Hence, we obtain the crystals of the more soluble component.

Sublimation
Certain organic solids on heating directly change from solid to vapour state without passing through a liquid state. These substances are sublimable. This process is sublimation.
We use this process for the separation of sublimable volatile compounds from non-sublimable impurities. We use this for the purposes of purification of camphor, naphthalene, anthracene, benzoic acid, Iodine and salicylic acid etc containing non-volatile impurities.

Simple Distillation
Distillation is the joint process of vapourisation and condensation. We use this method for the purification of liquids which boil without decomposition and contain non-volatile impurities. We can also use this method for separating liquids having sufficient difference in their boiling points

Fractional Distillation
We can use this process to separate a mixture of two or more miscible liquids which have boiling points close to each other. We carry out this process by using fractionating columns. The fractionating column is a special type of long glass tube that has obstructions to the passage of the vapour upwards and that of liquid downwards. This method can separate a mixture of acetone (b. p. 330 K) and methyl alcohol (b. p. 338 K) or a mixture of benzene and toluene.

Distillation under Reduced Pressure

We use this method for the purification of high boiling liquids and liquids which decompose at or below their boiling points. Practical examples include the crude oil industry, sugarcane industry etc.

Steam Distillation
This method is applicable for the separation and purification of those organic compounds (solids or liquids) which:

are insoluble in water

are volatile in steam

possess a high vapour pressure (10-15 mm Hg) at 373 K and

contain non-volatile impurities.

Azeotropic Distillation
An azeotropic mixture is a mixture having a constant boiling point. The most familiar example is a mixture of ethanol and water in the ratio of 95.87: 4.13 (a ratio present in rectified spirit). It boils at 78.13oC. We can’t separate the constituents of an azeotropic mixture by fractional distillation. Hence, we have to use a special type of distillation (azeotropic distillation) for separating the constituents of an azeotropic mixture.

In this method, we use the third compound in distillation. The process uses the fact that dehydrating agents like diethyl ether etc. depress the partial pressure of one of the original components. As a result, the boiling point of that component raises sufficiently and thus, the other component will distil over.






MARK ME AS A BRAINLIST IF YOU THINK ITS HEPFUL