chemistry

METHODS OF PURIFICATION OF ORGANIC COMPOUNDS

Organic compounds obtained either from natural sources or synthetic processes in the laboratories are contaminated with impurities. These are purified by different methods.

I.            Methods of purification of solids:

1.Crystallisation:-

This method is useful to purify the solid organic compounds.

The principle involved in this method is that the compound should be insoluble at low temperature but soluble at higher temperatures in the given solvent. Impurities are either insoluble or soluble and go into filtrate.

The insoluble impurities are removed by filteration in hot condition.

The crystals of the compound are seperated by filtering under reduced pressure using Buckner funnel.

Repeated crystallisation is required and the coloured impurities are removed by adsorbing them with activated charcoal.

2.Sublimation : -

It is used to purify activated solid organic compounds which undergoes sublimation.

If the compound is sublimating the impurity should not sublimate.

If the compound to be purified has high vapour pressure below its melting point and sublimates readily on heating and the impurities don’t sublimate.

The pure compound is seperated by scratching the watch glass.

If the sublimating substances have a low vapour pressure or decompose on heating before sublimation, then the sublimation is carried out under low pressure.

II.         Methods of purification of liquids:-

1.Simple distillation: - The vapourisation of a liquid by heating and subsequent condensation of vapours by cooling is known as distillation.

This process is useful for purification of liquids contaminated with non volatile impurities.

The liquids that have boiling point difference greater than 40⁰ C can be purified by this method.

2.Fractional distillation:-

This process is useful for the purification of liquids having boiling point difference less than 40⁰C.

In this process, liquid with high b.p. is collected in the condenser and liquid with low b.p. is collected in the receiver.

3.Distillation under reduced pressure:-

This method is useful to purify liquids that have very high boiling points and those which decompose at or below their boiling points.

At reduced pressure the boiling point of a liquid is also reduced. Hence its decomposition is prevented.

4. Steam distillation:-

The liquids insoluble in water possess high boiling point and steam volatile and the impurities are not steam volatile are purified by this method.

Principle in steam distillation is - sum of the vapour pressure of organic liquid (P₁) and that of

water (P₂) is equal to atmospheric pressure (P) and mixture boils.

The water layer and the organic liquid layer are seperated using separating funnel. Ex:- Aniline is purified by this method from Aniline - water mixture

Relative masses obtained in steam distillation process is calculated by a formula =

W

A

=

n

M

A

=

p0 M

A

; Where W

and W  are the masses of A and B

A

A

W

n M

p0 M

B

B

A

B

B

B

B

M _Aand M _Bare the molecular masses of A and B P_A⁰and P_B⁰are the vapour pressures of A and B

At 98.5⁰ C water and aniline have vapour pressures 717 torr and 43 torr. In steam distallation at

	0	WH 2O		717 ×18
	98.5 C , the relative masses obtained are		=	43×93 = 3.23
		WAniline
5.	Solvent extraction( differential extraction) :-
	In this method, organic compound is separated from its aqueous solution by using its solubility
	which is different with organic solvent and water.
6.	Chromatography:-
	It was discoverd by Tswett in (1906). He separated chlorophyll and Xanthophyll and other
	compounds by using adsorbent CaCO3  .
	It is classified into three types based on the physical states of stationary phase and mobile phase.

Chromatography involves the three steps

a)   Adsorption and retention of a mixture of substances on the stationary phase and separation of adsorbed substances by the mobile phase to different distance on the stationary phase.

b)     Recovery of the substances separated by a continuous flow of the mobile phase (known as elution)

c)      Qualitative and quantitative analysis of the eluted substances

	S.No.	Chromatography Process	Stationary Phase	Mobile Phase
	1.	Column chromatography (Adsorption)	Solid	Liquid
	2.	Liquid – liquid partition	Liquid	Liquid
		chromatography
	3.	Paper chromatography	Liquid	Liquid
	4.	Thin layer chromatography (TLC)	Liquid (or) solid	Liquid
	5.	Gas – liquid chromatography (GLC)	Liquid	Gas
	6.	Gas – solid chromatography (GSC)	Solid	Gas
	7.	Ionic change chromatography	Solid	Liquid

Important chromatography techniques are

1)   Adsorption chromatography : a) Column chromatography, b) Thin layer chromatography

2)   Partition chromatography :  a) Paper chromatography

a)   Column Chromatography

The principle used in this method is differential adsorption.

In the column chromatography the components of a mixture are separated by a column of adsorbent (stationary phase) packed in a glass tube.

The mixture to be adsorbed on the adsorbent is placed at the top of the stationary phase.

A suitable eluant, either a single solvent or a mixture of solvents is allowed to flow down the column slowly.

The most readily adsorbed substances are retained near the top and others come down

accordingly to various distances. b)Thin layer chromatography (TLC):

This also based on the adsorption differences.

The glass plate which is coated with adsorbent (Eg : Silica gel, alumina) as a thin layer (0.2mm thick) is called TLC plate or chromoplate.

The plate is then kept in a closed jar containing the eluant.

The relative adsorption of a component of the mixture is expressed in terms of RETARDATION FACTOR (R_f) value.

_{Rf =} Distance moved by thesubstance from base line(X)

                Distance moved by thesolvent from base line (Y)

The colourless compounds which fluorescence are detected with ultraviolet light.

Spots of compounds are even detected by allowing them to adsorb iodine, when they give brown spots.

Some times an appropriate reagent is sprayed, as ninhydrin solution to detect aminoacids.

c)  Partition Chromatography

This is based on continuous differential partitioning of components of a mixture between the stationary phase and the mobile phase.

In paper chromatography, a special paper called chromatography paper contains water trapped in it which acts as the stationary phase.

The chromatography paper spotted with the solution of the mixture at the base is suspended in a suitable solvent or a mixture of solvents. This solvent (s) acts as the mobile phase.

The solvent rises up the paper by capillary action and moves over the spot.

The paper selectively retains different components as per their differing partition in mobile and stationary phases and is known as chromatogram.

The spots of the separated coloured compounds are detected.