CLASS-11th LAKSHYA INSTITUTE CHEMISTRY
(ORGANIC CHEMISTRY) Ch-7 Organic chemistry
Purification of organic compounds:-
Purification of organic compounds are based on the nature of the compound and the impurity
present in it.
Techniques used for purification:-
(i) Sublimation
(ii) Crystallisation
(iii) Distillation
(iv) Differential extraction
(v) Chromatography
Sublimation:-
Solid change in vapour without going to liquid.
It is used to separate Sublimable compounds from non-Sublimable
impurities.
Ex:- Sublime compounds iodine, Camphor, Naphthalene,
Ammonium chloride etc
Crystallisation:-
• The difference in solubilities of compounds and the impurities in a suitable solvent.
• It is one of the most commonly used technique for Purification of Solid organic compound.
• The impure compound is dissolved in a solvent in which it is sparingly soluble at room
temperature but appreciably soluble at higher temperature. On cooling the solution pure
compound crystallized out and removed by filtration.
• If the Compound is highly soluble in one solvent and very little soluble in another solvent.
• Crystallisation can be carried out in mixture.
• Impurities which impart colour to the solution are removed by adsorbing over activated charcoal.
• Repeated crystallisation becomes necessary for purification of compound containing impurities of
comparable Solubilities.
Distillation:-
Distillation involves conversion of a liquid into vapours by heating followed by condensation of
vapour produced by cooling.
Types of distillation:-
(a). Simple distillation:- when liquid sample has non-volatile impurities
or when the difference in boiling point of component is 25 K or more.
Technique: - Mixture is heated in a round-bottom flask fitted with a water
condenser. Vapour of lower boiling components are formed first which
are collected on condensation. Vapour of higher boiling components are
formed later and condensed in the same way.
Ex:- Chloroform (b.p 334 K) and aniline (b.p. 457 K) Benzene (b.p 353 K) and Chloroform (b.p 334
K)
(b) Distillation under reduced pressure (vacuum):-
when Liquid boils at higher temperature and it may decompose
before boiling Point is attained distillation under reduced Pressure
is done
Ex:- Recovery of glycerol from spent-lye, concentration of sugar juice
in the soap industries.
Technique:- A liquid boils at a temperature at which its vapour
pressure is equal the external pressure and liquid can be made
to boil at lower temperature (to prevent its decomposition) by the
application of reduced pressure with the help of water pump or
vacuum pump.
(c) Steam distillation:-
• When the substance is immiscible with water and steam-volatile.
• It can be purified by distillation in a current of steam.
• The liquid boils when the sum of vapour pressures
due to the organic liquid (p₁) and due to water (p₂)
becomes equal to the atmospheric pressure (p),
p =p₁+p₂. Since p₁ is lower than p, the organic
liquid evaporates at lower temperature than
its boiling point.
Ex:- (i). Aniline is separated by this technique from
aniline water mixture
(ii). Ortho nitrophenol and para nitro phenol is
separated by this method
Technique:- Steam from a steam generator is fasted through a heated Flask containing the
liquid. Mixture boils close to but below 373 K (b.p of water). The mixture of steam and volatile
organic compounds is condensed and collected.
The compound is later separated from water using a separating funnel.
(d) Fractional distillation:-
when the difference in boiling points of two liquid is not too much,
vapours of both liquids are formed simultaneously and also vapours
are formed at the same time. In this case fractional distillation is
adopted.
Technique:- Fractionating Columns of various types are used.
Ex:- Crude oil in the Petroleum industry.
Differential extraction:-
when organic substance (Liquid or Solid) is present in aqueous solution.
It can be extracted into another organic liquid by a method called solvent
extraction.
Based on:-
• Organic solvent forms a distinct (separatable) layer with water.
• Solubility of the organic substance to be extracted in organic
solvent should be larger than water
• Organic solvent is volatile so that it can be removed by
distillation or evaporation.
• Greater the number of extraction by the solvent, larger
the amount extracted.
• For the solubility of organic substance in organic solvent
and water at equilibrium
𝑆𝑜𝑙𝑢𝑏𝑖𝑙𝑖𝑡𝑦 𝑖𝑛 𝑜𝑟𝑔𝑎𝑛𝑖𝑐 𝑆𝑜𝑙𝑣𝑒𝑛𝑡
𝐾= 𝑆𝑜𝑙𝑢𝑏𝑖𝑙𝑖𝑡𝑦 𝑖𝑛 𝑤𝑎𝑡𝑒𝑟
If K >1, thus repeated extraction will extract organic substance
from water to organic substance solvent.
Chromatography:-
Chromatography is an important technique extensively used to separate mixtures into their
components, purify compounds and also to test the purity of compounds.
This technique is used for separation of colour two or more substance mixtures in different phases.
Mobile Phase:- which moves relative to the other.
Stationary phase:- The substance amount is higher but moves slowly.
Cs / Cm = k,
K → Partition coefficient of substance.
Cs → Concentration of substance in stationary
Cm → Concentration of substance in mobile
A pure solvent, a mixture of solvents, or a gas is allowed to move slowly over the stationary phase.
Types of Chromatography:-
(a) Adsorption chromatography:- Stationary phase is a solid on which the sample component are
absorbed. The mobile phase may be liquid (liquid-solid chromatography) or a gas (gas-solid
chromatography). Commonly used adsorbents are silica gel and alumina. When a mobile phase
is allowed to move over a stationary phase (adsorbent), the components of the mixture move by
varying distances over the stationary phase.
Two main types of chromatographic techniques based on the principle of differential
adsorption:-
(i) Column chromatography:-
The mixture adsorbed on adsorbent is placed on the top of the
adsorbent column packed in a glass tube.
• An appropriate eluent which is a liquid or a mixture of liquids
is allowed to flow down the column slowly.
• Depending upon the degree to which the compounds are adsorbed,
complete separation takes place.
• The most readily adsorbed substances are retained near the top
and others come down to various distances in the column.
(ii) Thin layer chromatography:-
A thin layer (about 0.2mm thick) of an adsorbent (silica gel or alumina)
is spread over a glass plate of suitable size. Plate is known as thin layer
chromatography plate or chromaplate.
• The solution of the mixture to be separated is applied as a small spot
about 2 cm above one end of the TLC plate.
• The glass plate is then placed in a closed jar containing the eluant.
• As the eluent rises up the plate, the components of the mixture move
up along with the eluent to different distances depending on their degree
of absorption and separation takes place.
• The relative adsorption of each component of the mixture is expressed in
terms of its retardation factor (Rf value).
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑒𝑑 𝑏𝑦 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑜𝑢𝑛𝑑
Rf value = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑒𝑑 𝑏𝑦 𝑡ℎ𝑒 𝑠𝑜𝑙𝑣𝑒𝑛𝑡
, Rf α 1/Polarity
Detection techniques:-
• The spots of coloured compounds are visible on the TLC plate due to their original colour.
• The spots of colourless compounds can be detected by putting the plate under ultraviolet light.
• By placing the plate in a covered jar containing a few crystals of iodine, spots of compounds which
absorb iodine will show up as brown spots.
• Sometimes. an appropriate reagent may also be sprayed on the plate.
Ex:- amino acids may be detected by spraying the plate with ninhydrin solution.
Rf of A = x/z
Rf of B = y/z
Rf of A >Rf of B (Adsorption power)
Col. chro. A → upper (Gravity action)
B → Lower
T.L.C A → Lower (Capillary action)
B → upper
(b) Partition chromatography:-
It is based on continuous differential partitioning of components of a
mixture between stationary and mobile phases.
• In paper chromatography, a special quality paper known as chromatography
paper is used. Chromatography paper contains water trapped in it, which
acts as the stationary phase.
• A strip of chromatography paper spotted at the base with the solution of
the mixture is suspended in a suitable solvent or a mixture of solvents.
• The solvent rises up the paper by capillary action and flows over the spot.
• The paper strip so developed is known as a chromatogram.
• The spots of the separated coloured compounds are visible at different heights
from the position of the initial spot on the chromatogram.
• The spots of the separated colourless compounds may be observed either/under
ultraviolet light or by the use of an appropriate spray reagent as discussed under
thin layer chromatography.
The main differences between TLC and paper chromatography are:
1. The principle behind thin-layer chromatography is based on adsorption. On the other hand, the
principle of paper chromatography is based on partition.
2. The stationary phase of thin-layer chromatography is the glass plates coated with silica gel
whereas the stationary phase of paper chromatography is the water trapped in the cellulose filter
paper.
3. In thin-layer chromatography, corrosive reagents can be used but not in the case of paper
chromatography, as the corrosive agents can destroy the paper.
