Purification and Characterisation of Organic Compounds Chapter Notes | Chemistry Class 12 - NEET PDF Download

 Page 1


 
             
 The word ‘organic’ signifies life. Therefore, all 
substances which were obtained directly or indirectly 
from living organisms, plants and animals were called 
organic compounds and the branch of chemistry which 
deals with these compounds was called organic 
chemistry. 
 Modern definition of organic chemistry : 
Organic chemistry is a chemistry of hydrocarbons and 
their derivatives in which covalently bonded carbon is 
an essential constituent. 
 Berzelius put forward a theory in 1815 known as 
vital force theory. According to this theory, "organic 
compounds could be prepared only by living organism 
under the influence of a mysterious force known as vital 
force".  Accidental synthesis of urea by Wohler and 
synthesis of acetic acid by Kolbe led to the fall of this 
theory. 
   
O
NH C NH CNO NH
| |
2 2 4
? ? ? ? ?
?
  
 
 
  COOH CH CHO CH
O
? ? ? ? ? ?
3
] [
3
  
 
 
  
 Berthelot prepared methane in laboratory and 
the most abundant organic compound is cellulose 
which is a polymer of glucose. Kekule and Couper 
proposed the tetravalency of carbon and wrote the first 
structural formula. In 1874, Van't Hoff and Le Bell 
suggested a tetrahedron model of carbon. 
Purification and Characterisation of organic  
compounds 
 The study of organic compounds starts with the 
characterisation of the compound and the 
determination of its molecular structure. The procedure 
generally employed for this purpose consists of the 
following steps :  
 (1) Purification of organic compounds 
 (2) Qualitative analysis of organic compounds 
 (3) Quantitative analysis of organic compounds 
 (4) Determination of molecular mass of organic 
compounds 
 (5) Calculation of Empirical formula and 
Molecular formula of organic compounds 
 (6) Determination of structure of organic 
compounds by spectroscopic and diffraction methods 
 (1) Purification of organic compounds :  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) 
and the type of impurities present in it. Following 
methods are commonly used for this purpose, 
 (i) Simple crystallisation  
 (ii) Fractional crystallisation, 
 (iii) Sublimation  
 (iv) Simple distillation  
 (v) Fractional distillation  
 (vi) Distillation under reduced pressure 
 (vii) Steam distillation  
 (viii) Azeotropic distillation 
 (ix) Chromatography  
 (x) Differential extraction 
 (xi) Chemical methods  
    
(Ammonium 
cyanate) 
(First organic compound 
synthesised in laboratory) 
Urea 
Acetic acid 
(First organic compound 
synthesised from its 
elements) 
Acetaldehyde 
 
 
 
Page 2


 
             
 The word ‘organic’ signifies life. Therefore, all 
substances which were obtained directly or indirectly 
from living organisms, plants and animals were called 
organic compounds and the branch of chemistry which 
deals with these compounds was called organic 
chemistry. 
 Modern definition of organic chemistry : 
Organic chemistry is a chemistry of hydrocarbons and 
their derivatives in which covalently bonded carbon is 
an essential constituent. 
 Berzelius put forward a theory in 1815 known as 
vital force theory. According to this theory, "organic 
compounds could be prepared only by living organism 
under the influence of a mysterious force known as vital 
force".  Accidental synthesis of urea by Wohler and 
synthesis of acetic acid by Kolbe led to the fall of this 
theory. 
   
O
NH C NH CNO NH
| |
2 2 4
? ? ? ? ?
?
  
 
 
  COOH CH CHO CH
O
? ? ? ? ? ?
3
] [
3
  
 
 
  
 Berthelot prepared methane in laboratory and 
the most abundant organic compound is cellulose 
which is a polymer of glucose. Kekule and Couper 
proposed the tetravalency of carbon and wrote the first 
structural formula. In 1874, Van't Hoff and Le Bell 
suggested a tetrahedron model of carbon. 
Purification and Characterisation of organic  
compounds 
 The study of organic compounds starts with the 
characterisation of the compound and the 
determination of its molecular structure. The procedure 
generally employed for this purpose consists of the 
following steps :  
 (1) Purification of organic compounds 
 (2) Qualitative analysis of organic compounds 
 (3) Quantitative analysis of organic compounds 
 (4) Determination of molecular mass of organic 
compounds 
 (5) Calculation of Empirical formula and 
Molecular formula of organic compounds 
 (6) Determination of structure of organic 
compounds by spectroscopic and diffraction methods 
 (1) Purification of organic compounds :  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) 
and the type of impurities present in it. Following 
methods are commonly used for this purpose, 
 (i) Simple crystallisation  
 (ii) Fractional crystallisation, 
 (iii) Sublimation  
 (iv) Simple distillation  
 (v) Fractional distillation  
 (vi) Distillation under reduced pressure 
 (vii) Steam distillation  
 (viii) Azeotropic distillation 
 (ix) Chromatography  
 (x) Differential extraction 
 (xi) Chemical methods  
    
(Ammonium 
cyanate) 
(First organic compound 
synthesised in laboratory) 
Urea 
Acetic acid 
(First organic compound 
synthesised from its 
elements) 
Acetaldehyde 
 
 
 
 
                                       Purification, Classification and Nomenclature of Organic compounds 971 
 (i) Simple crystallisation : This is the most 
common method used to purify organic solids. It is 
based upon the fact that whenever a crystal is formed, 
it tends to leave out the impurities. For crystallisation, 
a suitable solvent is one (a) which dissolves more of 
the substance at higher temperature than at room 
temperature (b) in which impurities are either 
insoluble or dissolve to an extent that they remain in 
solution (in the mother liquor) upon crystallisation, 
(c) which is not highly inflammable and (d) 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.        
 Examples : (a) Sugar having an impurity of 
common salt can be crystallized from hot ethanol since 
sugar dissolves in hot ethanol but common salt does 
not. 
 (b) A mixture of benzoic acid and naphthalene can 
be separated from hot water in which benzoic acid 
dissolves but naphthalene does not. 
 (ii) Fractional crystallisation : The process of 
separation of different components of a mixture by 
repeated crystallisations is called fractional 
crystallisation. The mixture is dissolved in a solvent in 
which the two components have different solubilities. 
When a hot saturated solution of this mixture is 
allowed to cool, 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 allowed to cool when the crystals of the more 
soluble component are obtained. The two components 
thus separated are recrystallized from the same or 
different solvent to yield both the components of the 
mixture in pure form.    
 Fractional crystallisation can be used to separate 
a mixture of 
3
KClO (less soluble) and KCl (more 
soluble). 
 (iii) Sublimation : Certain organic solids on 
heating directly change from solid to vapour state 
without passing through a liquid state, such substances 
are called sublimable and this process is called 
sublimation.  
                          
 
 The sublimation process is used for the separation 
of sublimable volatile compounds from non sublimable 
impurities. The process is generally used for the 
purification of camphor, naphthalene, anthracene, 
benzoic acid 
2 4
, HgCl Cl NH , solid 
2
SO , Iodine and 
salicylic acid etc containing non-volatile impurities.  
 (iv) Simple distillation : Distillation is the joint 
process of vapourisation and condensation. This 
method is used for the purification of liquids which boil 
without decomposition and contain non-volatile 
impurities. This method can also be used for separating 
liquids having sufficient difference in their boiling 
points. This method can be used to separate a mixture 
of 
 (a) chloroform (b. p. 334 K) and aniline (b. p. 457 
K) 
 (b) ether (b. p. 308 K) and toluene (b. p. 384 K) 
 (v) Fractional distillation : This process is used 
to separate a mixture of two or more miscible liquids 
which have boiling points close to each other. Since in 
this process, the distillate is collected in fractions 
under different temperatures, it is known as fractional 
distillation. This process is carried out by using 
fractionating columns.  Fractionating column is a 
special type of long glass tube provided with 
obstructions to the passage of the vapour upwards and 
that of liquid downwards. This method may be used to 
separate a mixture of acetone (b. p. 330 K) and methyl 
alcohol (b. p. 338 K) or a mixture of benzene and 
toluene.  One of the technological applications of 
fractional distillation is to separate different fractions 
of crude oil in petroleum industry into various useful 
fractions such as gasoline, kerosene oil, diesel oil, 
lubricating oil etc. 
 (vi) Distillation under reduced pressure : This 
method is used for the purification of high boiling 
liquids and liquids which decompose at or below their 
boiling points. 
 The crude liquid is heated in distillation flask 
fitted with a water condenser, receiver and vacuum 
pump. As the pressure is reduced, the liquid begins to 
boil at a much lower temperature than its normal 
boiling point. The vapour is condensed by water 
condenser and the pure liquid collects in the receiver. 
 Glycerol which decomposes at its boiling point 
(563 K) under atmospheric pressure can be distilled 
without decomposition at 453 K under 12 mm of Hg. 
Similarly, sugarcane juice is concentrated in sugar 
industry by evaporation under reduced pressure which 
saves a lot of fuel. 
 (vii) Steam distillation : This method is 
applicable for the separation and purification of those 
organic compounds (solids or liquids) which (a) are 
insoluble in water (b) are volatile in steam (c) possess 
a high vapour pressure (10-15 mm Hg) at 373 K and (d) 
contain non-volatile impurities. 
 Aniline (b. p. 457 K) can be purified by steam 
distillation since it boils at a temperature of 371.5 K in 
Solid 
Vapour 
Heat 
Cool 
Page 3


 
             
 The word ‘organic’ signifies life. Therefore, all 
substances which were obtained directly or indirectly 
from living organisms, plants and animals were called 
organic compounds and the branch of chemistry which 
deals with these compounds was called organic 
chemistry. 
 Modern definition of organic chemistry : 
Organic chemistry is a chemistry of hydrocarbons and 
their derivatives in which covalently bonded carbon is 
an essential constituent. 
 Berzelius put forward a theory in 1815 known as 
vital force theory. According to this theory, "organic 
compounds could be prepared only by living organism 
under the influence of a mysterious force known as vital 
force".  Accidental synthesis of urea by Wohler and 
synthesis of acetic acid by Kolbe led to the fall of this 
theory. 
   
O
NH C NH CNO NH
| |
2 2 4
? ? ? ? ?
?
  
 
 
  COOH CH CHO CH
O
? ? ? ? ? ?
3
] [
3
  
 
 
  
 Berthelot prepared methane in laboratory and 
the most abundant organic compound is cellulose 
which is a polymer of glucose. Kekule and Couper 
proposed the tetravalency of carbon and wrote the first 
structural formula. In 1874, Van't Hoff and Le Bell 
suggested a tetrahedron model of carbon. 
Purification and Characterisation of organic  
compounds 
 The study of organic compounds starts with the 
characterisation of the compound and the 
determination of its molecular structure. The procedure 
generally employed for this purpose consists of the 
following steps :  
 (1) Purification of organic compounds 
 (2) Qualitative analysis of organic compounds 
 (3) Quantitative analysis of organic compounds 
 (4) Determination of molecular mass of organic 
compounds 
 (5) Calculation of Empirical formula and 
Molecular formula of organic compounds 
 (6) Determination of structure of organic 
compounds by spectroscopic and diffraction methods 
 (1) Purification of organic compounds :  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) 
and the type of impurities present in it. Following 
methods are commonly used for this purpose, 
 (i) Simple crystallisation  
 (ii) Fractional crystallisation, 
 (iii) Sublimation  
 (iv) Simple distillation  
 (v) Fractional distillation  
 (vi) Distillation under reduced pressure 
 (vii) Steam distillation  
 (viii) Azeotropic distillation 
 (ix) Chromatography  
 (x) Differential extraction 
 (xi) Chemical methods  
    
(Ammonium 
cyanate) 
(First organic compound 
synthesised in laboratory) 
Urea 
Acetic acid 
(First organic compound 
synthesised from its 
elements) 
Acetaldehyde 
 
 
 
 
                                       Purification, Classification and Nomenclature of Organic compounds 971 
 (i) Simple crystallisation : This is the most 
common method used to purify organic solids. It is 
based upon the fact that whenever a crystal is formed, 
it tends to leave out the impurities. For crystallisation, 
a suitable solvent is one (a) which dissolves more of 
the substance at higher temperature than at room 
temperature (b) in which impurities are either 
insoluble or dissolve to an extent that they remain in 
solution (in the mother liquor) upon crystallisation, 
(c) which is not highly inflammable and (d) 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.        
 Examples : (a) Sugar having an impurity of 
common salt can be crystallized from hot ethanol since 
sugar dissolves in hot ethanol but common salt does 
not. 
 (b) A mixture of benzoic acid and naphthalene can 
be separated from hot water in which benzoic acid 
dissolves but naphthalene does not. 
 (ii) Fractional crystallisation : The process of 
separation of different components of a mixture by 
repeated crystallisations is called fractional 
crystallisation. The mixture is dissolved in a solvent in 
which the two components have different solubilities. 
When a hot saturated solution of this mixture is 
allowed to cool, 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 allowed to cool when the crystals of the more 
soluble component are obtained. The two components 
thus separated are recrystallized from the same or 
different solvent to yield both the components of the 
mixture in pure form.    
 Fractional crystallisation can be used to separate 
a mixture of 
3
KClO (less soluble) and KCl (more 
soluble). 
 (iii) Sublimation : Certain organic solids on 
heating directly change from solid to vapour state 
without passing through a liquid state, such substances 
are called sublimable and this process is called 
sublimation.  
                          
 
 The sublimation process is used for the separation 
of sublimable volatile compounds from non sublimable 
impurities. The process is generally used for the 
purification of camphor, naphthalene, anthracene, 
benzoic acid 
2 4
, HgCl Cl NH , solid 
2
SO , Iodine and 
salicylic acid etc containing non-volatile impurities.  
 (iv) Simple distillation : Distillation is the joint 
process of vapourisation and condensation. This 
method is used for the purification of liquids which boil 
without decomposition and contain non-volatile 
impurities. This method can also be used for separating 
liquids having sufficient difference in their boiling 
points. This method can be used to separate a mixture 
of 
 (a) chloroform (b. p. 334 K) and aniline (b. p. 457 
K) 
 (b) ether (b. p. 308 K) and toluene (b. p. 384 K) 
 (v) Fractional distillation : This process is used 
to separate a mixture of two or more miscible liquids 
which have boiling points close to each other. Since in 
this process, the distillate is collected in fractions 
under different temperatures, it is known as fractional 
distillation. This process is carried out by using 
fractionating columns.  Fractionating column is a 
special type of long glass tube provided with 
obstructions to the passage of the vapour upwards and 
that of liquid downwards. This method may be used to 
separate a mixture of acetone (b. p. 330 K) and methyl 
alcohol (b. p. 338 K) or a mixture of benzene and 
toluene.  One of the technological applications of 
fractional distillation is to separate different fractions 
of crude oil in petroleum industry into various useful 
fractions such as gasoline, kerosene oil, diesel oil, 
lubricating oil etc. 
 (vi) Distillation under reduced pressure : This 
method is used for the purification of high boiling 
liquids and liquids which decompose at or below their 
boiling points. 
 The crude liquid is heated in distillation flask 
fitted with a water condenser, receiver and vacuum 
pump. As the pressure is reduced, the liquid begins to 
boil at a much lower temperature than its normal 
boiling point. The vapour is condensed by water 
condenser and the pure liquid collects in the receiver. 
 Glycerol which decomposes at its boiling point 
(563 K) under atmospheric pressure can be distilled 
without decomposition at 453 K under 12 mm of Hg. 
Similarly, sugarcane juice is concentrated in sugar 
industry by evaporation under reduced pressure which 
saves a lot of fuel. 
 (vii) Steam distillation : This method is 
applicable for the separation and purification of those 
organic compounds (solids or liquids) which (a) are 
insoluble in water (b) are volatile in steam (c) possess 
a high vapour pressure (10-15 mm Hg) at 373 K and (d) 
contain non-volatile impurities. 
 Aniline (b. p. 457 K) can be purified by steam 
distillation since it boils at a temperature of 371.5 K in 
Solid 
Vapour 
Heat 
Cool 
 
           972 Purification, Classification and Nomenclature of Organic compounds 
presence of steam. Other compounds which can be 
purified by steam distillation are: nitrobenzene, 
bromobenzene,  o-nitrophenol, salicylaldehyde, o-
hydroxyacetophenone, essential oils, turpentine oil etc. 
 (viii) Azeotropic distillation : Azeotropic mixture 
is a mixture having 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 78.13
o
C. The constituents of an 
azeotropic mixture can't be separated by fractional 
distillation. Hence a special type of distillation 
(azeotropic distillation) is used for separating the 
constituents of an azeotropic mixture. 
 In this method a third compound is used in 
distillation. The process is based on the fact that 
dehydrating agents like 
4 , 6 6
CCl H C , diethyl ether etc. 
depress the partial pressure of one of the original 
components. As a result, the boiling point of that 
component is raised sufficiently and thus the other 
component will distil over. 
 Dehydrating agents having low boiling point (e.g. 
, 4 , 6 6
CCl H C ether) depress the partial pressure of 
alcohol more than that of water; on the other hand, 
dehydrating agents having high boiling point (glycerol, 
glycol) depress the partial pressure of water more than 
that of alcohol.                  
 (ix) Chromatography : This is a modern method 
used for the separation of mixtures into its 
components, purification of compounds and also to test 
the purity of compounds. The name chromatography is 
based on the Greek word 'chroma' meaning colour and 
'graphy' for writing because the method was first used 
for the separation of coloured substances found in 
plants. This method was described by Tswett in 1906.   
 (a) Principle of chromatography : The technique of 
chromatography is based on the difference in the rates 
at which the components of a mixture move through a 
porous medium (called stationary phase) under the 
influence of some solvent or gas (called moving phase). 
Thus, this technique consists of two phases- one is a 
stationary phase of large surface area while the second 
is a moving phase which is allowed to move slowly over 
the stationary phase. The stationary phase is either a 
solid or a liquid while the moving phase may be a liquid 
or a gas.     
 (b) Types of chromatography : Depending upon 
the nature of the stationary and the mobile phases, the 
different types of chromatographic techniques 
commonly used are in a given table, 
Table : 22.1 
Type of 
Chromatography 
Mobile/Sta
tionary 
Phase 
Uses 
Adsorption or 
column 
chromatography  
Liquid/Soli
d 
Large scale separations  
Thin-layer 
chromatography   
Liquid/Soli
d 
Qualitative analysis 
(identification and 
characterization of 
organic compounds) 
High performance 
liquid 
chromatography  
Liquid/Soli
d 
Qualitative and 
quantitative analysis 
Gas-liquid 
chromatography 
(GLC) 
Gas/Liquid Qualitative and 
quantitative analysis 
Partition 
chromatography or 
ascending paper 
chromatography 
Liquid/Liqu
id 
Qualitative and 
quantitative analysis of 
polar organic 
compounds (sugars, ?-
amino acids and 
inorganic compounds) 
 It is constant for a given substance (component) 
under a given set of conditions. Therefore, it is possible 
to identify the various components by determining 
their 
f
R values. 
    (x) Differential extraction : This method is used 
for the separation of an organic compound (solid or 
liquid) from its aqueous solution by shaking with a 
suitable solvent (e.g. ether, benzene, chloroform, 
carbon tetrachloride etc.) in a separating funnel. The 
selected solvent should be immiscible with water but 
should dissolve the organic compound to an appreciable 
extent. 
 It is important to note that extraction is more 
efficient (i.e., more complete) when a given volume of 
the extracting solvent is used in several installments.  
 This method is normally applied to nonvolatile 
compounds. For example, benzoic acid can be extracted 
from its water solution using benzene. 
 (xi) Chemical methods : Besides these physical 
methods, a number of chemical methods have also been 
used to separate a mixture of organic compounds. 
These methods are based upon the distinguishing 
chemical properties of one class of organic compounds 
from the others. For example, 
 (a) Phenols can be separated from carboxylic 
acids on treatment with an aqueous solution of 
3
NaHCO . Since carboxylic acids dissolve in 
3
NaHCO solution evolving 
2
CO but phenols usually do 
not react.  
 (b) Destructive distillation of wood gives 
pyroligneous acid which contains acetic acid (10%), 
Page 4


 
             
 The word ‘organic’ signifies life. Therefore, all 
substances which were obtained directly or indirectly 
from living organisms, plants and animals were called 
organic compounds and the branch of chemistry which 
deals with these compounds was called organic 
chemistry. 
 Modern definition of organic chemistry : 
Organic chemistry is a chemistry of hydrocarbons and 
their derivatives in which covalently bonded carbon is 
an essential constituent. 
 Berzelius put forward a theory in 1815 known as 
vital force theory. According to this theory, "organic 
compounds could be prepared only by living organism 
under the influence of a mysterious force known as vital 
force".  Accidental synthesis of urea by Wohler and 
synthesis of acetic acid by Kolbe led to the fall of this 
theory. 
   
O
NH C NH CNO NH
| |
2 2 4
? ? ? ? ?
?
  
 
 
  COOH CH CHO CH
O
? ? ? ? ? ?
3
] [
3
  
 
 
  
 Berthelot prepared methane in laboratory and 
the most abundant organic compound is cellulose 
which is a polymer of glucose. Kekule and Couper 
proposed the tetravalency of carbon and wrote the first 
structural formula. In 1874, Van't Hoff and Le Bell 
suggested a tetrahedron model of carbon. 
Purification and Characterisation of organic  
compounds 
 The study of organic compounds starts with the 
characterisation of the compound and the 
determination of its molecular structure. The procedure 
generally employed for this purpose consists of the 
following steps :  
 (1) Purification of organic compounds 
 (2) Qualitative analysis of organic compounds 
 (3) Quantitative analysis of organic compounds 
 (4) Determination of molecular mass of organic 
compounds 
 (5) Calculation of Empirical formula and 
Molecular formula of organic compounds 
 (6) Determination of structure of organic 
compounds by spectroscopic and diffraction methods 
 (1) Purification of organic compounds :  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) 
and the type of impurities present in it. Following 
methods are commonly used for this purpose, 
 (i) Simple crystallisation  
 (ii) Fractional crystallisation, 
 (iii) Sublimation  
 (iv) Simple distillation  
 (v) Fractional distillation  
 (vi) Distillation under reduced pressure 
 (vii) Steam distillation  
 (viii) Azeotropic distillation 
 (ix) Chromatography  
 (x) Differential extraction 
 (xi) Chemical methods  
    
(Ammonium 
cyanate) 
(First organic compound 
synthesised in laboratory) 
Urea 
Acetic acid 
(First organic compound 
synthesised from its 
elements) 
Acetaldehyde 
 
 
 
 
                                       Purification, Classification and Nomenclature of Organic compounds 971 
 (i) Simple crystallisation : This is the most 
common method used to purify organic solids. It is 
based upon the fact that whenever a crystal is formed, 
it tends to leave out the impurities. For crystallisation, 
a suitable solvent is one (a) which dissolves more of 
the substance at higher temperature than at room 
temperature (b) in which impurities are either 
insoluble or dissolve to an extent that they remain in 
solution (in the mother liquor) upon crystallisation, 
(c) which is not highly inflammable and (d) 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.        
 Examples : (a) Sugar having an impurity of 
common salt can be crystallized from hot ethanol since 
sugar dissolves in hot ethanol but common salt does 
not. 
 (b) A mixture of benzoic acid and naphthalene can 
be separated from hot water in which benzoic acid 
dissolves but naphthalene does not. 
 (ii) Fractional crystallisation : The process of 
separation of different components of a mixture by 
repeated crystallisations is called fractional 
crystallisation. The mixture is dissolved in a solvent in 
which the two components have different solubilities. 
When a hot saturated solution of this mixture is 
allowed to cool, 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 allowed to cool when the crystals of the more 
soluble component are obtained. The two components 
thus separated are recrystallized from the same or 
different solvent to yield both the components of the 
mixture in pure form.    
 Fractional crystallisation can be used to separate 
a mixture of 
3
KClO (less soluble) and KCl (more 
soluble). 
 (iii) Sublimation : Certain organic solids on 
heating directly change from solid to vapour state 
without passing through a liquid state, such substances 
are called sublimable and this process is called 
sublimation.  
                          
 
 The sublimation process is used for the separation 
of sublimable volatile compounds from non sublimable 
impurities. The process is generally used for the 
purification of camphor, naphthalene, anthracene, 
benzoic acid 
2 4
, HgCl Cl NH , solid 
2
SO , Iodine and 
salicylic acid etc containing non-volatile impurities.  
 (iv) Simple distillation : Distillation is the joint 
process of vapourisation and condensation. This 
method is used for the purification of liquids which boil 
without decomposition and contain non-volatile 
impurities. This method can also be used for separating 
liquids having sufficient difference in their boiling 
points. This method can be used to separate a mixture 
of 
 (a) chloroform (b. p. 334 K) and aniline (b. p. 457 
K) 
 (b) ether (b. p. 308 K) and toluene (b. p. 384 K) 
 (v) Fractional distillation : This process is used 
to separate a mixture of two or more miscible liquids 
which have boiling points close to each other. Since in 
this process, the distillate is collected in fractions 
under different temperatures, it is known as fractional 
distillation. This process is carried out by using 
fractionating columns.  Fractionating column is a 
special type of long glass tube provided with 
obstructions to the passage of the vapour upwards and 
that of liquid downwards. This method may be used to 
separate a mixture of acetone (b. p. 330 K) and methyl 
alcohol (b. p. 338 K) or a mixture of benzene and 
toluene.  One of the technological applications of 
fractional distillation is to separate different fractions 
of crude oil in petroleum industry into various useful 
fractions such as gasoline, kerosene oil, diesel oil, 
lubricating oil etc. 
 (vi) Distillation under reduced pressure : This 
method is used for the purification of high boiling 
liquids and liquids which decompose at or below their 
boiling points. 
 The crude liquid is heated in distillation flask 
fitted with a water condenser, receiver and vacuum 
pump. As the pressure is reduced, the liquid begins to 
boil at a much lower temperature than its normal 
boiling point. The vapour is condensed by water 
condenser and the pure liquid collects in the receiver. 
 Glycerol which decomposes at its boiling point 
(563 K) under atmospheric pressure can be distilled 
without decomposition at 453 K under 12 mm of Hg. 
Similarly, sugarcane juice is concentrated in sugar 
industry by evaporation under reduced pressure which 
saves a lot of fuel. 
 (vii) Steam distillation : This method is 
applicable for the separation and purification of those 
organic compounds (solids or liquids) which (a) are 
insoluble in water (b) are volatile in steam (c) possess 
a high vapour pressure (10-15 mm Hg) at 373 K and (d) 
contain non-volatile impurities. 
 Aniline (b. p. 457 K) can be purified by steam 
distillation since it boils at a temperature of 371.5 K in 
Solid 
Vapour 
Heat 
Cool 
 
           972 Purification, Classification and Nomenclature of Organic compounds 
presence of steam. Other compounds which can be 
purified by steam distillation are: nitrobenzene, 
bromobenzene,  o-nitrophenol, salicylaldehyde, o-
hydroxyacetophenone, essential oils, turpentine oil etc. 
 (viii) Azeotropic distillation : Azeotropic mixture 
is a mixture having 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 78.13
o
C. The constituents of an 
azeotropic mixture can't be separated by fractional 
distillation. Hence a special type of distillation 
(azeotropic distillation) is used for separating the 
constituents of an azeotropic mixture. 
 In this method a third compound is used in 
distillation. The process is based on the fact that 
dehydrating agents like 
4 , 6 6
CCl H C , diethyl ether etc. 
depress the partial pressure of one of the original 
components. As a result, the boiling point of that 
component is raised sufficiently and thus the other 
component will distil over. 
 Dehydrating agents having low boiling point (e.g. 
, 4 , 6 6
CCl H C ether) depress the partial pressure of 
alcohol more than that of water; on the other hand, 
dehydrating agents having high boiling point (glycerol, 
glycol) depress the partial pressure of water more than 
that of alcohol.                  
 (ix) Chromatography : This is a modern method 
used for the separation of mixtures into its 
components, purification of compounds and also to test 
the purity of compounds. The name chromatography is 
based on the Greek word 'chroma' meaning colour and 
'graphy' for writing because the method was first used 
for the separation of coloured substances found in 
plants. This method was described by Tswett in 1906.   
 (a) Principle of chromatography : The technique of 
chromatography is based on the difference in the rates 
at which the components of a mixture move through a 
porous medium (called stationary phase) under the 
influence of some solvent or gas (called moving phase). 
Thus, this technique consists of two phases- one is a 
stationary phase of large surface area while the second 
is a moving phase which is allowed to move slowly over 
the stationary phase. The stationary phase is either a 
solid or a liquid while the moving phase may be a liquid 
or a gas.     
 (b) Types of chromatography : Depending upon 
the nature of the stationary and the mobile phases, the 
different types of chromatographic techniques 
commonly used are in a given table, 
Table : 22.1 
Type of 
Chromatography 
Mobile/Sta
tionary 
Phase 
Uses 
Adsorption or 
column 
chromatography  
Liquid/Soli
d 
Large scale separations  
Thin-layer 
chromatography   
Liquid/Soli
d 
Qualitative analysis 
(identification and 
characterization of 
organic compounds) 
High performance 
liquid 
chromatography  
Liquid/Soli
d 
Qualitative and 
quantitative analysis 
Gas-liquid 
chromatography 
(GLC) 
Gas/Liquid Qualitative and 
quantitative analysis 
Partition 
chromatography or 
ascending paper 
chromatography 
Liquid/Liqu
id 
Qualitative and 
quantitative analysis of 
polar organic 
compounds (sugars, ?-
amino acids and 
inorganic compounds) 
 It is constant for a given substance (component) 
under a given set of conditions. Therefore, it is possible 
to identify the various components by determining 
their 
f
R values. 
    (x) Differential extraction : This method is used 
for the separation of an organic compound (solid or 
liquid) from its aqueous solution by shaking with a 
suitable solvent (e.g. ether, benzene, chloroform, 
carbon tetrachloride etc.) in a separating funnel. The 
selected solvent should be immiscible with water but 
should dissolve the organic compound to an appreciable 
extent. 
 It is important to note that extraction is more 
efficient (i.e., more complete) when a given volume of 
the extracting solvent is used in several installments.  
 This method is normally applied to nonvolatile 
compounds. For example, benzoic acid can be extracted 
from its water solution using benzene. 
 (xi) Chemical methods : Besides these physical 
methods, a number of chemical methods have also been 
used to separate a mixture of organic compounds. 
These methods are based upon the distinguishing 
chemical properties of one class of organic compounds 
from the others. For example, 
 (a) Phenols can be separated from carboxylic 
acids on treatment with an aqueous solution of 
3
NaHCO . Since carboxylic acids dissolve in 
3
NaHCO solution evolving 
2
CO but phenols usually do 
not react.  
 (b) Destructive distillation of wood gives 
pyroligneous acid which contains acetic acid (10%), 
 
                                       Purification, Classification and Nomenclature of Organic compounds 973 
acetone (0.5%) and methanol (3%). Acetic acid can be 
separated from this mixture by treating it with milk of 
lime when acetic acid forms the calcium salt. The 
reaction mixture on distillation gives a mixture of 
acetone and methanol (which can be further separated 
by fractional distillation into individual components as 
mentioned above) while the calcium salt remains as 
residue in the flask. The calcium salt is then 
decomposed with dil HCl and distilled to afford acetic 
acid.        
 (c) A mixture of 1
o
, 2
o
 and 3
o
 amines can be 
separated using either benzenesulphonyl chloride 
(Hinsberg's reagent) or diethyl oxalate (Hoffmann's 
method). 
 (d) Purification of commercial benzene : 
Commercial benzene obtained from coal-tar distillation 
contains 3-5% thiophene as an impurity which can be 
removed by extraction with conc. 
4 2
SO H . This 
purification is based upon the fact that thiophene 
undergoes sulphonation much more easily than 
benzene. Thus, when commercial benzene is shaken 
with conc. 
4 2
SO H in a separating funnel, thiophene 
undergoes sulphonation to form thiophene-2-sulphonic 
acid which dissolves in conc. 
4 2
SO H while benzene 
does not.  
 
                    ? ? ? ? ? ? ?
temp Room
4 2
SO H                        
O H
2
?  
 
 
 
 
 After this treatment, the benzene layer is 
removed, washed with water to remove unreacted 
4 2
SO H , dried over anhyd. 
2
CaCl and then distilled to 
give pure benzene. 
 (e) Absolute alcohol from rectified spirit : The 
rectified spirit (ethanol : 13 . 4 : 87 . 95 ,
2
O H by weight) is 
kept over a calculated amount of active quick lime 
(CaO) for few hours and then refluxed. During this 
process, water present in rectified spirit combines with 
CaO to form 
2
) (OH Ca . When the resulting mixture is 
distilled, absolute alcohol distils over leaving behind, 
2
) (OH Ca . 
 Drying of Organic Substances. (1) For solids : 
Most solids are dried first by pressing them gently 
between folds of filter papers. Compounds which 
neither decompose on heating nor melt below 100
o
C are 
dried by keeping them in steam or oven maintained at 
110
o
C. Substances, which decompose on heating are 
dried by keeping them in a vacuum desiccator 
containing a suitable dehydrating agent like fused 
2
CaCl , conc. 
4 2
SO H , ,
10 4
O P solid KOH or NaOH, etc 
(desiccant). 
 (2) For liquids : Organic liquids are generally 
dried by keeping them over night in contact with a 
dehydrating (desiccating) agent which does not react 
chemically with the liquid to be dried. Commonly used 
dehydrating agents are quick lime, anhydrous 
2
CaCl , 
fused 
4
CuSO or KOH CaSO ,
4
, metallic sodium or 
potassium, etc. 
 Criteria of purity of organic compounds : The 
purity of an organic compound can be ascertained by 
determining its some physical constants like m.p., b.p., 
specific gravity, refractive index and viscosity. In usual 
practice, sharp m.p. (in case of solids) and boiling point 
(in case of liquids) are used as criteria for purity 
because their determination is feasible in the 
laboratory. A pure organic solid has a definite and sharp 
(sudden, rapid and complete) melting point, while an 
impure substance has a lower and indefinite melting 
point. 
 (1) Mixed melting point : The melting point of 
two thoroughly mixed substances is called mixed 
melting point. This can also be used for ascertaining 
the purity of a compound . 
 The substance, whose purity is to be tested, is 
mixed with a pure sample of the same compound. The 
melting point of the mixture is determined. If the 
melting point of the mixture is sharp and comes out to 
be the same as that of pure compound, it is sure that 
the compound under test is pure. On the other hand, if 
the melting point of the mixture is less than the 
melting point of the pure compound, the compound in 
question is not pure. 
 (2) Qualitative analysis : (Detection of Elements 
) 
 The qualitative analysis of an organic compound 
involves the detection of all the elements present in it. 
  Carbon is an essential constituent of an organic 
compound whereas hydrogen is nearly always present. 
On heating the organic compound with dry cupric 
oxide, carbon is oxidized to 
2
CO and hydrogen to O H
2
. 
2
CO is detected by lime water which turns milky while 
O H
2
 is detected by anhydrous 
4
CuSO (white) which 
turns it blue. This method is known as copper oxide 
test. 
  Cu CO CuO C 2 2
2
Heat
? ? ? ? ? ? ; 
  O H CaCO CO OH Ca
2
Milk y
3 2
 water Lime
2
) ( ? ? ? ? ? 
  Cu O H CuO H ? ? ? ? ? ?
2
Heat
2
 ; 
  
(Hydrated)
 Blue
2 4 2
) (Anhydrous
Colourles
4
5 . 5 O H CuSO O H CuSO
s
? ? ? ? 
Thiophen
e 
(Conc.
) 
S 
H SO
3
 
Thiophene-2-sulphonic 
acid  
(Dissolves in conc. 
) 
S 
Page 5


 
             
 The word ‘organic’ signifies life. Therefore, all 
substances which were obtained directly or indirectly 
from living organisms, plants and animals were called 
organic compounds and the branch of chemistry which 
deals with these compounds was called organic 
chemistry. 
 Modern definition of organic chemistry : 
Organic chemistry is a chemistry of hydrocarbons and 
their derivatives in which covalently bonded carbon is 
an essential constituent. 
 Berzelius put forward a theory in 1815 known as 
vital force theory. According to this theory, "organic 
compounds could be prepared only by living organism 
under the influence of a mysterious force known as vital 
force".  Accidental synthesis of urea by Wohler and 
synthesis of acetic acid by Kolbe led to the fall of this 
theory. 
   
O
NH C NH CNO NH
| |
2 2 4
? ? ? ? ?
?
  
 
 
  COOH CH CHO CH
O
? ? ? ? ? ?
3
] [
3
  
 
 
  
 Berthelot prepared methane in laboratory and 
the most abundant organic compound is cellulose 
which is a polymer of glucose. Kekule and Couper 
proposed the tetravalency of carbon and wrote the first 
structural formula. In 1874, Van't Hoff and Le Bell 
suggested a tetrahedron model of carbon. 
Purification and Characterisation of organic  
compounds 
 The study of organic compounds starts with the 
characterisation of the compound and the 
determination of its molecular structure. The procedure 
generally employed for this purpose consists of the 
following steps :  
 (1) Purification of organic compounds 
 (2) Qualitative analysis of organic compounds 
 (3) Quantitative analysis of organic compounds 
 (4) Determination of molecular mass of organic 
compounds 
 (5) Calculation of Empirical formula and 
Molecular formula of organic compounds 
 (6) Determination of structure of organic 
compounds by spectroscopic and diffraction methods 
 (1) Purification of organic compounds :  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) 
and the type of impurities present in it. Following 
methods are commonly used for this purpose, 
 (i) Simple crystallisation  
 (ii) Fractional crystallisation, 
 (iii) Sublimation  
 (iv) Simple distillation  
 (v) Fractional distillation  
 (vi) Distillation under reduced pressure 
 (vii) Steam distillation  
 (viii) Azeotropic distillation 
 (ix) Chromatography  
 (x) Differential extraction 
 (xi) Chemical methods  
    
(Ammonium 
cyanate) 
(First organic compound 
synthesised in laboratory) 
Urea 
Acetic acid 
(First organic compound 
synthesised from its 
elements) 
Acetaldehyde 
 
 
 
 
                                       Purification, Classification and Nomenclature of Organic compounds 971 
 (i) Simple crystallisation : This is the most 
common method used to purify organic solids. It is 
based upon the fact that whenever a crystal is formed, 
it tends to leave out the impurities. For crystallisation, 
a suitable solvent is one (a) which dissolves more of 
the substance at higher temperature than at room 
temperature (b) in which impurities are either 
insoluble or dissolve to an extent that they remain in 
solution (in the mother liquor) upon crystallisation, 
(c) which is not highly inflammable and (d) 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.        
 Examples : (a) Sugar having an impurity of 
common salt can be crystallized from hot ethanol since 
sugar dissolves in hot ethanol but common salt does 
not. 
 (b) A mixture of benzoic acid and naphthalene can 
be separated from hot water in which benzoic acid 
dissolves but naphthalene does not. 
 (ii) Fractional crystallisation : The process of 
separation of different components of a mixture by 
repeated crystallisations is called fractional 
crystallisation. The mixture is dissolved in a solvent in 
which the two components have different solubilities. 
When a hot saturated solution of this mixture is 
allowed to cool, 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 allowed to cool when the crystals of the more 
soluble component are obtained. The two components 
thus separated are recrystallized from the same or 
different solvent to yield both the components of the 
mixture in pure form.    
 Fractional crystallisation can be used to separate 
a mixture of 
3
KClO (less soluble) and KCl (more 
soluble). 
 (iii) Sublimation : Certain organic solids on 
heating directly change from solid to vapour state 
without passing through a liquid state, such substances 
are called sublimable and this process is called 
sublimation.  
                          
 
 The sublimation process is used for the separation 
of sublimable volatile compounds from non sublimable 
impurities. The process is generally used for the 
purification of camphor, naphthalene, anthracene, 
benzoic acid 
2 4
, HgCl Cl NH , solid 
2
SO , Iodine and 
salicylic acid etc containing non-volatile impurities.  
 (iv) Simple distillation : Distillation is the joint 
process of vapourisation and condensation. This 
method is used for the purification of liquids which boil 
without decomposition and contain non-volatile 
impurities. This method can also be used for separating 
liquids having sufficient difference in their boiling 
points. This method can be used to separate a mixture 
of 
 (a) chloroform (b. p. 334 K) and aniline (b. p. 457 
K) 
 (b) ether (b. p. 308 K) and toluene (b. p. 384 K) 
 (v) Fractional distillation : This process is used 
to separate a mixture of two or more miscible liquids 
which have boiling points close to each other. Since in 
this process, the distillate is collected in fractions 
under different temperatures, it is known as fractional 
distillation. This process is carried out by using 
fractionating columns.  Fractionating column is a 
special type of long glass tube provided with 
obstructions to the passage of the vapour upwards and 
that of liquid downwards. This method may be used to 
separate a mixture of acetone (b. p. 330 K) and methyl 
alcohol (b. p. 338 K) or a mixture of benzene and 
toluene.  One of the technological applications of 
fractional distillation is to separate different fractions 
of crude oil in petroleum industry into various useful 
fractions such as gasoline, kerosene oil, diesel oil, 
lubricating oil etc. 
 (vi) Distillation under reduced pressure : This 
method is used for the purification of high boiling 
liquids and liquids which decompose at or below their 
boiling points. 
 The crude liquid is heated in distillation flask 
fitted with a water condenser, receiver and vacuum 
pump. As the pressure is reduced, the liquid begins to 
boil at a much lower temperature than its normal 
boiling point. The vapour is condensed by water 
condenser and the pure liquid collects in the receiver. 
 Glycerol which decomposes at its boiling point 
(563 K) under atmospheric pressure can be distilled 
without decomposition at 453 K under 12 mm of Hg. 
Similarly, sugarcane juice is concentrated in sugar 
industry by evaporation under reduced pressure which 
saves a lot of fuel. 
 (vii) Steam distillation : This method is 
applicable for the separation and purification of those 
organic compounds (solids or liquids) which (a) are 
insoluble in water (b) are volatile in steam (c) possess 
a high vapour pressure (10-15 mm Hg) at 373 K and (d) 
contain non-volatile impurities. 
 Aniline (b. p. 457 K) can be purified by steam 
distillation since it boils at a temperature of 371.5 K in 
Solid 
Vapour 
Heat 
Cool 
 
           972 Purification, Classification and Nomenclature of Organic compounds 
presence of steam. Other compounds which can be 
purified by steam distillation are: nitrobenzene, 
bromobenzene,  o-nitrophenol, salicylaldehyde, o-
hydroxyacetophenone, essential oils, turpentine oil etc. 
 (viii) Azeotropic distillation : Azeotropic mixture 
is a mixture having 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 78.13
o
C. The constituents of an 
azeotropic mixture can't be separated by fractional 
distillation. Hence a special type of distillation 
(azeotropic distillation) is used for separating the 
constituents of an azeotropic mixture. 
 In this method a third compound is used in 
distillation. The process is based on the fact that 
dehydrating agents like 
4 , 6 6
CCl H C , diethyl ether etc. 
depress the partial pressure of one of the original 
components. As a result, the boiling point of that 
component is raised sufficiently and thus the other 
component will distil over. 
 Dehydrating agents having low boiling point (e.g. 
, 4 , 6 6
CCl H C ether) depress the partial pressure of 
alcohol more than that of water; on the other hand, 
dehydrating agents having high boiling point (glycerol, 
glycol) depress the partial pressure of water more than 
that of alcohol.                  
 (ix) Chromatography : This is a modern method 
used for the separation of mixtures into its 
components, purification of compounds and also to test 
the purity of compounds. The name chromatography is 
based on the Greek word 'chroma' meaning colour and 
'graphy' for writing because the method was first used 
for the separation of coloured substances found in 
plants. This method was described by Tswett in 1906.   
 (a) Principle of chromatography : The technique of 
chromatography is based on the difference in the rates 
at which the components of a mixture move through a 
porous medium (called stationary phase) under the 
influence of some solvent or gas (called moving phase). 
Thus, this technique consists of two phases- one is a 
stationary phase of large surface area while the second 
is a moving phase which is allowed to move slowly over 
the stationary phase. The stationary phase is either a 
solid or a liquid while the moving phase may be a liquid 
or a gas.     
 (b) Types of chromatography : Depending upon 
the nature of the stationary and the mobile phases, the 
different types of chromatographic techniques 
commonly used are in a given table, 
Table : 22.1 
Type of 
Chromatography 
Mobile/Sta
tionary 
Phase 
Uses 
Adsorption or 
column 
chromatography  
Liquid/Soli
d 
Large scale separations  
Thin-layer 
chromatography   
Liquid/Soli
d 
Qualitative analysis 
(identification and 
characterization of 
organic compounds) 
High performance 
liquid 
chromatography  
Liquid/Soli
d 
Qualitative and 
quantitative analysis 
Gas-liquid 
chromatography 
(GLC) 
Gas/Liquid Qualitative and 
quantitative analysis 
Partition 
chromatography or 
ascending paper 
chromatography 
Liquid/Liqu
id 
Qualitative and 
quantitative analysis of 
polar organic 
compounds (sugars, ?-
amino acids and 
inorganic compounds) 
 It is constant for a given substance (component) 
under a given set of conditions. Therefore, it is possible 
to identify the various components by determining 
their 
f
R values. 
    (x) Differential extraction : This method is used 
for the separation of an organic compound (solid or 
liquid) from its aqueous solution by shaking with a 
suitable solvent (e.g. ether, benzene, chloroform, 
carbon tetrachloride etc.) in a separating funnel. The 
selected solvent should be immiscible with water but 
should dissolve the organic compound to an appreciable 
extent. 
 It is important to note that extraction is more 
efficient (i.e., more complete) when a given volume of 
the extracting solvent is used in several installments.  
 This method is normally applied to nonvolatile 
compounds. For example, benzoic acid can be extracted 
from its water solution using benzene. 
 (xi) Chemical methods : Besides these physical 
methods, a number of chemical methods have also been 
used to separate a mixture of organic compounds. 
These methods are based upon the distinguishing 
chemical properties of one class of organic compounds 
from the others. For example, 
 (a) Phenols can be separated from carboxylic 
acids on treatment with an aqueous solution of 
3
NaHCO . Since carboxylic acids dissolve in 
3
NaHCO solution evolving 
2
CO but phenols usually do 
not react.  
 (b) Destructive distillation of wood gives 
pyroligneous acid which contains acetic acid (10%), 
 
                                       Purification, Classification and Nomenclature of Organic compounds 973 
acetone (0.5%) and methanol (3%). Acetic acid can be 
separated from this mixture by treating it with milk of 
lime when acetic acid forms the calcium salt. The 
reaction mixture on distillation gives a mixture of 
acetone and methanol (which can be further separated 
by fractional distillation into individual components as 
mentioned above) while the calcium salt remains as 
residue in the flask. The calcium salt is then 
decomposed with dil HCl and distilled to afford acetic 
acid.        
 (c) A mixture of 1
o
, 2
o
 and 3
o
 amines can be 
separated using either benzenesulphonyl chloride 
(Hinsberg's reagent) or diethyl oxalate (Hoffmann's 
method). 
 (d) Purification of commercial benzene : 
Commercial benzene obtained from coal-tar distillation 
contains 3-5% thiophene as an impurity which can be 
removed by extraction with conc. 
4 2
SO H . This 
purification is based upon the fact that thiophene 
undergoes sulphonation much more easily than 
benzene. Thus, when commercial benzene is shaken 
with conc. 
4 2
SO H in a separating funnel, thiophene 
undergoes sulphonation to form thiophene-2-sulphonic 
acid which dissolves in conc. 
4 2
SO H while benzene 
does not.  
 
                    ? ? ? ? ? ? ?
temp Room
4 2
SO H                        
O H
2
?  
 
 
 
 
 After this treatment, the benzene layer is 
removed, washed with water to remove unreacted 
4 2
SO H , dried over anhyd. 
2
CaCl and then distilled to 
give pure benzene. 
 (e) Absolute alcohol from rectified spirit : The 
rectified spirit (ethanol : 13 . 4 : 87 . 95 ,
2
O H by weight) is 
kept over a calculated amount of active quick lime 
(CaO) for few hours and then refluxed. During this 
process, water present in rectified spirit combines with 
CaO to form 
2
) (OH Ca . When the resulting mixture is 
distilled, absolute alcohol distils over leaving behind, 
2
) (OH Ca . 
 Drying of Organic Substances. (1) For solids : 
Most solids are dried first by pressing them gently 
between folds of filter papers. Compounds which 
neither decompose on heating nor melt below 100
o
C are 
dried by keeping them in steam or oven maintained at 
110
o
C. Substances, which decompose on heating are 
dried by keeping them in a vacuum desiccator 
containing a suitable dehydrating agent like fused 
2
CaCl , conc. 
4 2
SO H , ,
10 4
O P solid KOH or NaOH, etc 
(desiccant). 
 (2) For liquids : Organic liquids are generally 
dried by keeping them over night in contact with a 
dehydrating (desiccating) agent which does not react 
chemically with the liquid to be dried. Commonly used 
dehydrating agents are quick lime, anhydrous 
2
CaCl , 
fused 
4
CuSO or KOH CaSO ,
4
, metallic sodium or 
potassium, etc. 
 Criteria of purity of organic compounds : The 
purity of an organic compound can be ascertained by 
determining its some physical constants like m.p., b.p., 
specific gravity, refractive index and viscosity. In usual 
practice, sharp m.p. (in case of solids) and boiling point 
(in case of liquids) are used as criteria for purity 
because their determination is feasible in the 
laboratory. A pure organic solid has a definite and sharp 
(sudden, rapid and complete) melting point, while an 
impure substance has a lower and indefinite melting 
point. 
 (1) Mixed melting point : The melting point of 
two thoroughly mixed substances is called mixed 
melting point. This can also be used for ascertaining 
the purity of a compound . 
 The substance, whose purity is to be tested, is 
mixed with a pure sample of the same compound. The 
melting point of the mixture is determined. If the 
melting point of the mixture is sharp and comes out to 
be the same as that of pure compound, it is sure that 
the compound under test is pure. On the other hand, if 
the melting point of the mixture is less than the 
melting point of the pure compound, the compound in 
question is not pure. 
 (2) Qualitative analysis : (Detection of Elements 
) 
 The qualitative analysis of an organic compound 
involves the detection of all the elements present in it. 
  Carbon is an essential constituent of an organic 
compound whereas hydrogen is nearly always present. 
On heating the organic compound with dry cupric 
oxide, carbon is oxidized to 
2
CO and hydrogen to O H
2
. 
2
CO is detected by lime water which turns milky while 
O H
2
 is detected by anhydrous 
4
CuSO (white) which 
turns it blue. This method is known as copper oxide 
test. 
  Cu CO CuO C 2 2
2
Heat
? ? ? ? ? ? ; 
  O H CaCO CO OH Ca
2
Milk y
3 2
 water Lime
2
) ( ? ? ? ? ? 
  Cu O H CuO H ? ? ? ? ? ?
2
Heat
2
 ; 
  
(Hydrated)
 Blue
2 4 2
) (Anhydrous
Colourles
4
5 . 5 O H CuSO O H CuSO
s
? ? ? ? 
Thiophen
e 
(Conc.
) 
S 
H SO
3
 
Thiophene-2-sulphonic 
acid  
(Dissolves in conc. 
) 
S 
 
           974 Purification, Classification and Nomenclature of Organic compounds 
 If the substance under investigation is a volatile 
liquid or gas, the vapours are passed over heated 
copper oxide kept in combustion tube and the gaseous 
products are tested as above. 
 Lassaigne method 
 This is used to detect nitrogen, halogen and 
sulphur. Organic compounds are fused with dry sodium 
in a fusion-tube and fused mass after extraction with 
O H
2
 is boiled and filtered. Filtrate called sodium 
extract (S.E.) is used to detect elements (other than C 
and H) and the tests are given in the table. 
 ? Organic compounds being covalents normally do 
not have ionisable groups, hence direct test is not 
possible. 
? Fusion with Na forms soluble salt (like 
NaCN NaCl, etc.) which can be easily detected. 
 ? This test fails in case of diazo compounds. 
 ?  Sometimes when the amount of nitrogen 
present is small, the prussian blue is present in 
colloidal form and the solution looks green. 
Table : 22.2  Lassaigne method (Detection of elements) 
Elemen
t 
Sodium Extract (S.E.) Confirmed Test Reaction 
Nitroge
n 
.) . ( E S
NaCN N C Na ? ? ? ? ?
?
 
S.E.+ NaOH FeSO ?
4
, boil 
and cool + HCl conc FeCl .
3
? 
Blue or green colour 
4 2 2 4
) ( 2 SO Na CN Fe FeSO NaCN ? ? ? ? ? 
de ferrocyani Sodium
6 4 2
] ) ( [ 4 ) ( CN Fe Na NaCN CN Fe ? ? ? ? 
NaCl CN Fe Fe FeCl CN Fe Na
HCl
12 ] ) ( [ 4 ] ) ( [ 3
 blue) Prussian (
de ferrocyani Ferric 
3 6 4 3 6 4
? ? ? ? ? ? 
Sulphu
r (S.E.)
2
2 S Na S Na ? ? ? ?
?
 
(i) S.E. + sodium nitro 
prusside 
(ii)S.E+
Pb CO CH H CO CH
2 2 3 2 3
) ( ? 
 A black ppt. 
(i) 
(Purple)
5 4
ide nitropruss Sodium
5 2 2
] . ) ( [ ] ) ( [ S NO CN Fe Na NO CN Fe Na S Na ? ? ? ?
 or   
                                                       
Violet) (
prusside thionitro Sodium
5 3
] ) )( ( [ CN ONSNa Fe Na 
(ii) COONa CH PbS Pb COO CH S Na
ppt black
COOH CH
3
.
2 3 2
2 ) (
3
? ? ? ? ? ? ? ? ? 
Haloge
n 
.) . ( E S
NaX X Na ? ? ? ?
?
 
(X = Cl, Br, I) 
S.E. 
3 3
AgNO HNO ? ? 
(i) White ppt soluble in aq 
3
NH confirms Cl . 
(ii) Pale yellow ppt 
partially soluble in aq. 
3
NH 
confirms Br.  
(iii) Yellow ppt insoluble in 
aq 
3
NH confirms I. 
ppt
HNO
AgX AgNO NaX ? ? ? ? ? ? ?
3
3
 
soluble
2 3 3
ppt White
] ) ( [ ) ( 2 Cl NH Ag aq NH AgCl ? ? ? ? 
 soluble Partially
2 3 3
ppt. Yellow
] ) ( [ ) ( 2 Br NH Ag aq NH AgBr ? ? 
Insoluble ) (
3
? ? ? ? aq NH AgI 
Nitroge
n and 
sulphur 
togethe
r 
(S.E.)
NaCNS S N C Na ? ? ? ? ? ?
?
with excess of Na the 
thiocyanate formed 
decomposes into 
cyanide and sulphide. 
NaCN Na NaCNS ? ? 2 
                             
S Na
2
? 
As in test for nitrogen; 
instead of green or blue 
colour, blood red 
colouration confirms 
presence of N and S both. 
NaCl Cl SCN Fe SCN Fe FeCl NaCNS 3 )] ( [ or ) ( [ 3
 colour) red Blood (
ide sulphocyan Ferric 
2 3 3
? ? ? ? ? 
Table : 22.3 Other methods for detection of elements 
Element Test 
Nitrogen  Soda lime test : A pinch of an organic compound is heated strongly with soda lime ) ( CaO NaOH ? in a 
test tube. If ammonia gas evolves, it indicates nitrogen. 
3 3
Acetamide
2 3
NH COONa CH NaOH CONH CH
CaO
? ? ? ? ? ? . 
This test is, however, not reliable since certain compounds like nitro, azo etc do not evolve 
3
NH when heated with soda lime.