NCERT Textbook: Some Basic Principles & Techniques | Chemistry Class 11 - NEET PDF Download

 Page 1


256 chemistry Organic chemistry – sOme Basic  
Princi Ples and Techniques
After studying this unit, you will 
be able to 
• understand reasons for 
tetravalence of carbon and 
shapes of organic molecules; 
• write structures of organic 
molecules in various ways;
• classify the organic compounds; 
• name the compounds according 
to IUPAC system of nomenclature 
and also derive their structures 
from the given names;
• understand the concept of 
organic reaction mechanism;
• explain the influence of electronic 
displacements on structure and 
reactivity of organic compounds;
• recognise the types of organic 
reactions;
• learn the techniques of 
purification of organic 
compounds;
• write the chemical reactions 
involved in the qualitative 
analysis of organic compounds;
• understand the principles 
involved in quantitative analysis 
of organic compounds.
In the previous unit you have learnt that the element 
carbon has the unique property called catenation due to 
which it forms covalent bonds with other carbon atoms. 
It also forms covalent bonds with atoms of other elements 
like hydrogen, oxygen, nitrogen, sulphur, phosphorus 
and halogens. The resulting compounds are studied 
under a separate branch of chemistry called organic 
chemistry. This unit incorporates some basic principles 
and techniques of analysis required for understanding the 
formation and properties of organic compounds.
8.1 General inTroduc Tion
Organic compounds are vital for sustaining life on earth 
and include complex molecules like genetic information 
bearing deoxyribonucleic acid (DNA) and proteins that 
constitute essential compounds of our blood, muscles and 
skin. Organic compounds appear in materials like clothing, 
fuels, polymers, dyes and medicines. These are some of 
the important areas of application of these compounds.
Science of organic chemistry is about two hundred 
years old. Around the year 1780, chemists began to 
distinguish between organic compounds obtained from 
plants and animals and inorganic compounds prepared 
from mineral sources. Berzilius, a Swedish chemist 
proposed that a ‘vital force’ was responsible for the 
formation of organic compounds. However, this notion 
was rejected in 1828 when F. Wohler synthesised an 
organic compound, urea from an inorganic compound, 
ammonium cyanate.
NH
4
CNO     NH
2
CONH
2
Ammonium cyanate          Urea
The pioneering synthesis of acetic acid by Kolbe 
(1845) and that of methane by Berthelot (1856) showed 
conclusively that organic compounds could be synthesised 
from inorganic sources in a laboratory.
u niT 8
Unit 8.indd   256 2/24/2023   16:52:17
Rationalised 2023-24
Page 2


256 chemistry Organic chemistry – sOme Basic  
Princi Ples and Techniques
After studying this unit, you will 
be able to 
• understand reasons for 
tetravalence of carbon and 
shapes of organic molecules; 
• write structures of organic 
molecules in various ways;
• classify the organic compounds; 
• name the compounds according 
to IUPAC system of nomenclature 
and also derive their structures 
from the given names;
• understand the concept of 
organic reaction mechanism;
• explain the influence of electronic 
displacements on structure and 
reactivity of organic compounds;
• recognise the types of organic 
reactions;
• learn the techniques of 
purification of organic 
compounds;
• write the chemical reactions 
involved in the qualitative 
analysis of organic compounds;
• understand the principles 
involved in quantitative analysis 
of organic compounds.
In the previous unit you have learnt that the element 
carbon has the unique property called catenation due to 
which it forms covalent bonds with other carbon atoms. 
It also forms covalent bonds with atoms of other elements 
like hydrogen, oxygen, nitrogen, sulphur, phosphorus 
and halogens. The resulting compounds are studied 
under a separate branch of chemistry called organic 
chemistry. This unit incorporates some basic principles 
and techniques of analysis required for understanding the 
formation and properties of organic compounds.
8.1 General inTroduc Tion
Organic compounds are vital for sustaining life on earth 
and include complex molecules like genetic information 
bearing deoxyribonucleic acid (DNA) and proteins that 
constitute essential compounds of our blood, muscles and 
skin. Organic compounds appear in materials like clothing, 
fuels, polymers, dyes and medicines. These are some of 
the important areas of application of these compounds.
Science of organic chemistry is about two hundred 
years old. Around the year 1780, chemists began to 
distinguish between organic compounds obtained from 
plants and animals and inorganic compounds prepared 
from mineral sources. Berzilius, a Swedish chemist 
proposed that a ‘vital force’ was responsible for the 
formation of organic compounds. However, this notion 
was rejected in 1828 when F. Wohler synthesised an 
organic compound, urea from an inorganic compound, 
ammonium cyanate.
NH
4
CNO     NH
2
CONH
2
Ammonium cyanate          Urea
The pioneering synthesis of acetic acid by Kolbe 
(1845) and that of methane by Berthelot (1856) showed 
conclusively that organic compounds could be synthesised 
from inorganic sources in a laboratory.
u niT 8
Unit 8.indd   256 2/24/2023   16:52:17
Rationalised 2023-24
257 organic chemistry – some basic principles and techniques 
The development of electronic theory of 
covalent bonding ushered organic chemistry 
into its modern shape.
8.2 tetra Valence OF car BOn:  
sha Pes OF Organic cOmPOunds
8.2.1 The Shapes of Carbon Compounds  
The knowledge of fundamental concepts of 
molecular structure helps in understanding 
and predicting the properties of organic 
compounds. You have already learnt theories 
of valency and molecular structure in Unit 4. 
Also, you already know that tetravalence of 
carbon and the formation of covalent bonds 
by it are explained in terms of its electronic 
configuration and the hybridisation of s and p 
orbitals. It may be recalled that formation and 
the shapes of molecules like methane (CH
4
), 
ethene (C
2
H
4
), ethyne (C
2
H
2
) are explained 
in terms of the use of sp
3
, sp
2
 and sp hybrid 
orbitals by carbon atoms in the respective 
molecules. 
Hybridisation influences the bond length 
and bond enthalpy (strength) in compounds. 
The sp hybrid orbital contains more s 
character and hence it is closer to its nucleus 
and forms shorter and stronger bonds than 
the sp
3
 hybrid orbital. The sp
2
 hybrid orbital 
is intermediate in s character between sp 
and sp
3
 and, hence, the length and enthalpy 
of the bonds it forms, are also intermediate 
between them. The change in hybridisation 
affects the electronegativity of carbon. The 
greater the s character of the hybrid orbitals, 
the greater is the electronegativity. Thus, a 
carbon atom having an sp hybrid orbital with 
50% s character is more electronegative than 
that possessing  sp
2
 or sp
3
 hybridised orbitals. 
This relative electronegativity is reflected in 
several physical and chemical properties of 
the molecules concerned, about which you 
will learn in later units. 
8.2.2  some characteristic Features of p 
Bonds
In a p (pi) bond formation, parallel orientation 
of the two p orbitals on adjacent atoms is 
necessary for a proper sideways overlap. 
Thus, in H
2
C=CH
2
 molecule all the atoms 
must be in the same plane. The p orbitals are 
mutually parallel and both the p orbitals are 
perpendicular to the plane of the molecule. 
Rotation of one CH
2
 fragment with respect 
to other interferes with maximum overlap 
of p orbitals and, therefore, such rotation 
about carbon-carbon double bond (C=C) is 
restricted. The electron charge cloud of the p 
bond is located above and below the plane of 
bonding atoms. This results in the electrons 
being easily available to the attacking  
reagents. In general, p bonds provide the most 
reactive centres in the molecules containing 
multiple bonds.
Problem 8.1
How many s and p bonds are present in 
each of the following molecules?
(a) HC=CCH=CHCH
3
 (b) CH
2
=C=CHCH
3 
solution
(a) s
C – C
: 4; s
C–H
 : 6; p
C=C
 :1; p C=C:2
(b) s
C – C
: 3; s
C–H
: 6; p
C=C
: 2.
Problem 8.2
What is the type of hybridisation of each 
carbon in the following compounds? 
(a) CH
3
Cl, (b) (CH
3
)
2
CO, (c) CH
3
CN, 
(d) HCONH
2
, (e) CH
3
CH=CHCN
solution
(a) sp
3
, (b) sp
3
, sp
2
, (c) sp
3
, sp, (d) sp
2
, (e) 
sp
3
, sp
2
, sp
2
, sp
Problem 8.3
Write the state of  hybridisation of carbon 
in the following compounds and shapes 
of each of the molecules. 
(a) H
2
C=O, (b) CH
3
F, (c) HC=N.
solution
(a) sp
2
 hybridised carbon, trigonal planar; 
(b) sp
3
 hybridised carbon, tetrahedral; (c) 
sp hybridised carbon, linear.
Unit 8.indd   257 10/10/2022   10:37:28 AM
Rationalised 2023-24
Page 3


256 chemistry Organic chemistry – sOme Basic  
Princi Ples and Techniques
After studying this unit, you will 
be able to 
• understand reasons for 
tetravalence of carbon and 
shapes of organic molecules; 
• write structures of organic 
molecules in various ways;
• classify the organic compounds; 
• name the compounds according 
to IUPAC system of nomenclature 
and also derive their structures 
from the given names;
• understand the concept of 
organic reaction mechanism;
• explain the influence of electronic 
displacements on structure and 
reactivity of organic compounds;
• recognise the types of organic 
reactions;
• learn the techniques of 
purification of organic 
compounds;
• write the chemical reactions 
involved in the qualitative 
analysis of organic compounds;
• understand the principles 
involved in quantitative analysis 
of organic compounds.
In the previous unit you have learnt that the element 
carbon has the unique property called catenation due to 
which it forms covalent bonds with other carbon atoms. 
It also forms covalent bonds with atoms of other elements 
like hydrogen, oxygen, nitrogen, sulphur, phosphorus 
and halogens. The resulting compounds are studied 
under a separate branch of chemistry called organic 
chemistry. This unit incorporates some basic principles 
and techniques of analysis required for understanding the 
formation and properties of organic compounds.
8.1 General inTroduc Tion
Organic compounds are vital for sustaining life on earth 
and include complex molecules like genetic information 
bearing deoxyribonucleic acid (DNA) and proteins that 
constitute essential compounds of our blood, muscles and 
skin. Organic compounds appear in materials like clothing, 
fuels, polymers, dyes and medicines. These are some of 
the important areas of application of these compounds.
Science of organic chemistry is about two hundred 
years old. Around the year 1780, chemists began to 
distinguish between organic compounds obtained from 
plants and animals and inorganic compounds prepared 
from mineral sources. Berzilius, a Swedish chemist 
proposed that a ‘vital force’ was responsible for the 
formation of organic compounds. However, this notion 
was rejected in 1828 when F. Wohler synthesised an 
organic compound, urea from an inorganic compound, 
ammonium cyanate.
NH
4
CNO     NH
2
CONH
2
Ammonium cyanate          Urea
The pioneering synthesis of acetic acid by Kolbe 
(1845) and that of methane by Berthelot (1856) showed 
conclusively that organic compounds could be synthesised 
from inorganic sources in a laboratory.
u niT 8
Unit 8.indd   256 2/24/2023   16:52:17
Rationalised 2023-24
257 organic chemistry – some basic principles and techniques 
The development of electronic theory of 
covalent bonding ushered organic chemistry 
into its modern shape.
8.2 tetra Valence OF car BOn:  
sha Pes OF Organic cOmPOunds
8.2.1 The Shapes of Carbon Compounds  
The knowledge of fundamental concepts of 
molecular structure helps in understanding 
and predicting the properties of organic 
compounds. You have already learnt theories 
of valency and molecular structure in Unit 4. 
Also, you already know that tetravalence of 
carbon and the formation of covalent bonds 
by it are explained in terms of its electronic 
configuration and the hybridisation of s and p 
orbitals. It may be recalled that formation and 
the shapes of molecules like methane (CH
4
), 
ethene (C
2
H
4
), ethyne (C
2
H
2
) are explained 
in terms of the use of sp
3
, sp
2
 and sp hybrid 
orbitals by carbon atoms in the respective 
molecules. 
Hybridisation influences the bond length 
and bond enthalpy (strength) in compounds. 
The sp hybrid orbital contains more s 
character and hence it is closer to its nucleus 
and forms shorter and stronger bonds than 
the sp
3
 hybrid orbital. The sp
2
 hybrid orbital 
is intermediate in s character between sp 
and sp
3
 and, hence, the length and enthalpy 
of the bonds it forms, are also intermediate 
between them. The change in hybridisation 
affects the electronegativity of carbon. The 
greater the s character of the hybrid orbitals, 
the greater is the electronegativity. Thus, a 
carbon atom having an sp hybrid orbital with 
50% s character is more electronegative than 
that possessing  sp
2
 or sp
3
 hybridised orbitals. 
This relative electronegativity is reflected in 
several physical and chemical properties of 
the molecules concerned, about which you 
will learn in later units. 
8.2.2  some characteristic Features of p 
Bonds
In a p (pi) bond formation, parallel orientation 
of the two p orbitals on adjacent atoms is 
necessary for a proper sideways overlap. 
Thus, in H
2
C=CH
2
 molecule all the atoms 
must be in the same plane. The p orbitals are 
mutually parallel and both the p orbitals are 
perpendicular to the plane of the molecule. 
Rotation of one CH
2
 fragment with respect 
to other interferes with maximum overlap 
of p orbitals and, therefore, such rotation 
about carbon-carbon double bond (C=C) is 
restricted. The electron charge cloud of the p 
bond is located above and below the plane of 
bonding atoms. This results in the electrons 
being easily available to the attacking  
reagents. In general, p bonds provide the most 
reactive centres in the molecules containing 
multiple bonds.
Problem 8.1
How many s and p bonds are present in 
each of the following molecules?
(a) HC=CCH=CHCH
3
 (b) CH
2
=C=CHCH
3 
solution
(a) s
C – C
: 4; s
C–H
 : 6; p
C=C
 :1; p C=C:2
(b) s
C – C
: 3; s
C–H
: 6; p
C=C
: 2.
Problem 8.2
What is the type of hybridisation of each 
carbon in the following compounds? 
(a) CH
3
Cl, (b) (CH
3
)
2
CO, (c) CH
3
CN, 
(d) HCONH
2
, (e) CH
3
CH=CHCN
solution
(a) sp
3
, (b) sp
3
, sp
2
, (c) sp
3
, sp, (d) sp
2
, (e) 
sp
3
, sp
2
, sp
2
, sp
Problem 8.3
Write the state of  hybridisation of carbon 
in the following compounds and shapes 
of each of the molecules. 
(a) H
2
C=O, (b) CH
3
F, (c) HC=N.
solution
(a) sp
2
 hybridised carbon, trigonal planar; 
(b) sp
3
 hybridised carbon, tetrahedral; (c) 
sp hybridised carbon, linear.
Unit 8.indd   257 10/10/2022   10:37:28 AM
Rationalised 2023-24
258 chemistry 8.3  structural r ePresen tati Ons 
OF Organic cOmPOund s 
8.3.1  complete, condensed and Bond-line 
structural Formulas
Structures of organic compounds are 
represented in several ways. The Lewis 
structure or dot structure, dash structure, 
condensed structure and bond line structural 
formulas are some of the specific types. The 
Lewis structures, however, can be simplified 
by representing the two-electron covalent 
bond by a dash (–). Such a structural formula 
focuses on the electrons involved in bond 
formation.  A single dash represents a single 
bond, double dash  is used for double bond 
and a triple dash represents triple bond. Lone-
pairs of electrons on heteroatoms (e.g., oxygen, 
nitrogen, sulphur, halogens etc.) may or may 
not be shown. Thus, ethane (C
2
H
6
), ethene 
(C
2
H
4
), ethyne (C
2
H
2
) and methanol (CH
3
OH) 
can be represented by the following structural 
formulas. Such structural representations are 
called complete structural formulas.
Similarly, CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
3
 
can be further condensed to CH
3
(CH
2
)
6
CH
3
. 
For further simplification, organic chemists 
use another way of representing the 
structures, in which only lines are used. 
In this bond-line structural representation 
of organic compounds, carbon and 
hydrogen atoms are not shown and the 
lines representing carbon-carbon bonds are  
drawn in a zig-zag fashion. The only atoms 
specifically written are oxygen, chlorine, 
nitrogen etc. The terminals denote methyl  
(–CH
3
) groups (unless indicated otherwise by 
a functional group), while the line junctions 
denote carbon atoms bonded to appropriate 
number of hydrogens required to satisfy the 
valency of the carbon atoms. Some of the 
examples are represented as follows:
(i) 3-Methyloctane can be represented in 
various forms as:
(a)   CH
3
CH
2
CHCH
2
CH
2
CH
2
CH
2
CH
3
                   
  |
                   
CH
3
 
These structural formulas can be further 
abbreviated by omitting some or all of the 
dashes representing covalent bonds and by 
indicating the number of identical groups 
attached to an atom by a subscript. The 
resulting expression of the compound is 
called a condensed structural formula. Thus, 
ethane, ethene, ethyne and methanol can be 
written as:
CH
3
CH
3
        H
2
C=CH
2
       HC=CH            CH
3
OH 
Ethane       Ethene         Ethyne      Methanol 
       Ethane                  Ethene               
   Ethyne              Methanol
(ii) Various ways of representing 2-bromo 
butane are:
(a)  CH
3
CHBrCH
2
CH
3 
 (b)  
(c)  
(b)
(c)
Unit 8.indd   258 10/10/2022   10:37:29 AM
Rationalised 2023-24
Page 4


256 chemistry Organic chemistry – sOme Basic  
Princi Ples and Techniques
After studying this unit, you will 
be able to 
• understand reasons for 
tetravalence of carbon and 
shapes of organic molecules; 
• write structures of organic 
molecules in various ways;
• classify the organic compounds; 
• name the compounds according 
to IUPAC system of nomenclature 
and also derive their structures 
from the given names;
• understand the concept of 
organic reaction mechanism;
• explain the influence of electronic 
displacements on structure and 
reactivity of organic compounds;
• recognise the types of organic 
reactions;
• learn the techniques of 
purification of organic 
compounds;
• write the chemical reactions 
involved in the qualitative 
analysis of organic compounds;
• understand the principles 
involved in quantitative analysis 
of organic compounds.
In the previous unit you have learnt that the element 
carbon has the unique property called catenation due to 
which it forms covalent bonds with other carbon atoms. 
It also forms covalent bonds with atoms of other elements 
like hydrogen, oxygen, nitrogen, sulphur, phosphorus 
and halogens. The resulting compounds are studied 
under a separate branch of chemistry called organic 
chemistry. This unit incorporates some basic principles 
and techniques of analysis required for understanding the 
formation and properties of organic compounds.
8.1 General inTroduc Tion
Organic compounds are vital for sustaining life on earth 
and include complex molecules like genetic information 
bearing deoxyribonucleic acid (DNA) and proteins that 
constitute essential compounds of our blood, muscles and 
skin. Organic compounds appear in materials like clothing, 
fuels, polymers, dyes and medicines. These are some of 
the important areas of application of these compounds.
Science of organic chemistry is about two hundred 
years old. Around the year 1780, chemists began to 
distinguish between organic compounds obtained from 
plants and animals and inorganic compounds prepared 
from mineral sources. Berzilius, a Swedish chemist 
proposed that a ‘vital force’ was responsible for the 
formation of organic compounds. However, this notion 
was rejected in 1828 when F. Wohler synthesised an 
organic compound, urea from an inorganic compound, 
ammonium cyanate.
NH
4
CNO     NH
2
CONH
2
Ammonium cyanate          Urea
The pioneering synthesis of acetic acid by Kolbe 
(1845) and that of methane by Berthelot (1856) showed 
conclusively that organic compounds could be synthesised 
from inorganic sources in a laboratory.
u niT 8
Unit 8.indd   256 2/24/2023   16:52:17
Rationalised 2023-24
257 organic chemistry – some basic principles and techniques 
The development of electronic theory of 
covalent bonding ushered organic chemistry 
into its modern shape.
8.2 tetra Valence OF car BOn:  
sha Pes OF Organic cOmPOunds
8.2.1 The Shapes of Carbon Compounds  
The knowledge of fundamental concepts of 
molecular structure helps in understanding 
and predicting the properties of organic 
compounds. You have already learnt theories 
of valency and molecular structure in Unit 4. 
Also, you already know that tetravalence of 
carbon and the formation of covalent bonds 
by it are explained in terms of its electronic 
configuration and the hybridisation of s and p 
orbitals. It may be recalled that formation and 
the shapes of molecules like methane (CH
4
), 
ethene (C
2
H
4
), ethyne (C
2
H
2
) are explained 
in terms of the use of sp
3
, sp
2
 and sp hybrid 
orbitals by carbon atoms in the respective 
molecules. 
Hybridisation influences the bond length 
and bond enthalpy (strength) in compounds. 
The sp hybrid orbital contains more s 
character and hence it is closer to its nucleus 
and forms shorter and stronger bonds than 
the sp
3
 hybrid orbital. The sp
2
 hybrid orbital 
is intermediate in s character between sp 
and sp
3
 and, hence, the length and enthalpy 
of the bonds it forms, are also intermediate 
between them. The change in hybridisation 
affects the electronegativity of carbon. The 
greater the s character of the hybrid orbitals, 
the greater is the electronegativity. Thus, a 
carbon atom having an sp hybrid orbital with 
50% s character is more electronegative than 
that possessing  sp
2
 or sp
3
 hybridised orbitals. 
This relative electronegativity is reflected in 
several physical and chemical properties of 
the molecules concerned, about which you 
will learn in later units. 
8.2.2  some characteristic Features of p 
Bonds
In a p (pi) bond formation, parallel orientation 
of the two p orbitals on adjacent atoms is 
necessary for a proper sideways overlap. 
Thus, in H
2
C=CH
2
 molecule all the atoms 
must be in the same plane. The p orbitals are 
mutually parallel and both the p orbitals are 
perpendicular to the plane of the molecule. 
Rotation of one CH
2
 fragment with respect 
to other interferes with maximum overlap 
of p orbitals and, therefore, such rotation 
about carbon-carbon double bond (C=C) is 
restricted. The electron charge cloud of the p 
bond is located above and below the plane of 
bonding atoms. This results in the electrons 
being easily available to the attacking  
reagents. In general, p bonds provide the most 
reactive centres in the molecules containing 
multiple bonds.
Problem 8.1
How many s and p bonds are present in 
each of the following molecules?
(a) HC=CCH=CHCH
3
 (b) CH
2
=C=CHCH
3 
solution
(a) s
C – C
: 4; s
C–H
 : 6; p
C=C
 :1; p C=C:2
(b) s
C – C
: 3; s
C–H
: 6; p
C=C
: 2.
Problem 8.2
What is the type of hybridisation of each 
carbon in the following compounds? 
(a) CH
3
Cl, (b) (CH
3
)
2
CO, (c) CH
3
CN, 
(d) HCONH
2
, (e) CH
3
CH=CHCN
solution
(a) sp
3
, (b) sp
3
, sp
2
, (c) sp
3
, sp, (d) sp
2
, (e) 
sp
3
, sp
2
, sp
2
, sp
Problem 8.3
Write the state of  hybridisation of carbon 
in the following compounds and shapes 
of each of the molecules. 
(a) H
2
C=O, (b) CH
3
F, (c) HC=N.
solution
(a) sp
2
 hybridised carbon, trigonal planar; 
(b) sp
3
 hybridised carbon, tetrahedral; (c) 
sp hybridised carbon, linear.
Unit 8.indd   257 10/10/2022   10:37:28 AM
Rationalised 2023-24
258 chemistry 8.3  structural r ePresen tati Ons 
OF Organic cOmPOund s 
8.3.1  complete, condensed and Bond-line 
structural Formulas
Structures of organic compounds are 
represented in several ways. The Lewis 
structure or dot structure, dash structure, 
condensed structure and bond line structural 
formulas are some of the specific types. The 
Lewis structures, however, can be simplified 
by representing the two-electron covalent 
bond by a dash (–). Such a structural formula 
focuses on the electrons involved in bond 
formation.  A single dash represents a single 
bond, double dash  is used for double bond 
and a triple dash represents triple bond. Lone-
pairs of electrons on heteroatoms (e.g., oxygen, 
nitrogen, sulphur, halogens etc.) may or may 
not be shown. Thus, ethane (C
2
H
6
), ethene 
(C
2
H
4
), ethyne (C
2
H
2
) and methanol (CH
3
OH) 
can be represented by the following structural 
formulas. Such structural representations are 
called complete structural formulas.
Similarly, CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
3
 
can be further condensed to CH
3
(CH
2
)
6
CH
3
. 
For further simplification, organic chemists 
use another way of representing the 
structures, in which only lines are used. 
In this bond-line structural representation 
of organic compounds, carbon and 
hydrogen atoms are not shown and the 
lines representing carbon-carbon bonds are  
drawn in a zig-zag fashion. The only atoms 
specifically written are oxygen, chlorine, 
nitrogen etc. The terminals denote methyl  
(–CH
3
) groups (unless indicated otherwise by 
a functional group), while the line junctions 
denote carbon atoms bonded to appropriate 
number of hydrogens required to satisfy the 
valency of the carbon atoms. Some of the 
examples are represented as follows:
(i) 3-Methyloctane can be represented in 
various forms as:
(a)   CH
3
CH
2
CHCH
2
CH
2
CH
2
CH
2
CH
3
                   
  |
                   
CH
3
 
These structural formulas can be further 
abbreviated by omitting some or all of the 
dashes representing covalent bonds and by 
indicating the number of identical groups 
attached to an atom by a subscript. The 
resulting expression of the compound is 
called a condensed structural formula. Thus, 
ethane, ethene, ethyne and methanol can be 
written as:
CH
3
CH
3
        H
2
C=CH
2
       HC=CH            CH
3
OH 
Ethane       Ethene         Ethyne      Methanol 
       Ethane                  Ethene               
   Ethyne              Methanol
(ii) Various ways of representing 2-bromo 
butane are:
(a)  CH
3
CHBrCH
2
CH
3 
 (b)  
(c)  
(b)
(c)
Unit 8.indd   258 10/10/2022   10:37:29 AM
Rationalised 2023-24
259 organic chemistry – some basic principles and techniques 
In cyclic compounds, the bond-line formulas 
may be given as follows:
Cyclopropane
Cyclopentane
chlorocyclohexane
Problem 8.4
Expand each of the following condensed 
formulas into their complete structural 
formulas.
(a) CH
3
CH
2
COCH
2
CH
3
(b) CH
3
CH=CH(CH
2
)
3
CH
3
solution
(a) 
(b)  
(b) 
solution
Condensed formula:
(a) HO(CH
2
)
3
CH(CH
3
)CH(CH
3
)
2
(b) HOCH(CN)
2
Bond-line formula:
Problem 8.5
For each of the following compounds, 
write a  condensed formula and also their 
bond-line formula.
(a) HOCH
2
CH
2
CH
2
CH(CH
3
)CH(CH
3
)CH
3
(b)
(a)
Problem 8.6
Expand each of the following bond-line 
formulas to show all the atoms including 
carbon and hydrogen
(a)
(b)
(c)
(d)
solution
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256 chemistry Organic chemistry – sOme Basic  
Princi Ples and Techniques
After studying this unit, you will 
be able to 
• understand reasons for 
tetravalence of carbon and 
shapes of organic molecules; 
• write structures of organic 
molecules in various ways;
• classify the organic compounds; 
• name the compounds according 
to IUPAC system of nomenclature 
and also derive their structures 
from the given names;
• understand the concept of 
organic reaction mechanism;
• explain the influence of electronic 
displacements on structure and 
reactivity of organic compounds;
• recognise the types of organic 
reactions;
• learn the techniques of 
purification of organic 
compounds;
• write the chemical reactions 
involved in the qualitative 
analysis of organic compounds;
• understand the principles 
involved in quantitative analysis 
of organic compounds.
In the previous unit you have learnt that the element 
carbon has the unique property called catenation due to 
which it forms covalent bonds with other carbon atoms. 
It also forms covalent bonds with atoms of other elements 
like hydrogen, oxygen, nitrogen, sulphur, phosphorus 
and halogens. The resulting compounds are studied 
under a separate branch of chemistry called organic 
chemistry. This unit incorporates some basic principles 
and techniques of analysis required for understanding the 
formation and properties of organic compounds.
8.1 General inTroduc Tion
Organic compounds are vital for sustaining life on earth 
and include complex molecules like genetic information 
bearing deoxyribonucleic acid (DNA) and proteins that 
constitute essential compounds of our blood, muscles and 
skin. Organic compounds appear in materials like clothing, 
fuels, polymers, dyes and medicines. These are some of 
the important areas of application of these compounds.
Science of organic chemistry is about two hundred 
years old. Around the year 1780, chemists began to 
distinguish between organic compounds obtained from 
plants and animals and inorganic compounds prepared 
from mineral sources. Berzilius, a Swedish chemist 
proposed that a ‘vital force’ was responsible for the 
formation of organic compounds. However, this notion 
was rejected in 1828 when F. Wohler synthesised an 
organic compound, urea from an inorganic compound, 
ammonium cyanate.
NH
4
CNO     NH
2
CONH
2
Ammonium cyanate          Urea
The pioneering synthesis of acetic acid by Kolbe 
(1845) and that of methane by Berthelot (1856) showed 
conclusively that organic compounds could be synthesised 
from inorganic sources in a laboratory.
u niT 8
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257 organic chemistry – some basic principles and techniques 
The development of electronic theory of 
covalent bonding ushered organic chemistry 
into its modern shape.
8.2 tetra Valence OF car BOn:  
sha Pes OF Organic cOmPOunds
8.2.1 The Shapes of Carbon Compounds  
The knowledge of fundamental concepts of 
molecular structure helps in understanding 
and predicting the properties of organic 
compounds. You have already learnt theories 
of valency and molecular structure in Unit 4. 
Also, you already know that tetravalence of 
carbon and the formation of covalent bonds 
by it are explained in terms of its electronic 
configuration and the hybridisation of s and p 
orbitals. It may be recalled that formation and 
the shapes of molecules like methane (CH
4
), 
ethene (C
2
H
4
), ethyne (C
2
H
2
) are explained 
in terms of the use of sp
3
, sp
2
 and sp hybrid 
orbitals by carbon atoms in the respective 
molecules. 
Hybridisation influences the bond length 
and bond enthalpy (strength) in compounds. 
The sp hybrid orbital contains more s 
character and hence it is closer to its nucleus 
and forms shorter and stronger bonds than 
the sp
3
 hybrid orbital. The sp
2
 hybrid orbital 
is intermediate in s character between sp 
and sp
3
 and, hence, the length and enthalpy 
of the bonds it forms, are also intermediate 
between them. The change in hybridisation 
affects the electronegativity of carbon. The 
greater the s character of the hybrid orbitals, 
the greater is the electronegativity. Thus, a 
carbon atom having an sp hybrid orbital with 
50% s character is more electronegative than 
that possessing  sp
2
 or sp
3
 hybridised orbitals. 
This relative electronegativity is reflected in 
several physical and chemical properties of 
the molecules concerned, about which you 
will learn in later units. 
8.2.2  some characteristic Features of p 
Bonds
In a p (pi) bond formation, parallel orientation 
of the two p orbitals on adjacent atoms is 
necessary for a proper sideways overlap. 
Thus, in H
2
C=CH
2
 molecule all the atoms 
must be in the same plane. The p orbitals are 
mutually parallel and both the p orbitals are 
perpendicular to the plane of the molecule. 
Rotation of one CH
2
 fragment with respect 
to other interferes with maximum overlap 
of p orbitals and, therefore, such rotation 
about carbon-carbon double bond (C=C) is 
restricted. The electron charge cloud of the p 
bond is located above and below the plane of 
bonding atoms. This results in the electrons 
being easily available to the attacking  
reagents. In general, p bonds provide the most 
reactive centres in the molecules containing 
multiple bonds.
Problem 8.1
How many s and p bonds are present in 
each of the following molecules?
(a) HC=CCH=CHCH
3
 (b) CH
2
=C=CHCH
3 
solution
(a) s
C – C
: 4; s
C–H
 : 6; p
C=C
 :1; p C=C:2
(b) s
C – C
: 3; s
C–H
: 6; p
C=C
: 2.
Problem 8.2
What is the type of hybridisation of each 
carbon in the following compounds? 
(a) CH
3
Cl, (b) (CH
3
)
2
CO, (c) CH
3
CN, 
(d) HCONH
2
, (e) CH
3
CH=CHCN
solution
(a) sp
3
, (b) sp
3
, sp
2
, (c) sp
3
, sp, (d) sp
2
, (e) 
sp
3
, sp
2
, sp
2
, sp
Problem 8.3
Write the state of  hybridisation of carbon 
in the following compounds and shapes 
of each of the molecules. 
(a) H
2
C=O, (b) CH
3
F, (c) HC=N.
solution
(a) sp
2
 hybridised carbon, trigonal planar; 
(b) sp
3
 hybridised carbon, tetrahedral; (c) 
sp hybridised carbon, linear.
Unit 8.indd   257 10/10/2022   10:37:28 AM
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258 chemistry 8.3  structural r ePresen tati Ons 
OF Organic cOmPOund s 
8.3.1  complete, condensed and Bond-line 
structural Formulas
Structures of organic compounds are 
represented in several ways. The Lewis 
structure or dot structure, dash structure, 
condensed structure and bond line structural 
formulas are some of the specific types. The 
Lewis structures, however, can be simplified 
by representing the two-electron covalent 
bond by a dash (–). Such a structural formula 
focuses on the electrons involved in bond 
formation.  A single dash represents a single 
bond, double dash  is used for double bond 
and a triple dash represents triple bond. Lone-
pairs of electrons on heteroatoms (e.g., oxygen, 
nitrogen, sulphur, halogens etc.) may or may 
not be shown. Thus, ethane (C
2
H
6
), ethene 
(C
2
H
4
), ethyne (C
2
H
2
) and methanol (CH
3
OH) 
can be represented by the following structural 
formulas. Such structural representations are 
called complete structural formulas.
Similarly, CH
3
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
3
 
can be further condensed to CH
3
(CH
2
)
6
CH
3
. 
For further simplification, organic chemists 
use another way of representing the 
structures, in which only lines are used. 
In this bond-line structural representation 
of organic compounds, carbon and 
hydrogen atoms are not shown and the 
lines representing carbon-carbon bonds are  
drawn in a zig-zag fashion. The only atoms 
specifically written are oxygen, chlorine, 
nitrogen etc. The terminals denote methyl  
(–CH
3
) groups (unless indicated otherwise by 
a functional group), while the line junctions 
denote carbon atoms bonded to appropriate 
number of hydrogens required to satisfy the 
valency of the carbon atoms. Some of the 
examples are represented as follows:
(i) 3-Methyloctane can be represented in 
various forms as:
(a)   CH
3
CH
2
CHCH
2
CH
2
CH
2
CH
2
CH
3
                   
  |
                   
CH
3
 
These structural formulas can be further 
abbreviated by omitting some or all of the 
dashes representing covalent bonds and by 
indicating the number of identical groups 
attached to an atom by a subscript. The 
resulting expression of the compound is 
called a condensed structural formula. Thus, 
ethane, ethene, ethyne and methanol can be 
written as:
CH
3
CH
3
        H
2
C=CH
2
       HC=CH            CH
3
OH 
Ethane       Ethene         Ethyne      Methanol 
       Ethane                  Ethene               
   Ethyne              Methanol
(ii) Various ways of representing 2-bromo 
butane are:
(a)  CH
3
CHBrCH
2
CH
3 
 (b)  
(c)  
(b)
(c)
Unit 8.indd   258 10/10/2022   10:37:29 AM
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259 organic chemistry – some basic principles and techniques 
In cyclic compounds, the bond-line formulas 
may be given as follows:
Cyclopropane
Cyclopentane
chlorocyclohexane
Problem 8.4
Expand each of the following condensed 
formulas into their complete structural 
formulas.
(a) CH
3
CH
2
COCH
2
CH
3
(b) CH
3
CH=CH(CH
2
)
3
CH
3
solution
(a) 
(b)  
(b) 
solution
Condensed formula:
(a) HO(CH
2
)
3
CH(CH
3
)CH(CH
3
)
2
(b) HOCH(CN)
2
Bond-line formula:
Problem 8.5
For each of the following compounds, 
write a  condensed formula and also their 
bond-line formula.
(a) HOCH
2
CH
2
CH
2
CH(CH
3
)CH(CH
3
)CH
3
(b)
(a)
Problem 8.6
Expand each of the following bond-line 
formulas to show all the atoms including 
carbon and hydrogen
(a)
(b)
(c)
(d)
solution
Unit 8.indd   259 10/10/2022   10:37:30 AM
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260 chemistry Framework model Ball and stick model
Space filling model
Fig. 8.2
8.3.2  Three-Dimensional 
Representation of Organic 
Molecules
The three-dimensional (3-D) structure of 
organic molecules can be represented on 
paper by using certain conventions. For 
example, by using solid ( ) and dashed 
( ) wedge formula, the 3-D image of a 
molecule from a two-dimensional picture 
can be perceived. In these formulas the 
solid-wedge is used to indicate a bond 
projecting out of the plane of paper, towards 
the observer. The dashed-wedge is used to 
depict the bond projecting out of the plane of 
the paper and away from the observer. Wedges 
are shown in such a way that the broad end of 
the wedge is towards the observer. The bonds 
lying in plane of the paper are depicted by 
using a normal line (—). 3-D representation of 
methane molecule on paper has been shown 
in Fig. 8.1 
Fig. 8.1 Wedge-and-dash representation of CH
4
Molecular Models
Molecular models are physical devices 
that are used for a better visualisation and 
perception of three-dimensional shapes 
of organic molecules. These are made of 
wood, plastic or metal and are commercially 
available. Commonly three types of molecular 
models are used: (1) Framework model, (2) 
Ball-and-stick model, and (3) Space filling 
model. In the framework model only the 
bonds connecting the atoms of a molecule 
and not the atoms themselves are shown. 
This model emphasizes the pattern of 
bonds of a molecule while ignoring the size 
of atoms. In the ball-and-stick model, both 
the atoms and the bonds are shown. Balls 
represent atoms and the stick denotes a 
bond. Compounds containing C=C (e.g., 
ethene) can best be represented by using 
springs in place of sticks. These models are 
referred to as ball-and-spring model. The 
space-filling model emphasises the relative 
size of each atom based on its van der Waals 
radius. Bonds are not shown in this model. 
It conveys the volume occupied by each atom 
in the molecule. In addition to these models, 
computer graphics can also be used for 
molecular modelling. 
Unit 8.indd   260 11/10/2022   15:19:57
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