NCERT Textbook - Organic Chemistry: Some Basic Principles & Technique | NCERT Textbooks (Class 6 to Class 12) - CTET & State TET PDF Download

 Page 1


326 CHEMISTRY
ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES
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.
12.1 GENERAL INTRODUCTION
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 chemicals 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.
422
Heat
NH CNO NH CONH ?? ? ? ?
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.
UNIT 12
Page 2


326 CHEMISTRY
ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES
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.
12.1 GENERAL INTRODUCTION
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 chemicals 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.
422
Heat
NH CNO NH CONH ?? ? ? ?
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.
UNIT 12
327 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES
The development of electronic theory of
covalent bonding ushered organic chemistry
into its modern shape.
12.2 TETRAVALENCE OF CARBON:
SHAPES OF ORGANIC COMPOUNDS
12.2.1 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 organic
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.
12.2.2 Some Characteristic Features of p p p p 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 12.1
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 12.2
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 12.3
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.
Page 3


326 CHEMISTRY
ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES
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.
12.1 GENERAL INTRODUCTION
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 chemicals 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.
422
Heat
NH CNO NH CONH ?? ? ? ?
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.
UNIT 12
327 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES
The development of electronic theory of
covalent bonding ushered organic chemistry
into its modern shape.
12.2 TETRAVALENCE OF CARBON:
SHAPES OF ORGANIC COMPOUNDS
12.2.1 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 organic
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.
12.2.2 Some Characteristic Features of p p p p 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 12.1
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 12.2
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 12.3
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.
328 CHEMISTRY
12.3 STRUCTURAL REPRESENTATIONS
OF ORGANIC COMPOUNDS
12.3.1Complete, 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)
Page 4


326 CHEMISTRY
ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES
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.
12.1 GENERAL INTRODUCTION
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 chemicals 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.
422
Heat
NH CNO NH CONH ?? ? ? ?
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.
UNIT 12
327 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES
The development of electronic theory of
covalent bonding ushered organic chemistry
into its modern shape.
12.2 TETRAVALENCE OF CARBON:
SHAPES OF ORGANIC COMPOUNDS
12.2.1 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 organic
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.
12.2.2 Some Characteristic Features of p p p p 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 12.1
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 12.2
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 12.3
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.
328 CHEMISTRY
12.3 STRUCTURAL REPRESENTATIONS
OF ORGANIC COMPOUNDS
12.3.1Complete, 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)
329 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES
In cyclic compounds, the bond-line formulas
may be given as follows:
Cyclopropane
Cyclopentane
chlorocyclohexane
Problem 12.4
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
(b)
Solution
Condensed formula:
(a) HO(CH
2
)
3
CH(CH
3
)CH(CH
3
)
2
(b) HOCH(CN)
2
Bond-line formula:
(a)
(b)
Problem 12.5
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 12.6
Expand each of the following bond-line
formulas to show all the atoms including
carbon and hydrogen
(a)
(b)
(c)
(d)
Solution
Page 5


326 CHEMISTRY
ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES
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.
12.1 GENERAL INTRODUCTION
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 chemicals 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.
422
Heat
NH CNO NH CONH ?? ? ? ?
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.
UNIT 12
327 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES
The development of electronic theory of
covalent bonding ushered organic chemistry
into its modern shape.
12.2 TETRAVALENCE OF CARBON:
SHAPES OF ORGANIC COMPOUNDS
12.2.1 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 organic
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.
12.2.2 Some Characteristic Features of p p p p 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 12.1
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 12.2
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 12.3
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.
328 CHEMISTRY
12.3 STRUCTURAL REPRESENTATIONS
OF ORGANIC COMPOUNDS
12.3.1Complete, 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)
329 ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES
In cyclic compounds, the bond-line formulas
may be given as follows:
Cyclopropane
Cyclopentane
chlorocyclohexane
Problem 12.4
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
(b)
Solution
Condensed formula:
(a) HO(CH
2
)
3
CH(CH
3
)CH(CH
3
)
2
(b) HOCH(CN)
2
Bond-line formula:
(a)
(b)
Problem 12.5
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 12.6
Expand each of the following bond-line
formulas to show all the atoms including
carbon and hydrogen
(a)
(b)
(c)
(d)
Solution
330 CHEMISTRY
 Framework model
 Ball and stick model
Space filling model
Fig. 12.2
12.3.2 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. 12.1.
Fig. 12.1 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.