12. ORGANIC CHEMISTRY – SOME BASIC PRINCIPLES AND TECHNIQUES
Organic chemistry is the branch of chemistry that deals with carbon compounds. But all carbon
compounds are not considered as organic compounds. (E.g. CO2, CO, metal carbonates, bicarbonates etc.). So
organic chemistry can be defined as the branch of chemistry that deals with hydrocarbons and their derivatives.
Hydrocarbons are the major class of organic compounds and they contain only carbon and hydrogen atoms. All
other organic compounds are formed by replacing one or more hydrogen atoms of hydrocarbons by other atoms
or groups (They are called hydrocarbon derivatives).
All carbon compounds present in plants and animals are organic compounds. E.g. Carbohydrates,
proteins, vitamins, nucleic acids, amino acids, fats and oils, natural polymers etc. petroleum and coal are the
major source of organic compounds (hydrocarbons).
In ancient times, it was believed that a vital force (living body) is necessary for the production of an
organic compound. But in 1828, Frederic Wohler proved that this belief was wrong. He prepared urea in the
laboratory, by heating ammonium cyanate (NH4CNO). It was the first organic compound prepared in the
laboratory.
NH4CNO ⎯⎯Heat⎯→ NH2CONH2
Ammonium cyanate Urea
Then another scientist Kolbe synthesized acetic acid and Berthelot synthesized methane in the
laboratory. Nowadays about 95% of the organic compounds are synthesized in the laboratory.
Chemistry behind the existance of large number Carbon compounds
Carbon is a unique element and it can form a large number of compounds due to the following reasons:
i) Tetravalency of carbon: In all of its compounds, the valency of carbon is four. Carbon has 4 electrons
in its valency shell and requires 4 more electrons to complete the octet. So it attains the octet
configuration by forming 4 covalent bonds.
ii) Ability to form single bond and multiple bonds: C can form single bond and multiple bond (double or
triple bond) with itself and also with other elements like oxygen, nitrogen etc. This is possible by sp3,
sp2 or sp hybridisation.
iii) Catenation: Carbon shows catenation. It is the self linking property of an element to form long chains
and rings.
iv) Isomerism: Carbon compounds can show isomerism. It is the phenomenon in which compounds
having same molecular formula but different structural formula or spatial arrangement of atoms.
Structural representation of organic compounds
An organic compound can be represented by the following ways:
1. Complete structural formula: Here all the bonds between atoms are denoted by dashes (----). A single
dash represents a single bond, a double dash represents a double bond and a triple dash represents a
triple bond. E.g.
2. Condensed stuctural formula: Here the carbon-hydrogen bonds or all the bonds are omitted except the
multiple bonds. It is a simplified representation of an organic compound.
E.g. ethane - CH3CH3, propane - CH3CH2CH3, butane - CH3CH2CH2CH3, ethene - CH2=CH2 etc.
XI Chemistry Notes By Anil Kumar K.L [HSSLiVE] Page 1
, The condensed formula can again simplified as follows:
Butane – CH3(CH2)2CH3, Hexane- CH3(CH2)4CH3, Decane – CH3(CH2)8CH3 etc.
3. Bond line representation: It is the simplest form of representation of an organic compound. Here 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 free terminals denote
methyl (–CH3) groups.
E.g. butane:
3-methyl pentane:
Heptanal: H
O
3-Methyl pentanoic acid: OH
O
4. Three-Dimensional Representation (Wedge Representation): Here the structure of an organic molecule
can be represented by using solid ( ) and dashed ( ) wedges. The solid-wedge is used to
indicate a bond projecting out of the plane of paper, towards the observer. The dashed-wedge indicates
the bond projecting out of the plane of the paper and away from the observer. The broad end of the
wedge is always towards the observer. The bonds lying in plane of the paper are depicted by using a
normal line (—).
E.g. methane
Classification of Organic compounds
Organic compounds can be broadly classified into two – Acyclic or open chain compounds and cyclic or ring
compounds.
1. Acyclic or open chain or aliphatic compounds: In these compounds, the carbon atoms are joined
together to form long chains which may be straight chain or branched chain. They are further classified as
saturated compounds and unsaturated compounds. Saturated compounds contain only carbon – carbon
single bonds. But unsaturated compounds contain atleast one carbon – carbon multiple bond (double or
triple bond. Saturated hydrocarbons are called alkanes and unsaturated hydrocarbons are of two types –
alkenes and alkynes.
XI Chemistry Notes By Anil Kumar K.L [HSSLiVE] Page 2
, 2. Cyclic or closed chain or ring compounds: In these compounds, the carbon atoms are joined together to
form rings. These rings may be homocyclic or heterocyclic. If the ring contains only carbon atoms, it is
called homocyclic compound and if it contains atoms other than carbon (like O, N, S etc), it is called
heterocyclic compound.
Homocyclic compounds are further classified into two – Alicyclic compounds and Aromatic
compounds. Alicyclic compounds contain atleast one carbo-cyclic ring. Alicyclic hydrocarbons are of three
types – cycloalkanes, cycloalkenes and cycloalkynes. Aromatic compounds are some special type of
compounds. These are of two types. Aromatic compounds containing benzene ring are called benzenoid
compounds and those which do not contain benzene ring are called non-benzenoid compounds. E.g. for a
non-benzenoid aromatic compound is tropolone.
Heterocyclic compounds may be alicyclic heterocyclic compounds or aromatic heterocyclic compounds.
The classification of organic compounds can be diagrammatically represented as follows:
Organic compounds
Acyclic or open chain or Aliphatic compounds Cyclic or ring or closed chain compounds
Saturated compounds Unsaturated compounds Homocyclic compounds Heterocyclic compounds
Alicyclic Aromatic Alicyclic Aromatic
Benzenoid Non-benzenoid
Functional groups: Atoms or group of atoms (except hydrogen) which are bonded to carbon atoms are called
functional groups. These groups are responsible for the characteristic chemical properties of the organic
compounds. Some important functional groups, their names and name of the compounds are listed below:
Functional group Name of the group Name of compound
-OH Hydroxyl group Alcohol
-NH2 Amino group Amine
-X Halo group Halo compound
-CHO Aldehydric (formyl) group Aldehyde
-CO- or >CO Carbonyl (keto) group Ketone
-COOH Carboxyl group Carboxylic acid
-O- Oxy group Ether
-CN Cyano group Nitrile
-NO2 Nitro group Nitro compound
Homologous series: A series or group of organic compounds in which adjacent members are differed by a –CH2
group is called a homologous series. The members of a homologous series are called homologues. They contain
same functional groups, have similar chemical properties and show gradation in physical properties. They can be
prepared by some general methods of preparation. E.g. for homologous series are alkanes, alkenes, alkynes,
alcohols, ethers, carboxylic acids, aldehydes, ketones, amines, halo compounds etc.
Nomenclature of organic compounds
An organic compound has two types of names – Common name and IUPAC name. The common name is
based on the source or some properties. For e.g. citric acid is named so because it is found in citrus fruits and the
acid found in red ant is named formic acid since the Latin word for ant is formica.
XI Chemistry Notes By Anil Kumar K.L [HSSLiVE] Page 3
Organic chemistry is the branch of chemistry that deals with carbon compounds. But all carbon
compounds are not considered as organic compounds. (E.g. CO2, CO, metal carbonates, bicarbonates etc.). So
organic chemistry can be defined as the branch of chemistry that deals with hydrocarbons and their derivatives.
Hydrocarbons are the major class of organic compounds and they contain only carbon and hydrogen atoms. All
other organic compounds are formed by replacing one or more hydrogen atoms of hydrocarbons by other atoms
or groups (They are called hydrocarbon derivatives).
All carbon compounds present in plants and animals are organic compounds. E.g. Carbohydrates,
proteins, vitamins, nucleic acids, amino acids, fats and oils, natural polymers etc. petroleum and coal are the
major source of organic compounds (hydrocarbons).
In ancient times, it was believed that a vital force (living body) is necessary for the production of an
organic compound. But in 1828, Frederic Wohler proved that this belief was wrong. He prepared urea in the
laboratory, by heating ammonium cyanate (NH4CNO). It was the first organic compound prepared in the
laboratory.
NH4CNO ⎯⎯Heat⎯→ NH2CONH2
Ammonium cyanate Urea
Then another scientist Kolbe synthesized acetic acid and Berthelot synthesized methane in the
laboratory. Nowadays about 95% of the organic compounds are synthesized in the laboratory.
Chemistry behind the existance of large number Carbon compounds
Carbon is a unique element and it can form a large number of compounds due to the following reasons:
i) Tetravalency of carbon: In all of its compounds, the valency of carbon is four. Carbon has 4 electrons
in its valency shell and requires 4 more electrons to complete the octet. So it attains the octet
configuration by forming 4 covalent bonds.
ii) Ability to form single bond and multiple bonds: C can form single bond and multiple bond (double or
triple bond) with itself and also with other elements like oxygen, nitrogen etc. This is possible by sp3,
sp2 or sp hybridisation.
iii) Catenation: Carbon shows catenation. It is the self linking property of an element to form long chains
and rings.
iv) Isomerism: Carbon compounds can show isomerism. It is the phenomenon in which compounds
having same molecular formula but different structural formula or spatial arrangement of atoms.
Structural representation of organic compounds
An organic compound can be represented by the following ways:
1. Complete structural formula: Here all the bonds between atoms are denoted by dashes (----). A single
dash represents a single bond, a double dash represents a double bond and a triple dash represents a
triple bond. E.g.
2. Condensed stuctural formula: Here the carbon-hydrogen bonds or all the bonds are omitted except the
multiple bonds. It is a simplified representation of an organic compound.
E.g. ethane - CH3CH3, propane - CH3CH2CH3, butane - CH3CH2CH2CH3, ethene - CH2=CH2 etc.
XI Chemistry Notes By Anil Kumar K.L [HSSLiVE] Page 1
, The condensed formula can again simplified as follows:
Butane – CH3(CH2)2CH3, Hexane- CH3(CH2)4CH3, Decane – CH3(CH2)8CH3 etc.
3. Bond line representation: It is the simplest form of representation of an organic compound. Here 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 free terminals denote
methyl (–CH3) groups.
E.g. butane:
3-methyl pentane:
Heptanal: H
O
3-Methyl pentanoic acid: OH
O
4. Three-Dimensional Representation (Wedge Representation): Here the structure of an organic molecule
can be represented by using solid ( ) and dashed ( ) wedges. The solid-wedge is used to
indicate a bond projecting out of the plane of paper, towards the observer. The dashed-wedge indicates
the bond projecting out of the plane of the paper and away from the observer. The broad end of the
wedge is always towards the observer. The bonds lying in plane of the paper are depicted by using a
normal line (—).
E.g. methane
Classification of Organic compounds
Organic compounds can be broadly classified into two – Acyclic or open chain compounds and cyclic or ring
compounds.
1. Acyclic or open chain or aliphatic compounds: In these compounds, the carbon atoms are joined
together to form long chains which may be straight chain or branched chain. They are further classified as
saturated compounds and unsaturated compounds. Saturated compounds contain only carbon – carbon
single bonds. But unsaturated compounds contain atleast one carbon – carbon multiple bond (double or
triple bond. Saturated hydrocarbons are called alkanes and unsaturated hydrocarbons are of two types –
alkenes and alkynes.
XI Chemistry Notes By Anil Kumar K.L [HSSLiVE] Page 2
, 2. Cyclic or closed chain or ring compounds: In these compounds, the carbon atoms are joined together to
form rings. These rings may be homocyclic or heterocyclic. If the ring contains only carbon atoms, it is
called homocyclic compound and if it contains atoms other than carbon (like O, N, S etc), it is called
heterocyclic compound.
Homocyclic compounds are further classified into two – Alicyclic compounds and Aromatic
compounds. Alicyclic compounds contain atleast one carbo-cyclic ring. Alicyclic hydrocarbons are of three
types – cycloalkanes, cycloalkenes and cycloalkynes. Aromatic compounds are some special type of
compounds. These are of two types. Aromatic compounds containing benzene ring are called benzenoid
compounds and those which do not contain benzene ring are called non-benzenoid compounds. E.g. for a
non-benzenoid aromatic compound is tropolone.
Heterocyclic compounds may be alicyclic heterocyclic compounds or aromatic heterocyclic compounds.
The classification of organic compounds can be diagrammatically represented as follows:
Organic compounds
Acyclic or open chain or Aliphatic compounds Cyclic or ring or closed chain compounds
Saturated compounds Unsaturated compounds Homocyclic compounds Heterocyclic compounds
Alicyclic Aromatic Alicyclic Aromatic
Benzenoid Non-benzenoid
Functional groups: Atoms or group of atoms (except hydrogen) which are bonded to carbon atoms are called
functional groups. These groups are responsible for the characteristic chemical properties of the organic
compounds. Some important functional groups, their names and name of the compounds are listed below:
Functional group Name of the group Name of compound
-OH Hydroxyl group Alcohol
-NH2 Amino group Amine
-X Halo group Halo compound
-CHO Aldehydric (formyl) group Aldehyde
-CO- or >CO Carbonyl (keto) group Ketone
-COOH Carboxyl group Carboxylic acid
-O- Oxy group Ether
-CN Cyano group Nitrile
-NO2 Nitro group Nitro compound
Homologous series: A series or group of organic compounds in which adjacent members are differed by a –CH2
group is called a homologous series. The members of a homologous series are called homologues. They contain
same functional groups, have similar chemical properties and show gradation in physical properties. They can be
prepared by some general methods of preparation. E.g. for homologous series are alkanes, alkenes, alkynes,
alcohols, ethers, carboxylic acids, aldehydes, ketones, amines, halo compounds etc.
Nomenclature of organic compounds
An organic compound has two types of names – Common name and IUPAC name. The common name is
based on the source or some properties. For e.g. citric acid is named so because it is found in citrus fruits and the
acid found in red ant is named formic acid since the Latin word for ant is formica.
XI Chemistry Notes By Anil Kumar K.L [HSSLiVE] Page 3
