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Isomers
We have seen how organic
molecules can exist in more than one structure.
A chemical reaction depends on the structure of the molecules that
interact. This is because the electronic configurations of the
molecule depend on the way the bonds are formed in any structure. It is
therefore important to note that structures of organic molecules play a very
important role in the chemical reactivity of the molecule.
Organic
compounds having similar chemical formula but different structures are
called isomers. For example, a molecule of butane C4H10 can
have two structures or configurations, as we have seen. They are n-butane,
which is a straight chain structure, and iso-butane, which is a branched
chain structure. Thus n-butane and iso-butane are isomers of each other. A
molecule of pentane C5H12 can have three different
structures; n-pentane, iso-pentane and neo pentane are all isomers of each
other.
| Characteristics
of isomers |
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Isomers
have same chemical formulae or molecular formulae
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Isomers
differ in their structural formulae
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Because
of difference in the structures and hence their electronic
configurations and bondings, isomers exhibit different physical and
chemical properties.
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| Points
to remember |
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Isomers
occur in organic compounds that have more than 3 carbon atoms
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Organic
compounds with 4 carbon atoms have 2 isomers, those with 5 carbon atoms
have 3 isomers, those with 6 carbon atoms have 4
isomers, etc.
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As
the number of carbon atoms increase, the way the atoms can be arranged
can become very complex. Straight chains with branches and branches with
sub-branches can increase the complexity. For example, an organic compound having 10 carbon atoms will have 75 isomers!
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To
simplify complexity of
organic compounds, they are classified into groups. Just as elements in a
group in a periodic table exhibit similar properties, organic compounds
showing similar structures and hence physical and chemical properties are
put together in a group or a series. This is called as the homologous
series. They are arranged in the increasing order of their molecular
weights. In general, a homologous series is a group of organic compounds
having similar structure and chemical properties. The members of the
homologous series differ from each their by the CH2 group only.
Homologous
series of alkane is methane, ethane, propane, n-butane, n-pentane.
Homologous series of alkenes is ethene, propene, butene, etc. Homologous
series of alkynes is ethyne, propyne, butyne, etc.
As we will study reactions of these hydrocarbons, you will see how
the homologous series show similar properties, similar substitution
reactions and how they can be prepared by similar methods.
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