Isomerism is the phenomenon in which two or more individual compounds have same molecular formula but different structures. Isomers are the compounds which have same molecular formula but they differ in the arrangement of atoms in the molecule.
Key words: Isomerism, Structural Isomers, Chain Isomers, Position Isomers, Functional Group, Isomers, Metamers, Tautomers, Ring Chain Isomers, Configurational Isomers, Enantiomers, Stereoisomers, Conformational isomers, Diastereomers
Introduction
The term isomer is derived from Greek word “isomeros” where, iso means “equal” and meros means “part”. In Chemistry there are various kinds of isomers which are categorized on the basis of arrangement of the atoms present in the molecules. In general isomers of the compounds have different physical properties (such as melting points, boiling points) and chemical properties.
Mainly
there are two types of isomers present in nature, namely Constitutional Isomers
and Stereo Isomers.
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| Figure: Types of Isomers |
1.
Constitutional
Isomers / Structural
Isomers
Constitutional
isomers consist of molecules where the atoms have different arrangement /
position / carbon chains etc. They can be further divided into five
subcategories. They are described as follows;
A. Chain Isomers:
Chain
isomers are the pair of molecule where carbon skeleton in the molecules is
different from each other. For example; pentane, isopentane and neopentane have identical molecular formula C5H12 but different carbon chain
skeleton. Hence they are called as chain isomers of each other.
B.
Position
Isomers :
Position
isomers are having different position of functional groups in the carbon chain
skeleton. For example; in “1-butene” the double bond is present at C1-C2
whereas in 2-butene the double bond is present at C2-C3.
Therefore they are known as position isomers.
Consider 1-butanol and 2-butanol, they are differing in position of hydroxyl group. Therefore they are called as position isomers.
C.
Functional
Group Isomers :
The
functional group isomers have same chemical formula but different functional
groups in the molecules. For example; methyl acetate is a ester compound and
propionic acid is a carboxylic acid. These two molecules have same chemical
formula C3H6O2, so they are termed as functional
group isomers.
Consider
another example, 1-butanol and diethyl ether has different functional groups
but they have same molecular formula of C4H10O. Hence
they are called as functional group isomer.
D.
Metamers :
Metamers
are the isomers which are having same functional groups but they have different
carbon chain skeleton on either side of functional group. For example; methyl
propyl ether and diethyl ether, they are differ in carbon chain distribution at
oxygen atom. Therefore they are known as metamer isomers.
Let’s
see one more example, Ethyl acetate and methyl propionate have difference in
carbon chain distribution of either side of the carboxylate (COO) functional
group. Therefore they are termed as metamers of each other.
E.
Tautomers :
Toutomers
are the isomers of compound which have same carbon skeleton but they differ in
the position of protons and electrons (or double bonds). For example, keto and
enol forms of the carbonyl compound are known as tautomers.
The pyrrole has three tautomers based upon the position of protons and double bonds; 1H-pyrrole, 2H-pyrrole and 3H-pyrrole.
F.
Ring
Chain Isomers :
Ring
chain isomers are the two individual compounds where one compounds consist of
cyclic structure whereas the other compound is having open chain structure. For
example, cyclobutane and 2-butene has same molecular formula of C4H8.
Here cyclobutane consist of cyclic ring and 2-butene is an open chain compound.
Consider
another example, cyclohexene and 2,4-hexadiene has chemical formula of C6H10.
Here cyclohexene is cyclic compound whereas 2,4-hexadiene is a linear compound.
Therefore they are known as Ring Chain isomers.
2.
Stereoisomers
Stereoisomers of a compound are differ in three dimensional orientations of their atoms in space. There are two types of stereoisomers; Conformational isomers and Configurational isomer.
The
conformational isomers of compound are formed due to rotation of single
covalent bond. They are interchangeable isomers which are differing in
energies. For example, boat conformation of cyclohexane is having higher energy
whereas chair conformation is having least energy. Both the conformations of
cyclohexane are interchangeable.
Consider
one more example, ethane molecule has two interchangeable conformers which are
known as staggered conformer where the hydrogens has dihedral angle 60 degrees
and eclipsed conformer where the hydrogens has dihedral angle 0 degrees.
B.
Configurational
Isomers:
Configurational
isomers are non-interchangeable stereoisomers. They are of two types: Enantiomers
and Diastereomers.
a)
Enantiomers
Enantiomers
are non-super imposable mirror images of each other. They have ability to
rotate plane polarised light in equal and opposite direction. This property is
known as optical activity of the
compound. In general a compound is optically active if it has asymmetric carbon
atom which is bonded to four different atoms / groups. The asymmetric carbon is
also known as chiral carbon. The enantiomer which rotates plane polarised
light in clockwise direction is known as dextrorotatory.
Whereas the enantiomer which rotates light in counter-clockwise is known as levorotatory. The mixture of enantiomers
where both the compounds present in equal amount does not rotate the light and
this mixture is known as racemic mixture.
The
configuration of a chiral carbon is denoted by R/S system. According to this
naming system, all the groups bonded to the chiral carbon are given priorities
based on atomic number. Highest priority is given as 1 which goes to the
highest atomic number substituent. Similarly, lowest priority is givens as 4
which goes to the lowest atomic number substituent. After that molecule is
viewed in such a way that lowest priority substituent (that is 4) is pointed
away from a viewer. Now a viewer can see two possibilities: if the direction of
priority 1à2à3
is counter-clockwise then the configuration is S-configuration. Whereas if the
priority 1à2à3
is clockwise then the configuration of the chiral carbon is R-configuration.
For example, (S)-lactic acid and
(R)-lactic acid are enantiomers of each other.
b)
Diastereomers
Diastereomers
are non-identical and non-mirror images of each other. If the compound consists
of two or more stereocenters then the diastereomers differ in one or more
stereocenters (but not all). The diastereomers which are differing at only one
stereocenter are known as epimers. For example, (2R,3R)-2,3-dibromopentane and (2R,3S)-2,3-dibromopentane
are non-mirror images and non-identical compounds hence they are known as
diastereomers.
The alkene compounds have diastereomers they are also known as Geometric Isomers. Due to restricted rotation of carbon –carbon double alkene compounds shows isomers. For example, cis-2-butene and trans-2-butene are geometric isomers of each other.
The
cis-isomer is having identical groups on same side, whereas trans-isomer have identical groups on opposite side of carbon-carbon
double bond. The cis / trans naming system is only for
symmetrical alkenes. There is different naming system for unsymmetrical alkenes
that is E/Z notations. The groups bonded to the carbon-carbon double bond are
given priorities as higher / lower
based on their atomic number. If the higher priorities groups present on
same side of the double bond then the alkene is known as Z-alkene. If the
higher priorities groups are on opposite side of the double bond then the
alkene is known as E-alkene. For
example, (Z)-1-chloro-1-propene has
higher priorities groups (Cl and CH3) are on same side of double
bond. In (E)-1-chloro-1-propene
higher priorities groups are on opposite side of the double bond.
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